Global Warming and Extreme Weather

introduction

For the past hundred-and-fifty years or so, the Earth’s atmosphere has been getting warmer. Human beings have caused this warming themselves—at least partly—by using “fossil fuels” such as coal and oil. When these fuels are burned, carbon dioxide is released into the air. Because carbon dioxide is a “greenhouse gas,” more heat is trapped in the atmosphere and the world becomes warmer.

part one: why climate scientists are predicting extreme weather

Climate scientists are predicting that during the 21st Century “global warming” will cause a worldwide increase in “extreme weather.” Because of global warming, they say, there will be more heavy rain and more severe droughts.

Global warming will bring more heavy rain because, with a warmer atmosphere, there will be more “evaporation” from the Earth’s oceans. Evaporation occurs when water molecules escape from an area of liquid water and turns into a gas called “water vapor.” The warmer the weather, the more quickly the molecules on the surface of the water move around and the more likely they are to escape into the air.

As the warm air near the Earth’s surface rises up to higher levels of the atmosphere, it carries water vapor with it. As the air rises, it cools, and this cooling eventually causes the water vapor to “condense.” Tiny “droplets” of liquid water collect around the dust particles that are suspended everywhere in the atmosphere. The clouds we see in the sky are made up of millions of these droplets. If the air is very moist, the droplets will grow until they become “raindrops” about 0.5 millimeters in diameter. These raindrops are too heavy to remain suspended in the air and, so they fall toward the Earth as rain.

Because heat causes evaporation to happen more quickly, as the atmosphere warms, it will contain more and more water vapor; in other words, it will be more “moist.” Climate scientists predict that this moister, warmer atmosphere will lead to more storms, and that, on average, these storms will be bigger and stronger. They will also bring more rain and, most importantly, more “downpours,” which will cause floods and landslides. The run-off water from these storms will also cause erosion, which will damage farm land and, in the long-term, destroy forests and allow deserts to grow.

Climate scientists believe that heavy rain is not the only kind of extreme weather that will be more common on a warmer Earth. They also say that global warming will likely make extreme “droughts”—long periods of very dry weather—more common than they have been in the past. This prediction seems at first to contradict the prediction of increased rainfall: very wet weather and very dry weather are opposites. How could they both be caused by the same thing?

The answer is that just as evaporation removes water molecules from the surface of the ocean, it also takes water away from land surfaces. In other words, evaporation “dries out” moist soil. And, on land too, the hotter the atmosphere, the more quickly evaporation works. Because of this connection between heat and evaporation, a quite small rise in average temperature can cause especially severe droughts with terrible consequences. Farm crops die. Food prices go up; farmers go bankrupt. In poor countries, people may starve.

part two: are the predictions coming true?

All these predictions about the future effects of global warming are based on solid scientific knowledge and on careful observation. However, they are only predictions and no one can be certain that they will come true. The atmosphere is a complicated system. It has only been under scientific observation for a relatively short period of time, and the sharp increase in carbon dioxide emissions has been going on for an even shorter period. We will not know for certain how our weather will be affected by global warming until more time has passed and more careful measurements have been made.

Despite the uncertainty about the future effects of global warming, many scientists feel that the threat of damage to the environment is so great that governments must “cut back” carbon dioxide emissions immediately. Many governments are reluctant to do this, though, because they feel their countries’ economies will be badly damaged if they force their industries to use less fossil fuel. This reluctance is frustrating for people who believe that global warming is an emergency. They think that something must be done immediately to slow it down. They realize that many governments will do nothing unless they are under public pressure. They also realize, however, that ordinary people will only put pressure on their governments if they think there is a problem “right now.” Floods and droughts that may happen sometime in the future are not likely to upset the public very much. So, global warming “activists” are eager to find evidence that the scientists’ predictions are already coming true. They are sometimes criticized for “jumping to conclusions” too quickly.

As well as predicting that global warming will cause heavy rain and drought, some climate scientists have predicted that it will bring more hurricanes, and that, on average, these hurricanes will be more powerful than in the past. In 2005, a huge hurricane, “Katrina,“ destroyed large parts of the American city of New Orleans. Many people saw this as evidence that global warming was already having an effect on weather. Of course one event, even a very terrible one, cannot indicate a long-term trend. But Katrina wasn’t the only serious hurricane to hit the United States in 2005. In fact, 2005 was a record-breaking year for storm activity in the North Atlantic, and four of these storms, more than ever before, were extremely strong “Category Five” hurricanes. This made it was easy to believe that global warming was already having a spectacular effect on the weather. In the years that followed, though, there were actually fewer hurricanes than usual. So it came to seem that, perhaps, the storms of 2005 were not a sign of global warming, after all.

There are, however, two other sorts of extreme weather that do seem to be occurring more frequently. These types of weather are heavy “precipitation”—rain or snow—and drought. In recent years, in at least three parts of the world, South Africa, Australia, and the United States, an unusually high percentage of total precipitation has come from “extreme weather events.” In other words, there have been many large moisture-laden storms. Across the US, between 1948 and 2006, there was a 25% increase in the number of downpours and heavy snowfalls. It is probably still too early to be certain that global warming is already causing extreme precipitation. But these facts make it seem likely that is happening.

Another event which seems to indicate that the predictions are coming true is the severe drought that struck Australia from 2002 through 2007. This drought has been particularly bad in the heavily-populated southeastern part of the country where Australia’s best farm land is located and so, the whole country’s economy has been damaged by the dry weather. In 2006, for example, mainly because of the drought, the “gross national product” declined by .07%. The world economy has suffered too. Australia is a major wheat exporter and, because of the very poor crops there, world wheat prices rose sharply during the drought.

Apart from Antarctica, Australia is the driest of all the continents. It has gone through many long droughts in the past. For that reason most Australians don’t believe that the latest drought is itself caused by global warming. But, they do blame global warming for making the drought worse than it would otherwise have been. This belief is understandable: the warming trend in Australia seems to be stronger than in other parts of the world. Between 1950 and 2001, the average temperature there rose by 0.7°C—more than it rose in the whole world during the entire 20th Century.

Because Australia is so dry, evaporation rates there are always high. Approximately 90% of the rainwater normally returns to the atmosphere through evaporation. The especially hot weather during the drought often pushed the evaporation rate up to nearly a hundred percent. So, when rain fell during this period, it was often of no use to farmers.

There was some relief from the drought during 2007. The south-eastern coast actually had quite a wet summer although farther inland there was not enough rain to help farmers much. And the hot weather has continued: The year 2007 was the hottest ever in the south-eastern state of New South Wales with an average temperature 1.13°C above normal. A government climate scientist quoted in an Australian paper said there was no doubt that Australia was getting warmer and that the change was possibly permanent. He said, “It’s equivalent to moving New South Wales 150 kilometers closer to the equator.” He added that at least 85% of the rise in temperature was caused by the "enhanced greenhouse effect."

Whether or not global warming wqs making Australia hotter and making Australian droughts worse, many Australians have obviously come to believe that was so. This belief had an effect on the elections there in November 2007. A new Prime Minister, Kevin Rudd, was elected. During his election campaign, he had spoken frequently of the dangers of global warming and had promised, if he were elected, to reduce carbon dioxide emissions.

- fl, 2008