Carbon dioxide is the main culprit.
The single human activity that is most likely to have a large impact on the climate is the burning of "fossil fuels" such as coal, oil and gas. These fuels contain carbon. Burning them makes carbon dioxide gas. Since the early 1800s, when people began burning large amounts of coal and oil, the amount of carbon dioxide in the earth's atmosphere has increased by nearly 30%, and average global temperature appears to have risen between 1° and 2°F.
Carbon dioxide gas traps solar heat in the atmosphere, partly in the same way as glass traps solar heat in a sunroom or a greenhouse. For this reason, carbon dioxide is sometimes called a "greenhouse gas." As more carbon dioxide is added to the atmosphere, solar heat has more trouble getting out. The result is that, if everything else stayed unchanged, the average temperature of the atmosphere would increase.
As people burn more fossil fuel for energy they add more carbon dioxide to the atmosphere. If this goes on long enough, the average temperature of the atmosphere will almost certainly rise. You can learn more about how this works, and uncertainties in scientific understanding, from Details Booklet Part 1.
If global warming occurs, not every day or every place will be warmer. But on average most places will be warmer. This will cause changes in the amount and pattern of rain and snow, in the length of growing seasons, in the frequency and severity of storms, and in sea level. Farms, forests, and plants and animals in the natural environment, will all be affected.
Other gases and dust also matter.
Carbon dioxide is not the only gas released by human activities that can cause warming. Human emissions of methane and nitrous oxide together contribute almost half as much warming.
Not all things that enter the atmosphere cause warming. Dust from volcanos, and from human activities, can reflect sunlight (like a window shade) and cool the earth.
Coal and oil contain sulfur. When they are burned the sulfur is transformed into fine particles in the atmosphere. This sulfur pollution contributes to various environmental problems. Most scientists think that sulfur particles cool the planet. In the northern hemisphere, this cooling has partly canceled some of the warming that should have come from the growing concentrations of greenhouse gases. However, since emissions of greenhouse gases continue to grow, and most countries are working hard to reduce emissions of sulfur air pollution, this canceling will probably not continue in the future. In that case, the average temperature may rise more rapidly.
How much warming will there be?
If things go on pretty much as they have been, scientists' best guess is that the amount of warming will be about 2.5°F (1.4°C) by the year 2050. The range of uncertainty stretches from almost no change to over 4°F (2.3°C). Details Booklet Part 1 tells you more about how scientists reach these estimates, how much confidence can be placed in them, and what actions could increase or decrease the size of the change.
The ozone hole is a different problem.
Many people confuse the hole in the ozone layer with climate change. However, these two problems are not closely related. The ozone layer protects the earth from harmful ultraviolet light that can cause skin cancer and damage plants and animals. The main cause of the hole in the ozone layer is chlorofluorocarbons (CFCs), gases that are used in refrigerators, air conditioners, and industrial applications. While CFCs alone cause warming, their ozone destruction can cause cooling. So far these warming and cooling influences have approximately balanced. Prior to 1978 CFCs were used as a propellant in aerosol spray cans, but that use has ended in the U.S. Under an international agreement most uses of CFCs are now being phased out to protect the ozone layer.
Is climate change just speculation?
No, as the drawings show, the earth's climate has been changing continually for millions of years. Scientists know many of the things that cause these changes. Some, like the amount of carbon dioxide in the atmosphere, are now being affected by human activities. You can learn more from Details Booklet Part 1.
Human activities that are...|
Climate is not weather.|
weather is the condition of the atmosphere at a particular place and time measured in terms of such things as wind, temperature, humidity, atmospheric pressure, cloudiness, and precipitation (rain, snow, etc.). In most places, weather can change from hour-to-hour, day-to-day, and season-to-season.
climate is the average pattern of weather in a place. For example, San Diego, California has a "Mediterranean climate" which means temperatures are generally moderate year round, there is limited rainfall, and humidity is typically low.