Climate Variability and Extremes
When it comes to exploring our vulnerability to the varying climate, it is the extreme events that provide the most important messages. Extreme events have a disproportionate impact on the lives we lead, and so represent a vital aspect of climate variability and change.
People around the world have measured climatic conditions for many years, so it is possible to define what is considered normal and what is an extreme event for any part of the world. Data gathered over the 30-year period from 1961 to 1990 define the latest “Normals” used for climate reference. At any given time of the year, an extreme high temperature might be defined as one that occurs only once in every 30 years. A cold winter or hot summer can be specified in a similar way, or in terms of the number of days below or above defined exceptional values. This means that when there is a succession of extremes, or more extreme events over a period such as a season, it is possible to estimate whether these extremes are part of the normal expectation for the locality, or are so unlikely that they can only be explained in terms of some more radical shift in climate. The image above is of extreme rainfall that caused flooding in Queensland Australia in 2008.
The basic properties of any data series, for example temperature, can be defined in terms of the mean over time and the amount of variance about the mean. Other meteorological variables require more complicated statistical calculations. For instance, rainfall is episodic. In many parts of the world, much of the annual rainfall falls in a short rainy season. In addition, most of that rain may be concentrated in a few heavy falls and small shifts in the large-scale weather patterns from year to year may significantly alter the amount and/or the distribution of seasonal rainfall. More complex techniques are usually needed to interpret variations in rainfall.
Some facts on global climate extremes
Extreme meteorological events may be good markers of climatic change or variability, so it is important to keep good records of such extremes. A worldwide collection of such events has been collected by WMO in conjunction with the University of Arizona. Data has been collected on all types of events such as temperature, pressure, rainfall, hail, aridity, wind, tornados and cyclones. Some of the information can be seen in the table below:
From climate extremes to disasters
On 10 February 1935, 35 cm of snow fell on Laghouat on the edge of the Algerian Sahara. While this was certainly an extreme event, it was no disaster. Disasters occur frequently as a result of extreme climatic events, and also as a result of the accumulation of extreme events that constitute climatic variability or change. The word disaster is used to describe such events when they cause human sickness, death or migration on a large scale, or when they cause severe economic damage.
Although human misery cannot be adequately represented by statistics, it is helpful to have some measure of the global scale of the impact of weather-related disasters. Many data have been collected by the International Federation of the Red Cross and Red Crescent Societies (IFRC).
WMO contributes to the Disaster Risk Reduction community by providing scientific and technical services. This includes observing, detecting, monitoring, predicting and early warning of a wide range of weather–, climate- and water-related hazards.
[More in depth information] on WMO and disasters risk reduction can be found on the WMO disaster risk reduction programme website.
Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. On average 80 tropical cyclones form every year. They are called differently depending on where they are formed: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region.
Tropical cyclones are also given individual names. From famous ones include Hurricane Katrina, Cyclone Tracy and Typhoon Tip. Tropical cyclones are given names rather than numbers to make them easier to remember and warn communities if required.
[Further information] about cyclone names and the history can be found in the world weather watch pages.
Mid-latitude winter storms
Heavy rain and snow are dangerous for vulnerable communities. They can inhibit rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.
As an example, the East Asian winter is dominated by cold and relatively dry winds. The Siberian anticyclone is a common climatic feature that blows Arctic air over Siberia and northern China. Part of the flow sweeps out toward the North Pacific and part southward through China to the equatorial regions. The winter monsoon throughout this region is characterized by successive outbursts of cold air, called cold waves, that produce sharp falls in temperature of more than 10°C and are accompanied by snow and, in the south, rain. Snowstorms can be especially violent over northern China with sub-zero temperatures and gales lasting many days. The snowstorms can be damaging to communities and are particularly disruptive to transport, including coastal shipping. Periods of frost following the cold outbreak last several days and are a major hazard over the south of China as they have far-reaching effects on agriculture, especially on plants and crops. The frequency of cold waves vary greatly from year to year; as many as 10 per year or as few as one have been experienced. Periods where numerous cold waves occur are associated with higher pressure within the Siberian anticyclone and an intense low pressure system near the Aleutian Islands.
Droughts and dust storms
The primary cause of any drought is a lack of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.
Dust storms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.
Floods can occur anywhere after heavy rains. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surges caused by tropical cyclones, tsunamis or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide.
[Technical information] WMO has a flood forecasting Initiative to help prevent disasters through flooding.
The term ‘monsoon’, of Arabic origins, refers to a steady seasonal wind, and became widely associated with the Indian subcontinent and the onset of the main rainfall season. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world, including India, East Asia, Australia, and both North and South America.
Monsoon forecasts have improved since the early 1980s. This is the result of a growing understanding of the empirical relationships between indicators around the world and the subsequent monsoon. One reason for these advances has been the rising quality of data. Recent satellite observations have also revived interest in Himalayan snow cover as a predictor of monsoons.
A relationship between monsoons and Himalayan snow cover was first identified by Blandford, but the satellite observations have shown that it is more like a useful guide than a strict predictor. The extent of the all-Eurasian winter snow cover is a better indicator, given the geographically uneven and variable nature of snow cover over the Himalayas, Tibet and Siberia.
Heatwaves and cold waves/frost
Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of Western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.
[More in depth information] WMO has a severe world weather information service tracking dangerous and severe weather systems
[More in depth information] The annual WMO statement on the Status of the Global Climate looks at the global climate, including extreme events over the last year and how it compares to previous years.
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