Extreme weather events Introduction The further a particular weather event lies from the typical range of that type of event, the more it is likely to be described as an extreme event, irrespective of whether it concerns a violent storm, unusual temperatures, heavy precipitation, drought or flood. 2012 seems to have been a year of extreme weather events (‘superstorm’ Sandy in the USA, high rainfall and floods in the UK, etc.). Other years in the last decade have also contained droughts and wildfires (in the USA and Australia), hurricane Katrina (USA), floods (Pakistan) and heat waves (Russia and France). At the same time it is becoming increasingly accepted that human activity, principally the burning of fossil fuels, is changing the global climate and causing the atmosphere to warm. The average global temperature of the lowermost atmosphere has increased markedly since about 1980. Are the two observations, which operate on different timescales1, connected? Are extreme weather events really becoming more common and/or more severe, or are they perhaps part of the climate’s natural variability? The aim of this document is to investigate these two questions. Some basic physics of a warmer atmosphere As air warms its humidity is able to rise and so the atmosphere carries more water vapour. For example, the water content of the atmosphere increases by 7% for each degree Centigrade rise in temperature, although globally precipitation is expected to rise by only about 2%/°C because relative humidity is typically not expected to change on the global scale.8 1 climate change is defined as changes occurring at least over a few decades whereas extreme weather typically lasts from days to months. In addition the energy content of the lower atmosphere depends on its temperature2, latent heat and kinetic energy. As the lower atmosphere (and the upper ocean) has gained heat so circulation patterns have become more intense in regions of enhanced latent heat release. Peterson et al (2011) have concluded that in recent decades the lower atmosphere has experienced a net gain in energy. Increases in temperature and water vapour provided approximately equal contributions while the observed reduction in kinetic energy, from lower average wind speeds, was over 100 times smaller.3 In very simple terms this means that recent rises in atmospheric temperature have increased the potential for greater precipitation and more energetic weather systems. Another important factor is the flow of the jet stream, a variable “river” of fast moving air that circumnavigates the planet in the upper troposphere, around 10-15 km altitude, in the northern and southern hemispheres. If the jet stream above any particular point is flowing from polar latitudes it will be relatively cold at the surface; if it is flowing from subtropical latitudes it will be relatively warm (and, potentially, relatively wet) at the surface.4 The position of the jet stream can be influenced by differential heating caused by differences in temperature between continents and oceans and hence is susceptible to climate change. Figure 1. The effect of changes in temperature distribution on extremes. Different changes in temperature distributions between present and future climate and their effects on extreme values of the distributions: (a) effects of a simple shift of the entire distribution toward a warmer climate; (b) effects of an increase in temperature variability with no shift in the mean; (c) effects 2 Strictly enthalpy, the total energy within a system, which can be derived from temperature. 3 Peterson, T. C., K. M. Willett, et al. (2011). “Observed changes in surface atmospheric energy over land.” Geophysical Research Letters 38(L16707): pp.6. 4 Palmer, T. N. (2013). “Climate extremes and the role of dynamics.” Proceedings of the National Academy of Sciences(26 March 2013): pp.2. of an altered shape of the distribution, in this example a change in asymmetry toward the hotter part of the distribution. From Field et al. (2012)5 A graphical explanation for extreme weather Given that some extreme weather is directly temperature dependent, another approach to explaining a possibly more frequent occurrence of extreme weather events is to consider what happens when there is an increase in global temperature. For example, in the simple case of a normal temperature distribution curveError! Bookmark not defined. (Fig.1a, bold line), if the average temperature rises the distribution curve will shift to the right (dashed line) and the area under the curve in a given band of extreme temperatures will increase (red and brown zones); this means that the probability of that temperature happening goes up and conversely the probability of extreme lower temperatures decreases (blue zone). This has be described as climate change ‘loading the dice’ in favour of more extreme weather.6 Other scenarios with slightly different consequences (such as a change to greater variability of temperature (Fig.1b) and a change in the shape or symmetry of the curve (Fig. 1c)) can also be envisaged. These simple, non-physical considerations help to throw light on why the Earth is experiencing more extreme temperatures. Field et al. (2012) summarise the situation as follows (comments in [ ] added) ‘A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events. Changes in extremes can be linked to changes in the mean [average], variance [spread], or shape of probability distributions, or all of these [as shown in Figure 1]. …… Many extreme weather and climate events continue to be the result of natural climate variability. Natural variability will be an important factor in shaping future extremes in addition to the effect of anthropogenic [i.e. man-made] changes in climate.’5 Further, Rummukainen (2012) notes that ‘the detection of trends in extremes is a challenge because of …… the sporadic nature of many extremes and data limitations.’8 In this subject the (observational) past may not necessarily be the key to future model- based predictions if the statistics of extreme weather are changing over time. Regional effects As recent experience has demonstrated, extreme weather events are not evenly distributed around the world. Some regions are more susceptible to floods, storms or droughts than others. Looking ahead, Field et al. (2012) show figures for 26 separate regions of the world based on computed projections for the rest of the 21st century.5 For example, they show that for Central Europe a maximum daily temperature that would have been expected to occur only once in 20 years in the late 20th century will occur 5 Field, C. B., V. Barros, et al. (2012). Managing the risks of extreme events and disasters to advance climate change adaptation: Summary for policy makers, Cambridge University Press, Cambridge, UK, and New York, NY, USA: 1-19. Likely means 66-100% probability; very likely means 90-100% probability; 6 Hansen, J., M. Sato, et al. (2012). “Perception of climate change.” Proceedings of the National Academy of Sciences 109(37): pp.9. every 2-10 years in the mid-21st century and every 1-7 years in the late 21st century. Similarly, the chances of a ‘daily precipitation event’ that previously happened only once in 20 years will increase in the 21st century to 1 in 10-15 years and to 1 in 8-18 years, respectively. Central Europe, and almost everywhere else, will get hotter and wetter but to different extents. An ‘emergent’ phenomenon, described by Lavers et al (2011), is the idea that atmospheric ‘rivers’ of water or moisture ‘conveyor belts’ can carry vast quantities of water-laden air from the subtropics to the mid-latitudes across oceans. The water is precipitated when the air rises and cools, for example on encountering a mountainous or hilly west-facing coastline. These authors demonstrated that an atmospheric ‘river’ occurred simultaneously with the ten largest winter flood events since 1970 in a range of British river basins.7 The relation of this phenomenon to climate change is presently unclear but it could intensify as the humidity of the warming atmosphere increases. Determining the causes of extreme weather events Rummukainen (2012) says ‘Climate and weather extremes are sporadically recurring events that may have major local or regional impacts on the society and the environment…… Extreme events are part of the overall climate and weather alongside average conditions and variability, and thus are not unexpected as such. Climate change is expected to affect not only means [averages] but also variability and extremes. Some inferences can be based on past and present observations, but analyses of especially rare events are hampered by the availability of long time series.’8 The study of the extent to which extreme weather can be associated with man-made climate change is called attribution. It is a relatively new subject that depends to some extent on the availability of weather observations made over long periods with which the frequency and nature of contemporary events can be compared. In a review of the subject Scheirmeier (2011) notes that climate scientists are beginning to say that, thanks to advances in statistical tools, climate models and computer power, attribution is no longer impossible.9 One approach is to use climate models to generate simulations of weather, which include natural climate cycles, with and without the influence of anthropogenic greenhouse gases. In several cases it has been shown that the presence of greenhouse gases greatly increases the probability of some extreme weather events in line with observations.10,11 In other words some extreme weather 7 Lavers, D. A., R. P. Allan, et al. (2011). “Winter floods in Britain are connected to atmospheric rivers.” Geophysical Research Letters 38(L23803): pp.8.