Role of Climate Change

Role of Climate Change

Oklahoma tornado
Thermometer reading 110 ° F

Is a particular weather event—such as a tornado outbreak or a record heat wave—caused by natural variability or global warming? There are no definitive answers yet, but scientists are beginning to provide insight into the effect climate change might have on the chances of an extreme event occurring.

Climate model grid, representing surface and atmospheric layers

Computer models simulate climate change by dividing the world into 3-dimensional grid boxes, measuring physical processes such as temperature at each grid point.

Using computer models and sophisticated statistical techniques, researchers in the field of attribution science are able to estimate the odds that global warming increased (or decreased) the chances for certain extreme events.

A temperature outlook, issued on March 15, 2012, that shows how human-produced greenhouse gases were expected to change the odds that April 2012 would be as warm as the top five Aprils in the reference period (1960–2011).

Forecast of the change in the odds of a warmer April (2012) due to greenhouse gas emissions

The science of attributing extreme water and weather events to climate variability and change is relatively new. Several recent case studies have shed light on unusual events such as the 2003 European heat wave and the 2010 Russian heat wave. And, as seen in this image, some scientists are even producing forecasts of the changes in the odds of occurrence for hotter, colder, wetter, and drier events.

Anomalous number of days with maximum surface temperatures reaching 34°C for June-August 2003. Anomalies are departures from the 1971–2000 base period daily means.

Number of days with temperatures hotter than normal during the 2003 European Heat Wave

The summer of 2003 was the warmest in at least the past 500 years in Europe. European Community scientists correlate the 2003 summer European heat wave with 70,000 deaths above what would normally be expected. Researchers wanted to know if this heat wave was simply an extremely unusual natural event or whether climate change created an environment that altered the pattern of natural variability. Using a climate model, they analyzed the probabilities of exceeding the 2003 seasonal mean temperature under 2003 conditions, including the build-up of greenhouse gases from human activities. They then ran the model again without those greenhouse gases. Their conclusion is that global warming probably at least doubled the chances of the heat wave.

Temperature anomalies for the Russian Federation from July 20–27, 2010, compared to temperatures for the same dates from 2000 to 2008.

In the case of the 2010 Russian heat wave, researchers found the magnitude of that event was primarily caused by natural factors. Other scientists, who used a different approach, found that the risk of these types of events in western Russia has increased greatly due to global warming. It appears that, even if natural factors were the main cause of this particular weather extreme, the added strength produced by climate change pushed it to a new and dangerous level.

These early studies show promise, but much more work needs to be done to expand the regional areas and the types of weather events for which these studies are conducted. Researchers hope that improvements in attribution techniques will eventually lead to developing improved early warning capabilities.