CLIMATE CHANGE - PRECIPITATION CHANGES
Precipitation and Storm Changes
Increasing temperatures tend to increase evaporation which leads to more precipitation (IPCC, 2001). As average global temperatures have risen, average global precipitation has also increased. According to the IPCC, the following precipitation trends have been observed:
- Precipitation over land has increased by about 2 percent globally since 1900 but trends have varied regionally and over time.
- Records of land-surface precipitation show an increase of about 0.5 to 1 percent per decade over much of the Northern Hemisphere's mid- and high latitudes. Parts of eastern Russia are a notable exception.
- Over much of the sub-tropical land areas, precipitation has slightly declined by about 0.3 percent per decade, although this trend has weakened in recent decades.
- There is evidence of precipitation increases over the tropical oceans and tropical land areas during the 20th century, although the increases are not as evident during the past few decades.
- Areas receiving more overall precipitation have primarily received this precipitation from events of greater intensity.
United States Precipitation Changes
Observations compiled by NOAA's National Climatic Data Center7 show that total annual precipitation over the U.S has increased by 6.1 percent in the last century, although there is significant regional variability (see Figure 1). Precipitation has increased by at least 10% in the East North Central (climate region 5) and the South (climate region 4). Decreases in precipitation have been observed in the Southwest (climate region 7) and Hawaii (climate region 11).
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, 2001).
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, 2001). 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 global warming. 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):
“No single high impact tropical cyclone event of 2004 and 2005 can be directly attributed to global warming, though there may be an impact on the group as a whole.”
“Whilst the existence of a large multi-decadal oscillation in Atlantic tropical cyclones is still generally accepted, some scientists believe that a trend towards more intense cyclones is emerging. This is a hotly debated area for which we can provide no definitive conclusion. It is agreed that there is no evidence for a decreasing trend in cyclone intensities.”
- IPCC, 2001: Climate Change 2001: The Scientific Basis.10 Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.
- WMO/CAS Tropical Meteorology Research Program, 2006: Statement on Tropical Cyclones and Climate Change 11 (PDF, 4 pp., 143 kb, About PDF12)