10 what is going on with precipitation locally. The accumulations depend greatly on the frequency, size and duration of individual storms, as well as the rate (Byers 1948) and these depend on static stability and other factors as well. In particular, the need to vertically transport heat absorbed at the surface is a factor in convection and baroclinic instability both of which act to stabilize the atmosphere. Increased greenhouse gases also stabilize the atmosphere. Those are additional considerations in interpreting model responses to increased greenhouse gas simulations. Another clearly important factor in interpreting observed and modeled changes, not explored here, is the changes in atmospheric circulation which can alter the location and intensity of storm tracks and thereby lead to dipole structures in precipitation changes, with decreases in rainfall in some areas and increases in others. For example, Trenberth and Guillemot (1996) show how storm tracks changed across North America to help bring about the spring-summer 1988 drought and 1993 floods. There is firm evidence that moisture in the atmosphere is increasing. In the Western Hemisphere north of the equator, annual mean precipitable water amounts below 500 mb are increasing over the United States, Caribbean and Hawaii by about 5% per decade as a statistically significant trend from 1973 to 1993 (Ross and Elliott 1996), and these correspond to significant increases in relative humidities of 2 to 3% per decade over the Southeast, Caribbean and subtropical Pacific. Precipitable water and relative humidities are not increasing significantly over much of Canada, however, and are decreasing slightly in some areas. In China, recent analysis by Zhai and Eskridge (1997) also reveals upward trends in precipitable water in all seasons and for the annual mean from 1970 to 1990. Earlier, Hense et al. (1988) revealed increases in moisture in the western Pacific. A claim for recent drying in the tropics by Schroeder and McGuirk (1998) using TOVS data is questionable owing to the changes in instruments and satellites. Clearly, there is a need to obtain more reliable atmospheric moisture trends over the entire globe. Moreover, there is clear evidence that rainfall rates have changed in the United States. The incidence of heavy rainfall events has steadily increased at the expense of moderate rainfall events throughout this century. This has been shown by an analysis of the percentage of the U.S. area with much above normal proportion of total annual precipitation from 1 day extreme events, where the latter are defined to be more than 2 inches (50.8 mm) amounts (Karl et al. 1996). The "much above normal proportion" is defined to be the upper 10%. This quantity can be reliably calculated from 1910, and the percentage has increased steadily from less than 9 to over 11%, a 20% change. Karl and Knight (1998) have provided further analysis of U.S. precipitation increases and show how it occurs mostly in the upper tenth 11 derived from extreme and heavy events is increasing at the expense of more moderate events. Other evidence for increasing precipitation rates occurs in Japan (Iwashima and Yamamoto 1993) and Australia (Suppiah and Hennessy 1996). It has been argued that increased moisture content of the atmosphere favors stronger rainfall and snowfall events, thus increasing risk of flooding. As noted, there is a pattern of heavier rainfalls observed in many parts of the world where the analysis has been done. However, flooding records are confounded by changes in land use and increasing settlement of flood plains. Moreover, because there is a disparity between the rates of increase of atmospheric moisture and precipitation, there are implied changes in the frequency of precipitation and/or efficiency of precipitation (related to how much moisture is left behind in a storm). These arguments may help to explain the exceptional rain and snow falls over the U.S. in the winter of 1996-97. These included heavy rains and flooding in the Pacific Northwest in early January, where observed increases in moisture content of the atmosphere at Hawaii and in the subtropical Pacific (Ross and Elliott 1996) are especially pertinent. Also, heavy snowfalls in the Great Plains and Upper Mississippi Basin led to extensive flooding in the spring of 1997 as snows melted, and heavy rains in the Ohio River Valley which, along with snow melt, also produced extensive flooding. Note that the primary argument here is not that these flooding events would not have occurred but that they have probably been enhanced, perhaps by as much as 10%, because of the increased moisture in the atmosphere, over what would have occurred two decades ago. The above arguments suggest that there is not such a clear expectation on how local total precipitation amounts should change, except as an overall global average. With higher average temperatures in winter expected, more precipitation is likely to fall in the form of rain rather than snow, which will increase both soil moisture and run off, as noted by the IPCC (1996) and found in many models. In addition, faster snow melt in spring is likely to aggravate springtime flooding. In other places, dipolelike structures of precipitation change should occur in places where storm tracks shift meridionally. 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