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Incomplete drought recovery may become the new normal

New Delhi : Droughts are getting common across the world, but a new study has thrown light on what it takes to recover from one.

The length of drought recovery depends on several factors, including the region of the world and the post-drought weather conditions, said the study, warning that more frequent droughts in the future may not allow time for ecosystems to fully recover before the next one hits.

Droughts can be defined in several ways. The first is meteorological, defined as a period of less than average precipitation. The second is agricultural, in which the lack of rainfall impairs the productivity of plants. The third is hydrological, when water sources such as lakes, reservoirs, and aquifers begin to dry up to below-average conditions.

But, the amount of time it takes for an ecosystem to recover from a drought is an important measure of its severity.

Although 20th Century trends in drought regimes are ambiguous, across many regions more frequent and severe droughts are expected in the 21st Century. Recovery time — how long an ecosystem requires to revert to its pre-drought functional state — is a critical metric of drought impact. Yet, the factors influencing drought recovery and its spatiotemporal patterns at the global scale are largely unknown.

During the 20th Century, the total area of land affected by drought increased, and longer recovery times became more common, according to the research study published in Nature by a group of scientists including Carnegie's Anna Michalak and Yuanyuan Fang and William Anderegg of the University of Utah, said that given anticipated 21st Century changes in temperature and projected increases in drought frequency and severity due to climate change, recovery times will be slower in the future.

A chronic state of incomplete drought recovery may be the new normal for the remainder of the 21st Century and the risk of reaching "tipping points" that result in widespread tree deaths may be greater going forward, they said.

"Research has usually focused on the amount of rain and other precipitation that ends the deficit of water that causes a drought, but assessments of drought-recovery need to account for the restoration of normal plant function," explained Michalak.

The team used measures of photosynthetic activity to assess drought recovery. Quantifying how long it took for plant productivity to return to normal gave the researchers a better understanding of the longevity of a drought's effects.

"If another drought arrives before trees and other plants have recovered from the last one, the ecosystem can reach a 'tipping point' where the plants' ability to function normally is permanently affected," Fang said.

"There was a broad presumption that ecosystems and plants recovered almost immediately when the weather got wetter," Anderegg says. "We didn't know what the patterns were globally, including which plants seemed to recover faster or slower and which variables influenced that recovery time."

The team found that the post-drought climate conditions were the most influential factors in drought recovery time. Wet conditions, such as those that slammed California after its long drought, hastened recovery.

Dry conditions and temperature extremes, lengthened recovery. However, Anderegg says, "there are likely to be places in California where the drought was so severe that the ecosystem will not recover to the previous level because so much of the vegetation has died."

Location mattered as well. In general, most areas of the world are able to recover from a drought in less than six months. Some areas need up to a year. But the high-latitude Arctic regions and the tropics of South America and Southeast Asia need more time - up to two years. "That's worrisome because those regions store the largest chunks of carbon in ecosystems across the globe," Anderegg says.

With climate models forecasting that the extent and severity of drought is likely to increase, also increasing the likelihood that ecosystems may be hit with new droughts before they have recovered from the previous one.

“That could have a double whammy effect,” Anderegg says. “A second drought could be harder on an ecosystem and have the potential to push it off a cliff.” Ecosystem collapse in the face of perpetual drought could change verdant forests into grass and shrubs.

Such a double whammy hit the Amazon rainforest in 2005 and 2010, when back-to-back droughts, each with a once-in-a-century severity, hit the region. “Satellites showed that forests hadn’t recovered from the 2005 drought by the time the 2010 drought struck,” Anderegg says.

The study underscores the importance of drought recovery time in assessing drought impacts on ecosystems. Rain brings relief, but doesn’t solve drought-related problems immediately, Anderegg says. “Often recovery is longer than the drought itself.”

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