The Earth’s climate is constantly changing. Seasonal changes throughout the year are one form of climate change, and the climate changes over hundreds of thousands of years as the Earth’s orbit changes the amount and intensity of energy received from the Sun. As the National Center for Atmospheric Research explains, climate change is different from the changes in weather from day to day. But when most people talk about climate change, they mean the changes in the climate from decade to decade that we’re seeing around us now. Since 1880, the average global temperature has increased 1.5°F (0.83°C). Because warmer water expands to take up more volume, and because more land-based ice has been melting into the sea, the ocean has risen 4 to 8 inches (10 to 20 cm) over the last hundred years.
In the Arctic, change is happening even more rapidly. The melting of ice in Greenland, for example, has more than doubled in recent years, with 100 billion tons (90.7 billion metric tons) of ice melting every year on average. Rainfall and other aspects of climate are changing as well, with some parts of the world facing record droughts and others dealing with excessive rainfall. Emerging evidence and projections suggest that hazards such as storm surges, wildfires, drought, and heat waves are becoming more frequent as the rates and extremes of weather and climate are altered by climate change.
Scientists study how the climate changes today by measuring temperature, humidity, wind speed, changing chemistry, and other data at stations around the world. Some are on land, others are carried by balloons and airplanes, and some go on boats and buoys at sea. Satellites let scientists look at temperatures and atmospheric gases across the world. From all of this evidence, scientists are detecting rapid changes in the Earth’s average temperature, as well as in patterns of temperature, of rainfall, and other aspects of climate. By looking at the evidence, such as changes left in rocks, ice cores, tree rings, and sediments at the bottoms of lakes and the ocean, scientists learn how the climate changed thousands and millions of years ago. By comparing the modern evidence to this ancient evidence, scientists find that today’s climate change is on the scale of periods that led to catastrophic mass extinctions in the Earth’s history.
Scientists are not the only ones to see evidence that our climate is changing rapidly. As the National Wildlife Federation has found, hunters notice changes in how and when animals migrate, hibernate, and mate. Birdwatchers and the Audubon Society see similar changes in migration, nesting, and other bird behavior. Fly fishers, whose lures mimic specific insects at specific times, find that fish are hatching out of cycle with the insects’ hatchings. Skiers see changes in expected seasonal snow. Gardeners and farmers see changes in when seeds sprout, when leaves and flowers start budding, and when fruit and leaves fall off. The Arbor Day Foundation has tracked shifts in growing zones for plants, and programs like Project Budburst and the National Phenology Network monitor changes in plant growth. To explore these changes and prepare for them in a scientific manner, many schools across the US and around the world participate in citizen science programs to measure climate change in their own neighborhoods.
From all these lines of evidence — and many more — scientists know that the climate is changing, changing significantly, and changing rapidly.
- The National Climate Assessment by the United States Global Climate Change Program, a US-government-wide body synthesizing information from NASA, NOAA, the EPA, and other agencies involved in climate change research and responses.
- Frequently Asked Questions about the scientific basis of climate change from the Intergovernmental Panel on Climate Change, an international organization that periodically brings together scientists to evaluate the state of climate science.
- NASA’s Paleoclimatology, a series of articles by Holli Riebeek on the scientists who investigate climates from the distant past and how they do their work.
- The Discovery of Global Warming — A History, by historian of science Spencer Weart on behalf of the American Institute of Physics, shows how knowledge about climate change has grown since the 19th century.