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. 8 Rummukainen, M. (2012). “Changes in climate and weather extremes in the 21st century.” Wiley

Interdisciplinary Reviews: climate change 3(2 DOI: 10.1002/wcc.160): 115-129. 9 Scheirmeier, Q. (2011). “Extreme measures: Can violent hurricanes, floods and droughts be pinned on climate

change? Scientists are beginning to say yes.” Nature 477(8 September 2011): 148-149. 10 Pall, P., T. Aina, et al. (2011). “Anthropogenic greenhouse gas contribution to flood risk in England and

Wales in autumn 2000.” Nature 470(doi:10.1038/nature09762): 382-386. 11 Min, S.-K., X. Zhang, et al. (2011). “Human contribution to more-intense precipitation extremes.” Nature

470: 378-381.

events are beginning to be attributed to climate change although distinguishing such events from natural variability is sometimes difficult.

Examples of extreme weather in the 21st century

Extreme weather can present itself either as heavy precipitation (rain or snow) leading to flooding, storminess, drought, heat wave or an unusually cold period, or even as sand and dust storms. Storms and/or heavy rain together with a rising sea level can also contribute to coastal flooding.

Heavy precipitation

There are many examples of heavy rain causing rivers to flood in the last 15 years. Record high sea-surface temperatures in 2010 provided a source of unusually abundant atmospheric moisture for nearby monsoon rains and flooding in Pakistan, Colombia and Queensland in the same year.12

2012 – England experienced the wettest year since records began in 1910 and for England and Wales it was the third wettest year since 1766. Severe floods occurred in the southwest and in the north.13

2010-2011 – ‘A series of floods hit Australia, beginning in December 2010, primarily in the state of Queensland including its capital city, Brisbane. The floods forced the evacuation of thousands of people from towns and cities. At least 70 towns and over 200,000 people were affected. Damage initially was estimated at around A$1 billion. … The Queensland floods were followed by the 2011 Victorian floods which saw more than fifty communities in western and central Victoria also grapple with significant flooding.’14

2010-2011 – In Colombia, ‘the extreme, prolonged rainy seasons in 2010 and 2011 offered a preview of the damage climate shifts may bring to urban areas in the future. In Medellín, heavy rains created rivers in the city center and led to devastating landslides. In one case, more than 100 people were killed or missing after an avalanche of mud buried dozens of homes on a hillside above the city. ….. More than three million Colombians – about seven percent of the total population – were displaced or suffered significant damage to their homes in 2011 alone as a result of flooding.’15 Such floods

are not restricted to Colombia. ‘Models predict that warming in the Andes is likely to contribute both to more flooding and more drought in the region as mountain environments change.’ 15

2010 – Pakistan suffered very heavy monsoon rains. ‘Devastating floods, driven by unprecedented monsoon rains, began late in July 2010, leaving one-fifth of Pakistan submerged. The rains in the Khyber Pakhtunkhwa, Sindh, Punjab and Balochistan

12

Trenberth, K. E. and J. T. Fasullo (2012). “Climate extremes and climate change: The Russian Heat Wave and other Climate Extremes of 2010.” Journal of Geophysical Research 117 (D17103): doi: 10.1029/2012JD018020. 13

http://www.metoffice.gov.uk/ 14

http://en.wikipedia.org/wiki/2010%E2%80%932011_Queensland_floods 15

http://www.scientificamerican.com/article.cfm?id=south-american-cities-face-flood-risk-due-to-andes-meltdown

regions of Pakistan directly affected 20 million people mostly by destruction of property, livelihood and infrastructure. It left 2,000 people dead and 11 million homeless.’ 16

2000 - England and Wales suffered the wettest autumn since records began in 1766. Nearly 10,000 properties were damaged across the region, services were severely disrupted and insured losses were estimated at £1.3 billion.17

Storms

Severe storms go by a variety of names such as hurricane, typhoon, tropical cyclone and tornado depending on where they occur in the world. Some parts of the world have suffered severe storms in recent years and some of these have been attributed to climate change. There is evidence to suggest that such storms, particularly those that are seeded over the warming oceans, are becoming more powerful. Trenberth (2011) says ‘Global warming is contributing to a changing incidence of extreme weather because the environment in which all storms form has changed from human activities.’18 However, predicted changes in storminess in the 21st century are said to be less certain that the predicted changes in temperature.Error! Bookmark not defined.

Donat et al. (2011) suggest that most parts of Europe were more stormy towards the end of the 20th century compared with previous years since 1871.19 In the west Atlantic there has been an average of 7.7 hurricanes and 3.6 major hurricanes since 1995 compared to 5 hurricanes and 1.5 major hurricanes from 1970-1994 i.e. they have become more frequent.23 Although the Northwest Pacific Ocean is the most active basin on the planet, accounting for one-third of all tropical cyclone activity, I could not find references (in English) comparing recent and earlier typhoon records.20

2012 – ‘superstorm’ Sandy originated as a hurricane in the Caribbean and came ashore near New York in November. It came about from a combination of circumstances. ‘Hurricane Sandy got large because it wandered north along the U.S. coast, where ocean water is still warm this time of year, pumping energy into the swirling system. But it got even larger when a cold Jet Stream made a sharp dip southward from Canada down into the eastern U.S. The cold air, positioned against warm Atlantic air, added energy to the atmosphere and therefore to Sandy, just as it moved into that region, expanding the storm even further.’21

2008 – Cyclone Nargis was a strong tropical cyclone that in May caused the worst natural disaster in the recorded history of Burma. It caused catastrophic destruction and at least 138,000 fatalities. Damage was estimated at over US$10 billion, which made it the most damaging cyclone ever recorded in the Bay of Bengal.22

16

http://www.boston.com/bigpicture/2011/08/pakistan_flood_damage_one_year_later.html 17

Pall, P., T. Aina, et al. (2011). “Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000.” Nature 470 (doi:10.1038/nature09762): 382-386. 18

Trenberth, K. E. (2011). “Attribution of climate variations and trends to human influences and natural variability.” Wiley Interdisciplinary Reviews: climate change 2(6): pp.6. 19

Donat, M. G., D. Renggli, et al. (2011). “Reanalysis suggests long-term upward trends in European storminess since 1871.” Geophysical Research Letters 38(L14703): pp.6. 20

http://en.wikipedia.org/wiki/Tropical_cyclone_basins#Northwestern_Pacific_Ocean 21

Fischetti, M. (2012). “Did Climate Change Cause Hurricane Sandy?” Scientific American (30 October 2012): 89. 22

http://en.wikipedia.org/wiki/Cyclone_Nargis

2005 – Hurricane Katrina, which struck New Orleans, Louisiana, and caused widespread devastation along the central Gulf Coast states, was one of the strongest storms to impact the coast of the United States during the last 100 years.23 It was the costliest natural disaster, as well as one of the five deadliest hurricanes, in the history of the United States. At least 1,833 people died in the hurricane and subsequent floods, and total property damage was estimated at US$81 billion.24 The hurricane was accompanied by a storm surge which caused extensive flooding when flood defences were breached.

Drought

The availability of water underpins life as we know it, both biologically and in supporting the society within which we live. Under the combined influences of human development and a warming climate, the supply of, and demand for, water for consumption and irrigation, mineral exploration and energy production are changing and leave some areas exposed to the risk of drought.

Droughts differ from storms and floods in that a drought is usually declared only when a region has suffered dryness for many months; a soil-moisture drought, which is of most relevance to agriculture, is induced by a deficit in the land water balance and is caused by lack of precipitation and/or excess evapotranspiration.26 Dai (2012) states that ‘Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas. Analyses of model-simulated soil moisture, drought indices and precipitation-minus-evaporation suggest increased risk of drought in the twenty-first century. There are, however, large differences in the observed and model-simulated drying patterns. Reconciling these differences is necessary before the model predictions can be trusted.’25 He goes on to note that changes in tropical sea surface temperature, which are partly natural, have a strong influence on rainfall on land and incomplete information about such changes is at the root of some of the differences. For example, the El Niño-Southern Oscillation in the eastern Pacific Ocean can be shown to have influenced droughts on land. He concludes that there will be severe and widespread droughts in the next 30–90 years over many land areas resulting from either decreased precipitation and/or increased evaporation. However these statements are not easy to reconcile with Seneviratne (2012) who says ‘A new assessment of drought trends over the past 60 years finds little evidence of an expansion of the area affected by droughts,’ and ‘modelling shows that there has been little change in drought from 1950 to 2008, contradicting previous work that suggested an increase in recent years.’ 26 Field et al. (2012) imply that there is no necessary correlation between temperature changes and long-term drought variations.5,26 Clearly, explaining droughts is more complicated than storms or floods. There are also differences in the methodologies used by different studies.

The dryness associated with drought, coupled with extreme heat and low humidity, also greatly increases the risk of forest and bush fires which may destroy property.

23

http://www.ncdc.noaa.gov/special-reports/katrina.html 24

http://en.wikipedia.org/wiki/Hurricane_Katrina 25

Dai, A. (2012). “Increasing drought under global warming in observations and models.” Nature Climate Change 3: 52-58. 26

Seneviratne, S. I. (2012). “Climate science: Historical drought trends revisited.” Nature 491(7424): 338-339.

2012 – In 2012 a drought affected most of the USA, parts of Mexico, and central and Eastern Canada. It has continued into 2013.27 It has been described as an expansion of the 2010–2012 Southern United States drought. The current drought has inflicted, and is expected to continue to inflict, catastrophic economic ramifications on the affected states. The drought has exceeded, in most measures, the 1988-1989 North American drought, the most recent comparable drought. It is on track to exceed that drought as the costliest natural disaster in US history.28 This drought has been blamed on the lack of winter precipitation in the USA caused by changes taking place in the Atlantic Ocean. It was followed by a summer heat wave causing strong evaporation thus creating a self-sustaining vicious circle of drought.

2012 – The Sahel of West Africa, a region not unfamiliar with drought, is currently undergoing a severe drought accompanied by famine. However, the current famine, which is expected to get much worse before it gets better, is even more serious than those in the past and aid organisations are struggling to cope. More than 18 million people are facing a food crisis and more than 1 million children under five risk severe acute malnutrition. As noted above, droughts, and the famines they cause, are rarely down to one factor. Here the causes lie in the low and erratic rainfall that the area experienced in 2011 which led to a poor harvest in 2011 and 2012. Man-made problems have also complicated the issue. Locusts are a huge threat to crops but the instability of several of the countries they’ve infested has prevented adequate treatment.29

2002-2012 - SE Australia experienced a prolonged drought (‘the Big Dry’) for around a decade. In 2007, Rachel Nowak wrote in the New Scientist that ‘water restrictions had all but destroyed urban gardens and agricultural production across the country had fallen by a fifth. [And then], when water storage fell so low that energy supplies were threatened, the sense of panic became palpable. Australia is facing a national crisis, one that promises to transform the country, inexorably changing where people live, what they eat, what they do in their spare time, and - most threatening of all - their future economic well-being.’30 By 2012 the Federal Government had provided $4.5 billion in drought assistance since 2001. It has also been suggested that as the driest inhabited continent, Australia cannot continue to sustain its current rate of population growth without becoming overpopulated and that climate change will lead to a deterioration of natural ecosystems through increased temperatures, extreme weather events and less rainfall in the southern part of the continent, thus reducing its capacity to sustain a large population even further.31

Heat waves

A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power cuts due to increased use of air conditioning. Some of the most damaging heat waves are listed

27

http://www.climatewatch.noaa.gov/article/2013/drought-impacts-continue-to-pile-up 28

http://en.wikipedia.org/wiki/2012_North_American_drought 29

http://metro.co.uk/2012/08/03/sahel-drought-in-west-africa-leading-to-crisis-as-millions-of-lives-at-risk-521534/ 30

http://www.newscientist.com/article/mg19426085.300-australia--the-continent-that-ran-dry.html 31

http://en.wikipedia.org/wiki/Drought_in_Australia#End_declared_in_2012

below but, since 2000, many less severe, but record breaking, heat waves have been experienced in North America, Europe, Australia and elsewhere.36

Field et al. (2012) state that ‘In many (but not all) regions over the globe with sufficient data, there is medium confidence that the length or number of warm spells or heat waves has increased.’ And that, looking forward to 2100, ‘It is very likely that the length, frequency, and/or intensity of warm spells or heat waves will increase over most land areas’. For urban areas of Europe they say there is ‘Medium confidence that the length or number of warm spells or heat waves has increased since the middle of the 20th century’ and that it is ‘Likely’ there will be ‘more frequent, longer, and/or more intense heat waves or warm spells’.5

2010 – In May record temperatures in northern India claimed hundreds of lives in what is believed to be the hottest summer in the country since records began in the late 1800s. In Rajasthan the temperature almost reached 50°C. The Indian Meteorological Department attributed the record heat to lack of atmospheric humidity, hot dry winds

blowing across the south-western Thar desert and the effects of the 2009 El Ni o cycle.32

2010 - The heat wave that struck western Russia in the summer of 2010, killing 55,000 people, broke July temperatures records, caused US$15 billion in damage and led to deadly wildfires. The latest research has found both natural and manmade causes. The high temperatures (over 40°C) were within the natural range for a Russian summer but due to human-induced climate change, the chance of such an extreme heat wave happening has tripled over the past several decades.33,34 The same event also affected northeast China and many other locations in the northern hemisphere (see above and ref.35). In the UK, October hit a record temperature of 30°C.36

2003 – The 2003 European heat wave was the hottest summer on record in Europe since at least 1540. France was hit especially hard. The heat wave led to health crises in several countries and combined with drought to create a crop shortfall in parts of Southern Europe. The death toll was 70,000; 14,802, mostly elderly, people died in France. Auxerre, in the department of Yonne, recorded temperatures of over 40°C seven times in July and August.37

Severe winters

Some scientists have established a link between climate change and an increase in severe winters in northern Europe including the UK. The region has been prone to bad winters after summers, such as in 2011 and 2007, when the area of Arctic sea ice has been very low. It is proposed that heat and water vapour from the warmer Arctic Ocean affects the position of the jet stream – the west-to-east winds that form the boundary between the cold Arctic and the warm mid-latitudes. The jet stream has moved north and slowed so that weather systems also tend to move more slowly. This may explain

32

http://www.guardian.co.uk/world/2010/may/30/india-heatwave-deaths 33

http://www.agu.org/news/press/pr_archives/2012/2012-10.shtml 34

Otto, F. E. L., N. Massey, et al. (2012). “Reconciling two approaches to attribution of the 2010 Russian heat wave.” Geophysical Research Letters 39 doi:10.1029/2011GL050422(4). 35

http://theweek.com/article/index/205871/the-2010-heat-wave-7-excruciating-climate-records 36

http://en.wikipedia.org/wiki/Heat_wave#2010.E2.80.93Present 37

http://en.wikipedia.org/wiki/2003_European_heat_wave

extreme events such as very cold winters (and very hot summers) in the region.38 A similar explanation has been proposed for heavy winter snow and rain elsewhere in Europe and in North America and east Asia.39

2013 - The UK experienced its joint, second coldest March since records began more than 100 years ago.

Combinations of severe weather events

Clearly severe weather events, with damaging effects, can also arise from combinations of the above effects. For example, droughts and heat waves provide conditions for damaging wildfires, and storms and heavy rain can lead to severe flooding.

Rising sea level: Although rising sea level is not itself an extreme weather event it can, in combination with one or more of heavy rainfall, a storm surge caused by strong winds, or high tides, give rise to coastal flooding. Sea level is rising because of thermal expansion of the oceans, as they warm, and the addition of fresh water from melting land ice and water extracted from aquifers. Globally it is rising at around 3.2 mm/yr today and a rise of at least 0.5 -1.0 m by 2100 is expected assuming a constant rise rate. Hansen and Sato (2012), on the basis of an exponential rate, have even argued for a multimetre sea level rise by the end of the century.40 Sea-level rise is likely to be limited to below 2 meters by 2300 only in the unlikely event that warming will be kept to well below 1.5°C.41 Locally, sea level will depend on latitude (it will be greater in the tropics), changes in wind and ocean currents caused by climate change, and warming of the nearby ocean. Thus coastal flooding by sea water is likely to increase in the years ahead with serious consequences for the underlying arable land and fresh-water aquifers among other impacts such as flooded cities

Other implications of severe weather events

Severe weather events can also have other consequences. For example, during a drought water supply may be threatened and crops can fail leading to poor harvests and a shortage of food or even famine. In turn poor harvests can impact world trade in certain crops and their global prices. Even the infrastructure of a country can be affected; for example, unexpectedly high temperatures may adversely affect railway lines and points and metal pipelines (through extreme thermal expansion), cause the tarmac to melt on roads and lead to a heavy demand for electricity for air conditioning.

Finally, the damage caused by such events is obviously economically and socially disruptive, expensive to put right and in any event will cause insurance premiums to increase.

38

http://www.guardian.co.uk/environment/2012/sep/14/arctic-sea-ice-harsh-winter-europe 39

E.g. Liu, J., J. A. Curry, et al. (2012). “Impact of declining Arctic sea ice on winter snowfall.” Proceedings of the National Academy of Sciences online(10.1073/pnas.1114910109) and several other papers. 40

Hansen, J. and M. Sato (2012). Update of Greenland Ice Sheet Mass Loss: Exponential? Pers. Comm. 41

Schellnhuber, H. J., W. Hare, et al. (2012). Turn Down the Heat: Why a 4 degreeC warmer world must be avoided. Washington DC, A Report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics: pp.106.

Conclusions and predictions

Although natural climatic variability is ever present, many severe weather events which have been experienced in the last decade or so are increasingly being attributed to the consequences of climate change. Some events are happening more frequently than in the past and/or are becoming more intense.

The severity of the events depends to some extent on where they occur; not all countries can expect to be equally affected but there is a tendency for most places to get hotter and wetter.

Rummukainen (2012) states ‘Overall, evidence for possible changes in tropical cyclones because of climate change remains elusive, apart from some intensification of associated maximum wind speeds and rainfall rates and, with less confidence, increasing frequency of the more intensive categories of tropical storms.’7 Similarly,

predictions for extra-tropical cyclones lack certainty although their storm tracks and associated strong wind conditions are predicted to move polewards. However, Schellnhuber et al. (2012) predict an increased frequency of high-intensity tropical cyclones41 although this prediction may be less certain than the predicted increase in average global surface temperature.5 Grinsted et al. (2013), in a study of Atlantic hurricanes, found that temperatures at the hurricane spawning site, and relevant regional and average global temperatures were the factors best able to explain past hurricanes.42 They estimate a doubling of Katrina magnitude events associated with 20th century warming and, even more worryingly, they predict a twofold to sevenfold increase in the frequency of Katrina magnitude events for each 1°C rise in global temperature in the 21st century. Others have suggested that Europe is becoming more stormy.19 These predictions are consistent with global models which suggest that the frequency of extreme wind events generally will decrease in and around the tropics (between 40°S and 40°N) but could increase around 60° latitude.Error! Bookmark not defined.

Droughts are caused by an imbalance between precipitation and evaporation which may be brought about by changing conditions in the oceans. Although many areas are getting dryer it is not yet clear that the areas affected by drought are actually increasing in size. Overall, climate models indicate more droughts in the 21st century especially in subtropical regions.Error! Bookmark not defined. Increasing aridity is making some areas more susceptible to wildfires.

Heat waves and extreme high temperatures are becoming more common in number, the length of time for which they persist and/or in intensity. This is expected to continue for the rest of this century including in Europe.

Globally there will be fewer cold extremes. However, more severe winters are expected to occur in Europe, North America and east Asia following summers when there has been a large reduction in the area of Arctic sea ice.

42 Grinsted, A., J. C. Moore, et al. (2013). “Projected Atlantic hurricane surge threat from rising

temperatures.” Proceedings of the National Academy of Sciences(18 March doi: 10.1073/pnas.1209980110).

Near equatorial countries are most at risk of coastal flooding on the back of rising sea level.

The effect on society of extreme weather events has not been reviewed here but the physical damage to buildings and infrastructure and the impacts on societies and economies, is potentially very large. In a broader sense the vulnerability and exposure to risk from such events needs to be considered.

Readers wishing to obtain a deeper understanding of this subject are recommended to read the review articles of Field et al. (2012) and Rummukainen (2012).