Climate Change & Extreme Events
The climate varies naturally on all time-scales. Variations may occur due to forces such as volcanic eruptions or changes in the Sun's output of energy. They may also be generated by interactions among the different components of the global climate system: the atmosphere, oceans, biosphere, ice cover, and land surface. These internal interactions may cause fairly regular fluctuations, such as the El Niño phenomenon, or apparently random changes in climate.
Natural variability often produces climate extremes and disasters. On time-scales of days, months, and years, variability in weather and climate can produce heat waves, frosts, flooding, droughts, severe storms, and other extremes. An important question which scientists are trying to answer is whether mankind's interference with the climate system through the enhancement of the natural greenhouse effect will increase the frequency or magnitude of extreme weather events. Given the large natural variability and the obvious rarity of extreme weather events it is hard to ascribe the observed phenomena to the enhanced greenhouse effect or even discern a definite trend in extreme event throughout this century. What can be said with certainty, however, is that any change in climate will affect society mainly through extreme weather events.
What are Climatic Extremes and Extreme Weather Events?
Some definitions of climatic extremes choose to separate the nature of the event from its social and economic consequences. A climate extreme, then, is a significant departure from the normal state of the climate, irrespective of its actual impact on life or any other aspect of the Earth's ecology. When a climate extreme has an adverse impact on human welfare, it becomes a climatic disaster. In some parts of the world climatic disasters occur so frequently that they may even be considered part of the norm. It is possible that greenhouse gas-induced climate change will alter the frequency, magnitude, and character of both climate extremes and climatic disasters.
Others have defined climatic extremes or extreme weather events as sufficiently anomalous to cause substantial socio-economic damage. In this second definition, natural and social factors are interpreted together. Thus, it is a socio-economic threshold, which is, for a suitably adapted society, rarely crossed. Rare is defined as the return period of the extreme event being substantially longer than the recovery period of the damage caused.
Types of Extremes
Extreme weather events that may be considered include:
- Droughts (due to increased evaporation and reduced precipitation);
- River floods (due to increased precipitation);
- Landslides (due to increased precipitation);
- Storms, cyclones and tornadoes (due to changing heat transport patterns and increased land-ocean temperature differential);
- Ocean and coastal surges and related flooding (due to atmospheric pressure changes and sea level rise);
- Heat spells and cold snaps.
As the average global temperature increases one would expect that the moisture content of the atmosphere to rise, due to increased rates of evaporation from the sea surface. For every 1°C sea surface temperature rise, atmospheric moisture over the oceans increases by 6-8%. Increases in atmospheric moisture may lead to increased precipitation rates in some parts of the world (causing floods and landslides), whilst decreases may be experienced in other parts (leading to droughts) due to changes in energy and moisture transport patterns in the atmosphere. In general, as more energy and moisture is put into the atmosphere, the likelihood of storms, hurricanes and tornadoes increases.
Return Periods of Extreme Events
Global climate change could well affect the frequency, magnitude and location of extreme events. Any shift in mean climate will almost inevitably result in a change in the frequency of extreme events. In general, more heat waves and fewer frosts could be expected as the mean temperature rises. Computer models which simulate the effects on the global climate of doubling atmospheric carbon dioxide concentrations have revealed some alarming results. A model used by the UK Meteorological Office, for example, has projected that the daily maximum rainfall in North Western Europe will increase by 40%. Consequently, a 1 in 10 year flood becomes a 1 in 3 year flood. Another model predicts that up to 89% of years will be warmer than 1997 in the UK by the 2050s, currently the third warmest year on record.
In the developed world, the return period of extreme events may still be substantially greater than the recovery period from the disasters which the events cause. For less adaptable societies in the developing world, however, a shorter return period of extreme events may not allow them to fully recover from the effects of one event before the next event strikes.
Are there more Extreme Events?
Every region of the world experiences record-breaking climate extremes from time to time. In 1989, for example, the "Big Wet" in eastern Australia brought torrential downpours and the worst flooding in two centuries. The same year also saw an extreme typhoon season in Southeast Asia. The Philippines was hit by three typhoons in October, including Typhoon Elsie with its peak winds of 200km/hr. More than 1,000 people drowned a month later when southern Thailand was struck by the most powerful storm in fifty years. Many people in England will remember the "Hurricane" of October 1987. Droughts are another devastating type of climate extreme. Early this century, a trend towards increased drought in the North American Midwest culminated in the "Dust Bowl" decade of the "dirty thirties", after which conditions eased. During nine of the years since 1970, annual rainfall over the Sahel zone of northern Africa dropped more than 20% below the average prevailing during this century's first seven decades; those previous 70 years saw only one extreme of this magnitude.
Frequent reports of record-breaking events suggest that climate extremes are becoming more common. There is only limited scientific evidence, however, that this may be the case at the global level. The most recent Intergovernmental Panel on Climate Change scientific assessment concluded that it was likely that higher maximum and minimum temperatures, more hot days and fewer cold days, and more intense precipitatin events have been observed in the latter half of the 20th century. Nevertheless, it is still plausable that increased human vulnerability to climate extremes (particularly in developing countries) is transforming extreme events into climatic disasters. This is because people in many parts of the world are being forced to live in more exposed and marginal areas. In other areas, high-value property is being developed in high-risk zones. This explains, for example, why Hurricane Hugo, which devastated the Caribbean and southern United States in 1989, proved the costliest hurricane in history, with an estimated $10 billion in damages. Finally, because the communications revolution has made news and information more widely available than ever before, people are much more aware of the occurrence of extreme events and of their impact.
Scientists cannot state with confidence that today's extreme events are the first signs of climate change. They simply do not understand the climate system and the effects of greenhouse gas emissions well enough to conclude that particular events are linked to the problem. Nevertheless, monitoring and studying extreme events, and learning how to predict and cope with them, must be a priority. Of all aspects of climate variability, extreme events are likely to have greatest effect on human well-being in the decades to come. What is most certain, however, is that it is likely to be the poorest and most vulnerable societies in the developing world which will be least able to adapt to any increase in the frequency and magnitude of extreme weather phenomena.