A study by the Carnegie Institution in collaboration with the Indian Institute of Science proposes that whitening clouds over the ocean to reflect sunlight could help counter the effects of global warming.
The researchers suggested that the whitening would be accomplished by reducing the size of the water droplets making up the clouds.
They said that altered atmospheric circulation under the scheme in fact could increase monsoonal rains and cause the continents to become wetter, not drier, on average.
"Rain clouds, which have big droplets, tend to be grey and absorb sunlight, whereas clouds with smaller droplets tend to be white and fluffy and reflect more sunlight to space. In practice this could be done by shooting a fine spray of seawater high into the air, where the tiny salt particles would create condensation nucleii to form small cloud droplets," said co-author Ken Caldeira of the Carnegie Institution's Department of Global Ecology.
To test the climate consequences of doing this, researchers used a computer simulation of the global climate system in which atmospheric carbon dioxide concentrations were set at approximately twice that of present day.
Cloud droplets over the oceans in the model were reduced in size to make the clouds more reflective.
Clouds over land were unaltered.
As expected, the whitened clouds reflected more solar radiation and offset the warming effect of the high carbon dioxide levels.
However, the researchers were surprised as the model showed that the oceanic clouds caused the land surface to become cooler and wetter on average.
In previous climate simulations diminishing solar radiation by geoengineering had reduced precipitation on land.
"The drying of the continents has been a major concern with regard to geoengineering," said Caldeira.
But in the model the runoff from the continents increased by 7.5 percent globally, with the effect being strongest in the tropics.
The researchers concluded that the increased precipitation over land was driven by changes in air circulation, similar to the monsoonal pattern that determines rainfall in parts of Asia.
"Monsoons occur when air masses over land are warmer than air masses over the ocean, and this draws in cool, moist air from over the ocean which then drops rain over the land," said Caldeira.
In the simulations, the reflective oceanic clouds preferentially cooled the air over the oceans relative to land, setting up a monsoonal airflow.
Caldeira stresses that their study, in which all marine clouds worldwide were uniformly whitened, cannot be used to predict the geographic patterns of rainfall that might develop as a result of geoengineering.