Precipitation and Storm Changes

Precipitation Changes

Increasing temperatures tend to increase evaporation which leads to more precipitation (IPCC, 2007). As average global temperatures have risen, average global precipitation has also increased. According to the IPCC, the following precipitation trends have been observed:

• Precipitation has generally increased over land north of 30°N from 1900-2005, but has mostly declined over the tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have widely by region and over time.
• It has become significantly wetter in eastern parts of North and South America, northern Europe, and northern and central Asia, but drier in the Sahel, the Mediterranean, southern Africa and parts of southern Asia.
• Changes in precipitation and evaporation over the oceans are suggested by freshening of mid- and high-latitude waters (implying more precipitation), along with increased salinity in low-latitude waters (implying less precipitation and/or more evaporation).
• There has been an increase in the number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics.

United States Precipitation Changes

Observations compiled by NOAA's National Climatic Data Center show that over the contiguous U.S., total annual precipitation increased at an average rate of 6.1 percent per century since 1900, although there was considerable regional variability. The greatest increases came in the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (-9.25 percent).

Figure 1: Annual Precipitation Trends 1901-2005. Click on Thumbnail for full size image. Data courtesy NOAA's National Climatic Data Center.

In the Northern Hemisphere's mid- and high latitudes, the precipitation trends are consistent with climate model simulations that predict an increase in precipitation due to human-induced warming. By contrast, the degree to which human influences have been responsible for any variations in tropical precipitation patterns is not well understood or agreed upon, as climate models often differ in their regional projections (IPCC, 2007).

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Storm Changes

There is large natural variability in the intensity and frequency of mid latitude storms and associated features such as thunderstorms, hail events and tornadoes. To date, there is no long-term evidence of systematic changes in these types of events over the course of the past 100 years (IPCC, 2007). Analyses of severe storms are complicated by factors including the localized nature of the events, inconsistency in data observation methods, and the limited areas in which studies have been performed.

The frequency and intensity of tropical storm systems have also varied over the 20th century on annual, decadal and multi-decadal time scales. For example, in the Atlantic basin, the period from about 1995-2005 was extremely active both in terms of the overall number of tropical storm systems including hurricanes as well as in storm intensity. However, the two to three decades prior to the mid-1990s were characterized as a relatively inactive period.

Following the Atlantic hurricane season of 2005, which set a record with 27 named storms, a great deal of attention has focused on the relationship between hurricanes and climate change. Numerous studies were published on possible linkages, with a range of conclusions. To provide an updated assessment of the current state of knowledge of the impact of global warming on tropical systems, the World Meteorological Organization’s hurricane researchers published a consensus statement. Their conclusions include (WMO, 2006):

“Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.”

There is general agreement that no individual events in [2004 and 2005] can be attributed directly to the recent warming of the global oceans…[but] it is possible that global warming may have affected the 2004-2005 group of events as a whole.”