Moises Exposito-Alonso

François Vasseur

Wei Ding

George Wang

Hernán A Burbano

Detlef Weigel

Genomic basis and evolutionary potential for extreme drought adaptation in Arabidopsis thaliana

Keywords: climate change, polygenic adaptation, GWA, environmental niche models, random forest, drought, Arabidopsis thaliana, image processing

Because earth is currently experiencing dramatic climate change, it is of critical interest to understand how species will respond to it. The chance of a species to withstand climate change will likely depend on the diversity within the species and, particularly, whether there are subpopulations that are already adapted to extreme environments. However, most predictive studies ignore that species comprise genetically diverse individuals. We have identified genetic variants in Arabidopsis thaliana that are associated with survival of an extreme drought event, a major consequence of global warming. Subsequently, we determined how these variants are distributed across the native range of the species. Genetic alleles conferring higher drought survival showed signatures of polygenic adaptation, and were more frequently found in Mediterranean and Scandinavian regions. Using geo-environmental models, we predicted that Central European, but not Mediterranean, populations might lag behind in adaptation by the end of the 21 st century. Further analyses showed that a population decline could nevertheless be compensated by natural selection acting efficiently over standing variation or by migration of adapted individuals from populations at the margins of the species' distribution. These findings highlight the importance of within-species genetic heterogeneity in facilitating an evolutionary response to a changing climate.

Ongoing climate change has already shifted latitudinal and altitudinal distributions of many plant species1. Future changes in distributions by local extinctions and migrations are most commonly inferred from niche models that are based on current climate across species ranges2,3. Such approaches, however, ignore that an adaptive response can occur also in situ if there is sufficient variation in genes responsible for local adaptation4-6. The plant Arabidopsis thaliana is found under a wide range of contrasting environments, making it distinctively suited for studying evolutionary adaptation to a changing climate7-9. For the next 50 to 100 years, extreme drought events, potentially one of the strongest climate change-related selective pressures10, are predicted to become pervasive across the Eurasian range of A. thaliana2,11. An attractive hypothesis is that populations from the Southern edge of the species' range12 provide a reservoir of genetic variants that can make individuals resistant to future, more extreme, climate conditions12,13. To investigate the potential of A. thaliana to adapt to extreme drought events, we first linked genetic variation to survival under an experimental extreme-drought treatment. By combining genome-wide association (GWA) techniques that capture signals of local and/or polygenic adaptation14 with environmental niche models8,15, we then predicted genetic changes of populations under future climate change scenarios. An unexpected result of our predictions is that populations at both the Northern and Southern margins of the species' range will likely more easily adapt to increased extreme drought events, due to these populations carrying a greater spectrum of drought survival alleles.

Results and Discussion

Differential survival to an extreme drought event

We began by exposing a high-quality subset of 211 geo-referenced natural inbred A. thaliana accessions16 to an experimental extreme drought event during the vegetative phase, which killed the plants before they could reproduce (Table S1). After two weeks of normal growth, plants were challenged by a terminal severe drought for over six weeks and imaged every 2-4 days (Fig. 1A) (see Supplementary Methods section 2). To quantify the rate of leaf senescence, a polynomial linear mixed model was fit to the time series of green pixels per pot (Fig. 1B-D, Video S1). The average genotype deviations from the mean quadratic-term in the model provided the best estimate of this survivorship trait in late stages of drought (Supplementary Fig. 3, see details in Supplementary Methods), ranging from -5 to +5 x 10 -4 green pixels/day 2 . The most sensitive genotypes survived only about 32 days, while the most resilient plants survived about 15 days longer. Genotype-dependent survival probably reflects both constitutive as well as induced drought responses, i.e., both environmentdependent and -independent behaviors of the tested accessions. Additional environments need to be examined in order to disentangle these two types of responses.

The amount of water available during our drought experiment translates to only about 30-40 mm of monthly rainfall, and as expected, accessions with higher survival come from regions with low precipitation during the warmest season (correlation with climate variable bio18 [www.worldclim.org, ref. 17]: Pearson correlation, r=-0.19, p=0.005), and specifically with low precipitation during May and June (r≤-0.19, p≤0.005) (see Fig. 2A). To further exploit current climatic data, we used 19 bioclimatic variables and random forest models18 for environmental niche modeling (ENM) to predict the geographic distribution of the droughtsurvival index across Europe (Fig. 1C). Surprisingly, we found that individuals with higher drought survival were not only likely to be present around the Mediterranean, but also at the opposite end of the species' range in Sweden19 (Fig. 1C, ENM cross-validation accuracy=89%, Table S10). In contrast to the warm-dry Mediterranean climate, Scandinavian dry periods occur on average at freezing temperatures (Supplementary Fig. 12). Consequently, precipitation might occur as snow and soil water content is frozen, thus water is not accessible to plants, producing a physiological drought response20.

Survival across geographically structured population lineages

We then studied whether the different genetic lineages of A. thaliana are locally adapted6 to low precipitation regimes via increased drought-survival. Using an extended panel of 762 A. thaliana accessions (Table S1) we carried out genetic clustering21 and studied population size trajectories22 (Fig. 2). This corroborated the existence of a so-called Mediterranean 'relict' group12 and ten other derived groups of relictual (e.g. Spanish groups) or other (e.g. Central Europe) origin, as an apparent result of complex migration and admixture processes23. A generalized linear model indicated that genetic group membership explained a significant amount of drought-survival variance (GLM: R 2 =12.8%; p=4 x 10 -5 ), with the North (N) Swedish and Northeastern (NE) Spanish groups each having on average higher survival than the other groups (t-test p≤0.01). A population graph estimated by Treemix24 suggested a gene flow edge between the Mediterranean and Scandinavian drought-resistant genetic groups, potentially indicative of historical sharing of drought survival alleles (Fig. 2D). Finally, an ENM of the genetic group membership with climatic variables from the accession's geographic origin confirmed that the most important predictive variable of genetic structure was precipitation during the warmest quarter (bio18), followed by mean temperature of the driest quarter (bio9), and minimum temperature of the coldest month (bio6) (ENM accuracy > 95%. Supplementary Fig. 8 and Table S10). As our results indicate that the deepest genetic split parallels contrasts in local precipitation regimes and ability to survive drought, we expect that decline in rainfall could lead to a future loss of certain genetic groups and/or to turnover of genetic diversity11 (see Fig. 12 Supplementary Fig. 8).

The genomic basis of survival

Because the potential of populations to adapt to drought will ultimately depend on specific genetic variants and the selected trait architecture, we identified drought-associated loci with EMMAX25, a genome-wide association (GWA) method. Although genotype-associated variance25 h 2 was relatively high, 50%, no individual SNP was significantly associated with drought survival (minimum p~10 -7 , after FDR or Bonferroni corrections p>0.05) (Supplementary Fig. 5, Table S3). Significant associations in multiple phenotypes have been detected in similarly powered A. thaliana experiments26. While multiple testing adjustment can over-correct p-values and obscure true associations, the absence of significant associations may also be due to (i) polygenic trait architecture, with many small-effect loci27, and/or (ii) confounding by strong population structure, consistent with the association of drought survival with genetic group membership.

Polygenic signal of adaptation

To test for polygenic adaptation, we repeated the GWA analyses with a model that specifically handles both oligo-and polygenic architectures, BSLMM28. BSLMM estimates, among other parameters, the probability that each SNP comes from a group of major-effect loci. Around half of the top non-significant EMMAX SNPs were found to have over 99% probability of belonging to such a major-effect group (Fisher's exact test of overlap, p=3x10 -7 ; see Supplementary Methods 3.3). We further tested the polygenic hypothesis using the population genetic approach of Berg & Coop14. The test is based on the principle that if populations diverge in a specific trait such as drought-survival that is due to many loci, there should be an orchestrated shift in their allele frequencies. After testing some 60 groups of EMMAX SNP hits of variable size and at different ranks, we detected the most significant signal of polygenic adaptation with the group that included the 151 top SNPs (Table S9). The signal was lost for ranks below the top 300-400 EMMAX SNPs (Table S9). We then compared summary statistics of the top 151 SNPs with background SNPs matched in frequency to avoid GWA discovery biases. The top 151 SNPs showed high F st values, consistent with allele frequency differentiation between populations (Supplementary Fig. 5). Tajima's D values were positive (U Mann-Whitney p<0.05), indicating intermediate allele frequencies at the GWA loci (Supplementary Fig. 5), which could be a result of selection favoring alternative alleles in different ecological niches of the species29. The genomic regions containing the top SNPs did not show any evidence for precipitous reductions of haplotypic diversity, as would be expected for hard selective sweeps30 (Supplementary Fig. 5). Together these patterns fit the expectations of local adaptation from a polygenic trait controlled by some hundred loci31 -a scenario that should enable a fast response to new environmental shifts.

Ancestry associations suggest a Mediterranean origin of survival alleles

During local adaptation, the relevant loci diverge due to natural selection across populations, which generates a statistical correlation with population groups32. In this situation, the default correction of population structure applied in GWA might obscure some of the true associations. There are cases where F st scans can be useful to identify overly divergent loci that could be involved in local adaptation. However, in cases of strong population structure, the mean genome-wide F st is high32, complicating outlier detection (Supplementary Fig. 4). One can recover relevant variants that are deeply divergent across populations and therefore invisible to conventional GWA by first assigning ancestry to each SNP. Using ChromoPainter33, which relies on linkage disequilibrium information, we segmented each genome in question into its different population ancestries (here 11 groups). The first outcome of this analysis was that individuals from NW and NE Spain and, to a lesser extent, the Southern Mediterranean (Fig. 2A), have inherited many DNA segments from relictual individuals (Supplementary Fig. 7). In a generalized linear model framework, we then tested whether the ancestries of individuals at a SNP coincided with the observed phenotypic differences in drought-survival. Performing this "ancestry" genome-wide association (aGWA) and using a permutation correction of p-values (see Supplementary Methods 3.6), we detected 8 distinct peaks (p<0.001, Fig. 3A) including over 1,000 significant SNPs (70 SNPs after linkage disequilibrium pruning) (Table S4). The most prominent peak was located on chromosome 5 and explained over 20% of the variance in drought survival (Table S4). There was no overlap in top SNPs between GWA and aGWA because they search for different association signals. Our aGWA resembles other admixture mapping techniques34, and might be most useful for associations in scenarios of adaptive introgression and local adaptation. Although we do not know yet whether our observations can be generalized, our work demonstrates the power of using alternative GWA approaches in situations where adaptive variation is expected to be tightly linked to population history and structure.

To understand the origin of aGWA-identified SNPs, we constructed trees for all concatenated aGWA SNPs and for genome-wide background SNPs. Although the individuals from both the warm (Iberia and relicts) and cold (Scandinavia) edges of the species distribution are far apart in genome-wide SNPs, they are closely related in droughtassociated SNPs (Fig. 3B). Overall, this is consistent with a common Mediterranean origin of drought-adaptive genetic variants of both Northern and Southern individuals (Fig. 2D, Fig. 3B), and highlights the relevance of populations at the latitudinal extremes of the species range as a possible genetic reservoir for future climate change adaptation12.

Drought survival is a resilience trait independent on phenology

Drought adaptation can be accomplished by diverse mechanisms, with cross-stress resistance being pervasive35. An annual life history enables drought survival through an escape strategy based on the acceleration of the life cycle from germination to flowering and seed production. An alternative strategy, the avoidance strategy, is employed by many xeric perennials with increased water efficiency36. Previous drought experiments with A. thaliana have shown that both strategies exist, although early flowering, which is associated with an escape strategy, was more favourable under water-limiting conditions37,38. In our experiment, drought-survival was not negatively correlated with flowering time in unstressed conditions39 (Pearson correlation, r=0.07, p=0.12). Although a correlation was not significant at the individual ecotype level, the GWA effect sizes of drought-survival for the top 151 SNPs were positively correlated with the ability of the same SNPs to delay flowering (Pearson correlation, r=0.51, p=1x10 -11 , see Supplementary Methods 3.4). Given the described trade-off between escape by flowering and water use efficiency in A. thaliana37,40,41, our drought-survival index might be related to the avoidance strategy, although this needs to be tested with specific physiological experiments (Supplementary Fig. 11, Table S6). Gene enrichment analysis revealed a weak signal for membrane transport (see Supplementary Methods 3.7). Adjustment of osmotic balance through cell membrane transport is a drought avoidance mechanism42 that might also confer cross-tolerance to other abiotic stresses43. Therefore, it might be of relevance for Scandinavian A. thaliana accessions or other populations in extreme environments (Supplementary Fig. 12)19.

Forecast of genetic changes to global warming reveals regional differences in evolutionary potential

It is expected that populations with increased survival to severe abiotic stresses should have an evolutionary advantage in face of the predicted increase in drought frequency and intensity both around the Mediterranean and in Europe, which will constitute a critical hazard for many plants2,11, including A. thaliana. Surprisingly, environmental niche models (ENM) of species distributions, which have been used to predict future changes of species' ranges 2,3, do not usually include information of within-species diversity that can lead to adaptation from standing variation44-46. This could in turn lead to overestimates of extinction rates47-49. By fitting ENMs of current climate with SNP data, using a similar rationale as for the "climate GWA" of Hancock and colleagues7, we attempted to forecast the most likely genetic makeup under current and future climate conditions. We trained one ENM for each of the 151 GWA and 70 aGWA drought-associated SNPs to predict which allele, either the high or the low survival one, is more likely, given a set of environmental variables (all ENM 5CV accuracy >92%; Table S3-4, Supplementary Fig. 13141516). Consequently, from each model, we geographically mapped the potential distribution of the high survival allele using available environmental datasets (www.worldclim.org, ref. 17). Finally, concatenating the resulting 221 maps, we inferred the most likely individual genotype at each location. At present, individuals from both northern and southern edges of the species' Eurasian and N. African range are predicted to harbor more drought-survival alleles than those located in between (Fig. 3C, Supplementary Fig. 1516, with the quadratic term in a regression of allele count on latitude being positive at p=10 -3 ), corroborating our previous observations. Using the trained ENM, we also forecast the distribution of the 221 drought-survival alleles in 2070 (rpc 8.5, IPCC, www.ipcc.ch, ref. 17). While it was expected that populations in the Mediterranean Basin need to become more drought resistant11, our predictions anticipate a greater increase in the total number of droughtsurvival alleles for Central Europe (Fig. 3, Supplementary Fig. 1415). This is because by 2070 rainfall in Central Europe will likely become more similar to that in the Mediterranean2,11 (Supplementary Fig. 12).

Because some drought-survival alleles are currently not present in Central Europe, we speculated that gene migration might be necessary to facilitate adaptation to future conditions50. An underlying assumption of the ENM is that alleles will be present wherever required by the environment, but this assumption of "universal migration" may not be realistic for future predictions if the presence of alleles is currently geographically restricted. We therefore included two geographic boundary conditions in the ENM to generate alternative models that were either more or less "migration-limited" (see Supplementary Methods 4.2). After fitting all possible models and predicting allele distributions with future climate, we calculated the difference of predicted allele presence per map grid cell between the naïve, free migration ENM and the two geographically constrained ones (Fig. 3D-E). If an allele has currently a narrow distribution or is specific to a certain genetic background, its future presence in an area might not be predicted by the constrained models, even though the climate variables coincide with the SNP's environmental range. Such a scenario seems to apply to Central Europe, as the deficit in drought-survival alleles predicted by the free over the constrained models was 8-30% (18-66 out of 221) (Fig. 3E; with the quadratic term in a regression of the allele count difference on latitude being negative at p<10 -10 ). Central European populations may therefore be under threat of lagging adaptation by the end of the 21 st century.

In the end, for a population to persist, not only must drought-survival alleles be present locally, but they also need to increase in frequency51. The chance of this occurring will depend on current local allele frequencies and the strength of natural selection favouring the drought-survival alleles. Therefore, we studied current allele frequencies at three representative locations with the highest sampling density in our dataset (40 samples within a 50 kilometer area): Madrid (Spain), Tübingen (Germany) and Malmö (Sweden), which are near the southern edge, center and northern edge of the Eurasian and N. African range, respectively. Based on ENM predictions, we calculated allele frequency changes from present to 2070. Frequencies are predicted to increase significantly only in the Tübingen population (Student's t test, p<10 -16 , Table S11), but not in Madrid and Malmö, indicating that these two populations might already be adapted to the future local climate. Although not all drought-associated alleles are found in Tübingen (32 of 70 aGWA SNPs and 136 of 151 GWA SNPs), increasing the number of the alleles in single genotypes should be feasible, since there are already single genotypes that have 24 (aGWA) and 123 (GWA) of these alleles (see Supplementary Methods 4.2). Running 50-generations simulations starting at the present Tübingen frequency of each of the drought-survival alleles and assuming a range of selection coefficients, we estimated that a 1-3% of fitness advantage on average would be necessary to increase frequencies to match those of the adapted Madrid and Malmö populations (Supplementary Fig. 17, see Supplementary Methods 4.2). Such selection could take place efficiently when populations are large, as is typical for highly-proliferative weeds51,52.

Conclusion

Leveraging the genetic resources available for A. thaliana, we have begun to address the question of how climate change will affect biodiversity. We provide evidence for the possibility of adaptive genetic variation to extreme drought events from standing variation. Specifically, we found that drought survival in A. thaliana has a polygenic basis and that favorable alleles are more abundant toward the edges of the species' distribution range. Extreme adaptation at range edges might thus be critical for a species' persistence under climate change. Although many aspects of future adaptation are not considered here, namely non-drought related or seasonal climate change51, biotic interactions, phenotypic plasticity, or novel adaptive mutations, our spatially explicit analyses emphasize the potential of adaptive evolution from standing variation to mitigate climate change's detrimental effects.

Methods

Study populations

211 natural inbred lines from the 1001 Genomes project16 were grown in a terminal drought experiment, and 762 lines were analyzed for genetic structure and genome-environment models. These two subsets were selected based on sequence quality and homogeneity of geographic distribution (see Supplementary Methods 1.1). We retrieved the genomes corresponding to the above natural lines from http://1001genomes.org/data/GMI-MPI/ releases/v3.1/ and extracted the biallelic SNPs with >95% calling rate. This resulted in keeping ~4M SNP.

Genetic structure

To understand the genetic structure of Arabidopsis thaliana we ran, on the 762 samples, the software ADMIXTURE v1. 2 (ref. 21) assuming two to 20 groups and using a 5-fold crossvalidation procedure. The number of groups with the smallest cross-validation error was 11 (Fig. 2, Fig. Supplementary Table 1, Supplementary Fig. 8). We computed a genomic PCA using PLINK v1.9 (ref. [START_REF] Purcell | PLINK: a tool set for whole-genome association and population-based linkage analyses[END_REF]. The three first PC axes explained 33.5% of the genomic variance (see Supplementary Methods 3).

We used genomes with probability >0.9 of assignment to one of the 11 ADMIXTURE groups to run MSMC v. 3 (ref. 22). This was done in quartets of genomes, i.e. four genomes for within-population coalescent mode, and two genomes of each of two populations for the cross-coalescent mode (Fig. 2, Supplementary Fig. 5). Using the 11 genetic groups as population lineages, we run Treemix assuming zero to five migration edges24 (Fig. 2, Supplementary Fig. 5)

Terminal drought experiment

Stratified seeds from the selected 211 natural lines were sown in greenhouse pots and abundantly watered every three days during two weeks. Thereafter watering only occurred every three weeks, which dramatically reduced soil water content (Fig. 1, Supplementary Methods 1.2). Top-view photographs of the potting trays were done at 20 timepoints during the whole experiment with a high resolution Panasonic DMC-TZ61 digital camera mounted in a closed black box setting to ensure image consistency (Supplementary Methods 2). Using customized Python scripts and the module Open Computer Vision, we segmented the green plant-leave pixels from the brown soil background to monitor plant area over time (Supplementary Video). Starting from the day with the largest rosettes areas, until the end of the experiment, we modeled the decay of green area (i.e. # pixels) using a polynomial generalized linear mixed model with Poisson link as described in the MCMCglmm R package v.2.25 (see Supplementary Methods 2). The random genotype effects captured the average deviation of each genotype from a general intercept, slope and quadratic curvature. After calculating the heritability of each of the three coefficient deviations and their correlation with the genotype's climate variables of origin, we understood that it was the quadratic curvature that was the most suitable to use as index of survival (Supplementary Methods 2).

Genome-Wide Association (GWA)

Using the index of survival per genotype as the trait and the SNPs with a minimum allele frequency > 5% as predictors (n=879,654 SNPs), we carried out associations using the linear mixed model implemented in EMMAX software25 to find SNPs that excessively contributed to the prediction of survival of genotypes (Supplementary Table 3) (see Supplementary Methods 3.3). To corroborate the identified top SNPs we also performed a Bayesian Sparse Linear Mixed Model (BSLMM) with GEMMA software28. Both approaches, EMMAX and GEMMA, fit a model as: Y = X i β + Zu + є, where Y is the vector of trait values, X is the alternative allele dosage at SNP i and β the allelic effect of SNP i on the trait. Population structure is corrected with a random genotype term (of 211 levels) represented by u, which follows a Multivariate Normal distribution where is A the relationship matrix between all individual genotypes built from SNP information and is the genotypeassociated variance. Different from EMMAX, the BSLMM model considers that the β coefficients follow a mixture of two distributions, one that expects many small effects and another that generates few strong effects.

To determine whether the top SNPs identified in the GWA might have been subject to polygenic adaptation, we used the method from Berg & Coop14. We did this for several groupings of top SNPs and reported the group that yielded the strongest signal (see all results in Supplementary Table 9).

Using painted chromosomes generated using ChromoPainter v. 2.0.7 (ref. [START_REF] Lawson | Inference of population structure using dense haplotype data[END_REF], we carried out another set of associations between the survival trait and the local ancestry category (11 groups) of a chunk of the genome. We used a linear model, Y = μ + Xβ + є, and reported the positions in the genome with the least mean square error (i.e. highest R 2 ) (Supplementary Table 4). To compute p-values, we took an empirical p-value distribution approach based on 1,000 random permutation runs (see Supplementary Methods 3.6). To understand the ancestry of the associated genomic positions, we concatenated the SNP genotypes of the top-associated positions, computed genetic distances between natural lines and generated a Neighbour Joining tree. This tree was compared with a tree built from an equal number of randomly-picked background SNPs.

Genome-wide diversity and selection summary statistics

We calculated genome-wide F st among the ADMIXTURE-defined groups and Tajima's D with PLINK v1.9 (ref. [START_REF] Purcell | PLINK: a tool set for whole-genome association and population-based linkage analyses[END_REF]) and likelihood of a selective sweep with SweeD (ref. [START_REF] Pavlidis | SweeD: likelihood-based detection of selective sweeps in thousands of genomes[END_REF]. We investigated the enrichment of the top SNPs in the upper tail of the distributions of those statistics by calculating a right-tailed t-test in contrast with genome-background SNPs with the same frequency values (Supplementary Fig. 4, Supplementary Table 3, rank columns).

Environmental Niche Models

We used classification and regression Random Forest models implemented in the randomForest R package, available environmental databases www.worldclim.com v.1.4 (ref.

17, 19 bioclimatic variables at 2.5 arc-minutes resolution), and geographic locations of GWA-identified alleles, to fit environmental niche models (ENM). To evaluate model's predictive ability for each allele, we used a 5-fold cross-validation procedure in which ⅘ parts of the data were used to train the model and ⅕ was used to test it. This enabled us to assign a percentage of successful assignment of an allele given the environmental variables at a location (Supplementary Tables 34). The fitted Random Forest model was used to generate potential geographic distributions of survival-associated alleles which, all overlapped, provided a geographic map of density of survival alleles. Using existing predictions of the same 19 bioclimatic variables to 2050 and 2070 under both low (2.6 rcp) and high (8.5 rcp) CO 2 accumulation scenarios, we re-predicted the distribution of alleles to the different future scenarios using the previously fitted Random Forest models. Because of the implicit assumption of free movement of alleles, we generated two additional models per SNP: (1) ENM including the latitude and longitude variables in the Random Forest models and (2) ENM including the three first PC axes geographically modeled with present day climate (see below). By repeating predictions with future climate data, but keeping the latitude, longitude and PC components constant, some alleles would not be predicted in areas where the appropriate environment exists but which are outside of the current geographic distribution (1) or current local genomic background (2) (see Supplementary Methods 4, Supplementary Fig. 13141516).

Apart from the potential distribution of putatively adaptive alleles, we also modeled the geographic distribution of continuous traits, namely the aforementioned PCA components of population structure or the index of survival under drought itself. In those cases the Random Forest was of the regression type and the predictive ability was computed for the test data calculating the squared Pearson's correlation coefficient between predicted and true values (see Supplementary Methods 4).

To complement observations of presence and absence of alleles from ENM predictions, we carried out Wright-Fisher simulations of single biallelic SNPs (for details see Supplementary Methods 4.2.4). We ran simulations for 50 discrete generations. The population size was assumed of 300,000 plants, as inferred from diversity data, and was constant over time. Fitness was only determined by the selection coefficient of the drought alleles, which varied from 0 to 20% in an array of simulation runs. The starting frequency of the allele was set equal to the present day frequency of all natural lines sampled in a given geographic area (e.g., Tübingen). These simulations could be extended in the future to incorporate joint fitness effects from multiple adaptive mutations and complex environment-driven demographic processes (Supplementary Methods 4.2.4).

One-sentence summary

"Future genetic changes in A. thaliana populations could be forecast by combining climate change models with genomic predictions based on experimental phenotypic data."

Figure 1 .

 1 Figure 1. Terminal drought treatment and phenotyping of 211 accessions. (A) Soil water content as measured by sensors in three well spaced experimental trays. Vertical lines indicate dates of image acquisition. (B) Trajectories of total rosette area of 200 randomly chosen pots (see Video S1). Color index according to quadratic parameter in (D). (C) Map projection of the environmental niche model prediction of the quadratic parameter (the drought-survival index) in (D). (D) Decay trajectory modeled with a polynomial regression, with genotypes as random factors, from the day of maximum number of green pixels until the end of the experiment. Each line corresponds to one genotype.

Figure 2 .

 2 Figure 2. Population structure and history of 762 high-quality genomes. (A) Geographic locations and 11 genetic clusters estimated by ADMIXTURE (k=11 having the lowest cross-validation error). Black indicates less than 40 mm of June rainfall (1960 to 1990 average), which corresponds to the amount of water provided in our drought experiment (Fig. 1). Note areas of very low June rainfall in the Mediterranean basin and along the coast in Scandinavia (partially obscured by colored circles). Cape Verde Islands are shown as inset. (B) Principal Component Analysis of genome-wide SNPs. (C) Effective

Figure 3 .

 3 Figure 3. Ancestry GWA of drought survival and environmental predictions.

(

  A) Manhattan plot of SNPs from ancestry GWA (aGWA) after permutation correction of pvalues. Dashed lines indicate significant thresholds at p<0.05, 0.01, and 0.001. (B) Top, Neighbour-joining phylogeny of 1,000 concatenated genome-wide SNPs compared with a phylogeny of all significant aGWA SNPs (ca. 1,000). Colors indicate population clusters (Fig. 2). Relicts and N. Swedish groups are highlighted. Bottom, genetic distances for genome-background SNPs or aGWA SNPs. (C) Environmental niche models of 70 top aGWA SNPs (after LD pruning), trained with climate averages from 1960-1990, and then (D) used to forecast gain or loss of alleles in 2070 under free migration. (E) Discrepancy of alleles that can be gained by 2070 between the geographically constrained (PCA control) model and the free migration model.

Nat Ecol Evol. Author manuscript; available in PMC 2018 June 18.

Acknowledgements

We thank R. Wedegärtner for assistance with the greenhouse drought experiment, I. Henderson for the recombination map, the Petrov, Coop, Ross-Ibarra, Gaut and Schmitt labs for discussions. We thank J. Lasky, X. Picó, A. Hancock, H. Thomassen, T. Mitchell-Olds, J. Mujica, P. Lang, and D. Seymour for comments and the Weigel and Burbano labs for discussion. This work was supported by the President's Fund of the Max Planck Society, project "Darwin" to HAB and by central Max Planck Society funds and the ERC (AdG IMMUNEMESIS) to DW.

designed and carried out the and oversaw the project.

statistical analyses. DW and HAB advised

Code availability

Code for the image analysis pipeline available at http://github.com/MoisesExpositoAlonso/ hippo with DOI: https://doi.org/10.5281/zenodo.1039888, code for ancestryGWA is available at https://github.com/MoisesExpositoAlonso/aGWA with DOI: https://doi.org/ 10.5281/zenodo.1039882, code for Wright-Fisher population simulations at http:// github.com/MoisesExpositoAlonso/popgensim with DOI: https://doi.org/10.5281/zenodo. 1039886.

Data availability

Phenotypic datasets available in the Supplementary Dataset. Processed genome matrices are available at http://1001genomes.org/data/GMI-MPI/releases/v3.1/. Raw reads are stored in the www.ncbi.nlm.nih.gov/sra archive under the ID number: SRP056687.

Supplementary Material

Refer to Web version on PubMed Central for supplementary material.


Leila Amgoud
email: amgoud@irit.fr

Srdjan Vesic
email: vesic@irit.fr

A formal analysis of the outcomes of argumentation-based negotiations (Extended Abstract)

Keywords: I.2.3 [Deduction and Theorem Proving]: Nonmonotonic reasoning and belief revision; I.2.11 [Distributed Artificial Intelligence]: Intelligent agents Human Factors, Theory Argumentation, Negotiation

This paper tackles the problem of exchanging arguments in negotiation dialogues, and provides first characterizations of the outcomes of such rich dialogues.

INTRODUCTION

Negotiation is a process aiming at finding some compromise or consensus on an issue between two or several agents. Since early nineties, the importance of exchanging arguments during negotiation dialogues has been emphasized and several works have been carried out (see [START_REF] Rahwan | Argumentation-based negotiation[END_REF] for a survey). The basic idea is to allow agents not only to exchange offers but also reasons that support these offers in order to mutually influence their preferences, and consequently the outcome of the dialogue. These works are unfortunately still preliminary. Before work [START_REF] Amgoud | A unified and general framework for argumentation-based negotiation[END_REF], it was not yet clear how new arguments may have an impact on the agent who receives them. In [START_REF] Amgoud | A unified and general framework for argumentation-based negotiation[END_REF], it has been shown that the theory of an agent may evolve when new arguments are received. However, there is still no characterization of the outputs of an argument-based negotiation. The notion of optimal solution in such dialogues is unclear. This makes it difficult to evaluate the quality of any dialogue protocol.

This paper characterizes the outputs of an argument-based negotiation dialogue. It distinguishes between local solutions which are optimal solutions at a given step in a dialogue and global solutions which are the ideal solutions. Cite as: A formal analysis of the outcomes of argumentation-based ne- gotiations (Extended Abstract), Amgoud and Vesic, Proc. of 10th Int. Conf. on Autonomous Agents and Multiagent Systems (AA-MAS 2011), Tumer, Yolum, Sonenberg and Stone (eds.), May, 2-6, 2011,

Definition 1. An agent's theory is a tuple T = (O, A = Ae ∪ Ao, R, ≥, F) where O is a set of offers, Ae is a set of epistemic arguments, Ao is a set of practical arguments, R ⊆ A × A is an attack relation, ≥ ⊆ A × A is a partial preorder on A and F : O → 2 Ao s.t. ∪F(oi) = Ao and for all oi, oj ∈ O, if oi = oj , then F(oi) ∩ F(oj ) = ∅.
Arguments are evaluated using a credulous semantics, like stable semantics proposed in [START_REF] Dung | On the acceptability of arguments and its fundamental role in nonmonotonic reasoning, logic programming and n-person games[END_REF].

Definition 2. A set E ⊆ A is a stable extension of a theory T = (O, A = Ae ∪ Ao, R, ≥, F) iff: i) a, b ∈ E s.t. aRb, ii) ∀a ∈ A \ E, ∃b ∈ E
such that bRa and not (a > b). Let Ext(T ) be the set of all stable extensions of T .

A status is associated to each offer as follows.

Definition 3. Let T = (O, A = Ae ∪ Ao, R, ≥, F) be an agent theory and o ∈ O. The offer o is acceptable iff ∃a ∈ F(o) s.t. a ∈ E, ∀E ∈ Ext(T ). It is rejected iff F(o) = ∅ and ∀a ∈ F(o), E ∈ Ext(T ) s.t. a ∈ E. It is non-supported iff F(o) = ∅. It is negotiable otherwise. Let Oa(T ) (resp.
Or(T ), Ons(T ), On(T )) denote the set of acceptable (resp. rejected, non-supported, negotiable) offers in theory T .

It is easy to check that O = Oa(T ) ∪ Or(T ) ∪ On(T ) ∪ Ons(T ). From this partition, a basic ordering on the set O (i.e.

⊆ O × O) is defined. The idea is that any acceptable offer is preferred to any negotiable offer, any negotiable offer is preferred to any non-supported offer which in turn is preferred to any rejected offer. We abuse notation and write for instance Oa(T ) On(T ). R,≥,F) be an agent theory. Oa(T ) On(T ) Ons(T ) Or(T ) hold.

Definition 4. Let T = (O, A = Ae ∪ Ao,

NEGOTIATION OUTCOMES

We assume that negotiation takes place between two agents, denoted by Ag1 and Ag2. Each agent Agi is equipped with a theory Ti = (O, Ai, Ri, ≥i, Fi) which is used for computing the preference relation i on the set O. The set Ai is a subset of a universal set AL of arguments built from a logical language L. Relation Ri is a restriction of RL on Ai where RL ⊆ AL × AL. However, we assume that ≥i is defined over the whole set AL. The two agents are supposed to share the same set of offers. In order to define the outcomes of a negotiation, we need to define the notion of dialogue. Note that at each step t of a dialogue, the theory of each agent may evolve. The original set of arguments is augmented by the new arguments received from the other party, and the attack relation is modified accordingly. We denote by

T t i = (O, A t i , R t i , ≥ t i , F t i )
the theory of agent i at a step t of a dialogue and T 0 her theory before the dialogue.

The following property shows that the theory of an agent does not change in case of non-argumentative dialogues.

Property 1. If a dialogue d = (m1, . . ., m l ) is non- argumentative, then ∀j ∈ {1, . . . , l} it holds that 0 1 = j 1 and 0 2 = j 2 .
Let us now analyze the different solutions of a dialogue. The best solution for an agent at a given step of a dialogue is that which suits best her preferences. Definition 6. An offer o ∈ O is an accepted solution for agent Agi at step t of a dialogue d iff o ∈ Oa(T t i ).

Note that an offer may be accepted for one agent but not for the other. Such offer is certainly not a solution of the dialogue. A local solution at a given step is an offer which is accepted for both agents at that step. We use the term "local" because such an offer is accepted locally in time -it may have been rejected before, or may become rejected after several steps. Such a solution does not always exist.

Definition 7. An offer o ∈ O is a local solution at a step t of dialogue d iff o ∈ Oa(T t 1 ) ∩ Oa(T t 2 ).
Note that a local solution is not necessarily reached in a dialogue i.e. it is not necessarily the dialogue outcome. In order to be so, an efficient dialogue protocol should be used. The following result characterizes the situation where there exists a local solution.

Property 2. There exists a local solution iff there exist sets of arguments

A 1 ⊆ A 0 1 and A 2 ⊆ A 0 2 s.t. Oa(O, A 0 1 ∪A 2 , R1, ≥1, F1) ∩ Oa(O, A 1 ∪A 0 2 , R2, ≥2, F2) = ∅.
The next result studies the situation when agents do not have to agree on everything but they agree on the arguments related to a given part of the negotiation, which is separated from other problems. If the first agent owns more information than the second, then there exists a dialogue in which the second will agree with the first one.

Property 3. Let A ⊆ A 0 1 ∪ A 0 2 be s.t. ≥1 | A =≥2 | A and let A be not attacked by arguments of (A 0 1 ∪ A 0 2 ) \ A . If A 0 1 ∩ A ⊇ A 0 2 ∩ A and ∃a ∈ F(o) ∩ A 0 1 ∩ A s.t. a is accepted in T 0
1 then there exists a negotiation dialogue d = (m1, . . . , m l ) s.t. o is a local solution at step t. 1 Let m = (x, y, z) be a move. If y = θ (resp. z = θ), this means that no argument (resp. no offer) is uttered.

The next result studies the case when ≥ is complete and antisymmetric. In this case, we provide a condition under which there exists a local solution.

Property 4. Let ≥1 and ≥2 be complete and antisymmetric preorders. If there exist sets The two previous solutions are time-dependent. An offer may, for instance, be a local solution at step t but not at step t + 1. In what follows, we propose two other solutions (one for a single agent and one for a dialogue) which are not timedependent. They represent respectively the optimal solution for an agent and the ideal solution of a dialogue. An offer is an optimal solution for an agent iff she would choose that offer if she had access to all arguments owned by all agents.

A 1 ⊆ A 0 1 and A 2 ⊆ A 0 2 , ∃o ∈ O, ∃a1 ∈ (A 0 1 ∪ A 2 ) ∩ F(o), ∃a2 ∈ (A 0 2 ∪ A 1 ) ∩ F(o
Definition 8. An offer o ∈ O is an optimal solution for agent Agi iff o ∈ Oa(T ) where T = (O, A 0 1 ∪ A 0 2 , Ri, ≥i, Fi) with Ri ⊆ (A 0 1 ∪ A 0 2 ) × (A 0 1 ∪ A 0 2
). The following property shows that if an offer is optimal for an agent, then there exists a dialogue in which that solution is accepted for that agent at a given step.

Property 5. If o is an optimal solution for an agent, then there exists a dialogue d = (m1, . . . , m l ) s.t. o is accepted for that agent at step l.

If both agents agree when all information has been exchanged, they can obtain an ideal solution.

Definition 9. An offer o ∈ O is an ideal solution iff o ∈ Oa(O, A 0 1 ∪ A 0 2 , R 0 1 ∪ R 0 2 , ≥1, F1) ∩ Oa(O, A 0 1 ∪ A 0 2 , R 0 1 ∪ R 0 2 , ≥2, F2).
The next property shows that if an ideal solution exists, then it is a local solution for a dialogue. It is natural to expect that for two agents with same beliefs and goals an exchange of arguments can ameliorate the chance of finding a solution. Moreover, if the first agent has more information, he can influence the second one.

Property 7. Let ≥1=≥2, A 0 1 ⊇ A 0 2 . If o is an accepted solution for Ag1 at step t = 0, then o is an ideal solution.

  Definition 5. A negotiation dialogue is a finite sequence of moves d = (m1, . . . , m l ) s.t. mi = (xi, yi, zi), where xi is either Ag1 or Ag2, yi ∈ AL ∪ {θ}, zi ∈ O ∪ {θ} 1 , and yi = θ or zi = θ. If ∀i = 1, . . . , l, yi = θ, then d is said non-argumentative. It is argumentative otherwise.

Property 6 .

 6 If o is an ideal solution then there exists a dialogue d = (m1, . . . , m l ) s.t. o is a local solution at step l.

  ), s.t. odd chain of attacks x1RLx2, x2RLx3, . . . , x 2k+1 RLa1 with x1, x2, . . . x 2k ∈ A 0 1 ∪ A 2 and x1 >1 x2 >1 . . . >1 a1 and odd chain of attacks y1RLy2, y2RLy3, . . . , y 2k+1 RLa2 with y1, y2, . . . y k ∈ A 0 2 ∪ A 1 and y1 >2 y2 >2 . . . >2 a 2k , then there exists a local solution.


domain: sde shs

text:
Shinichiro Asayama
email: asayama.shinichiro@nies.go.jp

Kari De Pryck
email: kari.depryck@unige.ch

Silke Beck

Béatrice Cointe

Paul Edwards

Hélène Guillemot

Karin Gustafsson

Friederike Hartz

Hannah Hughes

Bård Lahn

Three institutional pathways to envision the future of the IPCC

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Three institutional pathways to envision the future of the IPCC Shinichiro Asayama, Kari De Pryck, Silke Beck, Béatrice Cointe, Paul N. Edwards, Hélène Guillemot, Karin M. Gustafsson, Friederike Hartz, Hannah Hughes, Bård Lahn, Olivier Leclerc, Rolf Lidskog, Jasmine E. Livingston, Irene Lorenzoni, Joanna Petrasek MacDonald, Martin Mahony, Jean Carlos Hochsprung Miguel, Marko Monteiro, Jessica O'Reilly, Warren Pearce, Arthur Petersen, Bernd Siebenhüner, Tora Skodvin, Adam Standring, Göran Sundqvist, Renzo Taddei, Bianca van Bavel, Mark Vardy, Yulia Yamineva and Mike Hulme

The IPCC has been successful at building its scientific authority but it will require institutional reform for staying relevant to new and changing political contexts. Exploring a range of alternative future pathways for the IPCC can help guide crucial decisions about redefining its purpose.

With the release of its Synthesis Report in March 2023, the IPCC concluded its Sixth Assessment Report (AR6)-its busiest, longest, and perhaps most challenging cycle given the major disruptions caused by the COVID-19 pandemic. The IPCC will soon commence its Seventh Assessment Report (AR7) cycle, beginning with the election of the new Bureau in July 2023. As with previous assessment cycles, this is a moment to rethink the work of the institution and discuss how it can be reformed to remain fit for purpose [1].

At the beginning of the AR6 cycle in 2015, the incoming chair, Hoesung Lee, stated his ambition for the IPCC to take a 'solution turn', shifting its focus from assessing the scientific basis of climate change to offering a range of policy solutions [2]. After July, the new chair of the IPCC will also have important choices to make in guiding its future direction during the AR7 cycle.

There has been no shortage of previous proposals for reforming the IPCC [3,4]. These have reflected different expectations for the role the IPCC should play in the complex web connecting science with policy and how it can best serve decision-makers. Due to institutional constraints, however, these proposals usually end up as a wish list. Structural inertia and power asymmetry within the IPCC make it hard to imagine the institution structured in radically different ways from its current form. Nevertheless, its 35 years of existence teach us that the IPCC's success hinges on its capacity to adapt its practices and procedures to changing political contexts and demands [5].

A critical decade of climate action awaits a new opportunity for IPCC reform. To avoid a situation in which its reports become out of date or not relevant to policy action by the time they are published, the IPCC needs to find ways to respond effectively to the rhythm of politics and help catalyse social change.

Here, we present three possible pathways for the future of the IPCC as it confronts new challenges that lie ahead. Whichever pathway is pursued, it will be faced with the challenge of maintaining scientific authority while pursuing policy relevance.

Five institutional pillars

The IPCC offers the most authoritative source of consolidated knowledge on climate change. Its success inspired the establishment of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) in 2012 and it is often seen as a 'gold standard' for global scientific assessment. There are frequent calls to create IPCC-like assessment bodies for issues such as artificial intelligence [6].

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This success was not assured when the IPCC was established in 1988. Its own history is filled with numerous controversies and frequent challenges to its credibility and legitimacy. In its first few assessment cycles, the IPCC gained its authority by carefully adapting its mandate to changing policy needs, managing the expectations of different audiences and responding to the rapidly evolving science and media landscape [7].

Drawing upon our recent collective work critically assessing the IPCC [5], we argue that the IPCC's authority rests on five institutional pillars: (1) comprehensive assessment; (2) policy neutrality; (3) diverse participation; (4) rigorous procedure; and (5) governmental ownership (Table 1). These have become the institutional backbone of the IPCC that structures its epistemic and diplomatic culture [8].

The IPCC's authority is also conditional on it being a 'learning organisation', able to adapt to changing social and political contexts. In each assessment cycle, the IPCC has reflected on its work and structure and this internal evaluation serves as a guide for its institutional learning. However, this learning by the IPCC has so far remained largely incremental rather than transformational.

For example, early in 2010, the IPCC was faced with an 'institutional crisis' caused by public criticism over errors in its Fourth Assessment Report (AR4) and a full external evaluation of its procedure was conducted by the InterAcademy Council (IAC). This could have been a 'constitutional moment' for the IPCC to revisit and redefine its mandate, assessment framework and institutional arrangements (ref. [5], Ch. 6). In response to the IAC recommendations, the IPCC did strengthen its rules of procedure, for example by improving the standardisation of uncertainty language and adopting a conflict-of-interest policy (ref. [5], Ch. 3). This helped the IPCC survive the crisis and retain its authority. And yet the IPCC missed an opportunity for more fundamental change.

The IPCC's tendency to incremental change is largely because every decision about its mandate, structure and work programme must be agreed upon by all member governments. For example, discussions at the 57th plenary session held in September 2022 raised the question of potential reforms to its structure for the next cycle [START_REF]Earth Negotiations Bulletin -Summary of the 57th Session of the Intergovernmental Panel on Climate Change[END_REF]. Despite bold suggestions from Bureau members and a strong appetite among many governments and observers for reforming the organisation, no convergence of views emerged because a few governments favoured the status quo, expressing their satisfaction with its current structure. This does not mean that the IPCC is doomed never to escape its past, but overcoming this structural bias to incrementalism is not easy. It requires pressure from inside and outside, going beyond conventional thinking to imagine alternative pathways toward more radical change.

Three future pathways

The coming decade brings new challenges to the IPCC. It will come under growing political pressure to inform the climate solution space [START_REF] Schipper | [END_REF]. The IPCC is expected to support the implementation of the Paris Agreement and in particular its periodic global stocktakes, the first round of which will close in 2023. If the IPCC aims to align its assessment cycle with each stocktake cycle, the current seven-year cycle will have to be cut short. This will place a significant burden on its assessment process, but might be necessary if the IPCC wants to strive for policy relevance.

To envision the future of the IPCC, it is useful to explore a range of alternative futures of the IPCC operating under a new mandate and with different structures. An exploration of such alternative futures can help stimulate the broader public debate about what kind of change is needed for the IPCC to enhance the science-policy interface.

In IPCC assessments, scenarios are used to explore a wide range of alternative future pathways of climate and development under different assumptions [START_REF] Chen | Climate Change 2021: The Physical Science Basis[END_REF]. This scenario approach to future thinking is also applicable to envisioning its own future. Here, we present three speculative scenarios for reforming the IPCC: (1) "Building on Success"; (2) "Diversifying Viewpoints"; and (3) "Advocating for Change" (Table 2). The latter two scenarios might be less likely to be realised due to structural inertia within the IPCC, yet articulating them helps to clarify some of the pathways the IPCC could alternatively follow. These scenarios each make different assumptions about the five key institutional pillars (Table 1). Each scenario therefore has different conceptions of where the credibility and legitimacy of the IPCC will come from-either from a continued striving for neutrality, a stronger emphasis on diversity, or an explicit recognition of responsibility. However, the role of social sciences and humanities in the assessment remains an open issue across all scenarios (see Box 1).

The characteristics of the IPCC in each scenario (Table 2, point c) can be described by different approaches to climate knowledge and three ideal types of science advice in policy [START_REF] Pielke | The Honest Broker: Making Sense of Science in Policy and Politics[END_REF]: a 'science-first' approach and a 'science arbiter' (Building on Success); a 'cosmopolitan' approach and an 'honest broker' (Diversifying Viewpoints); and a 'transformative' approach and an 'issue advocate' (Advocating for Change).

It must be noted that there is no 'right way' for reforming the IPCC. Each scenario poses new kinds of political challenges to the IPCC, which might undermine the public perceptions of its authoritative status or policy relevance. These scenarios offer some light on the challenges that the IPCC will face when seeking to keep a delicate balance between retaining scientific authority and pursuing policy relevance.

A fork in the road

The institutional life of the IPCC has been and will continue to be shaped by changing politics and cultural shifts in societies. This embeddedness of the IPCC's existence can be portrayed using the metaphor of 'a ship on the ocean'-a ship that is set afloat and that navigates an ocean with dynamic currents by regularly adapting its sailing techniques and finding new pathways on a sea chart. This metaphor suggests that the IPCC (the ship) cannot be isolated from the wider social and political context in which it evolves (the ocean). It also tells us that changing ocean dynamics may need a new ship design.

The success of the IPCC has led to a situation in which it is increasingly called upon to meet a growing number of diverging expectations from different audiences. But the IPCC cannot be all things for all people. This is because of its limited human resources and institutional capacity, but also because of competing worldviews on the role of the IPCC, namely, an unresolvable conflict between conducting neutral assessments and engaging in policy advocacy and the incommensurability of reaching consensus among different value systems and knowledges.

Regardless of the future pathway that the IPCC takes, these trade-offs are impossible to ignore.

Table 1. Five institutional pillars of the IPCC's authority

Institutional pillar

Description

Comprehensive assessment

The IPCC's mandate is to conduct comprehensive assessments of the scientific literature on the scientific basis of the risk of human-induced climate change, its potential impacts and options for adaptation and mitigation. This mandate serves as the basis of its institutional structure with three independent Working Groups (WGs). Thousand-pages-long comprehensive Assessment Reports (ARs) and dedicated Special Reports (SRs) on specific issues are key products of the IPCC and the embodiment of its scientific credibility.

Policy neutrality

Since its inception, the IPCC's work has been governed by the principle of 'policy relevant but not policy prescriptive'. This neutrality principle is considered by the Panel of central importance to maintaining its credibility and legitimacy, and to securing public trust. Any statement by the IPCC that is perceived to be prescriptive could be accused of breaching this principle and of therefore undermining its authority. The guise of policy neutrality thus acts as a 'firewall' to protect the IPCC against such criticisms.

Diverse participation

From its early days, the IPCC has sought to facilitate the involvement of developing countries by financially supporting their participation as authors or government representatives. The IPCC has expressed an increasingly strong desire to secure diverse representation among its authors across expertise, gender, geography and experience. More recently, for the purpose of capacity-building, it has also promoted the inclusion of early-career researchers. Although meaningful participation from developing countries is mixed and uneven, advancing diversity is considered essential to the legitimacy of the IPCC as the voice of global climate knowledge.

Rigorous procedure

Clear, meticulously defined rules of procedure are widely regarded as a bedrock of the credibility of the IPCC. In responding to criticisms, the IPCC has routinely strengthened its assessment procedures to enhance the robustness of its reports. The highlight of this process is a two-tier peer review, both by experts outside the IPCC and by government representatives. An extensive, open and transparent review of the scientific literature underlies the scientific rigour of its assessment. The IPCC also takes great care in calibrating the use of uncertainty language to communicate the main conclusions of its report.

Governmental ownership

The IPCC is governed by its member states who contribute to the assessment process by electing members of the Bureau, approving the report outline, nominating authors, and approving the reports' Summaries for Policymakers (SPMs). The line-by-line approval of SPMs is a unique institutional feature of the IPCC. While this gives state governments considerable influence over its work, it is widely recognised as a key mechanism for ensuring the IPCC's legitimacy and generating a shared ownership of its report.

Note: The description of five institutional pillars is based on the relevant chapters (Ch. 3,5,6,7,8,[START_REF]Earth Negotiations Bulletin -Summary of the 57th Session of the Intergovernmental Panel on Climate Change[END_REF][START_REF] Schipper | [END_REF][START_REF] Chen | Climate Change 2021: The Physical Science Basis[END_REF][START_REF] Pielke | The Honest Broker: Making Sense of Science in Policy and Politics[END_REF][START_REF] Klinsky | [END_REF]17,20 and 21) in ref. [5].

Table 2. Three scenarios of future pathways for reforming the IPCC 162

Scenario Description of underlying logic (a), institutional form (b), main characteristics and challenges (c)

Building on Success

(a) This is what some may call a 'business as usual' scenario. It assumes that the IPCC is an already well-functioning institution. By reason of past successful experience, it is considered to be able to retain its scientific authority by adapting to new political demands without major institutional reform.

(b) While there are some minor reforms in its assessment framework or work programme, the institutional structure largely remains intact. The IPCC continues to put a high priority on comprehensive ARs and SRs produced by the three existing WGs. It preserves policy neutrality as its governing principle, with a focus on communicating consensual and universal statements. It also continues to commit to improving (though, incrementally and instrumentally) the disciplinary, regional and gender balance in all aspects of its work, as well as to involving more earlycareer researchers. No major procedural change is introduced to the review process. The approval of SPMs follows current diplomatic practices, leaving government representatives with substantial influence over its outcome.

(c) Overall, this scenario is characterised by the institutional form that favours the status quo, inherited from past configurations. It is beneficial for those who already have established tenure within the institution, but it perpetuates uneven participation and knowledge hierarchy with marginal engagement in social sciences and humanities. This is called a 'science-first' approach to climate knowledge, taking the role of a 'science arbiter' that answers a specific policy question. The major challenge for this scenario is the growing policy irrelevance of the IPCC's mega-assessments. In this crucial decade for action, a large comprehensive assessment published every seven or more years is of little use for policymaking.

Diversifying Viewpoints

(a) This scenario assumes that the IPCC recognises the need to reflect different social values in its assessment for providing policy-relevant knowledge to a wide range of stakeholders and rights holders. As the issues of equity and justice are of growing political significance [START_REF] Klinsky | [END_REF], the pursuit of diversity, equity and inclusion (DEI) is understood as fundamental for its credibility.

(b) While the mandate of comprehensive review and the three WGs structure remain unchanged, DEI is espoused as a new governing principle in tandem with policy neutrality. The IPCC expresses a strong commitment to fostering greater inclusion of experts from diverse backgrounds across age, gender, race, religion, socioeconomic status and region. It also introduces procedures aimed at better incorporating social sciences and humanities, and various forms of knowledge from Indigenous communities, civil society and the private sector to promote 'epistemic equality' [14]. While governmental ownership remains key, a wider range of societal stakeholders is allowed to offer their own voices during SPM approvals.

(c) Overall, the institutional form of the IPCC in this scenario is characterised by what is called a 'cosmopolitan' approach to climate knowledge with a strong emphasis on epistemic and ontological diversity [15]. The focus is more on agreeing to disagree than on seeking a consensus above all. Abiding by the ethos of value pluralism, the IPCC plays the role of an 'honest broker' to expand a range of policy options.

The big challenge for this scenario lies in shifting away from the existing procedures of peer review and establishing new ones to better capture non-peer-reviewed, non-English and grey literature. A question is also raised about who are considered 'legitimate' stakeholders in the SPM approval and how to prevent the approval process from being dominated by a few powerful state governments and interest groups.

Advocating for Change

(a) The assumption of this scenario is that under new political circumstances, the IPCC in its current form is not fit for purpose. It therefore requires institutional transformation to guide transformational change in societies [16]. The IPCC's legitimacy is considered to rest on driving real change in policy.

(b) The mandate of comprehensive assessment is no longer a priority; it rather privileges the production of SRs (or similar) on specific issues to help effective policy implementations at different levels. The WGs are dissolved and replaced by several (both standing and ad-hoc) expert groups that cover a range of policy-relevant topics and are tasked to produce reports in a nimble and timely manner. It relinquishes the neutrality principle and does not shy away from engaging in evidencebased policy advocacy by embracing social responsibility [17] as a new governing principle. It recognises diverse and inclusive participation as essential for catalysing political change. The review procedure is renewed to better integrate practical forms of knowledge based on realworld experiences. Finally, it shifts governmental ownership towards a broader ownership model in which government representatives approve the SPMs with non-governmental stakeholders on an equal footing.

(c) Overall, this scenario characterises the IPCC as a 'transformative' approach to climate knowledge, directly linked to political actions [18]. The role of the IPCC becomes an 'issue advocate' for transformational and equitable change. It requires a far-reaching overhaul of its institution, redefining its purpose as responsible policy advocacy. But such institutional overhaul is a tall order. A grave challenge for this scenario is overcoming both institutional and psychological resistance from governments and experts to radical change, as well as finding common ground among diverse stakeholders for the SPM approval.

Note: The narrative description of each scenario consists of three elements: (a) the underlying logic of scenario assumptions; (b) the description of the IPCC's institutional form in scenarios; and (c) the main characteristics and challenges of scenarios.

Box 1. Social sciences and humanities in the IPCC

The IPCC has long been criticised for creating an epistemological hierarchy placing physical science at the top of the pyramid as the 'strongest' type of knowledge, largely ignoring contributions from social sciences (except economics) and humanities [19]. The dominance of physical science in the IPCC is reflected in the structure of three WGs: starting from WGI on the physical science basis to WGII on impacts, adaptation and vulnerability, and WGIII on mitigation. This structural linearity implies that advances in climate science lead to better understandings of potential impacts, which is a prerequisite for better adaptation and mitigation policies.

To respond to the narrow scope of IPCC assessments, social sciences and humanities can help pluralise epistemic viewpoints and expose the ethical, cultural and political dimensions of climate change. These are crucial for understanding what drives or obstructs effective adaptation and mitigation action [START_REF] Schipper | [END_REF].

To more explicitly recognise the social science and humanities contributions, it has been proposed to add a fourth WG on 'historical, cultural, and social contexts' [19]. While this could provide more space for such knowledge, it might also perpetuate the division between WGs and further increase bureaucratic bungling. A better integration of social sciences and humanities needs to come with a recognition of the uneven distribution of capacities across regions and of the inherent worldviews underpinning such knowledge.

Also, tight governmental oversight allows only politically non-controversial statements to make it through the SPM approval [20]. Without major reform to loosen government control over its assessment process, there is little room to address the most controversial policy-relevant questions within the IPCC (e.g., the governance of solar geoengineering technologies or the role of state and non-state actors in climate disinformation and obstruction).

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This work partly originates from a workshop organised by National Institute for Environmental Studies in April 2023, where S.A., K.D.P. and M.H. participated. S.A. acknowledges the financial support from the Japan Society for the Promotion of Science, Grants-in-Aid for Early-Career Scientists [20K20022]. M.H. acknowledges the financial support from the Department of Geography, University of Cambridge.

Author information

The authors declare no competing interests.