14 June 2012

Volcanoes lessen global rain — by more than climate model predicts

Posted by kramsayer

New research examines the effect of volcanic eruptions on rainfall, comparing model predictions with observations from the last century. Shown here is Mount Pinatubo in the Philippines, which erupted in 1991. (Credit: USGS)

GeoSpace is in Selfoss, Iceland this week, reporting from AGU’s Chapman Conference on Volcanism and the Atmosphere. Check back for posts on the science presented at the meeting, as well as field trips to nearby volcanoes and geologic features.

Selfoss, Iceland — Large volcanic eruptions that eject fine particles into the stratosphere can cause global temperatures to temporarily cool – leading some scientists to consider adding similar human-made aerosols to the atmosphere to temper the effects of climate change. But scientists are still working to understand how these explosive eruptions – and potential geoengineering efforts – would affect rain worldwide. And climate models might be underestimating how much precipitation decreases after eruptions.

“Precipitation is an area that hasn’t been looked at quite so much yet,” said Carley Iles, a climate science PhD student at the University of Edinburgh, United Kingdom. She’s investigating the rain-reducing effect of volcanoes using both observational data from the last century and a climate model’s reconstructions of roughly the last 600 years, from 1400 to 2000.

Teasing out the effects of volcanic eruptions on rainfall is a challenge, she said, because precipitation naturally varies from year to year and season to season. So scientists scrutinize the years after multiple eruptions, and look across the globe to average out some of the natural ups and downs.

Using a climate model commonly used for looking at the effects of volcano on climate, Iles and her colleagues found that average daily global precipitation over land declines after a major eruption. It reaches its lowest level one year post-eruption when precipitation dips by 0.05 millimeter (0.002 inch) below the norm of 2.1 mm per day (0.08 in per day), she said. The observational data for the last century, however, indicated a significantly larger decrease in precipitation compared to the model projections – about twice as much, Iles said.

“The climate model seemed to underestimate the size of the response,” Iles said. “The observations and model agree that precipitation decreases on a global scale, but the observation is bigger.”

Iles took a new look at how the rain response to volcanoes changes in different areas. She found that, the wet tropical regions (between the equator and 40 degrees north or south), as well as latitudes above and below the tropics get drier, on average. But the dry tropical regions actually become a little wetter. Circulation patterns and regional variability complicate the situation.

The precipitation effect from volcanoes takes place as those eruptions’ sulfur-rich aerosols spread through the stratosphere across the globe, lowering temperatures. The aerosol haze reduces energy reaching Earth’s surface – so there is less evaporation and, overall, less input into the hydrological cycle, Iles explained.

With the rain response to volcanoes varying across different regions, it’s difficult to say what effects, if any, the change might have on people or ecosystems, Iles said. But it’s an effect climate modelers and geoengineers can’t ignore.

Next, Iles is planning to take a look at post-eruption stream flows and runoff into rivers, to see if the small declines in precipitation add up to bigger volcanic impacts.

-Kate Ramsayer, AGU Science Writer