When it rains after a drought, it seems that everything returns to normal and that vegetation recovers. However, some of the consequences of the lack of water can persist during the following year. This is the conclusion of a study carried out on a global scale and recently published in Nature Communications, with the participation of the CSIC and CREAF. The results show that, if the soil continues to lose moisture throughout the year following a drought, the temperature increases more than it decreases when it recovers the same amount of water it had lost. Specifically, when the soil loses a certain amount of water, the temperature rises by 0.13°C and when the soil recovers the same amount of water it lost, the temperature falls by only 0.06°C. This is partly because plants do not recover at the same rate as the water in the soil: after a drought, they may have accumulated damage to their plant tissues and their ability to transpire and absorb CO₂ takes time to recover. As a consequence, the cooling effect of vegetation decreases and global warming increases.
“We observed the scars left by drought one year after it has passed in the study,” explains Josep Peñuelas, CSIC researcher at CREAF. According to the researcher, the reason is that plants absorb water from the soil and transport it to the leaves, where small pores called stomata regulate gas exchange and water loss. When there is enough water, the stomata remain open and allow CO₂, which is necessary for photosynthesis, to enter, while some of the water leaves the leaf as vapour through transpiration. This water vapour helps dissipate heat and cool the vegetation and the air surrounding it.
The problem is that, after severe water stress, the plant machinery is damaged: embolisms occur in the water-conducting vessels and carbon reserves are diverted towards repairing damage instead of creating new leaves. That is why, even when the soil receives water again, transpiration does not recover at the same rate and the cooling effect is reduced.
Josep Peñuelas, CSIC researcher at CREAF
This prolonged effect of drought has been observed across more than 60% of the Earth’s land surface. Regarding the reasons why the soil may take longer to recover its moisture, the researcher points out that the drought may have depleted water reserves in the deeper layers, which need time and continuous rainfall to recharge. In addition, when the soil is very dry and temperatures are high, some of the water may evaporate quickly or may not infiltrate into the deeper layers. The loss of vegetation also leaves the soil more exposed to the sun and favours evaporation.
The Amazon, one of the most vulnerable areas
The data indicate that the most affected areas are located in North America, eastern China, sub-Saharan Africa and the western Amazon. “The Amazon literally acts as the air conditioning system for the entire planet thanks to the enormous amount of water it transpires into the atmosphere,” highlights Peñuelas. “So, when an extreme drought causes humidity to fall in the western Amazon, the trees sharply reduce this transpiration and the rainforest not only stops cooling the world, but also slows down its vital function as a major carbon sink, bringing the ecosystem closer to critical tipping points,” he warns.
To reach these conclusions, the team analysed climate and soil moisture data from 1980 to 2024. They combined this information with 17 Earth system models (ESMs), which simulate the interactions between the atmosphere, soil, vegetation and other elements, with the aim of identifying the effect of drought one year later on temperature, vegetation, transpiration and carbon uptake.
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Refining future projections
The results of the research can help improve future projections of global warming. According to Peñuelas, “models should take into account that the effects of a drought can persist during the following months and affect the ability of ecosystems to regulate temperature and absorb CO₂”. In this way, the response of ecosystems to extreme climate events would be better represented.
The publication also contributes to the European CONCERTO project, which studies how forests and other terrestrial ecosystems respond to global change and extreme climate events.
