Guest essay by Eric Worrall

According to a Swiss study, the Tropics are not warming as rapidly as expected because of large scale agricultural irrigation.

Irrigation helps beat the heat of global warming 

17 JANUARY 2020

Large-scale irrigation can help alleviate and even reverse hot extremes driven by human activity and other drivers of global warming, a new international study has found.

The research, led by ETH Zurich in Switzerland, showed that irrigation dampens and in some cases offsets the combined effects of global warming on hot days. 

The researchers compared the climate effects of irrigation with other natural and human drivers, including greenhouse gas emissions, across intensely irrigated areas in Southern Europe, North Africa, South Asia, and the United States.

Dr Annette Hirsch, from the ANU Research School of Earth Sciences, was part of the research team.

She said while not a “watershed”, the study showed that irrigation offered some hope when it came to dealing with more frequent and intense hot extremes caused by global warming. 

“There’s no doubt that as the planet warms, we will face even hotter extremes more frequently,” Dr Hirsch said. “Many studies show that under global warming, the chances of hot extremes across the planet are increasing.

“But what we found was that while global warming has increased the chances of hot extremes across the planet, in some regions expanding irrigation reduces that effect or can even reverse it.

“Our results show heat extremes are partly or completely offset by the cooling effects of irrigation.

“In this case, irrigation has the same effect for a hotter planet as pumping up the evaporative cooler in your house.”

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Read more: https://www.anu.edu.au/news/all-news/irrigation-helps-beat-the-heat-of-global-warming

The abstract of the study;

Warming of hot extremes alleviated by expanding irrigation

Wim Thiery, Auke J. Visser, Erich M. Fischer, Mathias Hauser, Annette L. Hirsch, David M. Lawrence, Quentin Lejeune, Edouard L. Davin & Sonia I. Seneviratne 1269 Accesses93 AltmetricMetrics details

Abstract

Irrigation affects climate conditions – and especially hot extremes – in various regions across the globe. Yet how these climatic effects compare to other anthropogenic forcings is largely unknown. Here we provide observational and model evidence that expanding irrigation has dampened historical anthropogenic warming during hot days, with particularly strong effects over South Asia. We show that irrigation expansion can explain the negative correlation between global observed changes in daytime summer temperatures and present-day irrigation extent. While global warming increases the likelihood of hot extremes almost globally, irrigation can regionally cancel or even reverse the effects of all other forcings combined. Around one billion people (0.79–1.29) currently benefit from this dampened increase in hot extremes because irrigation massively expanded throughout the 20𝑡ℎthcentury. Our results therefore highlight that irrigation substantially reduced human exposure to warming of hot extremes but question whether this benefit will continue towards the future.

Read more: https://www.nature.com/articles/s41467-019-14075-4

If man made irrigation can have such a dampening impact on regional temperature, makes you wonder whether there might be an emergent form of natural irrigation which helps keep the tropics from overheating, by performing a similar temperature moderation role to agricultural irrigation.