Introduction

Frequently Asked Questions about Global Warming
(Union of Concerned Scientists)

What does the greenhouse effect have to do with global warming?

The "greenhouse effect" refers to the natural phenomenon that keeps the Earth in a temperature range that allows life to flourish. The sun's enormous energy warms the Earth's surface and its atmosphere. As this energy radiates back toward space as heat, a portion is absorbed by a delicate balance of heat-trapping gases in the atmosphere -- among them carbon dioxide and methane -- which creates an insulating layer. With the temperature control of the greenhouse effect, the Earth has an average surface temperature of 59°F (15°Celsius). Without it, its average surface temperature would be 0°F (-18°Celsius), a temperature so low, that the Earth would be frozen and could not sustain life.

"Global warming" refers to the rise in the Earth's temperature resulting from an increase in heat-trapping gases in the atmosphere.


What is causing global warming?

Scientists have concluded that human activities are contributing to global warming by adding large amounts of heat-trapping gases to the atmosphere. Our fossil fuel use is the main source of these gases. Every time we drive a car, use electricity from coal-fired power plants, or heat our homes with oil or natural gas, we release carbon dioxide and other heat-trapping gases. The second most important source of greenhouse gases is deforestation, mainly in the tropics, and other land-use changes.

Since pre-industrial times, the atmospheric concentration of carbon dioxide has increased by 31 percent. Over the same period, atmospheric methane has risen by 151 percent, mostly from agricultural activities like growing rice and raising cattle.

As the concentration of these gases grows, more heat is trapped by the atmosphere and less escapes back into space. This increase in trapped heat changes the climate, causing altered weather patterns that can bring unusually intense precipitation or dry spells and more extreme severe storms.


What is the best source of scientific information on global warming?

In 1988 the United Nations Environment Programme and the World Meteorological Organization set up the Intergovernmental Panel on Climate Change (IPCC) to examine the most current scientific information on global warming and climate change. More than 2,500 of the world's leading climate scientists, economists, and risk experts have contributed to the panel's most recent report, Climate Change 2001: The IPCC Third Assessment Report.

Scientists from about 100 countries were involved in this new report -- more than in any previous report and with greater participation from developing countries. These scientists reviewed all the published and peer-reviewed scientific information produced over the last few years to assess what is known about the global climate, why and how it changes, what it will mean for people and the environment, and what can be done about it.

The Third Assessment Report is the most comprehensive and up-to-date evaluation of global warming. As the new benchmark, it serves as the basis for international climate negotiations.


Is global warming already happening?

Yes. The Intergovernmental Panel on Climate Change concluded in its Third Assessment Report, "An increasing body of observations gives a collective picture of a warming world and other changes in the climate system." The kinds of changes already observed that create this consistent picture include the following.

Examples of observed climatic changes:

Increase in global average surface temperature of about 1 degree Fahrenheit in the 20th century

Decrease of snow cover, sea-ice ice extent and the retreat of mountain glaciers in the latter half of the 20th century

Rise in global average sea level and the increase in ocean water temperatures

Likely increase in average precipitation over the mid- and high-latitudes of the Northern Hemisphere, and over tropical land areas

Increase in the frequency of extreme precipitation events in some regions of the world

Examples of observed physical and ecological changes:

Thawing of permafrost
Lengthening of the mid- and high-latitude growing season
Poleward and upward shift of plant and animal ranges
Declines of some plant and animal species
Earlier flowering of trees
Earlier emergence of insects
Earlier egg-laying in birds


Are humans contributing to global warming?

For the first time in 1995, the world's climate experts in the Intergovernmental Panel on Climate Change concluded in a cautious consensus, "The balance of evidence suggests that there is a discernible human influence on the global climate."

In its 2001 assessment, the IPCC strengthened that conclusion considerably saying, "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

Scientists have found significant evidence that lead to this conclusion:

The observed warming over the past 100 years is unlikely to be due to natural causes alone; it was unusual even in the context of the last 1,000 years.

There are better techniques to detect climatic changes and attribute them to different causes.

Simulations of the climate response to natural causes (sun, volcanoes etc.) over the second half of the 20th century alone cannot explain the observed trends.

Most model simulations that take into account greenhouse gas emissions and sulphate aerosols (which have a cooling effect) are consistent with observations over the last 50 years.


How much warmer is the Earth likely to become?

The Third Assessment Report from the Intergovernmental Panel on Climate Change projects that the Earth's average surface temperature will increase between 2.5 and 10.4°F (1.4 to 5.8°C) between 1990 and 2100 if no major efforts are undertaken to reduce the emissions of greenhouse gases (the "business-as-usual" scenario). This is significantly higher than what the Panel predicted in 1995 (1.8-6.3°F or 1.0-3.5°C), mostly because scientists expect a reduced cooling effect from tiny particles (aerosols) in the atmosphere.

Scientists predict that even if we stopped emitting heat-trapping gases immediately, the climate would not stabilize for many decades because the gases we have already released into the atmosphere will stay there for years or even centuries. So while the warming may be lower or at a slower rate than predicted if we reduce emissions significantly, global temperatures cannot quickly return to today's averages. And the faster and more the Earth warms, the greater are the chances for some irreversible climate changes.


Would a temperature rise of a couple degrees really change the global climate?

An increase of a few degrees won't simply make for pleasantly warmer temperatures around the globe. Even a modest rise of 2° to 3°F (1.1-1.7°C) could have dramatic effects. In the last 10,000 years, the Earth's average temperature hasn't varied by more than 1.8°F (1.0°C). Temperatures of only 5° to 9°F cooler than those today prevailed at the end of the last Ice Age, in which the Northeast US was covered by more than 3,000 feet of ice.

Scientists predict that continued global warming on the order of 2.5-10.4 degrees Fahrenheit over the next 100 years (as projected in the IPCC's Third Assessment Report) is likely to result in:

a rise in sea level between 3.5 and 34.6 in. (9-88cm) leading to more coastal erosion, flooding during storms and permanent inundation

severe stress on many forests, wetlands, alpine regions, and other natural ecosystems

greater threats to human health as mosquitoes and other disease-carrying insects and rodents spread diseases over larger geographical regions

disruption of agriculture in some parts of the world due to increased temperature, water stress and sea-level rise in low-lying areas such as Bangladesh or the Mississippi River delta.