Projected Changes in Climate Variability

As in other highly developed nations, U.S. communities and industries have made substantial efforts to reduce their vulnerability to normal weather and climate fluctuations. However, adaptation to potential changes in weather extremes and climate variability is likely to be more difficult and costly. Unfortunately, projections of such changes remain quite uncertain, especially because variations in climate differentially affect different regions of the country. Perhaps the best-known example of a natural variation of the climate is caused by the El Niño -- Southern Oscillation (ENSO), which is currently occurring every several years. ENSO has reasonably well-established effects on seasonal climate conditions across the country. For example, in the El Niño phase, unusually high sea-surface temperatures (SSTs) in the eastern and central equatorial Pacific act to suppress the occurrence of Atlantic hurricanes (Figure 6-4) and result in higher-than-average wintertime precipitation in the southwestern and southeastern United States, and in above-average temperatures in the Midwest (Figure 6-5). During a strong El Niño, effects can extend into the northern Great Plains.

During the La Niña phase, which is characterized by unusually low SSTs off the west coast of South America, higher-than-average wintertime temperatures prevail across the southern half of the United States, more hurricanes occur in the tropical Atlantic, and more tornadoes occur in the Ohio and Tennessee valleys (Figures 6-4 and 6-5). During the summer, La Niña conditions can contribute to the occurrence of drought in the eastern half of the United States.

Other factors that affect the inter-annual variability of the U.S. climate include the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO).

The PDO is a phenomenon similar to ENSO, but is most apparent in the SSTs of the North Pacific Ocean. The PDO has a periodicity that is on the order of decades and, like ENSO, has two distinct phases -- a warm phase and a cool phase. In the warm phase, oceanic conditions lead to an intensification of the storm-generating Aleutian Low, higher-than-average winter temperatures in the Pacific Northwest, and relatively high SSTs along the Pacific Coast. The PDO also leads to dry winters in the Pacific Northwest, but wetter conditions both north and south of there. Essentially, the opposite conditions occur during the cool phase.

The NAO is a phenomenon that displays a seesaw in temperatures and atmospheric pressure between Greenland and northern Europe. However, the NAO also includes effects in the United States. For example, when Greenland is warmer than normal, the eastern United States is usually colder, particularly in winter, and vice-versa.

Given these important and diverse interactions, research is being intensified to improve model simulations of natural climate variations, especially to improve projections of how such variations are likely to change. Although projections remain uncertain, the climate model of the Max Planck Institute (MPI) for Meteorology in Germany, which is currently considered to provide the most realistic simulation of the ENSO cycle, calculates stronger and wider swings between El Niño and La Niña conditions as the global climate warms (Timmermann et al. 1999), while other models simply project more El Niño-like conditions over the eastern tropical Pacific Ocean (IPCC 2001d). Either type of result would be likely to cause important climate fluctuations across the United States.

Using the selected model scenarios as guides, but also examining the potential consequences of a continuation of past climate trends and of the possibility of exceeding particular threshold conditions, the National Assessment focused its analyses on evaluating the potential environmental and societal consequences of the climate changes projected for the 21st century, as described in the next section.