The way we humans are affecting the climate is changing rainfall patterns over land and sea, scientists at Lawrence Livermore National Laboratory (LLNL) in California have found. Kate Marvel and Céline Bonfils compared precipitation ‘fingerprints’ in satellite data against what climate models showed would result from actions like adding greenhouse gases to the atmosphere. “Everyone knows that temperatures are rising, but figuring out how that affects other aspects of the climate is tricky,” Kate told me. “We’ve shown that global precipitation is changing in the way climate scientists expect it to. The odds of the observed trends being due to natural climate variability are very low.”
Changes to rain, snow and all the other forms of falling wetness collectively known as precipitation are undeniably important, given their power to bring floods and droughts. Scientists have already shown that, over land, wet areas are getting wetter and dry areas are getting drier. These studies rely on data measured directly on land, reaching back almost a century. The long record gives scientists a lot of data to test, making it easier to tell human influences from the many natural rainfall patterns. Yet Kate and Céline wanted to use satellite data instead. Though these have only been recorded since 1979, each measurement is more reliable, and the satellites also cover the oceans.
“With such a short record, it’s often difficult to identify the ‘signal’ of climate change against the background of completely natural variability,” Kate explained. For example, the wet-gets-wetter, dry-gets-dryer strengthening of the Earth’s water cycle happens because warmer air can hold more water vapour. But that can be caused by the El Niño climate pattern, as well as by increasing greenhouse gases. Our activities can also change how air circulates above the planet, pushing dry regions and storm tracks toward the poles – but so can the La Niña pattern.