Just a few degrees can change an ecosystem

The United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement of December 2015 aims “to hold the increase in the global average temperature to below 2°C above preindustrial levels and to pursue efforts to limit the temperature increase to 1.5°C.…” For many regions of the world, achieving the global 2°C target would still imply substantially higher average temperatures, with daily maxima reaching extreme values (1). Recent ~1°C warming has caused damage in many systems already today (2). The degree to which future damage could be avoided by ambitious warming thresholds is uncertain. Regional temperatures in the Mediterranean basin are now ~1.3°C higher than during 1880–1920, compared with an increase of ~0.85°C worldwide (Fig. 1). The difference between (global) warming of 1.5° and >2°C above preindustrial levels is critically important for adaptation policies in the Mediterranean region, notably with respect to land-use systems and the conservation of biodiversity. Simulations with impact models have shed some light on the risks and sensitivities to climate change, but they face limitations when applied at the regional scale and for low levels of warming (3).

To assess the regional effects of different Paris Agreement thresholds for the Mediterranean basin, the Holocene reconstruction of spatiotemporal ecosystem dynamics from pollen allows the development of reliable scenarios of climate-driven change in land ecosystems (4). Despite the effects of human land use, broader-scale past ecosystem dynamics have been mostly driven by regional climate change.

Mediterranean basin ecosystems are a hot spot of the world’s biodiversity (5) and supply numerous services to people, including clean water, flood protection, carbon storage, and recreation. Thus, the broad-scale vulnerability of ecosystems to climate change can be used as an indicator of the importance of the warming thresholds identified by the Paris Agreement for the environment and human well-being. Given the confidence with which past ecosystems and climate change can be reconstructed from numerous pollen profiles, the development and validation of more reliable numerical models for the ecosystem-climate relationship have become possible. We apply such an approach to future climate conditions, using simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5) for three different greenhouse gas (GHG) forcings [see table S2 and (6) for details]. An analysis of the annual mean temperatures for these three scenarios for all climate models at the global and Mediterranean scale, an observational time series, and a spatiotemporal historical reconstruction at the Mediterranean scale (Fig. 1) indicate that (i) the projected warming in the Mediterranean basin exceeds the global trend for most simulations; (ii) the first decade of the 21st century has already surpassed the Holocene temperature variability; (iii) the global warming, but also the regional warming, is approximately a linear function of the CO₂concentration; and (iv) only a few simulations provide global warming lower than 2°C at the end of the 21st century.

Mediterranean land ecosystems are sensitive not only to warming but also to changes in water availability. Even if past variations in precipitation and their projections for the future are spatially more heterogeneous than temperature fields (7), for most scenarios, the changes in both fields will combine to reduce water availability and trigger losses of Mediterranean ecosystems and their biodiversity during the coming decades (8–10). During the Holocene (especially in the second half of this epoch), periods of precipitation deficits have occurred, but in contrast to the 21st-century situation, temperatures did not rise above the present average (Fig. 1) (4). These periods of precipitation deficits [~6 to ~5.2, ~4.2 to ~4, and ~3.1 to ~2.9 thousand years before the present (yr B.P.)] have been identified as possible causes of declines or collapses in civilization in the eastern Mediterranean region (11–13). A recent study (14) has attributed important crop failures in Syria to two strong drought episodes—characterized by a lack of precipitation (reduced by up to 30% in the 6-month winter season) and high temperatures (warming of 0.5° to 1.0°C in the annual mean relative to the 20th-century average)—in the eastern Mediterranean between 1998 and 2010. The 1998–2012 period was the driest of the last 500 years (15). Even if societal factors have likely been the primary causes of these crises (16, 17), Holocene droughts and the associated variability in land productivity may have also played a role, indicating the potential effect of climate change on agriculture-based economies.

To relate the past variability of climate and ecosystems with possible future conditions, we use the process-based ecosystem model BIOME4 (6), which, when compared with correlation techniques, allows a more reliable reconstruction of past climate-vegetation equilibria. Direct human impacts, such as the cultivation of crops or degradation processes, are not taken into account. For the Holocene, BIOME4 was inverted to generate gridded climate patterns by time steps of 100 years and associated ecosystems (“biomes”) from pollen records (4). For the future, the forward application of the same model yields ecosystem distributions from climate projections (6). The limitations of a relatively simple ecosystem model are largely offset by two factors. First, this method directly relates the physical environment, including its seasonal variability, and atmospheric CO₂ to plant processes and thereby avoids the strong assumptions made by niche models (18). Second, past observations are analyzed with the same process-based model that is used for the future projections, thus providing a more coherent framework for the assessment.

Figure 2 indicates reconstructed and estimated shifts in the distribution of major Mediterranean biomes (temperate conifer forest, deciduous forest, warm mixed forest, xerophytic shrubland, and steppe) over time for the past and future, relative to their current distribution. During the Holocene, only up to 15% of the land has had different ecosystems from those existing today at any moment. The 10% level was exceeded only during eight 100-year time slices, and all of these periods occurred before 4200 yr B.P. All of these were relatively humid periods, which became less frequent after this date (4). The past four millennia, and notably the past century, were generally dry compared with the first half of the Holocene.

The future is represented by the following three classes of simulated time-space fields of temperature and rainfall, which are derived from the CMIP5 project (table S2): Representative Concentration Pathway 8.5 (RCP8.5) (22 simulations), RCP4.5 (23 simulations), and RCP2.6 (16 simulations). RCP2.6 approximates the 2°C target. To assess the 1.5°C target, we created a fourth class (denoted RCP2.6L) from selected CMIP5 scenarios [see (6) for details]. Up to 2030, all four classes generate similar ecosystem distributions and generally remain within the bounds of Holocene fluctuations. By the end of the 21st century, RCP2.6L remains in the range of the 80th and 99th percentiles of the Holocene, whereas RCP2.6 simulates Mediterranean ecosystems as they were during the most extreme period of the Holocene (at 4700 yr B.P.), with a change of 12 to 15%.

The limits of the Mediterranean vegetation types defined by biogeographers (19) broadly coincide with our simulated warm mixed forest biome (Fig. 3A), with the following two exceptions: (i) Simulated warm mixed forests also extend to the Atlantic Ocean in the west of France, indicating the inability of BIOME4 to distinguish Atlantic pine forests, and (ii) the narrow strip of Mediterranean vegetation on the Libyan and Egyptian coast, which is below the spatial resolution of our climatic data. The reconstruction for the period of the Holocene (4700 yr B.P.) (Fig. 3B) with the greatest difference from today (Fig. 3A) shows that the main differences are related to the further regression of forested areas in the southern Mediterranean associated with expanding desert areas.

The data behind the figure

This figure shows the modeled biomes for each of the grid points across the Mediterranean Basin land mass. The data come from the BIOME4 model, run in two different ways. The first two were generated from the “inverse” mode, with pollen core data being the input. The next five maps were generated using the “forward” mode, putting in climate data to get out the biome types.

The last map was generated by comparing the differences between certain maps. Note that the colors do not indicate the same information as in the earlier maps.

This figure has a lot of information; see the next three tabs to break it down.

Past and present

The first set of maps to look at are the past and present maps.

There are two “present” maps. The first, 3A, was generated using the “inverse” BIOME4 model where the input is the pollen cores. This is the same way the 4700 B.P. map was generated. The second map, 3C, was generated using the “forward” BIOME4 model, putting in climate data to generate the biomes. This is the method used for the future scenarios, and this is the data set that is compared to the future scenarios for the last map.

How similar are the past and present maps? Are there patterns to where the differences are?

The future

Maps 3D-3G all show future scenarios of what the biomes would look like in the last decade of the century. These cover a range of average global temperature increases from 1.5°C to about 4°C.

How similar are these to each other? To the past and present maps? Are there patterns to where the differences are?

Points of change

The last map shows differences between maps.

What are the patterns of change? What does this indicate about the location and type of changes that might be expected?

Simulations for the RCP2.6L and RCP2.6 scenarios do not change biome distributions much at the end of the 21st century (Fig. 3, D to E). However, for the same period, the RCP4.5 scenario induces desert extension toward North Africa, the regression of alpine forests, and the extension of Mediterranean sclerophyllous vegetation. Under the RCP8.5 scenario, all of southern Spain turns into desert, deciduous forests invade most of the mountains, and Mediterranean vegetation replaces most of the deciduous forests in a large part of the Mediterranean basin. Figure 3H illustrates the variation from areas without any changes, regardless of scenario (stable white areas), to areas in which changes from scenario RCP2.6 already appear (red areas). As expected, the most-sensitive areas are those located at the limit between two biomes—for example, in the mountains at the transition between temperate and montane forest or in the southern Mediterranean at the transition between forest and desert biomes. The map for 4700 yr B.P., in which the past changes were among the highest (Fig. 3B), has the largest changes in the southwest, eastern steppe areas, and the mountains, but these changes are relatively sparse.

Our analysis shows that, in approximately one century, anthropogenic climate change without ambitious mitigation policies will likely alter ecosystems in the Mediterranean in a way that is without precedent during the past 10 millennia. Despite known uncertainties in climate models, GHG emission scenarios at the level of country commitments before the UNFCCC Paris Agreement will likely lead to the substantial expansion of deserts in much of southern Europe and northern Africa. The highly ambitious RCP2.6 scenario seems to be the only possible pathway toward more limited impacts. Only the coldest RCP2.6L simulations, which correspond broadly to the 1.5°C target of the Paris Agreement, allow ecosystem shifts to remain inside the limits experienced during the Holocene.

This analysis does not account for other human impacts on ecosystems, in addition to climate change (i.e., land-use change, urbanization, soil degradation, etc.), which have grown in importance after the mid-Holocene and have become dominant during the past centuries. Many of these effects are likely to become even stronger in the future because of the expanding human population and economic activity. Most land change processes reduce natural vegetation or they seal or degrade the soils, representing additional effects on ecosystems, which will enhance, rather than dampen, the biome shifts toward a drier state than estimated by this analysis. This assessment shows that, without ambitious climate targets, the potential for future managed or unmanaged ecosystems to host biodiversity or deliver services to society is likely to be greatly reduced by climate change and direct local effects.

Supplementary Materials

www.sciencemag.org/content/354/6311/465/suppl/DC1

Materials and Methods

Table S1 and S2

References (21–28)

References and Notes

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29. Acknowledgments: The authors are members of the Observatoire des Sciences de l’Univers Pytheas Institute and the ECCOREV network. This research has been funded by Labex OT-Med (project ANR-11-LABEX-0061), the “Investissements d’Avenir” French government project of the French National Research Agency (ANR), AMidex (project 11-IDEX-0001-02), and the European Union FP7-ENVIRONMENT project OPERAs (grant 308393). We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (table S2) for producing and making their model outputs available. For CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led the development of software infrastructure, in partnership with the Global Organization for Earth System Science Portals. R. Suarez and S. Shi have extracted and preprocessed the model simulations. Holocene climate reconstructions are available at http://database.otmed.fr/geonetworkotmed/srv/eng/search - |54b9bf34-57ae-45ea-b455-9f90351e538f. Future climate projections are available at http://cmip-pcmdi.llnl.gov/cmip5/