However, several gases in Earth’s atmosphere — such as carbon dioxide, methane and water vapor —work like a blanket to retain much of this heat. That helps to warm our atmosphere. The gases do this by absorbing the heat and radiating it back to Earth’s surface. Such gases are nicknamed “greenhouse gases” because of their heat-trapping effect. Without this so-called greenhouse effect, Earth would be too cold to support most forms of life.

But you can have too much of a good thing. Carbon dioxide is released when people use fossil fuels, such as coal, oil and natural gas. These fuels come from the ancient remains of plants and animals. Products of these fossil fuels, such as gasoline and diesel fuel, power most of the engines that drive cars, airplanes and ships. Coal and other fossil fuels are also burned to run electricity-generating plants that power factories, homes and schools.

By examining air bubbles in ice cores taken from Antarctica, scientists can go back and calculate what the concentrations of carbon dioxide in the atmosphere have been throughout the last 650,000 years. The amount of carbon dioxide in the atmosphere has been climbing to where today it is 30 percent greater than 650,000 years ago.

That rise in carbon dioxide “is essentially entirely due to the burning of fuels,” Susan Solomon says. An atmospheric chemist at the Massachusetts Institute of Technology, outside Boston, she studies factors that affect Earth’s atmosphere and climate.

People have further increased the levels of greenhouse gases in the air by changing the landscape. Plants take up carbon dioxide to make food in a process called photosynthesis. Once cut down, plants can no longer take in carbon dioxide. One result: This gas can begin building up in the air instead of fueling the growth of plants. So by cutting down trees and forests for farmland and other human uses, more carbon dioxide also enters the atmosphere.

“We’ve always had some greenhouse gases in the atmosphere,” Solomon says. “But because we’ve burned a lot of fossil fuels and deforested parts of the planet, we’ve increased the amount of greenhouse gases, and as a result have changed the temperature of the planet.”

El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have also linked unusual weather events — including ice storms, droughts and mudslides — to the arrival of an El Niño.

Today, researchers use the term El Niño only for those periods when the surface water around the equator in the eastern and central Pacific warms for an extended period of time.  Scientists declare the development of an El Niño when they observe a temperature increase of at least 0.4 degree Celsius (0.72 degree Fahrenheit) for five months in a row in the eastern Pacific near the equator.

At other times, the surface water in the eastern Pacific instead may cool for long stretches of time. When the average temperature drops by at least 0.4° C (0.72° degree F), climate scientists will announce the arrival of a La Niña (lah NEEN yah). This is Spanish for “the girl.” In general, effects of a La Niña run opposite to those triggered by an El Niño: Now, Central and South America may face severe droughts while Australia floods.

Much of the rest of the world, including large portions of Africa and North America, may also see substantial climate impacts from ENSO events.