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, particularly in recent decades.

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 nighttime 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 nighttime warming is expected to diminish, in part because the slow heating of the oceans causes a uniform atmospheric warming over time. In any event, society and ecosystems would experience nighttime warming differently than daytime warming, but may be no less problematic. For example, cold nighttime temperatures currently limit the northward spread of certain infectious diseases. The most dangerous consequences of U.S. heat waves, such as the 1995 episode in Chicago that resulted in 500 deaths, arise when nighttime temperatures remain above 80 degrees.

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