Dangerous climate change
Increasing interest in abrupt or dangerous climate change is reflected in the publication of a number of excellent publications over the past 12 months, most notably two reports from the Hadley Centre of the UK Met Office: Uncertainty, risk and dangerous climate change²⁵ and Stabilising climate to avoid dangerous climate change²⁶, and a special issue of the journal Global and Planetary Change, focusing on Extreme climatic events, edited by M. Beniston, and D.B. Stephenson³. One of the principal abrupt climate change concerns addressed in the Met Office publications is the potential slowdown or shutdown of the Gulf Stream and associated ocean currents that keep the UK and western Europe several degrees warmer than they would otherwise be. With the probability of at least a significant slowdown being put as high as 45 percent with a 3º C global temperature rise, the Met Office has modelled the effects of a sudden shutdown in the system of currents known as the ocean conveyor (of which the Gulf Stream is a part), which is responsible for transporting heat from the tropics to higher latitudes. In the 10 years after shutdown, this model output shows a cooling of the entire northern hemisphere, which is especially pronounced in the North Atlantic region. Within a few years, Summer temperatures in Central England are significantly depressed, with Winter temperatures suffering a dramatic fall to between – 10 to - 20º C.

Temperature trends and the human influence on climate change
Notwithstanding the potential for global warming to bring about a cooling trend, attention has continued to focus on rising temperatures. Jürg Luterbacher³⁹ at the University of Bern and colleagues, for example has looked back at European seasonal and annual temperature variability, trends and extremes over the last 500 years, and determined that 2003 was by far the hottest of the last 500 years (figure 13). Furthermore, the
Figure 13. July 2003 day-time land-surface temperatures collected by the Moderate Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite are compared to temperatures for July 2001. Over large areas, particularly in France, the UK, Germany and eastern Europe, temperatures are up to 10º C higher. 2003, as a whole, was by far the hottest in Europe for the last 500 years. Image credit: Reto Stockli and Robert Simmon, NASA Earth Observatory Team.

nine warmest European years on record have occurred since 1989, with the decade 1994 to 2003 appearing to be the warmest for half a millennium. Making a deterministic link between a specific meteorological event, such as the 2003 European heat wave, and anthropogenic global warming, remains problematical, because such an event can always occur by chance in an unmodified climate. Peter Stott⁶⁰ of the UK Met Office Hadley Centre and co-workers have, however, managed to estimate the contribution of human-induced increases in atmospheric concentrations of greenhouse gases and other pollutants to the risk of occurrence of unusually high mean summer temperatures across much of continental Europe (figure 14). Through
Figure 14. June-August temperature anomalies (relative to the 1961-90 mean, in ºC) across the region shown in the inset. Shown in the graph are observed temperatures (black line, with low-pass filtered temperatures as heavy black line), modeled temperatures from four HadCM3 simulations including both anthropogenic and natural forcings to 2000 (red, green, blue and turquoise lines), and estimated HadCM3 response to purely natural forcings (yellow line). The observed 2003 temperature is shown as a star. Also shown (red, green and blue lines) are three simulations (initiated in 1989) including changes to greenhouse gas and sulphur emissions according to SRES (Special Report on Emissions Scenarios) A2 scenario to 2100. The inset shows observed summer 2003 temperature anomalies in ºC. Courtesy: Peter Stott.

consideration of a range of simulations generated by the Hadley Centre HadCM3 climate model, which take account of human plus various natural forcings, the authors estimate that it is very likely (confidence level > 90 percent) that human influence has at least doubled the risk of heat waves as hot as 2003. Stott and his colleagues conclude that with the likelihood of such events projected to increase 100-fold over the next 40 years, it is difficult to avoid the conclusion that potentially dangerous anthropogenic interference in the climate system is already underway.

The effects of human activities have also been detected in the oceans, as reported in Science by Tim Barnett² of the University of California and colleagues. Large-scale increases in the heat content of the world’s oceans have been observed to occur over the last 40 years, with a huge 84 percent of the total heating of the Earth System (oceans, atmosphere, continents and cryosphere) going into their warming. The authors examined this warming on an ocean-by-ocean basis, using a recently upgraded and much expanded set of observational ocean data. The data were compared to simulations from two independent climate models, a so-called Parallel Climate Model (PCM) and the UK Hadley Centre HadCM3 Model. The results of numerical experiments run with these models were then used to attribute the causes of the observed warming. For both models, observations matched well with what would be expected if human activities were the primary cause of ocean warming. The signal of human-induced warming in the oceans is complex and has a vertical ‘structure’ that varies from ocean to ocean. In places, such as the Atlantic, the warming extends down to a depth of 700m or more, but in others – such as the northern Pacific and Indian Oceans – it is confined to the top 100m or so. Barnett and his colleagues stress that because past warming has been well simulated, credence can be attached to future changes predicted by the global models used, at least out to 20 to 30 years in the future.

While there is little doubt that European Summers are going to become considerably hotter, and the oceans are clearly warming, what does recent research say about prospects for the planet as a whole? In its comprehensive Third Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) forecast in 2001 that globally averaged surface temperatures will rise by between 1.4 and 5.8º C by the end of the century. A study addressed in HRSR2004, proposed that if our atmosphere continues to get cleaner as a result of less sooty, particulate matter, then temperatures could be 7 - 10º C (worst case) higher by 2100. The latest study, published in Nature by D. A. Stainforth⁵⁷ of Oxford University and co-workers, also suggests that the world, by the century’s end could be far warmer than suggested by the IPCC. The paper reports the results of the first multi-thousand ‘grand ensemble’ simulation of the global climate, known as Climateprediction.net. Individual simulations were carried out using idle processing capacity on personal computers volunteered by the general public (more than 90,000 participants in over 140 countries); a ‘distributed-computing’ method that led to a huge and continually expanding data set of results. Each participant downloaded an executive version of a full Global Climate Model (GCM) based upon a version of the Hadley Centre’s Unified Model. They were then allocated a particular set of parameter perturbations and initial conditions enabling them to run one simulation (one member of the grand ensemble). Their personal computer then carried out 45 years of simulation and returned the results to the projects servers. The sets of parameters – or model versions – proved to be as realistic as other state-of-the art climate models, but were far more sensitive. In response to a doubling of carbon dioxide, for example, model versions predicted temperature rises from less than 2º C to more than 11º C. Stainforth and his colleagues point out that the great success of the project has been in discovering GCM versions with comparatively realistic control climates and with sensitivities covering a much wider range than has ever been seen before. This, they stress, is a critical step forward in developing an improved understanding of potential responses to increasing levels of greenhouse gases, regional and seasonal impacts.

In attempting to predict how the climate is going to change going forward, determining its sensitivity to forcing agents, whether natural or anthropogenic, is critical. One way of doing this is to look back at the sensitivity of the climate of the recent past. Hans von Storch⁵⁹ of Germany’s Institute for Coastal Research, and colleagues, have tackled this issue for the period of the last thousand years, concluding that decadal or longer temperature variations over this period may have been at least a factor of two greater than previously thought. This has extremely important ramifications for the future, as stressed by Tim Osborn and Keith Briffa⁴⁷ of the University of East Anglia in the UK. They point out that greater long-term climate variability is likely to imply greater sensitivity of the climate to, for example, greenhouse gas concentrations. In effect, greater past climate variations imply greater future climate changes.

Climate change implications for Atlantic hurricanes and European windstorms
Direct links between climate change and specific hazards that show natural trends over periods of decades or longer, are very difficult to determine and quantify unequivocally and nowhere more so than in relation to windstorms where the issue is especially contentious,. Two new papers address the issue, in relation to both Atlantic hurricanes and European windstorms. Kevin Trenberth⁶² of the US National Center for Atmospheric Research examines uncertainties in relation to hurricanes and global warming. He points out that hurricane activity naturally varies widely on an inter-annual and multi-decadal scale, which makes it difficult to pick out any signal due to anthropogenic global warming. Over the course of the 20th century, however, a non-linear upward trend in Atlantic sea-surface temperatures (SST) has been recorded, becoming most pronounced in the last 35 years (figure 15). As SST is an important factor in hurricane formation, will this
Figure 15. Annual mean sea-surface temperature (SST) anomalies relative to 1961-1990 for 1870 to 2004, averaged over the tropical Atlantic (10º N to 20º N), excluding the Caribbean west of 80º W) (top) and the extratropical North Atlantic (30º N to 65ºN) (bottom). Heavy lines are 10-year running means. Will warming of the Atlantic mean more hurricanes?

lead to more hurricanes as the sea continues to warm? Trenberth indicates that higher SSTs will favour enhanced convection and therefore more thunderstorms, but not necessarily their organisation into the tropical storms that are hurricane precursors. Once a tropical storm forms, however, the warmer seas and other environmental conditions associated with global warming will provide more energy to fuel the storm, thereby increasing its potential intensity and amount of associated precipitation, both hurricane properties that already appear to be occurring. Looking ahead, Trenberth reports that there is no guarantee that we will see more hurricanes in the future, but we are likely to see a shift towards more extreme – and therefore more damaging – storms. It should be pointed out here that Trenberth’s conclusions have been challenged in a paper soon to be published in the Bulletin of the American Meteorological Society by Roger Pielke of the University of Colorado and colleagues, who argue that there is no proven link between greenhouse-gas emissions and hurricane behaviour. The paper will be discussed in HRSR 2006; meanwhile, a pre-print can be accessed online at: http://sciencepolicy.colorado.edu/admin/publication_files/resourse-1762-hurricanes_global_w.pdf

As addressed in HRSR2004, the jury remains out on whether climate change will result in an increase in European windstorm numbers and intensity. The issue is tackled again by Gregor Leckebusch and Uwe Ulbrich³⁶ at the University of Cologne. In a paper in Global and Planetary Change, they analyse the relationship between cyclones and extreme wind events over Europe under climate change conditions, using both global and regional climate model simulations. For a ‘business as usual’ scenario (based upon assumptions about future population growth, GDP per capita and carbon intensity of energy supply) that would see greenhouse gas emissions reach around 850 ppm by 2100 the results of the study suggest western parts of central Europe will see a tendency towards higher extreme wind speeds linked to an increase of more intense depressions, providing the potential for greater resulting damage. Similar, although less pronounced, changes in windstorm activity, are predicted with respect to a greenhouse gas emissions scenario that envisages concentrations reaching 600 ppm by the end of the century.

Future changes in UK extreme rainfall
With respect to flood hazard and risk, forecasts of future changes in extreme rainfall are particularly important, although difficult to accomplish at the regional small scales. In a paper in the Journal of Hydrology, M. Ekström¹⁷ of the University of East Anglia (UK) and colleagues address the problem in relation to the UK. Using the UK Met Office Hadley Centre HadRM3H model and adopting the IPCC emissions scenario A2 (a medium-high greenhouse gas emissions scenario), the authors assess the consequences for future (2070-2100) UK rainfall. In broad terms, the study predicts (figure 16) that for short-duration (1-2 day) precipitation events,

Figure 16. Top: Percentage change in 1-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year return period, (b) HadRM2, 50-year return period, (c) HadRM3H, 10-year return period and (d) HadRM3H, 50-year return period. Bottom: Percentage change in 10-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year return period, (b) HadRM2, 50-year return period, (c) HadRM3H, 10-year return period, and (d) HadRM3H, 50-year return period. Courtesy: Hayley Fowler.

event magnitude at a given return period will increase by 10 percent across the UK. For longer duration (5-10 days) events, event magnitudes at given return periods show large (up to 30 percent) rises in Scotland, with greater relative change at higher (25-50 years) return periods In the rest of the UK, there are small increases in the magnitude of more frequent events (up to 10 percent) but reductions (up to 20 percent) at higher return periods. Ekström and co-workers stress that the study focuses on annual changes in extreme rainfall, and note that, as far as flood risk is concerned, changes in seasonal extremes are likely to be more important. They note that there are trends towards increases in heavy rainfall events during winter and autumn months and that inappropriate seasonal changes in extreme rainfall may further increase the frequency and severity of UK flood events under future enhanced greenhouse conditions.

Accelerating melting in Antarctica and sea-level rise
The IPCC Third Assessment Report, forecasts sea-level rises of between 9 and 88 cm by 2100. This, however, is largely attributable to the melting of small glaciers and ice caps and to the thermal expansion of warmer oceans, and does not take into account potential catastrophic melting of the polar ice sheets. The loss and break up of ice shelves in western Antarctica – including the Luxembourg-sized Larsen-B Ice Shelf in 2002 – has, however, increased concern over the stability of the West Antarctic Ice Sheet (WAIS) and its potential future collapse. This would add a further 5m or so to a future rise. While collapse and melting of the WAIS is not regarded as being imminent, the chances have been put as high as 1 in 20 in the next 200 years. Furthermore, a recent paper by R. Thomas⁶¹ of NASA’s Goddard Space Flight Center, and others, highlights increased thinning of the glaciers in the Amundsen Sea sector of West Antarctica, and a resulting acceleration in the contribution of West Antarctica to contemporary sea-level rise. The authors show that every year, local glaciers are discharging around 250 km3 of ice into the ocean, almost 60 percent more than is accumulating within their catchment basins. This discharge is sufficient to raise sea level by more than 0.2 mm per year. Thomas and his colleagues also warn that the catchment region of the Amundsen Sea contains enough ice to raise sea level by 1.3 m. Furthermore, even though the glaciers here are the fastest moving in Antarctica, they are likely to flow much faster still once the buttressing floating ice shelves break away and melt.

In conclusion, the reader feeling in need of an accessible document that summarises contemporary ideas on climate change impacts, including those that may arise from abrupt climate change, is referred to a recent comprehensive report written by S. Retallack⁵² for the UK’s Institute for Public Policy Research. The work provides an excellent synthesis of current consensus, not only in relation to climate change and its potential impacts, but also to a range of possible solutions.