What’s Behind This Summer’s Severe Weather Around the World?

BY Talea Miller  August 16, 2010 at 4:50 PM EDT

Pakistani flood survivors; AFP/Getty
It’s been a summer of extreme weather: monsoon flooding is wreaking havoc in Pakistan and a persistent heat wave is fueling wildfires and creating health hazards in Russia.

The United States has also not been immune, as sweltering high temperatures have plagued the East Coast, and deadly floods have hit the Midwest.

This time of year always brings some treacherous weather to the northern hemisphere, but the events in Eastern Europe and Asia are “quite exceptional,” said Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colo.

“We expect monsoons in the region in general but they’ve been much more intense this year,” he said, in part because of La Nina, which causes cooling of surface temperatures in the Pacific Ocean and can bring wetter conditions to Asia.

A ridge forming in the atmosphere above Pakistan that is blocking the moisture from circulating out of the region is exacerbating the problem, said Ghassem Asrar, director of the World Climate Research Program at the U.N.’s World Meteorological Organization.

Trenberth and Asrar also point to climate change and the long-term warming of the oceans and the atmosphere as contributing factors.

“The Asian monsoon is generally enhanced under La Nina conditions so that is one factor,” Trenberth said, “but the fact that the Indian Ocean is so warm means it’s also providing a lot of moisture into the atmosphere, which is further fueling the heavy rain.”

Some climate-change skeptics argue there is not enough information about weather patterns over long periods of history to draw conclusions about extreme weather events and climate change.

CNN meteorologist Brandon Miller wrote in a recent piece “one extreme weather event, or even a series of weather events, is not caused by global warming or climate change. Weather extremes such as floods or heat waves happen every year, all over the globe.”

While many climate scientists agree that tying a specific event to climate change is hard to do, there is also growing consensus that some combination of warming and natural weather variability is probably increasing the frequency of these events.

“In a warming climate, we expect to see more of these extremes, we expect the magnitude be greater and the frequency to be greater,” said Asrar.

The extreme events seen this summer are also likely connected to each other by the currents in the atmosphere, Asrar said. Under normal conditions, the Asian monsoon season helps create the Mediterranean’s hot dry summer weather, said Trenberth, but that may have shifted north into Russia this summer.

“What is probably happening is there is a wave in the atmosphere, a large-scale wave that is going across the Russian sector that is being ‘paused,’” he said, in part by the heat released into the atmosphere by the heavy rains of the monsoons. That wave is encouraging the formation of anticyclones, areas of high atmospheric pressure that are blocking the arrival of less punishing weather, he said.

The below satellite image shows the distribution of above-average temperatures in Russia this summer versus average temperatures for the same dates between 2000 and 2008. Click for a larger view:

_slideshow.jpg” title=”Russia heatwave NASA” alt=”Russia heatwave July 2010; NASA image” class=”blog_main_horizontal” />

Normal seasonal changes should bring some relief to the region soon, Trenberth said, but when exactly remains an unknown. Also unknown is whether these types of events will become the new norm in coming years.

Current WMO models can predict a major departure from normal weather conditions with about a 60- to 70-percent confidence. Asrar said it is crucial this area of research continue growing and improving.

“Clearly it’s important in being able to see if these extreme events are going to happen, but more importantly for agriculture, water availability planning, you name it,” he said. “We live in such a global system you can no longer think this is an isolated case that affects a certain country, it really affects us all.”