Temperature:

Since the record hot year of 1998, six of the last ten years (1998-2007) have had annual average temperatures that fall in the hottest 10% of all years on record for the U.S. Accompanying a general rise in the average temperature, most of North America is experiencing more unusually hot days and nights. The number of heat waves (extended periods of extremely hot weather) also has been increasing over the past fifty years. However, the heat waves of the 1930s remain the most severe in the U.S. historical record.

There have been fewer unusually cold days during the last few decades. The last 10 years have seen fewer severe cold snaps than for any other 10-year period in the historical record, which dates back to 1895. There has been a decrease in frost days and a lengthening of the frost-free season over the past century. In summary, there is a shift towards a warmer climate with an increase in extreme high temperatures and a reduction in extreme low temperatures. These changes have been especially apparent in the western half of North America.

Precipitation Extremes:

Extreme precipitation episodes (heavy downpours) have become more frequent and more intense in recent decades over most of North America and now account for a larger percentage of total precipitation. For example, intense precipitation (the heaviest 1% of daily precipitation totals) in the continental U.S. increased by 20% over the past century while total precipitation increased by 7%.

The monsoon season is beginning about 10 days later than usual in Mexico. In general, for the summer monsoon in southwestern North America, there are fewer rain events, but the events are more intense.

Drought:

Drought is one of the most costly types of extreme events and can affect large areas for long periods of time. Drought can be defined in many ways. The assessment in this report focuses primarily on drought as measured by the Palmer Drought Severity Index, which represents multi-seasonal aspects of drought and has been extensively studied. Averaged over the continental U.S. and southern Canada the most severe droughts occurred in the 1930s and there is no indication of an overall trend in the observational record, which dates back to 1895. However, it is more meaningful to consider drought at a regional scale, because as one area of the continent is dry, often another is wet. In Mexico and the U.S. Southwest, the 1950s were the driest period, though droughts in the past 10 years now rival the 1950s drought. There are also recent regional tendencies toward more severe droughts in parts of Canada and Alaska.

Storms:

Hurricanes and Tropical Storms:

Atlantic tropical storm and hurricane destructive potential as measured by the Power Dissipation Index (which combines storm intensity, duration, and frequency) has increased. This increase is substantial since about 1970, and is likely substantial since the 1950s and 60s, in association with warming Atlantic sea surface temperatures.

There have been fluctuations in the number of tropical storms and hurricanes from decade to decade and data uncertainty is larger in the early part of the record compared to the satellite era beginning in 1965. Even taking these factors into account, it is likely that the annual numbers of tropical storms, hurricanes and major hurricanes in the North Atlantic have increased over the past 100 years, a time in which Atlantic sea surface temperatures also increased. The evidence is not compelling for significant trends beginning in the late 1800s. Uncertainty in the data increases as one proceeds back in time. There is no observational evidence for an increase in North American mainland land-falling hurricanes since the late 1800s. There is evidence for an increase in extreme wave height characteristics over the past couple of decades, associated with more frequent and more intense hurricanes.

Hurricane intensity shows some increasing tendency in the western north Pacific since 1980. It has decreased since 1980 in the eastern Pacific, affecting the Mexican west coast and shipping lanes. However, coastal station observations show that rainfall from hurricanes has nearly doubled since 1950, in part due to slower moving storms.

Other Storms:

There has been a northward shift in the tracks of strong low-pressure systems (storms) in both the North Atlantic and North Pacific over the past fifty years. In the North Pacific, the strongest 7 storms are becoming even stronger. Evidence in the Atlantic is insufficient to draw a conclusion about changes in storm strength.

Increases in extreme wave heights have been observed along the Pacific Northwest coast of North America based on three decades of buoy data, and are likely a reflection of changes in cold season storm tracks.

Over the 20th century, there has been considerable decade-to-decade variability in the frequency of snow storms (six inches or more). Regional analyses suggest that there has been a decrease in snow storms in the South and Lower Midwest of the U.S., and an increase in snow storms in the Upper Midwest and Northeast. This represents a northward shift in snow storm occurrence, and this shift, combined with higher temperature, is consistent with a decrease in snow cover extent over the U.S. In northern Canada, there has also been an observed increase in heavy snow events (top 10% of storms) over the same time period. Changes in heavy snow events in southern Canada are dominated by decade-to-decade variability.

The pattern of changes in ice storms varies by region. The data used to examine changes in the frequency and severity of tornadoes and severe thunderstorms are inadequate to make definitive statements about actual changes.