Could geoengineering affect the global water cycle?

This new study is based on an atmospheric general circulation model developed at the US National Center for Atmospheric Science at Boulder. The researchers coupled the model to a mixed-layer ocean model for performing climate change experiments. They then estimated the short-term response of the climate using the so-called fixed-SST forcing method proposed by James Hansen of NASA.

Radiative forcing is a means of measuring the impact of any mechanism that causes climate change; scientists have developed models that measure this forcing. "For example, adding CO₂ to the climate system has a positive radiative forcing, which leads to global warming," explains lead author Govindasamy Bala. "We can then use these models to precisely determine the change in the amount of solar radiation that would have the same radiative forcing as that caused by a doubling of CO₂ in the atmosphere."

The climate system responds on a variety of timescales. "For example, consider the response to an increase in solar flux: the radiative balance changes within milliseconds while leaf temperatures increase within minutes," says Caldeira. "Atmospheric stability decreases within days and soils begin to dry over a matter of weeks." There are then other changes that occur over months, decades, centuries, millennia and even tens of millennia.

In this study, however, short-term responses refer to changes in the climate system that occur before the global mean surface temperature changes. The findings could have implications for so-called solar radiation management (SRM) geoengineering, say the researchers. These schemes counter the effects of global warming from anthropogenic greenhouse gases by reducing the amount of solar radiation that is absorbed by the Earth. Examples include placing reflecting mirrors in space, injecting aerosols into the stratosphere, enhancing the albedo of marine clouds by seeding them with cloud condensation nuclei, and painting rooftops white.

Because the global water cycle is more sensitive to changes in solar radiation than to equivalent increases in CO₂, SRM geoengineering strategies could lead to reduced rainfall and a reduction in the intensity of the global water, or hydrological, cycle while mitigating surface warming, say the researchers.

"However, critics argue that a reduction in the intensity of the global water cycle is generally not valid for such SRM schemes," Bala told environmentalresearchweb. "They say that cloud brightening, for example, would not lead to a reduced intensity of the hydrological cycle but we are now investigating this hypothesis using climate models."

The researchers reported their work in Climate Dynamics.