Explainer: How scientists know Earth is warming

With satellites and ice cores, scientists can calculate the planet’s temperature
Nov 24, 2015 — 7:00 am EST
ice core

Scientists can use ice cores, such as this one from Greenland, to “see” past temperatures on Earth.

Jihong Cole-Dai

Temperatures on Earth can soar above 40° Celsius (104° Fahrenheit) and drop well below 0 °C (32 °F). Despite that variability, scientists can calculate a global average. Countries around the world have had reliable weather monitoring stations on land and sea since about 1880. In the 1960s, researchers also began taking the Earth's temperature with the help of satellites. Satellites don't measure temperature directly. Instead they measure radiation given off by oxygen that's in Earth's atmosphere. The intensity of this radiation is linked to air temperatures. Computer programs take all of these data and turn them into a global average.

Scientists use temperature readings from both satellites and climate monitoring stations to show that 2014 was officially the warmest year on record — at least through 2014.

The temperatures have not stopped rising, however. By the close of 2015, temperatures were yet again about to hit a new record high.

People have been keeping detailed temperature readings only as far as 1880. So scientists have to use other methods to calculate what earlier temperatures would have been. With these methods, they now can see that global temperatures have at times in the distant past been even higher than they are today. 

But the historical record also shows that over the past 100 years or so, temperatures have been rising exceptionally fast, notes Allegra LeGrande. She is a climate scientist at the NASA Goddard Institute for Space Studies and the Center for Climate Systems Research, in New York City.

Isotopes are different forms of the same element that vary in their weight. LeGrande finds clues about past and present climates in oxygen and hydrogen isotopes. Warmer temperatures and drier conditions in the atmosphere lead to the water in rain and snow having more heavy isotopes of both oxygen and hydrogen. Cooler, wetter conditions lead to greater numbers of the lighter isotopes of oxygen and hydrogen.

Glaciers and ice sheets trap samples of oxygen and hydrogen isotopes from Earth’s atmosphere going back about one million years. As more snow piles up, the snow on the bottom gets compacted into ice. A new layer accumulates each year.

Scientists drill and remove ice cores — long tubes of ice — from glaciers. Each layer of the ice core contains hydrogen and oxygen going back further in Earth's history. The amount of oxygen and hydrogen isotopes in each layer can tell us how warm or cold the climate was back then.

Those ice cores show current warming is happening faster now than in the past. In the last million years or so, the planet has gone through a series of ice ages. The Earth got cooler, then warmed up. Research by the National Aeronautics and Space Administration shows that during the periods of warming, global temperatures rose 4 °C to 7 °C (7.2 °F to 12.6 °F) over about 5,000 years. But in the past century, the temperature has climbed roughly 10 times faster!

Temperatures are rising quickly now because of greenhouse gases, such as carbon dioxide and methane. When the planet's ground and water are warmed by the sun, they radiate heat back out into the atmosphere. Greenhouse gases trap that heat close to the planet’s surface. That's good, up to a point. Life on Earth needs that warmth to survive. But pollution is adding additional greenhouse gases to the atmosphere. This happens every time people burn fossil fuels such as coal, oil or gas.

During the Industrial Revolution, people started to burn large amounts of fossil fuels. We are burning even more today. Since about 1750, carbon dioxide levels have increased nearly 38 percent (at least through 2009), according to data from the National Aeronautics and Space Administration. Methane concentrations have increased 148 percent. And levels of both greenhouse gases continue to rise.

That has been trapping a lot of heat. As a result, Earth is warming.

Power Words

(for more about Power Words, click here)

atmosphere   The envelope of gases surrounding Earth or another planet.

average    (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

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. The abbreviation for carbon dioxide is CO2.

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

core     In geology, Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.

fossil fuel    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, plants or animals.

glacier    A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

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 as greenhouse 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.

hydrogen    The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

ice age    Earth has experienced at least five major ice ages, which are prolonged periods of unusually cold weather experienced by much of the planet. During that time, which can last hundreds to thousands of years, glaciers and ice sheets expand in size and depth. The most recent ice age peaked 21,500 years ago, but continued until about 13,000 years ago.

ice sheet    A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

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

isotopes   Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.

methane   A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

National Aeronautics and Space Administration    Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.

oxygen    A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

satellite    A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space. 


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Further Reading

A. Biskup. “Explainer: Global warming and the greenhouse effect.” Science News for Students. May 7, 2010.