As I walked from my house to the train station during last week’s torrential downpour in New York, I found myself in need of a 21st-century Sir Walter Raleigh (you know, he who spread his cloak over a puddle in 1581 so Queen Elizabeth I would not get her feet wet). A cloak wasn’t going to do it for me, though: the intersection I needed to cross to reach the street climbing up to the station had become a 2-foot-deep lake, judging by where the water reached on cars intrepid enough to try to get through. A garbage can bobbed along in the current. I flagged down a passing car, who drove me around the block rather than through the flood.
This comes to mind on yet another morning of torrential rain in the city because of a new study, to be published later this week in the journal Nature (subscription required). Scientists compared global rainfall records from 1925 to 1999 to various models of precipitation: those that include only natural causes (normal variability in the planet’s climate system and changes due to volcano eruptions, for instance), those that include only human effects (release of greenhouse gases into the atmosphere) and those that include both. Their conclusion: human activity “has had a detectable influence on observed changes in average precipitation,” they write, and these changes “cannot be explained by internal climate variability or natural forcing.” (Natural forcing includes things like changes in solar output as well as volcanoes.) In fact, “the estimated contribution of natural forcing to observed zonal precipitation trends is small in relation to the estimated contribution from anthropogenic [that is, manmade] forcing,” the scientists conclude.
Specifically, greenhouse emissions have contributed to increased rainfall here in the mid-latitudes of the northern hemisphere (40 to 70 degrees N), where human influence is responsible for 50 to 85 percent of the extra precip. In contrast, human activity is responsible for only 20 to 40 of the observed drying trend in the northern tropics and subtropics, from the equator to 30 degrees N. Human activity is behind more than 75 percent of the 20th-century’s extra raininess in the southern hemisphere’s tropics and subtropics, from the equator to 30 degrees S.
Those who accuse climate modelers of Cassandra-ism, take note: this is another example where the models of human-induced climate change fall short of the observed changes. That is, the models forecast smaller changes in rainfall than has been documented.
We have seen the same thing with Arctic sea ice, where models forecast less sea-ice loss than has been observed. In May, for instance, scientists writing in the journal Geophysical Research Letters found that, over the last 50 years, summer sea ice in the Arctic had shrunk at three-times the rate that the best climate simulations had forecast. That’s something that greenhouse contrarians downplay. They are happy to point out uncertainties and imprecision in the models, without mentioning that those uncertainties cut both ways: rather than overstate the effect of greenhouse emissions, they may understate them, meaning climate change will be even worse than the models indicate.