Jerusalem – The warming effect of human-induced greenhouse gases is a given, but to what extent can we predict its future influence? That is an issue on which science is making progress, but the answers are still far from exact, say researchers from The Hebrew University of Jerusalem, the US and Australia who have studied the issue and whose work which has just appeared in the journal Science.
Indeed, one could say that the picture is a “cloudy” one, since the determination of the greenhouse gas effect involves multifaceted interactions with cloud cover.
To some extent, aerosols –- particles that float in the air caused by dust or pollution, including greenhouse gases – counteract part of the harming effects of climate warming by increasing the amount of sunlight reflected from clouds back into space. However, the ways in which these aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models, say the researchers. As a result, the radiative forcing (that is, the disturbance to the earth’s “energy budget” from the sun) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming.
And while advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, further progress is hampered by limited observational capabilities and coarse climate models, says Professor Daniel Rosenfeld of the Fredy and Nadine Herrmann Institute of Earth Sciences at The Hebrew University of Jerusalem, author of the article in Science. Rosenfeld wrote this article in cooperation with Dr. Steven Sherwood of the University of New South Wales, Sydney, Dr. Robert Wood of the University of Washington, Seattle, and Dr. Leo Donner of the US National Oceanic and Atmospheric Administration.
Their recent studies have revealed a much more complicated picture of aerosol-cloud interactions than considered previously. Depending on the meteorological circumstances, aerosols can have dramatic effects of either increasing or decreasing the cloud sun-deflecting effect, the researchers say. Furthermore, little is known about the unperturbed aerosol level that existed in the preindustrial era. This reference level is very important for estimating the radiative forcing from aerosols.