Currently, around 50% of man-made annual CO2 emissions are absorbed in roughly equal proportions by land and oceans. In parallel with the rising CO2 emissions in the last few decades, these so-called land and ocean carbon sinks have also grown steadily, and have thus prevented an acceleration of climate change. However, more recent studies assume that these important carbon sinks will weaken in the coming decades, although there is still no consensus on the underlying mechanisms.

Example European Drought and Heatwave in 2003

Plants play a crucial role in the land-based carbon sink, as they remove CO2 from the atmosphere through photosynthesis and bind it in the biomass. However, this important function of plants can be compromised by extreme weather events such as heat waves, droughts or heavy rain. A well-known example is the drought and heat wave that Europe experienced in 2003, during which plant growth decreased by 30 percent, cancelling the carbon sink of four years in this region.

Climate Extremes and Plant Growth

The increasing evidence of more frequent and intense climatic extremes and the corresponding negative effects on ecosystems in the recent past raises the question of whether such shifts in climatic extremes have already led to a systematic reduction in plant growth at both regional and global levels.

How plant growth in connection with climate extremes changed over time within roughly the last three decades was investigated in a new international study spearheaded by Dr. David Gampe from the Institute for Geography at the Augsburg University and published in the journal ‘Nature Climate Change’. The researchers analyzed three different data sets of global gross primary production (GPP), which were created using different approaches. The GPP describes the uptake of carbon by an ecosystem through photosynthesis, i.e. how strongly plants grow by absorbing CO2.

"In order to evaluate systematic shifts in gross primary production in response to observed changes in climatic extremes, the 1000 largest negative GPP extreme events were identified in the period from 1982 to 2016 and assigned to extreme climatic events," explains Augsburg geographer Dr. David Gampe, who currently acts as a substitute for the professorship for physical geography and soil science at the Ludwig-Maximilians-University in Munich.

Increase by 10,6 Percent

The key results indicate that the northern mid-latitudes in particular - including Europe among others - show a significant increase (10.6%) in negative GPP extremes between the periods examined. Regional hotspots are East Asia and Central North America. In addition, the study also finds that the months with exceptionally reduced plant growth due to GPP extremes have shifted from June/July to July/August, more towards the peak and later portion of the growing season. In the tropical regions, it is predominantly the Amazon region that shows a sharp increase in negative GPP extremes.

The majority of the identified extreme events of reduced plant growth (around 70% globally) can be attributed to climate extremes, which result directly from combinations of anomalous rainfall and temperature values. The remaining portion could have been caused by factors such as fire, pest infestation or wind, but these were not part of the study. The results showed that warm droughts in particular (characterized by coinciding water deficits and high temperatures) have increased significantly and are largely responsible for reduced plant growth in large parts of the northern hemisphere.

Grass and agricultural land covers particularly affected

Exceptional increases in negative extremes in gross primary production due to warm droughts were identified in the comparison period, especially for grasslands (increase of 95%) and croplands (+ 84%). They are more prone to drought than forested regions because these ecosystems generally do not possess the deep roots needed to reach water in the deeper soil layers. The study also shows that the negative impact of climate extremes on agricultural land can only be mitigated to a limited extent by management options such as irrigation.

The negative extremes of plant growth identified in the study counteract the general positive trend in global plant productivity (as an important component of the rising land-based carbon sink). The results of the study show that this actually positive GPP trend, which varies from region to region, has weakened considerably due to the increase in negative GPP extremes. In other words: Extreme weather events such as droughts lead to lower CO2 uptake, sometimes to the extent that the actually increasing productivity trend is weakened or stopped.

Risks for agriculture and food security

“Previous studies projected a sharp increase in negative GPP extremes by the end of the 21st century. The current study suggests that these adverse impacts of climate extremes and especially warm droughts on plant productivity are already ongoing” elaborates geographer Prof. Dr. Wolfgang Buermann, a co-author of the study. However, less plant productivity also means less CO2 uptake and thus a weakening of the land-based carbon sink, which further accelerates climate change. The fact that grasses and crop lands in particular are disproportionally impacted by climate extremes underscores the need for swift adaptation measures, especially given the current expansion in agricultural land. Fewer monocultures, better use of existing resources, different types of plants and social rethinking are strategies that can help the agricultural sector to better deal with the consequences of climate change.

Publication:

David Gampe, Jakob Zscheischler, Markus Reichstein, Michael O’Sullivan, William K. Smith, Stephen Sitch and Wolfgang Buermann: Increasing impact of warm droughts on northern ecosystem productivity over recent decades, Nature Climate Change, DOI: 10.1038/s41558-021-01112-8
 

Go to Nature Climate Change:

https://www.nature.com/articles/s41558-021-01112-8