Carbon accounting may be inaccurate due to impacts of floods

A novel global analysis has revealed that floods may be as influential on plants’ carbon uptake as droughts, signifying that current carbon accounting may be inaccurate.

The investigation, conducted by researchers at Stanford University, has indicated that carbon uptake – the process in which plants convert atmospheric carbon dioxide into carbohydrates through the process of photosynthesis – may potentially be affected by floods as much as droughts, meaning that traditional carbon accounting methods may be erroneous.

The research is published in Environmental Research Letters.

The power of plants

In combatting the devastating effects of climate change, plants are our most valued ally, employing photosynthesis to soak up around one-third of the carbon dioxide produced from human activities – carbon uptake – storing it in the soil to prevent it from becoming a heat-trapping gas that exacerbates climate change.

Periods of extreme weather can greatly affect this ecosystem process; however, until now, research and resources have primarily focused on how carbon uptake is manipulated by periods of drought, with floods being studied significantly less. This new study outlines how floods need to be analysed comprehensively in future carbon accounting and why it is critical to incorporate plant responses to severe rainfall in modelling vegetation dynamics and soil carbon storage.

Alexandra Konings, the senior author of the study and an assistant professor of Earth system science in Stanford’s School of Earth, said: “These wet extremes have basically been ignored in this field, and we’re showing that researchers need to rethink it when designing schemes for future carbon accounting. Specific regions might be much more important for flood impacts than previously thought.”

Redesigning carbon accounting

Increasing the amount of photosynthesis that takes place on Earth could significantly help mitigate the effects of climate change. When combined with other factors, a higher frequency of photosynthesis would allow more considerable amounts of carbon to be stored in the soil over the long term. To measure the performance of photosynthesis, the team employed publicly available satellite data from between 1981 to 2015, which allowed them to examine plant greenness.

Because conventional methods of carbon accounting are focused on the impacts of droughts, the Stanford researchers were astounded to discover that photosynthesis is actually impacted by flooding quite frequently – in around 50% of the regions investigated. This inferred that although periods of drought are understood to mitigate photosynthesis, floods can instigate considerable fluctuations in the process.

“I think the drought side is probably something that many of us understand clearly because we can see soils drying out – we know that plants need water to be able to function normally,” said lead study author Caroline Famiglietti, a PhD student in Earth system science.

The team utilised statistical analysis to divide the planet into regions, isolating periods in which the plants’ photosynthesis will not have spiked from other factors, for example, because of changes in sunlight or temperature. They subsequently employed long-term soil moisture datasets to distinguish which particular locations were more prone to extreme floods than severe droughts, discovering that multiple regions in eastern Africa, central Mexico, and northern latitudes should be investigated comprehensively.

“Everything that is observed in this master dataset reflects the behaviour of the broader climate system,” Famiglietti said. “This paper identified something surprising, but it didn’t answer all the questions we still have.”

In parts of the world with warmer climates, periods of extreme weather are more common, extensive, intensive, and persistent. However, the processes controlling drought responses in plants are better understood than in floods, with the study suggesting that there is an opportunity to redesign carbon accounting methods to predict the future of climate change better and understand how it links to ecosystem carbon storage.

“If we can better understand these processes, we can improve modelling and better prepare for the future,” Famiglietti said.

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