Warming’s role in extreme weather

Devastatingly hot weather. Torrential downpours. Scientists have long suspected that global warming can cause extreme weather events. Now experts have numbers to support that idea.

Climate change is to blame for about 3 out of every 4 major spikes in daily temperature. The same goes for nearly 1 in 5 exceptionally heavy rainfalls and other bouts of extreme precipitation. That’s the finding of a new study. Researchers offered details of how they came to these conclusions April 27 in Nature Climate Change.

The burning of fossil fuels has been leading to a buildup of carbon dioxide in Earth’s atmosphere. This CO₂ is a greenhouse gas. That means it can trap heat in the air. As a result, our planet’s temperature has been rising. And this global warming will account for a growing share of ever more frequent extreme weather events, the new study finds.

Many governments hope to limit global warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above temperatures that were typical before the Industrial Revolution. It will be hard for nations to achieve that. But even if they do, global warming still will be responsible for nearly all heat extremes, the new study finds. Global warming also will be to blame for about 2 out of every 5 extreme rains and snowfalls.

“This is a considerable fraction” of major weather events, says climate scientist Peter Stott. He works for the United Kingdom’s national weather service at the Met Office Hadley Centre in Exeter, England. Stott was not involved with the new study.

So far, Earth has warmed about 0.85 degree C (1.53 degree F) since 1750. That’s about when the Industrial Revolution got its start. That’s also when fossil fuel use took off, spewing lots of CO₂ into the air. Overall, the new study shows that “relatively small rises in global temperature translate into large increases in the likelihood of extremes,” Stott says.

What the new climate analysis did

Erich Fischer and Reto Knutti work at the Swiss Federal Institute of Technology in Zurich. These climate scientists analyzed 25 different computer models of global climate. These analyses looked at different climate periods, from preindustrial times to the present. The analyses also projected what weather events were likely to be like in the future. This period was projected to be warmer by 2 and 3 degrees C (3.6 and 5.4 degrees F).

The computer looked at when extreme heat and precipitation occurred during each climate period. It focused on instances of unusual heat or heavy precipitation. Before the Industrial Revolution, such extremes typically would have occurred only once in every 1,000 days.

But since the globe has been warming, such extremes are no longer nearly as rare, Fischer and Knutti find. And their mathematical analyses show that changes in the atmosphere due to human activities have been largely behind this warming.

These new findings provide “a global statement,” Fischer says. They show that across the planet there will be more extremes of heat, rain or snow due to global warming. However, he warns, effects may differ a lot from place to place. Indeed, some regions actually may see fewer extreme weather events.

The findings can’t pin any specific weather event on climate change, says Sebastian Sippel. This climate scientist likens it to lung cancer: A doctor can never say which cigarette caused it.

“You can still get the biggest heat that you have ever seen without any human changes,” says Sippel, who works at the Max Planck Institute for Biogeochemistry in Jena, Germany. Still, he notes, the new study does show that warming is having an effect.

Power Words

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cancer  Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

carbon dioxide  A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate   The weather conditions prevailing in an area in general or over a long period.

climate change  Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

computer model  A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

fossil fuels  Any fuel — such as coal, petroleum (crude oil) or natural gas —  that has developed in the Earth over millions of years from the decayed remains of bacteria, plant or animals.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse gas   A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

greenhouse effect   The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants asgreenhouse gases. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk.

Industrial Revolution  A period of time beginning around 1750 marked by new manufacturing processes and a switch from wood to coal and other fossil fuels as a main source of energy.

simulate  To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

weather  Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.