14 February 2024

Drought limits carbon uptake more than expected


A Chinese study challenges methods for detecting compound drought events affecting vegetation. The impact of these events is likely to increase in the future

by Matteo Cavallito


Drought phenomena characterized by low soil moisture and a large vapor pressure deficit, or the difference between the amount of moisture in the air and the amount the air can hold when saturated, “pose significant threats to terrestrial carbon sink and agricultural production.” The concurrence of events, in particular, would limit the ability of vegetation to absorb carbon more than previously thought. This is supported by a study published in the journal Climate and Atmospheric Science.

“We find frequent and severe vegetation compound droughts and their considerable adverse impacts on carbon uptake in mid- and low-latitude regions, particularly in drylands,” the authors explain.

Compound drought phenomena are underestimated

The investigation, by a group of researchers at Beijing Normal University, challenges the methods of analysis used so far and based  on quantiles arguing that the these have led scientists to underestimate the phenomenon. Compound events, in particular, have typically been defined as phenomena characterized by the simultaneous crossing of some extreme measurable threshold by multiple dependent variables involved.

However, the authors remark, the negative impacts of these events can actually occur even when not all variables are reporting values above the critical thresholds.

To simplify: the commonly used method does not seem to work by being unable to capture the true incidence of the observed phenomena. “The widely adopted quantile-based approach identifies only 11% of vegetation compound droughts (VCDs) and 26% of global gross primary productivity (Or , according to UN-REDD Program, “The total amount of carbon compounds produced by photosynthesis of plants in an ecosystem in a given period of time”, ed.) anomalies due to VCDs”

A new method

As an alternative to the traditional method, therefore, the authors created a new framework for defining compound droughts. This framework is based on the response of primary carbon compound productivity to low soil moisture and high vapor pressure deficit.

This impact-based approach is combined with analysis of measurements available between 1981 and 2017. And, most importantly, with historical simulations for the past (1930-2014) and future (2016-2100). “We evaluate the absolute changes in the frequency, intensity, and GPP anomalies of vegetation compound droughts between the two periods,” the authors explain.

The phenomenon will increase in 81% of territories

“To facilitate comparison, historical thresholds of soil moisture and high vapor pressure deficit extremes are used to define vegetation compound droughts for the two future scenarios, assuming unchanged vegetation response to drought,” the researchers point out. In future simulations, they add, “we find more frequent vegetation compound droughts due to climate change over more than 81% of assessed land regions, except in the tropical Africa and South Asia with higher soil moisture.”

When comparing with historical data, moreover, the frequency of the phenomenon on a global scale marks increases between 10.9 and 16.5 percent depending on the different scenarios considered.

Finally, the study warns how carbon loss caused by drought events may contribute to accelerating future climate change. Therefore, better mitigation strategies need to be developed, such as reducing fossil fuel use, improving land management practices and restoring forests. Also paying more attention to dryland ecosystems, which are more vulnerable to the phenomenon.