Climate Change the 2015 Paris Agreement Thresholds and Mediterranean Basin Ecosystems

Given the importance of Mediterranean ecosystems as a hotspot for global biodiversity and the provision of many services to the population, including drinking water, flood protection, carbon storage and recovery, this further increase in temperature is essential. As a result of the Paris Agreement and the resulting climate discussion, the focus has been on two thresholds: limiting the average global temperature increase to 2°C or the more ambitious 1.5°C. But what would these changes mean for different parts of the world? Researchers studied plant ecosystems in the Mediterranean and used pollen grains to create ecosystem models since the last major ice age. They then assessed the impacts of four future climate scenarios, ranging from an increase of 1.5°C to more than 4°C. How different was this region in the past and what changes can be expected in the future? This image shows the modelled biomes for each of the grid points on the landmass of the Mediterranean basin. The data comes from the BIOME4 model, which is run in two different ways. The first two were generated from the “reverse” mode, with pollen core data entered. The following five maps were generated in “before” mode, with climate data inserted to obtain biome types. The Holocene is known for its climatic stability. How does that reflect in this group? Comparison of changes in average temperature and precipitation between 1990 and 2005 (Fig. Global warming above 1.5° Celsius, the ideal limit of the 2015 Paris Agreement, will transform the Mediterranean, producing ecosystems never seen in the past 10,000 years, a new study reports. Distribution of Mediterranean biomes (A) reconstructed (rec) from pollen for the time being; (B) rebuilt from pollen for 4700 years B.P.; (C) currently simulated with the BIOME4 model; and (D to G) for scenarios RCP2.6L, RCP2.6, RCP4.5 and RCP8.5 respectively at the end of the 21st century. For simulations, each point indicates the most common biome in all CMIP5 climate simulations.

The map (H) shows for each point the number of scenarios that differ from the present simulated. Yellow areas indicate when only RCP8.5 is different; the orange zones indicate when RCP4.5 and RCP8.5 are different; the red zones indicate when RCP2.6, RCP4.5 and RCP8.5 are different; and blue circles mark areas where the biome type differs from the current biome type at 4700 BCE. In this graph, the X axis shows the time, but note that the scale changes. It is centered at present, marked as 0 on the axis that represents 2000-2010. On the left, in negative years, are the last 10,000 years of the Holocene, divided into periods of 100 years. On the right, there is the future, which only spans 100 years. The United Nations` December 2015 Paris Framework Agreement on Climate Change aims to keep average global warming well below 2°C above pre-industrial levels. In the Mediterranean region, recent pollen-based reconstructions of climate and ecosystem variability over the past 10,000 years provide information on the effects of warming thresholds on biodiversity and land use potential. We compare scenarios of future climate-related changes in terrestrial ecosystems with the dynamics of ecosystems reconstructed over the past 10,000 years.

Only a warming scenario of 1.5°C allows ecosystems to remain in the variability of the Holocene. At a warming of 2°C or more, climate change will cause changes in the Mediterranean terrestrial ecosystem that are unmatched in the Holocene, a period characterized by recurrent rainfall deficits rather than temperature anomalies. There are two cards “present”. The first, 3A, was generated using the “inverse” BIOME4 model, into which the pollen nuclei entered. This is the same way that the 4700 B.P. card was generated. The second map, 3C, was generated using the “advanced” BIOME4 model, which provided climate data to generate the biomes. This is the method used for future scenarios, and it is the data set that is compared to future scenarios for the last map. The Y axis is the ratio of variation of the five main biomes (temperate coniferous forest, deciduous forest, warm mixed forest, xerophytic bush and steppe) compared to the current distribution.

The simulations for the RcP2.6L and RCP2.6 scenarios do not significantly alter the biome distribution at the end of the 21st century (Fig. 3, D to E). However, for the same period, the RCP4.5 scenario induces the expansion of the desert towards North Africa, the regression of alpine forests and the expansion of Mediterranean sclerophyll vegetation. According to the RCP8.5 scenario, all of southern Spain turns into desert, deciduous forests invade most mountains, and Mediterranean vegetation replaces most deciduous forests in much of the Mediterranean. Figure 3H illustrates the variation from regions without change, regardless of the scenario (stable white areas), to areas where changes from RCP2.6 are already occurring (red zones). As expected, the most sensitive areas are those located on the border between two biomes – for example, in the mountains to the transition between temperate and montane forests, or in the southern Mediterranean to the transition between forest and desert biomes. The map of 4700 BC, in which past changes were among the highest (Fig. 3B), shows the greatest changes in the southwest, eastern steppes and mountains, but these changes are relatively rare. Mediterranean ecosystems are a hotspot for global biodiversity (5) and provide many services to people, including drinking water, flood protection, carbon sequestration and recreation.

Thus, the high vulnerability of ecosystems to climate change can be used as an indicator of the importance of the warming thresholds set in the Paris Agreement for the environment and human well-being. Given the confidence with which past ecosystems and climate change can be reconstructed from many pollen profiles, the development and validation of more reliable numerical models for the ecosystem-climate relationship has become possible. We are applying such an approach to future climate conditions, using simulations from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) for three different greenhouse gas (GHG) propulsion systems [see Tables S2 and (6) for more details]. An analysis of the annual average temperatures for these three scenarios for all climate models on a global and Mediterranean scale, an observational time series and a historical spatio-temporal reconstruction on the Mediterranean scale (Fig. 1) indicate that (i) projected warming in the Mediterranean exceeds the global trend for most simulations; (ii) the first decade of the 21st century. Century has already surpassed the temperature variability of the Holocene; (iii) global warming, but also regional warming, is roughly a linear function of CO2 concentration; and (iv) few simulations provide global warming below 2°C at the end of the 21st century. To link past climate and ecosystem variability to possible future conditions, we use the ecosystem model based on BIOME4 processes (6), which allows a more reliable reconstruction of past climate-vegetation balances compared to correlation techniques. Direct human influences, such as crop cultivation or degradation processes, are not taken into account. For the Holocene, BIOME4 was reversed to generate pixelated climate models in 100-year temporal stages and associated ecosystems (“biomes”) from pollen records (4). For the future, the advanced application of the same model translates into ecosystem distributions based on climate projections (6). The limitations of a relatively simple ecosystem model are more than offset by two factors. First, this method directly links the physical environment, including its seasonal variability, and atmospheric CO2 to plant processes, thus avoiding the strong assumptions of niche models (18).

Second, past observations are analyzed using the same process-based model used for future projections, creating a more coherent evaluation framework. Our analysis shows that in about a century, without ambitious mitigation measures, anthropogenic climate change is likely to transform Mediterranean ecosystems in a way unprecedented in the last 10 millennia. Despite the uncertainties known in climate models, GHG emission scenarios at the level of countries` commitments prior to the UNFCCC Paris Agreement are likely to lead to significant desert expansion in much of Southern Europe and North Africa. The very ambitious RCP2.6 scenario seems to be the only possible way to have a more limited impact. Only the coldest RCP2.6L simulations, which largely meet the 1.5°C target of the Paris Agreement, allow ecosystem changes to remain within the boundaries of the Holocene. .

Tjip de Jong

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