RESEARCH:

Geoengineering could weaken vital monsoon rainfall -- study

Geoengineering, or, as some enthusiasts call it, "hacking the planet" in an attempt to avert extreme changes in global precipitation, could come with some serious side effects, a new study has found.

A team of international researchers modeling the effects of geoengineering experiments have found that in a severe global warming scenario, using atmospheric injections of seawater or other materials that might dim sunlight to bring the world back to preindustrial temperatures would come with a downside.

It might also decrease precipitation below preindustrial levels, with an especially significant impact on regional monsoon seasons that could lead to crop failures and, perhaps, famine. These effects would in part be due to decreased sunlight causing decreased evaporation.

"Monsoons weaken in response to geoengineering, and weaken considerably," said co-lead author John Fasullo.

The study modeled the effects on precipitation after sunlight dimming is used to bring global temperatures back to preindustrial levels after a fourfold increase in atmospheric CO2. Fasullo and colleague Simone Tilmes, from the National Center for Atmospheric Research -- with the help of 25 other scientists -- ran the scenario through 12 models used by the international Geoengineering Model Intercomparison Project (GeoMIP).

The scientists found that bringing global temperatures back to preindustrial levels via sunlight dimming would reduce global precipitation by around 4.5 percent, with significant reductions over monsoonal land regions in East Asia (6 percent reduction) and North America (7 percent reduction). The frequency of months with heavy precipitation would decrease up to 20 percent, they found.

In contrast, they modeled that a fourfold increase in atmospheric CO2 would increase global precipitation by around 6.9 percent, with the frequency of months with heavy precipitation increasing by more than 50 percent.

Cooler with a chance of drought

The results raised the question common to geoengineering debates of what would be more damaging: to keep pouring CO2 into the atmosphere, or to try to fix it with massive geoengineering experiments.

Some of the findings could make geoengineering look more attractive, Fasullo said, particularly the 20 percent decrease in extreme precipitation events.

And while a geoengineering-induced 4.5 percent decrease in global precipitation might also not sound too devastating, the GeoMIP study found that some of the most populated and agriculturally productive areas in the world would be hit hardest.

"Half of the population of the world live in these regions, and half of the world's agricultural production comes from these regions," Fasullo said.

"The larger concern is we get global surface temperature back to near-normal conditions, but in doing so we increase the risk of drought," he added.

Exploring for a 'middle road'

The results left Fasullo wondering whether there was a potential "middle road" that didn't let CO2 levels increase so much, allowing for minor -- and potentially less disruptive -- geoengineering.

"Can you strike a happy medium between warming and drought?" he asked.

The difficulty, at least as far as monsoons are concerned, is that even the slightest changes in rainfall -- up or down -- could have big impacts, according to Fasullo.

"It's definitely something we can study, and it deserves to be looked at."

He added, however, that there are "major reasons to be cautious about implementing any geoengineering approach."

"You don't know what you don't know, so there's likely to be unforeseen impacts," he said.

Ben Kravitz, a climate modeler at the Pacific Northwest National Laboratory who ran one of the models for the study, agreed.

"Solar geoengineering is not a permanent solution," he said. "The only permanent solution to stopping the problem of climate change is we need to stop putting CO2 in the atmosphere."

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