Extreme weather, such as hurricanes and floods, are some of the most destructive forces of nature, and may cause several hundreds of billions of dollars of damage worldwide. In the Intergovernmental Panel on Climate Change (IPCC) 2007 report, there were strong indications that extreme weather is on the increase. Following the IPCC’s 34th session in Kampala, Uganda, the link with climate change is now considered stronger.
The IPCC meeting
Amongst other tasks, the IPCC meeting in Kampala, held 18-19 November 2011, agreed on the contents of a report prepared by the joint session of Working Groups I and II in preparation for the IPCC 5th Assessment Report. Working Group I addresses scientific issues relating to climate change. Working Group II covers impacts, adaptation and vulnerability of countries to climate change. A third Working Group will report on the mitigation of climate change.
The 5th Assessment itself is planned for 2013-2014 and will represent, when published, the latest understanding of climate change for the past and for the next century or more. There is still considerable work to be done by scientists worldwide before the first drafts of the separate working group reports are available.
The Origins of Extreme Weather Changes
In the summary for policymakers of the joint working group, the origin of changes in extreme weather is explained. Changes can arise from any of three different factors (Figure 1). If a climate mean (e.g. temperature) shifts upwards, then high temperatures are recorded more often but low temperatures are experienced less often. In a second type of variation, there is increased climate variability, leading to more climate extremes, both high and low. A third type of variation could arise from a change in the shape of the probability of a given climate measurement.
It is likely that all three of these types of variability are changing due to greenhouse gas increases. Clearly, although there is a statistical chance in many situations of increased extreme weather, it is not possible to allocate a particular event (flood or hurricane) to greenhouse gas changes.
Changes in Observed Extremes
By definition, extreme events are rare, and it is difficult to be certain about trends in their frequency. Nonetheless, significant changes in extremes have been observed and are expected in the future.
Temperature. For most land areas with enough data, temperature has followed the first type of climate variability, with a shift upwards in the mean, leading to more frequent hot periods, and less frequent cold periods. By the end of the century, the IPCC expects that a 1 in 20 year hottest day is likely to become a 1 in 2 year occurrence.
Precipitation. Both heavy rain and drought have increased in frequency (Min et al., 2011; Medvigy and Beaulieu, 2011), following the second type of variability. Although it is likely that these extremes will increase in the future, the IPCC is not sufficiently confident of the climate simulations to put a precise figure on the changes at this stage.
Sea Level Rise. Extreme coastal high waters are expected to increase in frequency in the future. Sea level rise likely follows the third type of climate variability. Combined with the problem of tropical storms, the sea level extremes are likely to be a particular problem for small tropical islands.
Wind. The mean wind also likely follows the third type of variability, with increases in hurricane frequency at the upper end, and little change at the lower end.
Changes in the Frequency of Severe Tropical Storms
Severe tropical storms are driven by warm oceans, and as the climate warms, so an increase in storm development takes place. Another factor is that these storms have a very large vertical extent and can be influenced by processes as high as 10 km altitude. In particular, the frequency of severe storms is very sensitive to temperature changes, increasing by as much as 30% for each 1C rise in surface temperature (Elsner et al., 2008). Increases in severe storm frequency have also been confirmed in observations by Mann et al. (2009). Climate model simulations also expect an increase in the frequency of severe storms in the future (Knutsen et al., 2010).
The IPCC report notes that the financial losses for the period 2001-2006 amount to about 1% of GDP, with the largest amount generally borne by middle income countries. However, for some small island developing countries, losses have been as much as 8% of GDP.
The report has produced a schematic illustration of the impacts of extreme weather (Figure 2). Natural variability in the climate, and human induced climate change, combine together to give the weather extremes observed and predicted (deep blue in the figure). The financial risk (disaster risk in the center of the figure) is the intersection of the vulnerability (the degree of risk to a given event) and exposure (the total cost if a given risk is realised) of an individual country or location to the weather event.
Once a disaster risk is established, a given disaster can in principle be managed either through short term expenditure, and in the longer term, adapting the local environment to climate change. For example by building larger levees or tidal defences to protect from storm surges.
Although it is, in principle, straightforward to add up the financial cost of extreme weather events, the extent of the human suffering is often not taken into consideration. It is not that this is unrecognized. Rather, these aspects are difficult, if not impossible, to quantify.
The IPCC report recognizes that in the USA itself “exposure and vulnerability are increasing due to growth in population and increase in property values, particularly along the Gulf and Atlantic coasts”. While some of the risk has been offset by improved building regulations, in common with more tropical countries, the USA is clearly not immune from the financial and human costs of future extreme weather changes.
Elsner, J. B. et al., (2008), The increasing intensity of the strongest tropical cyclones. Nature, 455, 92-95. Accessed November 20, 2011.
Knutsen, T.R, et al. (2010), Tropical cyclones and climate change. Nature Geoscience 3, 157 – 163, doi:10.1038/ngeo779. Accessed November 20, 2011.
Mann, M.E. et al., (2009), Atlantic hurricanes and climate over the past 1,500 years. Nature 460, 880-883, doi:10.1038/nature08219. Accessed November 20, 2011.
Medvigy, D. and C. Beaulieu (2011), Trends in daily solar radiation and precipitation coefficients of variation since 1984. Journal of Climate, doi: 10.1175/2011JCLI4115.1. Accessed November 20, 2011.
Min, S-K., et al., Human contribution to more-intense precipitation extremes. Nature 470,378–381, doi:10.1038/nature09763, 2011. Accessed November 20, 2011.
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