what We Know: Underlying Processes.

The earth receives a tremendous amount of energy from the sun. The land, sea, and air absorb some of this energy and reflect some of it back into space. The overall description of this process is called the earth’s energy budget. (See “Global Reflected Shortwave Solar Radiation” and “Global Outgoing Longwave Heat Radiation” on this site to learn more.)
The “greenhouse effect” is one aspect of this energy budget. Just as the glass walls of a greenhouse keep the interior temperature higher than that outside, the earth’s atmosphere traps some of the energy radiated from the earth near the planets surface. The presence of “greenhouse gases” (like water vapor and carbon dioxide), keeps the planet’s average temperature at a hospitable 15°C. (With no greenhouse effect, the earth’s average temperature would stabilize at about -18°C). Not all components of the atmosphere are greenhouse gases, however; in fact, oxygen and nitrogen, which together make up more than 95% of our atmosphere, are not greenhouse gases.

This diagram shows the processes that make up the planet’s energy budget. The earth’s surface absorbs shortwave radiation (red arrows) and re-radiates longwave infrared radiation (blue arrow). The numbers are percentages: For example, 30% of the solar radiation shining on the earth is reflected away.

The greenhouse effect is not in dispute—but it lies at the heart of the study of global climate change.


There’s no doubt that increases in the atmospheric concentration of carbon dioxide and other greenhouse gases strengthens the greenhouse effect and contributes to global warming.
What remains uncertain are the precise effects of a strengthened greenhouse effect on global temperatures. Because there is still much to be learned about how the world’s climate will react to increased greenhouse gas concentrations, the range of possible climate futures projected by the IPCC is an indication of uncertainty about how much the world will warm over the coming century—not of whether that warming is happening.


Aerosols are another key component of the earth’s atmosphere. These are suspended liquid and solid particles, including things like soot from fires and volcanic eruptions, sea salt, bacteria, and viruses. Aerosols affect the earth’s energy budget by scattering and absorbing radiation:


Overall, aerosols likely exert a cooling effect, because many of these particles tend to prevent radiation from reaching the planet’s surface (although due to their size and shape, some aerosols may help trap heat near the ground).

This satellite image shows a dust plume from the Sahara Desert blowing across the Atlantic Ocean. The green to red colors in the dust plume image represent increasing densities of tiny airborne particles known as aerosols.

Evidences and Uncertainties

Measurements from a variety of sources have suggested that the earth’s average atmospheric temperature has risen over the last several hundred years—but by how much? Taking the average temperature of the earth’s atmosphere is a very difficult measurement problem. First, measurements must be taken in a large and diverse enough range of locations to ensure that their average is truly a measure of global temperature and is not biased toward one region or another. Second, those locations must be chosen so that individual measurements are not thrown off by sources of unusually high or low temperatures, such as cities (which tend to be “heat islands” warmer than the surrounding landscape). Third, no measuring device is perfect—all measurements include some amount of error, or “noise.” Understanding the kinds of errors associated with different measurement techniques is a key element in evaluating the accuracy of a given temperature value. In addition, the study of climate requires measurements over very long time periods, so sources of paleoclimate data (data on climate from the distant past) are key to understanding climate change. (See “Global Stratospheric and Tropospheric Temperature Anomalies (1979–2001)” on this site to learn more about the problems of measurement.)

How much will global atmospheric temperatures change over the next century? Two kinds of problems make this an exceptionally difficult question to answer. (See “Sample Forecasts of Future Temperature Change” on this site for some possible answers.) First, the enormous complexity of the earth’s dynamic climate system—including the interacting air masses, winds and, ocean currents, and patterns of evaporation and precipitation—makes long-term climate prediction extremely problematic. Estimates drawn from reports by the Intergovernmental Panel on Climate Change (IPCC) project increases in average global temperatures ranging from 1.4 degrees to 5.8 degrees C by the year 2100. These numbers may seem small, but because average global temperatures are actually remarkably stable over long periods, this range actually represents a very significant rise in the earth’s temperature over a very short time.

A second problem complicating the picture is the unpredictability of human behavior. At what rate will the human population—and its production of carbon dioxide—grow? As formerly undeveloped countries expand their industry, often using cheaper (and more polluting) fossil-fuel technology, their contributions to greenhouse gases will rise and add to the problem—but by how much? To what extent will new, cleaner technologies (such as cars powered by hydrogen fuel cells) be developed and adopted by countries around the world? These kinds of uncertainties make the tough problem of predicting climate change all the more difficult. Even moderate increases in atmospheric temperatures could alter precipitation levels, making some areas wetter and others drier, and affecting agriculture worldwide. Warmer temperatures could increase the frequency and strength of storm systems, leading to more powerful and destructive hurricanes and subsequent flooding. Small shifts in the earth’s temperature could result in more powerful and destructive hurricanes. This satellite image shows Hurricane Isaac on the afternoon of September 29, 2000.

Slight changes in temperature may lead to higher ozone levels near the earth’s surface. This could significantly increase smog problems in large cities—bad for all of us, but especially serious for many elderly, ill, or otherwise physically vulnerable citizens.

Small increases in atmospheric temperatures could also change the way clouds form and dissipate. Warmer temperatures near the ground could cause lower clouds to evaporate, letting heat rise farther into the atmosphere.

As this heated air rises and cools, higher clouds form. But lower clouds usually reflect sunlight back into space while higher clouds tend to absorb more heat. More high clouds mean more heat trapped near the earth’s surface—so small increases in temperature could set off a cycle in which the atmosphere holds more and more heat over time.

(This is an example of a positive feedback loop—a system in which small changes in one direction may set the stage for later, larger changes in the same direction. But we don’t yet know whether positive feedback loops like this will dominate future climate, or whether other factors will prevent patterns like this from unfolding. )