Extreme weather is a weather event such as snow, rain, drought, flood, or storm that is rare for the place where it occurs. For example, normal temperatures at the equator would constitute a heat wave if they occurred at the North Pole. The Intergovernmental Panel on Climate Change (IPCC) suggests that “rare” means in the bottom 10% or top 10% of severity for a given event type in a given location. Because extreme weather is by definition rare, it is difficult to assess the risk of such events, including changes in risk with global warming.
Also, since extreme weather events have always occurred, even before anthropogenic (human-caused) climate change began to be unequivocally present starting in about 1980, it is impossible to attribute any one extreme event to climate change. Climate change is likely to increase the frequency of heat waves or other extreme weather events, but it will never be possible to point to one such event and say that it was caused by climate change.
Nevertheless, it is possible to estimate the effects of climate change on the frequency and average magnitude (strength) of extreme weather events. There is evidence that some weather extremes have already shifted: cold nights have decreased globally, for example, while warm nights have increased (associated with heat waves). Droughts, storm intensity, and heat waves have increased and will continue to do so. Most categories of extreme weather events, with the exception of cold waves, are predicted to continue to increase with global warming.
Historical Background and Scientific Foundations
Historical data enable us to see whether global warming has already changed the frequency of various extreme weather events such as heat waves, droughts, floods, and hurricanes. Weather observations of uniform quality for the whole globe have only been available since about 1970, when satellite data were first gathered. Data from earlier decades on tropical cyclones in some parts of the world are spotty, and even after 1970 data on cyclone intensity are not always of good quality
However, during the twentieth century, a global network of weather stations gradually came into being, and adequate data are available for most regions for the last half-century or more. Since 2000, global data collection has improved greatly, with the collection of continent-scale daily data sets, the placement of more closely spaced instruments, and the recovery of data from national archives.
These data show that from 1950 to the early 2000s, the number of heat waves has increased, along with the incidence of warm nights. In most places the number of heavy precipitation events (unusually heavy rainfalls) has increased, along with flood frequency. The intensity of tropical cyclones (called hurricanes when they occur in the Atlantic) has probably increased since 1995, but the frequency of such storms has probably not increased.
The observational record for several important categories of extreme weather events follows. In the final section, projections for changes in these types of extreme weather events during the twenty-first century are described.
On the scale of days rather than of seasons, there is evidence that cold nights have become less common and warm nights more common. This is a global pattern, but specific regions show various patterns. In Central America and northern South America, for example, extremes of both cold and warmth have become more common; in southern South America, cold nights have become rarer and warm nights more common, but there has been no widening of extremes. That is, the more-frequent warm nights are not any warmer, on average, than warm nights used to be in that region.
Beginning in the second half of the twentieth century, heat waves have increased in duration. Although climate scientists frequently caution that no particular weather event can be said to be caused by climate change (or purely natural causes), individual events can illustrate what sorts of event are likely to become more common because of climate change. The 2003 heat wave in Europe is one such event. That June-July-August period was the hottest in Europe since instrumental record-keeping in the region began in 1780, beating the previous record-holder, the summer of 1807, by 2.52°F (1.4°C). Heat waves have become more frequent and longer-lasting in Europe and other areas of the world since the beginning of the twentieth century.
Warming speeds up the drying of soil and so tends to increase the frequency and severity of droughts even apart from decreases in rainfall. Soil moisture has been found to be decreasing over the Northern Hemisphere since the mid–1950s, especially in Eurasia, northern Africa, Canada, and Alaska. Trends have been smaller and more erratic in the Southern Hemisphere. Only extreme decreases in soil moisture correspond to drought. Drought is not strictly defined: agricultural drought refers to low moisture in the topmost yard (meter) or so of soil, which affects crops, while meteorological drought refers to a long period of low precipitation, and hydrologic drought refers to below-normal levels in streams, lakes, and groundwater.
A widely accepted scientific measure of drought is the Palmer Drought Severity Index (PDSI), which assesses soil moisture by combining data on precipitation, temperature, and locally available water. Global PDSI has varied greatly from year-to-year over the last century, but the overall trend has been upward. Most of the world has seen significant increase in PDSI, that is, more drought. Africa, particularly in the Sahel (the east-west band just south of the Sahara desert), has shown particularly strong increases in drought. The only large areas with moistening trends are central South America, western Russia, Scandinavia, and the east-central United States. This pattern is caused by a combination of slightly lower precipitation with higher temperatures due to global warming. According to one study cited by the IPCC, “very dry” zones worldwide have more than doubled in area since the 1970s due to a combination of natural (El Niño–Southern Oscillation) forcings and anthropogenic global warming.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.
CLIMATOLOGIST: Scientist who specializes in the study of climate.
DROUGHT: A prolonged and abnormal shortage of rain.
PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, the water droplets slowly being pulled down by gravity. Because their water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m) and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.
SAHEL: The transition zone in Africa between the Sahara Desert to the north and tropical forests to the south. This dry land belt stretches across Africa and is under stress from land use and climate variability.
TROPICAL CYCLONES: Large rotating storm systems characterized by a clear, low-pressure center surrounded by spiral arms of thunderstorms. Such storms form in the tropics because they are powered by the thermal energy of warm surface ocean waters. In the Atlantic, tropical cyclones are termed hurricanes.
One result of global warming is increased evaporation. Because evaporation acts to cool land, areas where moisture has increased, such as eastern North and South America, have warmed less than other parts of the world. One result of increased evaporation is that there is more moisture available for extreme precipitation events— unusually massive downpours or snowstorms. The former can be especially destructive, causing flooding. The IPCC finds that there has been a shift toward more precipitation coming from very wet days (upper 5% of wet-day rainfall intensities) in recent decades. There will likely be a 2–4% increase in the number of extreme precipitation vents in middle and high latitudes in the coming century. One study (Palmer and Raäisaänen, 2002) has found that the frequency of extreme precipitation events will increase by about a factor of five over parts of the United Kingdom by 2100. Data are still not adequate to form a consistent account of such changes in the tropics and subtropics.
A 2002 study by P. C. D. Milly and colleagues investigated changes in great floods, that is, floods above 100-year levels from 29 drainage basins larger than 77,220 square mi (200,000 square km). The scientists found that there has been a substantial increase in such floods since the beginning of the twentieth century, that this increase is consistent with computer climate models, and that the models predict such floods will continue to increase in frequency with global warming.
Tropical cyclones (called hurricanes when they occur in the Atlantic Ocean) have probably increased in intensity and duration since about 1995. They have probably not increased in frequency (about 60 occur each year worldwide, for reasons that are not yet understood). Warming of sea surface temperatures is expected by some scientists to increase hurricane intensity and duration because hurricanes draw their energy from warm surface waters. Climate scientists are divided over the question of whether the observed and predicted increases in intensity and duration of tropical cyclones are real.
Impacts and Issues
Climate scientists project that continued warming of Earth's atmosphere will lead to increased summer drying of soils with increased risk of drought over most of the world's land area. In the business-as-usual (most pessimistic) IPCC projection scenario, the percentage of world land area experiencing extreme drought at any one time increases from 1% today to 30% by 2100. Actual changes will depend on whether efforts to mitigate greenhouse-gas emissions are successful and on the uncertainties involved in predicting Earth's behavior as a physical system.
Although it may seem paradoxical, risk of extreme precipitation and flooding increases even as risk of drought increases. Warmer air has greater water-holding capacity; precipitation will occur in more concentrated events with longer dry periods in between. During the dry periods, soils dry; during the intense precipitation events, water runs off rather than soaking in: dry soils absorb water more slowly than moist soils and sudden bursts of water tend to run off faster than they can be absorbed even under the best conditions.
Heat waves will continue to become more common. The European heat wave of 2003 is representative of the type of heat waves that will become more common as the climate warms. Extreme weather around the world in 2007 was unusually common. Omar Baddour, a climatologist employed by the United Nations's World Meteorological Organization, said that “When we observe such extremes in individual years, it means that this fits well with current knowledge from the IPCC report on global trends” (Associated Press, 2007).
Not all forms of extreme weather will increase under global warming. Computer models project a 50–100% decrease in the number of cold waves (also called cold-air outbreaks or cold snaps) in the Northern Hemisphere relative to rates in the early 2000s. There is not enough evidence, as of late 2007, to say whether tornadoes, hail, lightning, and dust storms are yet occurring at greater frequency or intensity because of global warming.
Primary Source Connection
This section from a 2007 Intergovernmental Panel on Climate Change (IPCC) report examines climate data from 1950 to present. A review of this scientific evidence indicates that there has been a noticeable and quantifiable change in the global climate over the last 50 years.
IN CONTEXT: DESERTIFICATION AND DEATH
Desertification claimed major international attention in the 1970s. This resulted from an extended period of severe drought in the Sahel region during 1968 to 1973, affecting six African countries on the southern border of the Sahara Desert. Although international relief measures were undertaken, millions of livestock died during that prolonged drought, and thousands of people suffered or died of starvation.
Arid lands in parts of North America are among those severely affected by desertification; almost 90% of such habitats are considered to be moderately to severely desertified. The arid and semi-arid lands of the western and southwestern United States are highly vulnerable to this kind of damage.
OBSERVATIONS: SURFACE AND ATMOSPHERIC CLIMATE CHANGE
Frequently Asked Question 3.3
Has there been a Change in Extreme Events like Heat Waves, Droughts, Floods and Hurricanes?
Since 1950, the number of heat waves has increased and widespread increases have occurred in the numbers of warm nights. The extent of regions affected by droughts has also
increased as precipitation over land has marginally decreased while evaporation has increased due to warmer conditions. Generally, numbers of heavy daily precipitation events that lead to flooding have increased, but not everywhere. Tropical storm and hurricane frequencies vary considerably from year to year, but evidence suggests substantial increases in intensity and duration since the 1970s. In the extratropics, variations in tracks and intensity of storms reflect variations in major features of the atmospheric circulation, such as the North Atlantic Oscillation.
In several regions of the world, indications of changes in various types of extreme climate events have been found. The extremes are commonly considered to be the values exceeded 1, 5 and 10% of the time (at one extreme) or 90, 95 and 99% of the time (at the other extreme). The warm nights or hot days (discussed below) are those exceeding the 90th percentile of temperature, while cold nights or days are those falling below the 10th percentile. Heavy precipitation is defined as daily amounts greater than the 95th (or for ‘very heavy’, the 99th) percentile.
In the last 50 years for the land areas sampled, there has been a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Decreases in the occurrence of cold days and increases in hot days, while widespread, are generally less marked. The distributions of minimum and maximum temperatures have not only shifted to higher values, consistent with overall warming, but the cold extremes have warmed more than the warm extremes over the last 50 years …. More warm extremes imply an increased frequency of heat waves. Further supporting indications include the observed trend towards fewer frost days associated with the average warming in most mid-latitude regions.
A prominent indication of a change in extremes is the observed evidence of increases in heavy precipitation events over the mid-latitudes in the last 50 years, even in places where mean precipitation amounts are not increasing. For very heavy precipitation events, increasing trends are reported as well, but results are available for few areas.
Drought is easier to measure because of its long duration. While there are numerous indices and metrics of drought, many studies use monthly precipitation totals and temperature averages combined into a measure called the Palmer Drought Severity Index (PDSI). The PDSI calculated from the middle of the 20th century shows a large drying trend over many Northern Hemisphere land areas since the mid-1950s, with widespread drying over much of southern Eurasia, northern Africa, Canada and Alaska, and an opposite trend in eastern North and South America. In the Southern Hemisphere, land surfaces were wet in the 1970s and relatively dry in the 1960s and 1990s, and there was a drying trend from 1974 to 1998. Longer-duration records for Europe for the whole of the 20th century indicate few significant trends. Decreases in precipitation over land since the 1950s are the likely main cause for the drying trends, although large surface warming during the last two to three decades has also likely contributed to the drying. One study shows that very dry land areas across the globe (defined as areas with a PDSI of less than -3.0) have more than doubled in extent since the 1970s, associated with an initial precipitation decrease over land related to the El Niño–Southern Oscillation and with subsequent increases primarily due to surface warming.
Changes in tropical storm and hurricane frequency and intensity are masked by large natural variability. The El Niño–Southern Oscillation greatly affects the location and activity of tropical storms around the world. Globally, estimates of the potential destructiveness of hurricanes show a substantial upward trend since the mid– 1970s, with a trend towards longer storm duration and greater storm intensity, and the activity is strongly correlated with tropical sea surface temperature. These relationships have been reinforced by findings of a large increase in numbers and proportion of strong hurricanes globally since 1970 even as total numbers of cyclones and cyclone days decreased slightly in most basins. Specifically, the number of category 4 and 5 hurricanes increased by about 75% since 1970. The largest increases were in the North Pacific, Indian and Southwest Pacific Oceans. However, numbers of hurricanes in the North Atlantic have also been above normal in 9 of the last 11 years, culminating in the record–breaking 2005 season.
Based on a variety of measures at the surface and in the upper troposphere, it is likely that there has been a poleward shift as well as an increase in Northern Hemisphere winter storm track activity over the second half of the 20th century. These changes are part of variations that have occurred related to the North Atlantic Oscillation. Observations from 1979 to the mid-1990s reveal a tendency towards a stronger December to February circumpolar westerly atmospheric circulation throughout the troposphere and lower stratosphere, together with poleward displacements of jet streams and increased storm track activity. Observational evidence for changes in small-scale severe weather phenomena (such as tornadoes, hail and thunderstorms) is mostly local and too scattered to draw general conclusions; increases in many areas arise because of increased public awareness and improved efforts to collect reports of these phenomena.
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