Between 1925 and 1999 precipitation between 40 and 70 degrees north latitudes increased at the rate of 62 millimeters (2.44 inches) per century. The northern tropics and subtropics, between 0 and 30 degrees, became drier at 98 millimeters (3.86 inches) a century, while it got wetter in the corresponding zone between the equator and 30 degrees south at a rate of 82 millimeters (3.23 inches) per century. The models, which factor in natural effects such as solar winds and volcanic eruptions, along with anthropogenic forcings like greenhouse gases and aerosols, match these precipitation variations accurately in trend and reasonably well in magnitude.
“A warmer globe means more water vapor in the atmosphere, which increases the potential for rainfall,” she says, explaining the increase in total global rainfall over the past several decades. “The way [the moisture] turns into rain is more complex, however,” she adds, which causes both increments and decrements in local rainfall. The step from moisture to clouds involves cooling, seed particles (including pollutant aerosols) and global wind patterns that blow the moisture from its place of origin to its place of condensation. There are even factors, like change in forest cover, that are known to influence local rainfall but are not very well represented in any of the models. All these complications have traditionally rendered attempts at modeling rainfall—which is much harder than modeling temperature changes—futile. “We were surprised by how well the results matched [real-life data],” Hegerl says.
This, however, is not the end-all of climate modeling. Almost all of the rainfall data available today are over land, whereas oceans cover 70 percent of Earth’s surface. Difficulty in measuring rainfall over the oceans has precluded any analysis of this immense area. Furthermore, for reasons still unknown, of the 10 or so models used, different ones make accurate predictions at different latitudes; no single model works over all latitudes, and the mean of all of them is closest to observed data. And lastly, although the models get the precipitation trends spot-on, they “significantly underestimate the magnitude of change [in rainfall],” Hegerl admits, explaining that better modeling is near the top of the agenda for the researchers.
So what’s next? Now that the link between shifting rain trends and increasing greenhouse gas and aerosol emissions has been confirmed, scientists are looking to explore connections between climate change and other atmospheric metrics such as cloud cover.
