Is the U.S. climate getting more extreme?

Is the world's climate getting more extreme, with hotter heat waves, colder cold events, heavier rains, and more extreme droughts? After seeing the unprecedented weather events of 2010 and the equally impressive floods of 2011, it's an important question to be asking. Unfortunately, it's an almost impossible question to answer objectively, because we simply don't have good enough long-term global weather records to do so. However, in the U.S., we do have good enough records to attempt this, and the NOAA National Climatic Data Center (NCDC) has developed a Climate Extremes Index to do so. For 2010, the Climate Extremes Index (CEI) showed that the U.S. had a near-average area experiencing extreme weather conditions. Averaged over decades-long time scales, the U.S. climate has been getting more extreme since 1970, but has not changed significantly over a century-long time scale. The Climate Extremes Index (CEI) is based upon three parameters:

1) Monthly maximum and minimum temperature
2) Daily precipitation
3) Monthly Palmer Drought Severity Index (PDSI)

The temperature data is taken from 1100 stations in the U.S. Historical Climatology Network (USHCN), a network of stations that have a long period of record, with little missing data. The temperature data is corrected for the Urban Heat Island effect, as well as for station and instrument changes. The precipitation data is taken from 1300 National Weather Service Cooperative stations. The Climate Extremes Index defines "much above normal" as the highest 10% of data, "much below normal" as the lowest 10%, and is the average of these five quantities:

1) The sum of: (a) the percentage of the United States with monthly maximum temperatures much below normal, and (b), the percentage of the United States with monthly maximum temperatures much above normal.

2) The sum of: (a) the percentage of the United States with monthly minimum temperatures much below normal, and (b), the percentage of the United States with monthly minimum temperatures much above normal.

3) The sum of: (a) the percentage of the United States in severe drought each month (equivalent to the lowest tenth percentile) based on the Palmer Drought Severity Index (PDSI), and (b), the percentage of the United States with severe moisture surplus (equivalent to the highest tenth percentile) based on the PDSI.

4) Twice the value of the percentage of the United States with a much greater than normal proportion of precipitation derived from extreme (equivalent to the highest tenth percentile) 1-day precipitation events.

5) The sum of: (a) percentage of the United States with a much greater than normal number of days with precipitation, and (b), percentage of the United States with a much greater than normal number of days without precipitation.


Figure 1. The Annual Climate Extremes Index (CEI), updated through 2010, shows that U.S. climate has generally been getting more extreme since the early 1970s, but that the 2010 climate was just slightly more extreme than average. On average since 1910, 21% of the U.S. has seen extreme conditions in a given year (thick black line), and in 2010 this number was about 24%. Image credit: National Climatic Data Center.

As summarized by Gleason et al. (2008), the National Climatic Data Center concludes that based on the Climate Extremes Index, the percentage of the U.S. seeing extreme temperatures and precipitation generally increased since the early 1970s. These increases were most pronounced in the summer. No trend in extremes were noted for winter. The annual CEI index plot averaged for all five temperature and precipitation indices (Figure 1) showed that four of the ten most extreme years on record occurred since 1996. However, some very extreme years also occurred in the 1910s through 1950s, in association with widespread extreme drought and above-average temperatures. The most extreme year in U.S. history was 1998, with 1934 a close second. The year 1998 was the hottest year in U.S. history, with a record 78% of the U.S. experiencing minimum temperatures much above normal. That year also had a record 23% of the U.S. with much greater than normal precipitation from extreme 1-day precipitation events. The 1934 extreme in CEI was due in large part because of the most widespread drought of the century--a full 52% of the U.S. was affected by severe or extreme drought conditions. That year also saw a record 64% of the U.S. with much above normal maximum temperatures.

Maximum and minimum temperatures
It is very interesting to look at the five separate indices that go into the Climate Extremes Index. Today I'll look at temperature, and focus on drought and precipitation in my next post. The portion of the U.S. experiencing month-long maximum or minimum temperatures either much above normal or much below normal has been about 10% over the past century (black lines in Figures 2 and 3.) However, over the past decade, about 20% of the U.S. has experienced monthly maximum temperatures much above normal, and less than 5% has experienced maximum temperatures much cooler than normal. Minimum temperatures show a similar behavior, but have increased more than the maximums (Figure 3). Climate models minimum temperatures should be rising faster than maximum temperatures if human-caused emissions of heat-trapping gases are responsible for global warming, which is in line with what we are seeing in the U.S. using the CEI.

While there have been a few years (1921, 1934) when the portion of the U.S. experiencing much above normal maximum temperatures was greater than anything observed in the past decade, the sustained lack of maximum temperatures much below normal over the past decade is unique. The behavior of minimum temperatures over the past decade is clearly unprecedented--both in the lack of minimum temperatures much below normal, and in the abnormal portion of the U.S. with much above normal minimum temperatures. Remember that these data ARE corrected for the Urban Heat Island effect, so we cannot blame increased urbanization for the increase in temperatures.


Figure 2. The Annual Climate Extremes Index (CEI) for maximum temperature, updated through 2010, shows that about 10% of U.S. had maximum temperatures much warmer than average during 2010. This was right near the average from the past 100 years (thick black line.) Image credit: National Climatic Data Center.


Figure 3. The Annual Climate Extremes Index (CEI) for minimum temperature, updated through 2010, shows that about 35% of U.S. had minimum temperatures much warmer than average during 2010. This was the 7th largest such area in the past 100 years. The mean area of the U.S. experiencing minimum temperatures much warmer than average over the past 100 years is about 10% (thick black line.) Image credit: National Climatic Data Center.

I'll have a new post on Tuesday, when I'll talk about how extremes of drought and precipitation have changed in the U.S. (I may talk instead about the developing winter storm for the Appalachians and Northeast coming Wednesday and Thursday this week, though.)

References
Gleason, K.L., J.H. Lawrimore, D.H. Levinson, T.R. Karl, and D.J. Karoly, 2008: "A Revised U.S. Climate Extremes Index", J. Climate, 21, 2124-2137.

The National Climatic Data Center has a more in-depth discussion of the Climate Extremes Index for the U.S. on a regional and seasonal basis during 2010. In some regions, such as the Southeast U.S., 2010 was a remarkably extreme year.

Jeff Masters