Many studies in recent years have examined these changes in various remote sensing datasets and found that approximately the land area of Poland is added as leaf area in global ecosystems each year. This phenomenon is referred to as the so-called “Earth's greening trend." These increases in leaf area and photosynthetic rates indicate that also more CO2 is being absorbed from the atmosphere and stored in ecosystems, representing the important land carbon sink of anthropogenic CO2 emissions.

However, changes in leaf area and atmospheric CO2 concentration are mutually dependent. On one hand more, leaf area means more photosynthesizing surface area and thus more CO2 uptake; on the other hand, increased CO2 fixation means more resources are available to plants, which will then be invested in the formation of more leaves. A scientific team led by Dr. Alexander Winkler (Max Planck Institute for Meteorology, MPI-M) used satellite observations and Earth system model simulations to investigate this bidirectional causal relationship in more detail and use it to reduce the high uncertainties in predictions of carbon uptake by plants in response to rising CO2 (Winkler et al., 2019).

In a follow-up study, Winkler et al. have now investigated in more detail the mechanisms underlying the observed greening patterns. Why is the leaf area of the Earth increasing in the first place? Scientists debate about which drivers are behind the Earth’s greening trend and in what proportion. Climatic changes induced by rising greenhouse gas concentrations are thought to play a key role. For example, in the high northern tundra and boreal forest ecosystems, amplified warming in recent decades appears to be stimulating higher plant activity. The productivity of ecosystems in the north is mainly limited by too short growing seasons and low temperatures. The warming of the Arctic seems to relax this limitation on productivity, manifested mainly in an increase in the length of the annual growing season, but also in a general increase in plant activity. Other debated drivers of the observed greening patterns are fertilization effects due to increased atmospheric nitrogen deposition, intensified land use practices, and the general re-growth of temperate forests. But the main driver being discussed is the "CO2 fertilization effect". As the atmospheric CO2 concentration increases, so does the CO2 concentration inside the leaves of plants. Plants can increase their light-use and water-use efficiency through various physiological mechanisms so that, in general, rising CO2 concentrations stimulate an increase in photosynthetic rates.


Figure 1: Natural vegetation shows opposing leaf area index trends with increasing CO2 (LAI short for: leaf area index/leaf area index). Green coloring indicates areas where vegetation density or leaf area index has increased over the period considered (1982-2017). Reddish colors, on the other hand, indicate areas where leaf area index has decreased with increasing CO2. Areas without significant changes are shown in gray. White areas represent agricultural land or ice sheets.

These drivers, in particular the effects of climate changes as well as the CO2 fertilization effect on ecosystems, need to be better understood globally in order to better assess the land sink of anthropogenic CO2 emissions and thus the future evolution of atmospheric CO2 concentrations. In collaboration with an international team of developers of the world's leading land surface models, all of which represent the key processes in one way or another, Winkler and his colleagues at MPI-M and Boston University disentangled the mechanisms underlying the satellite-observed greening in Earth's major ecosystems. Using so-called "factorial" experiments, in which the drivers of greening can be turned on and off, it is possible to estimate the individual contribution of the two postulated main drivers, i.e., the effect of climate changes and the CO2 fertilization effect. In addition to the simulations with the land surface models, these factorial experiments were performed with the fully coupled Earth system model of the MPI-M (MPI-ESM) on the high-performance computer Mistral at the German Climate Computing Center (DKRZ). This approach also allowed Winkler et al. to assess how strongly the internal variability of the climate system which only occurs in a coupled Earth system model affects greening.

The analyses of the latest and updated satellite datasets show that Earth’sgreening trend seems to stagnate, and in some ecosystems even a decline in leaf area is observed. In particular, leaf area is decreasing in the pan-tropical green belt of dense vegetation. The models, whether as stand-alone land surface models driven by observed climate forcing, or as interactive sub-models in an Earth system model, do not reproduce the extent of observed leaf area loss. However, in the factorial experiments, where the CO2 fertilization effect has been "turned off" but climate changes are "turned on", the models simulate leaf area losses in the tropical forests comparable to observations. Accordingly, climate changes in tropical forests lead to a loss of leaf area, which is compensated in the models by leaf area gain caused by the CO2 fertilization effect.

An examination of rainfall patterns indicates that leaf area loss is most likely associated with long-term drying and recurrent droughts. Central African humid forests emerge as a hotspot in this context. Since the 1970s a long-term drying trend has been observed with steady declines of precipitation in both dry and wet seasons. The cause of the precipitation decline is still controversial. It is hypothesized that rainfall in equatorial Africa may increase under climate change; therefore, it is suspected that this trend could also be related to multi-decadal climate oscillations and/or changes in the West African monsoon system. Regardless of the causes, this long-term water shortage has most likely led to the most pronounced leaf area loss in the world's tropical forests.

The height of the land area displayed indicates, for the time interval from 1982 to 2016, the LAI that was determined on the basis of the satellite measurements. The annual cycle can clearly be depicted. The color shading of the different region indicates the change in the LAI on the basis of a runnung 5-years mean. Green shading indicates increasing LAI, while brown shading indicates regions where the vegetation is shrinking.

The animation created by Michael Böttinger (DKRZ) in cooperation with Alexander Winkler shows how the trend of the leaf area index evolved over the decades, from an initial greening to a decline of leaf area in tropical Central Africa during the recent two decades of the satellite era.

Overall, the slow-down of Earth’s observed greening trend and the loss of leaf area in the highly productive ecosystems of the tropics may indicate a weakening of the terrestrial carbon sink. This could mean that a larger amount of future CO2 emissions will actually remain in the atmosphere and accelerate climate change.

Publication and References:

Winkler, A.J., Myneni, R.B., Alexandrov, G.A., Brovkin, V., 2019. Earth system models underestimate carbon fixation by plants in the high latitudes. Nature Communications 10, 885.

Winkler, A.J., Myneni, R.B., Hannart, A., Sitch, S., Haverd, V., Lombardozzi, D., Arora, V.K., Pongratz, J., Nabel, J.E.M.S., Goll, D.S., Kato, E., Tian, H., Arneth, A., Friedlingstein, P., Jain, A.K., Zaehle, S., Brovkin, V., 2021. Slow-down of the greening trend in natural vegetation with further rise in atmospheric CO2. Biogeosciences Discussions 1–36.

Author and scientific contact:

Alexander Winkler, Max Planck Institute for Meteorology (MPI-M) in Hamburg: YWxleGFuZGVyLndpbmtsZXJAbXBpbWV0Lm1wZy5kZQ== and/or: Max Planck Institut for Biogeochemistry (MPI-BGC) in Jena:  YXdpbmtsZXJAYmdjLWplbmEubXBnLmRl

Further information: