Are extreme weather events—severe storms, flooding, droughts, heat waves or extremely violent cyclones—becoming more common? The answer appears to be 'yes'. Trends towards more powerful storms and hotter, longer dry periods have been observed, according to the IPCC's Fourth Assessment Report, and this trend is projected to continue. On the other hand, extreme cold-weather events are expected to decline in frequency, and for some regions of the world this will produce a positive impact, reducing health and infrastructure costs associated with freezing events.
Global warming is expected to cause an increase in weather extremes because it will change the distribution of heat and thus the flow of energy through the climate system. This will in turn alter the circulation patterns of the atmosphere and the oceans, and it will also modify the hydrological cycle by which water is circulated between the earth's surface and the atmosphere. Warmer temperatures mean greater evaporation, and a warmer atmosphere is able to hold more moisture—hence there is more water aloft that can fall as precipitation. However, many regional factors (ocean temperatures and currents, the location of mountain ranges, etc.) moderate this broad correlation. The frequency of heavy precipitation events has increased over most land areas, particularly over eastern parts of North and South America, in northern Europe, and northern and central Asia. The Rhine floods of 1996 and 1997, the Chinese floods of 1998, the East European floods of 1998 and 2002, the Mozambique and European floods of 2000, and the monsoon-based flooding of 2004 in Bangladesh (which left 60 per cent of the country under water), are examples of more powerful extreme events. There is also empirical evidence of more intense tropical cyclone activity in the North Atlantic since about 1970.
Rain falling in heavier bursts does not necessarily equate to more rain over a year—although this may occur in some regions. In fact, greater evaporation as the weather gets hotter may exacerbate droughts and desertification in dry regions. Climate change for these areas may mean that, when rain does come, it could fall in heavier bursts, which may be less useful than regular small amounts. Drying has been observed over the Sahel, the Mediterranean, southern Africa and parts of southern Asia. In Africa's large catchment basins of Niger, Lake Chad, and Senegal, total available water has decreased by 40 to 60 per cent, and desertification has been worsened by lower than average annual rainfall, runoff, and soil moisture, especially in southern, northern, and western Africa. Some regions of the earth have experienced 50-year drying trends, with some evidence linking the drying with lower rainfall associated with changing weather patterns in response to increasing sea surface temperature.
Australia is a naturally dry continent, which has been subject to periodic droughts throughout human history. One of the biggest questions for Australia is whether climate change will cause an increase in the severity, duration, frequency or distribution of droughts. CSIRO modelling suggests that the incidence of drought in some areas could triple by 2070, based on scenarios using different levels of carbon dioxide emissions. If accurate, such predictions indicate dire challenges for Australia's agricultural sector. However, CO2 concentrations are not the only factor in determining Australia's climate. The effects of the El Niño-Southern Oscillation (ENSO) on Pacific Ocean circulation and sea surface temperature, and variability in the Southern Annular Mode, which brings the rain-bearing westerly frontal systems across southeastern Australia, are key determinants of rainfall over the continent. Whether and how these phenomena might vary in a warmer world is currently the subject of intense research.
Tropical storms and hurricanes are potentially sensitive to global warming because their formation is restricted to ocean areas where the sea surface temperature is greater than 27°C. Since warming may cause a greater area of ocean to reach this temperature more often, it follows that the zone of hurricane activity could expand. There is mounting theoretical modelling and observational evidence that tropical cyclones have and will continue to become more intense (and hence more destructive) due to higher sea surface temperatures. However, the IPCC does not project an increase in the number of tropical cyclones globally, although there could be an increase in the number of cyclones outside the tropics. In the Australasian region, cyclone frequency and intensity are also strongly associated with ENSO.
It needs to be understood, however, that an individual cyclone or hot spell cannot be attributed to climate change. There have always been extremes in weather. Climate change deals with long-term trends, not individual events. In addition, we should bear in mind that monitoring, measurement and reporting arrangements have improved over time, which complicates identification of trends in these events, and so a longer period of data collection is required to improve our understanding of change. However, a directional long term climatic trend (e.g., an increase in temperature) increases the probability that an extreme event (e.g., a heat wave) could occur. For example, the central European heat wave of 2003 that caused over 30,000 excess deaths was made about three times more likely by the underlying trend of increasing temperature.
It is also true that extreme events are likely to have greater impacts today than, say, a century ago. This is because of the greater population density in vulnerable areas, the increase in urbanised areas, and the more extensive networks of sophisticated and costly infrastructure (e.g. transport and energy systems). The higher sea level means that storms will cause greater surges of waves onto land.
The effects of storm surges are particularly devastating when they make landfall in heavily populated and low-lying areas adjacent to shallow sea-bed. The most significant recent example is the storm surge in Burma associated with Cyclone Nargis—a storm surge of at least 4 metres high that accumulated over the shallow seabed off Burma and inundated the Irrawaddy Delta region, drowning over 100,000 people.
Recent CSIRO research suggests that modest to moderate increases in average and maximum cyclone intensities are expected in the Australian region in a warmer world. On the Australian coast, impacts of sea-level rise and storm surges could be expected along the full length of the tropical coast. On the NSW coast, where a narrow continental shelf limits the size of storm surges, large wind-driven waves can have significant impacts.
The IPCC Fourth Assessment Report (Climate change 2007—the physical science basis, Chapter 3, pp. 310–16) provides examples of some recent notable extreme events. As single extreme events cannot simply and directly be attributed to anthropogenic climate change, the list consists of patterns of extreme weather persisting for some time.