By attempting to pin down the huge economic cost of climate change, UK economist – Sir Nicholas Stern⁸⁷ – has nailed the myth that tackling greenhouse gas emissions is bad for the global economy. The essence of the Stern Review, published as The Economics of Climate Change, is that unchecked, climate change will result in a 20 percent contraction of the world’s economy by 2100. Taking action now, however, would require just one percent of the global GDP. The Stern Review has been criticised, on the one hand for unjustified scaremongering, and on the other for underestimating the true impact of global warming in the coming century. If nothing else, however, it draws attention to the potentially devastating consequences of climate change for the global market, and highlights the outcome of doing nothing.

In the first half of 2007 the IPCC³⁷ published its 4th Assessment Report (http://www.ipcc.ch/), presenting the findings of three study groups, which focused on the Physical Science Basis; Impacts, Adaptation and Vulnerability; and Mitigation of Climate Change. The reports are conservative, for example they do not address the melting behaviour of the polar ice sheets, and have suffered both from being strongly consensus-based and from political interference. Nevertheless, they make bleak reading, with a best-estimate temperature rise for a high emissions scenario of 4 degrees C by 2100, and ‘dangerous’ (defined as in excess of 2 degrees C) temperature rises for lower emissions scenarios (Figure 1). The assessment predicts that it is likely that tropical cyclones will become more intense, with higher peak wind velocities and heavier associated precipitation, while extra-tropical storms are projected to move pole-ward, resulting in new wind, precipitation and temperature patterns. The assessment also forecasts that it is very likely that hot extremes, heat waves and heavy precipitation events will become more frequent.




Figure 1. Forecast global average surface temperature rises (relative to 1980 – 1999) for various emissions scenarios. The orange line relates to greenhouse gas concentrations held constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for six emissions scenarios. Courtesy IPCC.


The IPCC reports do not address certain critical issues, most notably the future behaviour of the polar ice sheets, which present the threat of catastrophic melting contributing to substantial and rapid sea-level rises in this century. An alternative and highly disturbing summary of climate change prospects appears in the Philosophical Transactions of the Royal Society. Put together by James Hansen³¹, Director of NASA’s Goddard Institute for Space Studies, and co-workers, the paper presents evidence from past climate data for positive feedback effects driving the rapid flipping of the Earth’s climate from one state to another. The authors warn that we are currently dangerously close to triggering such a ‘flip’, which would see climate change begin to run out of control, with devastating consequence for humankind. Hansen and his colleagues stress that only urgent and effective measures to bring greenhouse gas emissions under control will keep our climate within or close to the range of the past million years.

Abrupt climate change: latest news

Rapid and major changes in our climate, along the lines of the aforementioned non-linear ‘flipping’, continues to form an important focus of climate change research (see the review by John Mitchell⁵⁷ and his UK Met Office colleagues in Philosophical Transactions of the Royal Society), with most attention paid to polar ice sheet collapse, catastrophically rising sea levels, and Atlantic Thermohaline circulation (Gulf Stream and associated currents) slowdown or shutdown. In relation to melting at the poles, recent satellite monitoring confirms that the Greenland ice sheet is continuing to suffer from the more rapidly rising temperatures at high latitudes. Writing in Science, Jianli Chen¹², and his colleagues, of the University of Texas at Austin, report the results of gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite, which reveal an estimated total ice melting rate across Greenland of 239 ± 23 km³ a year. This is up from around 80 km³ a year between 1997 and 2003. In a second paper, also published in Science, S. B. Luthcke⁵³ of NASA’s Goddard Space Flight Center, and co-researchers, use data from the same satellite to show that from a situation of near balance during the 1990s, the Greenland Ice Sheet experienced an excess ice loss of around 101 (± 16) gigatons a year between 2003 and 2005 (Figure 2).


Figure 2. Trends in ice gain (above 2000 m) and ice loss (below 2000m) for the Greenland Ice Sheet between 2003 and 2005. From a position of near balance in the mid- 1990s, the Greenland Ice Sheet experienced an excess ice loss of around 101 (± 16) gigatons a year between 2003 and 2005. While ice mass is accumulating above 2000 m, this is more than counterbalanced by ice loss at lower altitudes. Courtesy: Science.

In a third Science paper, Andrew Shepherd of Edinburgh University, and Duncan Wingham⁷⁵ of UCL, show that both Greenland and Antarctica are losing mass overall, with an estimated combined imbalance of around 125 gigatons of ice a year. While this is currently sufficient to raise sea levels by just 0.35 mm a year (the total annual rise is ~ 3 mm/y), the authors caution that, with polar glacier accelerations of 20 – 100 percent occurring in just the last decade, accelerated ice discharge could result in a significantly increasing contribution to sea level rise. Writing in the journal, Sedimentary Geology, Nick Eyles²² of the University of Toronto, reveals how quickly sea levels can rise during periods of rapidly climbing temperatures. From its maximum low stand (130m lower than today) at the end of the last ice age, around 20,000 years ago, sea level climbed at an average rate of 15 m per thousand years. There were periods, however, when this rate rose to 20 m per millennium and even as high as 4m in a hundred years. Such rises were at least partly driven by meltwater facilitating ice sheet break-up by lubricating ice sheet motion, a mechanism that seems to be driving the current rapid acceleration of polar glaciers.

The stability of the Gulf Stream and associated currents (also known as the Atlantic Conveyor and the Atlantic Meridional Overturning Circulation or AMOC), which keep the UK and European climates considerably balmier than otherwise, remains an important research focus. Surprise, not to say mild panic, resulted from a 2005 paper published in Science by Harry Bryden⁸ and his team at the UK’s National Oceanography Centre, who found a 30 percent fall in the northward flow of the AMOC. It now appears, however, that the AMOC is not (yet) slowing. This is revealed by the first continuous observations of the circulation, gathered by a series of instrumented buoys stretching from the Bahamas to West Africa (Project RAPID) A news report by Richard Kerr⁴¹ in Science summarises the findings presented at the first RAPID conference, held at Birmingham in the UK, which showed that annual variations are as great as those from one decade to the next. Watch this space for future results from RAPID.

Looking further ahead, however, many climate scientists expect some slowdown in the AMOC, as the circulation is disrupted by huge volumes of fresh meltwater from the Greenland Ice Sheet. In a paper in Climatic Change, Kirsten Zickfeld¹⁰⁰ of the University of Victoria (Canada), and colleagues, report the results of an elicitation exercise, in which 12 leading climate scientists were questioned about the response of the AMOC to global warming. All twelve anticipate a weakening of the circulation as greenhouse gas concentrations in the atmosphere climb. For a 4 degree C rise by 2100, eight of the twelve assess the probability of triggering collapse of the AMOC as significantly higher than zero, with three suggesting a figure higher than 40 percent. Elicited consequences of such an event include large changes in temperature, precipitation and sea level in the North Atlantic region. This continued concern for the stability of the AMOC as greenhouse gas concentrations in the atmosphere, and global temperatures, climb, is also reflected in the IPCC 2007 Physical Science Basis report, which proposes that a slowdown of the AMOC is very likely in this century, perhaps by 25 percent or more, although an abrupt transition is regarded as very unlikely.

Climate change and future storminess

While, as mentioned earlier, climate change is expected to encourage more powerful storms and higher peak wind velocities, the picture is not always this clear. Writing in the Journal of Climate, for example, Lennart Bengtsson² of Reading University (UK), and colleagues, find that there is no indication that climate change will lead to more intense storms in the future, either in the tropics or at mid-latitudes. They do, however, predict a small reduction in the numbers of weaker storms, together with significant changes in storm track location and intensity at regional scales. The authors also forecast pole-ward storm track shifts in both hemispheres with, in the north, a weakening of the Mediterranean storm track and a strengthening of the storm track north of the British Isles. Tropical storm tracks are predicted to weaken in the Atlantic, while strengthening in the eastern Pacific. As will become apparent in the course of reading this review, there is currently little consensus on exactly how and when climate change will impact upon storm activity across all regions.

Tropical cyclones

The latest news in the current debate on whether or not we are already experiencing a climate change driven rise in tropical cyclone (TC) activity is addressed in the later section on meteorological hazards. Here, however, the focus is on the likelihood of a future global warming influence on tropical cyclones. In order to make forecasts about the future, it is often useful to look back. This Jeffrey Donnelly and Jonathan Woodruff¹⁸ of the Woods Hole Oceanographic Institution in Massachusetts do, in order to examine patterns of hurricane activity over the last 5,000 years. Writing in Nature, the authors report the results of a survey of storm deposits preserved in coastal lagoon sediments in Puerto Rico, which provide a record of intense Atlantic hurricane landfalls. The results of the survey show that intense hurricane activity has varied over the period on centennial and millennial timescales, apparently modulated by variations in the El Niño – Southern Oscillation (ENSO) and the strength of the West African Monsoon. Donnelly and Woodruff point out that a better understanding of how global warming might affect these climate patterns is needed before we can understand how climate change will affect intense Atlantic hurricane activity. They also note that periods of intense activity in the past have coincided with times when seasurface temperatures (SST) were not as elevated as they are today, suggesting that factors other than SST are important.

Johan Nyberg⁶⁰ of the Geological Survey of Sweden, and co-workers, have also examined past Atlantic hurricane activity in order to make predictions about future behaviour. In the Journal Nature, they present a record of Atlantic hurricane activity for the past 270 years, using proxy measurements of vertical wind shear and SST derived from corals and marine sediments. They note that the average frequency of major hurricanes decreased gradually from the 1760s to the early 1990s, reaching anomalously low values in the 1970s and 1980s. They suggest that the period of high activity since 1995 is not unusual compared to episodes of past hurricane activity, and regard the current hike as a return to ‘normal’ hurricane activity. The authors propose that vertical wind shear, which hinders hurricane growth and development, is the main determinant of hurricane frequency over the past 270 years, rather than SST.

Nyberg and colleagues suggest that wind shear has offset recent global warming related SST rises to cap the level of recent Atlantic activity. Looking ahead, however, they forecast that the future possibility of lower vertical wind shear may result in longer storm lifetimes leading to higher hurricane frequencies and greater storm intensities.

In a paper published in Geophysical Research Letters, Gabriel Vecchi of the US National Oceanographic and Atmospheric Administration (NOAA), and Brian Soden⁹² of the University of Miami, also address the issue of vertical wind shear and hurricane activity. Vecchi and Soden utilise climate models to project future changes in vertical wind shear over the tropical North Atlantic during the hurricane season, and find that this increases substantially, during the course of this century, in both the tropical Atlantic and eastern Pacific. While accepting that other important factors also need to be taken into account, the authors note that a future increasing trend in vertical wind shear could translate into the capping of hurricane genesis and intensification in the Atlantic and East Pacific.

Mid-latitude storms

While less effusive than the current debate on the consequences of climate change for tropical cyclone activity, research interest in how global warming might result in changes in the characteristics of extra-tropical storms remains strong. In a paper published in the Journal of Climate, Jing Jiang of China’s Nanjing University and William Perrie⁴⁰ of Canada’s Bedford Institute of Oceanography, Dartmouth, examine the impact of climate change on autumn North Atlantic mid-latitude cyclones. For high atmospheric greenhouse gas concentrations, the authors predict that Atlantic midlatitude storms will increase in radius and may also tend towards becoming more severe and faster travelling, along slightly more pole-ward tracks.

With peak wind speeds being a critical determinant of damage potential, any prediction of future wind speeds would be particularly useful. This issue is addressed by Burkhardt Rockel and Katja Woth⁷², of the GKSS Research Centre in Geesthacht, Germany, in a paper in the journal Climatic Change. Using an ensemble of regional climate models (RCMs), the authors examine extremes of near surface wind over Europe and make predictions about future changes. They estimate a future increase of up to 20 percent in the number of storm peak events over central Europe (Figure 3), as well as a possible rise in future mean daily wind speed during the winter months, with a fall in the autumn in areas influenced by North Atlantic extratropical cyclones.



Figure 3. Change in the total number of storm peaks in the European Region from 1961–1990 to 2071– 2100, as simulated by two different Regional Climate Models. Courtesy: Climate Change.

Of greatest importance to the market, in a climate change context, is constraining how modifications to European windstorm behaviour will be reflected in future damage, and therefore in future claims. This issue is addressed by Gregor Leckebusch⁴⁹ of Germany’s Freie Universitat in Berlin, and his team, in a paper in Geophysical Research Letters. Leckebusch and colleagues determine loss potentials using an ensemble of climate models and show that for the UK and Germany, ensemble-mean storm-related losses are predicted to increase by up to 37 percent. In addition, the authors find that the inter-annual variability of extreme events also increases, raising the spectre of a higher risk of extreme storm activity and related losses. Zeroing in on the market’s primary interest, Leckebusch is joined by J. G. Pinto⁶⁷ of the Universität zu Köln, and others, in a second paper published in Natural Hazards and Earth System Sciences, which focuses on future insured losses from European windstorms. The authors find that, on average, insured loss potentials rise for all European regions by 2100, with changes being largest for France and Germany and smallest for Portugal and Spain, although the in-country spread is large, depending upon the scenario. Increased losses result from greater surface wind maxima across western and central Europe, which are in turn linked to more and more powerful extreme cyclones over the UK and North Sea. In a third co-authored paper, Leckebusch⁴⁸ utilises four global climate models (GCMs) and four regional climate models (RCMs) to analyse European winter storm events, and their changes in the light of increased anthropogenic gas concentrations. The results using the GCMs show a reduced storm track density across central Europe under climate change conditions, while if just the strongest 5 percent of storms is considered, increasing cyclone activity is predicted for western parts of Europe. Significant increases in the intensity and frequency of extreme wind speeds are observed across significant parts of Europe, dependent upon the model used.

Prospects for a wetter (and drier) world

While many parts of the world will become drier, some will become a great deal wetter. This is already happening, and – for the first time – the human influence has been detected at the global scale. Writing in Nature, Xuebin Zhang⁹⁹ of Environment Canada, and colleagues from the UK, US and Japan, compare observed changes in land precipitation during the 20th century with changes simulated by climate models. The results show that human activities are responsible for observed changes in average precipitation that can’t be explained by internal climate variability or natural forcing (Figure 4). In particular, the human influence is charged with making a significant contribution to observed increases in precipitation in northern hemisphere mid-latitudes, to drying in the northern hemisphere sub tropics, and to moistening in the southern hemisphere sub-tropics and tropics. The authors note that the observed changes are greater than predicted by models and suggest that these may already have had a noticeable impact on agriculture, ecosystems and human health.


Figure 4. 1925–1999 changes in observed and simulated precipitation anomalies. Time series (left) of observed annual zonal mean precipitation anomalies in 10° latitude bands (thin black line) together with ensemble mean annual zonal mean precipitation anomalies (thin blue trace). Straight dashed black and red lines indicate the trends. Green (or yellow) shading identifies latitude bands with increasing (or decreasing) trends in both observations and models; grey shading indicates disagreement between observed and simulated trends. The map (right) indicates the different 10° latitude bands and whether trends agree in sign. Areas with insufficient data are shown in white. Only land precipitation data are used. Courtesy: Nature.

A second paper in the journal Science also focuses on future precipitation, and suggests that climate change will bring more rain more quickly than previously thought. Frank Wentz⁹⁶, and colleagues, at Remote Sensing Systems in the US, note that while climate models and satellite observations both indicate that the total amount of water in the atmosphere will increase at a rate of seven percent per one degree C, climate models predict that precipitation will increase at a much slower (1 – 3 percent per 1 degree C) rate. The authors reveal, however, that recent satellite observations indicate that total atmospheric water and precipitation have increased at about the same rate over the past two decades. If maintained such a trend could have enormous implications, in particular for future extreme precipitation and flood events.

Concentrating on the European region, the implications of future precipitation for flood and drought risks are addressed in Climatic Change by Bernhard Lehner⁵⁰, and co-workers, of Germany’s University of Kassel. The authors present an integrated analysis of possible impacts of climate change on future flood and drought frequencies across the continent. Perhaps not surprisingly, northern to north-eastern Europe is predicted to become most prone to a rise in flood frequencies, while southern and south-eastern Europe is forecast to show a significant increase in drought frequency. Most importantly, today’s 100-year flood and drought events in the most critical regions, where the biggest changes in flood and drought risk are expected, are likely to happen every 10 – 50 years by the 2070s.

The impact of climate change on coastal flood risk in the UK forms the focus of a paper in Philosophical Transactions of the Royal Society by Jim Hall³⁰ of Newcastle University, UK, and colleagues. On the basis of scenarios of changing climate, society and economy, during the course of this century, and assuming no adaptation to increasing coastal flood risk, the authors expect the annual cost of damage due to coastal flooding in England and Wales to climb from £0.5 billion to between £1.0 and £13.5 billion. Over the same period, the proportion of the national flood risk borne by coastal flooding will rise from around 50 percent to between 60 and 70 percent. Hill and his colleagues note that adaptation could dramatically reduce future annual costs, but that this would require significant capital expenditure.