MYTH: Although thermometers located at Earth�s surface indicate that the planet�s average temperature is higher today than it has been for at least 130 years, satellite measurements of the temperature of the atmosphere thousands of feet above the surface indicate a slight cooling since 1979. The surface temperature data is unreliable due to the heat-trapping effect of urban areas. Therefore, there is no compelling evidence that warming has occurred.

FACT: Since thermometers and satellites measure temperatures at two different places in the atmosphere, it is not surprising that the trends sometimes differ. At higher altitudes, temperatures fluctuate more than at the surface due to natural climate influences like sunlight-reflecting particles from volcanoes. This variability or noise in the satellite record obscures the warming trend due to the buildup of the greenhouse gases which is apparent in the global surface temperature data. Furthermore, the depletion of the ozone layer, which has occurred mostly since 1979, has had a cooling effect on the atmosphere which is more marked at higher altitudes than at the surface.

Earth�s surface has warmed over both the northern and southern hemispheres, and the warming is apparent in data taken both on land and at sea. Therefore, the urban "heat island" effect could not be causing much of the observed warming.

MYTH: Changes in temperature and changes in greenhouse gas emissions over the past century did not occur simultaneously. Therefore, measured warming cannot be due to the greenhouse gases.

FACT: Many factors have influenced climate in addition to the buildup of greenhouse gases, so there is no reason to expect the two trends to match exactly. However, computer models indicate that warming due to the greenhouse gas buildup will dominate the other factors over the coming decades. These factors include small changes in the output of the sun, a haze of particles arising from volcanic eruptions and from fossil fuel burning which reflects sunlight, and natural variations of climate.

In addition, the slow heating of the oceans leads to a lag between emissions and their effect on temperature. Therefore, a simple overlay of greenhouse gas emissions and temperature data is deceptive. When global temperatures are simulated with a computer model which accounts for most of the additional influences, the result is consistent with the observed warming, and the buildup of greenhouse gases is a dominant factor.

MYTH: Computer models are unreliable as a guide to future climate change because they do not reproduce past changes. In particular, they fail to account for the one-half degree warming over the past century.

FACT: When changes in the haze of particles as well as greenhouse gases are taken into account, the models simulate a global temperature trend over the past century which is consistent with the observed warming. In addition, there is increasing agreement between the geographical pattern of climate change predicted by the models and the measured pattern of temperature change. Models reproduce other key features of global climate, including the magnitude of the temperature variation from winter to summer and aspects of ancient climates inferred from ice cores, pollen and fossil data.

MYTH: Carbon dioxide is removed from the atmosphere fairly quickly, so if global warming turns out to be a problem, society can wait until after consequences occur to reduce greenhouse gas emissions.

FACT: Carbon dioxide, which is emitted largely by combustion of fossil fuels, is the most important human-made greenhouse gas. If emissions of carbon dioxide were halted today, it would take more than a century for the atmospheric level of carbon dioxide to approach its pre-industrial level. Furthermore, about 15% of all the carbon dioxide which has been emitted already would remain airborne for thousands of years, causing warming indefinitely. In addition, the slow warming of the ocean creates a lag between emissions and their full effect on temperature. In others words, the consequences of past emissions are not yet entirely apparent.

MYTH: Rapid warming occurred at early times without catastrophic consequences, so society and ecosystems can adapt readily to any foreseeable warming.

FACT: Rapid warming occurred in some places at the end of the last glacial period, but since then, global climate has been stable for ten thousand years. During this time, agriculture and civilization arose, leading to fixed settlements, infrastructure, and large populations in areas which are highly vulnerable to climate shifts, such as low-lying coastal zones.

Furthermore, the existence of rapid climate changes in the distant past provides only limited insight into the fate of natural ecosystems. Previously, rapid climate changes may not have occurred everywhere at once, as they are expected to in the future. Furthermore, there were no human-made barriers to inhibit adaptation by ecosystems. In contrast, human settlements, highways, and farmland will block migration of species adjusting to the warming climate of the coming century.

MYTH: Warming has been occurring largely at night, which is less of a problem than daytime warming.

FACT: Warming over the past century has occurred both during the day and at night, but night-time warming has been the greater of the two. The buildup of the haze of particles from combustion discussed above has probably contributed to this asymmetric warming. But as warming continues, the tendency toward night-time warming is expected to diminish, in part because the slow heating of the oceans causes a uniform atmospheric warming over time. In any event, night-time warming would be experienced by society and ecosystems differently than daytime warming, but may be no less problematic. For example, the northward spread of certain infectious diseases is currently limited by cold night-time temperatures.

MYTH: The IPCC has reduced its projected global warming by 1/3. If we wait a little longer, the threat of human-induced climate change may disappear altogether.

FACT: Computer models of climate have been adjusted to account for the cooling effect of particle haze, which has resulted in lower projections of global average warming by year 2100. The reflection of sunlight by particles partially masks, but does not eliminate, the buildup of the greenhouse effect. When developing countries like China move to reduce the emissions of sulfur dioxide which generate these particles (as the U.S., Europe and Japan already have in order to reduce health effects and acid rain), the hidden warming inevitably will be revealed.

MYTH: Human activities only contribute 4% of CO₂ emissions; the rest comes from natural sources like decaying vegetation and forest fires started by lightning. The human contribution is too small to have a significant effect on climate, particularly since the oceans absorb most of the extra CO₂ emissions.

FACT: Before human beings began to affect the level of CO₂ in the atmosphere, the natural emissions of CO₂ were nearly exactly balanced by natural process which remove CO₂. As a result, the amount of CO₂ in the atmosphere had changed very little for 10,000 years. The additional source of CO₂ from human activity like burning coal and oil for energy, has thrown the system out of balance. Although the oceans and forests absorb about half the CO₂ emitted by industry, the rest builds up in the atmosphere. As a result, CO₂ levels are now 30% above what they were in pre-industrial times. Similar changes of CO₂ occurred naturally tens of thousands of years ago and they were generally accompanied by large shifts in global average temperature.

MYTH: The buildup of CO₂ will lead to a "greening" of the earth because plants can utilize the extra CO₂ to speed their growth.

FACT: Under the controlled conditions which occur in a greenhouse with ample water and fertilizer, plants grow more rapidly in an atmosphere enriched by CO₂. The extent that this effect carries over into natural systems like forests is unknown. Some plants, potentially including weeds, may benefit while others may not. The consequence for forests and other ecosystems is uncertain, but it is unlikely to counteract the adverse impacts of a rapid climate change.

MYTH: If Earth has warmed since pre-industrial times, it is because the intensity of the sun has increased.

FACT: The intensity of the sun continuously changes, but there were no direct measurements of the size of these shifts before 1979. Indirect evidence suggests that the Sun�s variations in the past may have been large enough over the course of centuries to affect Earth�s climate significantly. However, a recent estimate indicates that the warming effect of solar variations since pre-industrial times is only about one-tenth as large as the warming effect of the greenhouse gas buildup. The Sun�s variations since 1979 have been very small.

MYTH: The IPCC now predicts a global warming of only 1 to 3.5 degrees Celsius over the coming century. This small change, much less than the current daily temperature range for most major cities, is hardly a cause for concern.

FACT: The global average temperature has changed by less than a degree Celsius up or down for ten thousand years. So the projected warming is expected to exceed any climate change which has occurred during the history of civilization. In addition, even apparently small global average temperature changes will be accompanied by much larger regional climate shifts. For example, a warming which is twice as large as the global average is projected to occur at high northern latitudes. Apparently small global average changes also led to large climate shifts in the past: Earth�s average temperature increased by only about 5 degrees Celsius between the end of the last ice age and today, but much of the Northern Hemisphere went from being buried under thousands of feet of ice to being ice-free.

MYTH: It is hard enough to predict the weather a few days in advance, so how can we have any confidence in projections of climate a hundred years from now?

FACT: Climate and weather are different. Weather refers to temperatures, precipitation, and storms on a given day at a particular place. Climate reflects a long term average, sometimes over a very large area, like a continent or even the entire Earth. Averages over large areas and periods of time are easier to estimate than the specific characteristics of weather. Nevertheless, scenarios for global warming always recognize a large range of uncertainty.