Global Ecosystem Variability/Change and Its Response to Elevated CO2, Global Warming, and Climate Variability—A Study Using the SSiB4/TRIFFID Model

The global ecosystem has experienced dramatic changes at different scales in past decades, especially after the 1980s: a substantial vegetation greening and increase in CO₂ sink (i.e., enhanced gross primary product, GPP) were observed. However, the causes of these changes remain unclear. In this study, we used a biophysical-dynamic vegetation model (SSiB4/TRIFFID) to comprehensively estimate global natural vegetation changes during the period of 1948-2007 and conducted a set of experiments to attribute those changes to elevated CO₂, global warming, and climate variability (mainly precipitation). The spatial and temporal characteristics of simulated natural vegetation fraction coverage, leaf area index (LAI) and GPP were analyzed and confirmed that the simulated climatology, spatial distributions, decadal variabilities, and trends were generally consistent with the available observations.

Our analyses based on observation and the attribution experiments revealed quite different characteristics in ecosystem changes and various causes over different climate zones and ecoregions before and after the 1980s. A most pronounced reverse of the trend of the LAI and GPP changes from negative before the 1980s to positive after the 1980s were found in West Africa, caused by significant precipitation deficit (increase) before (after) the 1980s, which was the dominant factor that caused the ecosystem change. In contrast, western U.S. & Mexico, South America Savanna, and East Africa had positive/negative trend before/after the 1980s, also consistent with the positive/negative precipitation anomalies before/after the 1980s. In the East Asian semi-arid area, the drying trend in precipitation dominated the negative trends of LAI and GPP since the 1950s. In all these regions, the elevated CO₂ effect seemed always to stimulate the growth, while warming temperature generally hampered the growth.

The elevated CO₂ and warming temperature predominated the ecosystem variability in some other areas. In the North America and Eurasia boreal forests areas, the elevated CO₂ and warming temperature enhanced the vegetation growth, producing the positive trend for GPP from 1950-2007 and for the LAI after the 1980s. In the warm tropical rainforest areas, the CO₂ fertilization effect was a dominant factor during 1950-2007 that contributed to the GPP and LAI increase since 1950.

In North Pole and Tibetan Plateau tundra areas, the positive effect of elevated CO₂ and warming temperature compensated the negative effect of precipitation anomaly, resulting in little change in LAI and GPP before the 1980s. After the 1980s, all three factors contributed to the positive GPP and LAI trend there.

Furthermore, we also investigated the change in vegetation fraction for each plant functional type (PFT). It has been found that the total vegetation fraction changes occurred mainly in the transition zones between semi-arid and arid regions as well as high latitudes. While the former areas were mainly controlled by precipitation regime, the high-latitudes was dominated by precipitation only before the 1980s and was controlled by the combination of CO₂ fertilization, increased precipitation, and warming temperature after the 1980s. Meanwhile, the competition of different PFTs also played a role in partitioning the fraction covers among different PFTs.