Dangerous climate change

Concerns about the stability of the Atlantic Thermohaline Circulation (ATHC) - the system of ocean currents that keeps the UK and western Europe several degrees C warmer than comparable latitudes (figure 13), reached a new high at the end of 2005. This followed publication of a paper in Nature by Harry Bryden ¹¹ and his team at the Southampton Oceanography Centre in the UK, in which they presented evidence for a 30 percent slowdown in the warm waters heading towards the Arctic in the North Atlantic Drift. These results are based upon indirect measurements of the circulation and based upon just five ‘snapshots’ of data acquired in 1957, 1981, 1992, 1998 and 2004, the latter by Bryden’s team. As a consequence, some oceanographers have questioned the validity and significance of the results, claiming that the observations may just represent a blip and that circulation may increase again. Having examined the earlier data, however, Bryden and co-workers point out that the flow was steady between 1957 and 1992, but dropped off before 1998 and has remained low since. While the seas off Europe currently show no sign of cooling - another argument used by scientists sceptical of the results - and are in fact slightly warmer than a decade ago, Bryden has commented elsewhere that if the slowdown persists, temperatures in the UK and Europe could be expected to fall by about 1 degree C over the coming decade. This may not sound like much, but it could bring winter conditions similar to those that gripped the region during the Little Ice Age, between the 15th and 19th centuries, which saw sea ice in the Channel, frost fairs on the Thames, and skating on the Dutch canals.



Figure 13. (Left) The Atlantic Thermohaline Circulation (ATHC) carries warm tropical waters (red) to high latitudes and returns cold, deep water (blue) that feeds the Global Conveyor. (Right). Here, warm waters are shown in light blue and cold waters in dark blue.

Courtesy: Greg Holloway (left) and Wikipedia (right).

More evidence also came to light, during the course of the last year, for the growing instability of the Greenland and West Antarctic Ice Sheets, offering the prospect of dramatically rising sea levels over the next few centuries. In the journal Science, Eric Rignot of NASA’s Jet Propulsion Laboratory in California, and Pannir Kanagaratnam ⁵⁵ of the University of Kansas, reveal that the amount of ice discharged into the sea from the Greenland Ice Sheet has more than doubled over the last decade, from 90 cubic km in 1996 to 222 cubic km in 2005. The enormous acceleration in melting seems to be related to a dramatic rise in the extent of summer melting, and is happening because meltwater is percolating down from the surface via crevasses, and lubricating the base of the glaciers that carry ice from the interior towards the sea. In another paper in Geophysical Research Letters, Adrian Luckman ³⁹ of Swansea University (UK), and co-workers, present evidence that, in just the last couple of years, two of Greenland’s major outlet glaciers – the Helheim and the Kangerdlugssuaq – have doubled their speed to around 14 km a year (figure 14). In 1998, the Jakobshavn glacier showed a similar acceleration, and together the three glaciers drain nearly a fifth of the entire ice sheet. With close to half the discharge from the entire ice sheet occurring via 12 glaciers, there is real concern that the remaining glaciers will follow, leading to the wholesale collapse of the ice sheet. This would ultimately lead to a rise in global sea levels of around 7.2 m.



Figure 14. Changes in the position of Greenland's Helheim glacier's 'calving front' from 2001 (Left) to 2005 (Right). The location of this margin, where the glacier breaks up into icebergs, has changed dramatically over the period, at a time when the glacier has doubled its rate of flow.

Courtesy: NASA

At the opposite end of the planet, a number of papers addressed the issue of accelerating ice loss along the margins of the giant West Antarctic Ice Sheet. These do not address specific hazard implications in the short term, but describe processes with the potential to lead to serious coastal inundation at longer time scales. Most notably, Isabella Velicogna, of the University of Colorado, and John Wahr ⁷², of NASA’s Jet Propulsion Laboratory, describe how they used very precise measurements of gravity, by means of the Gravity Recovery and Climate Experiment satellites, to determine the amount of ice lost from Antarctica every year. This, they estimate, is now occurring at the rate of 152 ± 80 cubic kilometres a year, mostly from the West Antarctic (rather than the larger East Antarctic) Ice Sheet (WAIS). In another Science paper, Jonathan Overpeck ⁵⁰ of the University of Arizona, and colleagues, warns that the level of warming over the poles by the year 2100 may be as high as it was around 130,000 years ago; a time when global sea levels were several metres above current levels. Overpeck and his co-workers recognise that both the Greenland Ice Sheet and parts of the Antarctic Ice Sheet may be vulnerable, and they caution that the record of past ice-sheet melting indicates that the rate of future melting may be faster than previously thought. Even more worryingly, the authors note that a threshold of melting, triggering many metres of sea-level rise, could be crossed well before the end of the century (figure 15). A more detailed summarisation of recent research into ATHC and sea-level rise aspects of dangerous climate change can be found in the BUHRC’s Issues in Risk Science 5, compiled by Bill McGuire⁴⁶.



Figure 15. (Left) Impact of a 7 m sea-level rise on the UK coastiline which would result from the complete collapse and melting of the Greenland Ice Sheet.

(Right) The new coastline that would result from a 13 m sea-level rise due to the melting of both the Greenland and West Antarctic Ice Sheets.

Courtesy: DisasterMan.

Climate change and extra-tropical windstorm activity

As was apparent from the discussion in the opening part of this review, exactly how climate change will be reflected in tropical cyclone activity remains a matter for debate. The situation is similar with respect to extra-tropical storms, although there is some consensus for the suggestion that in a warmer world, such storms will become more intense, if not more numerous. Three papers published in Geophysical Research Letters, and addressed here, examine various issues of extra-tropical storminess in the context of climate change, the first looking at how storm tracks may shift during the course of this century. Jeffrey Yin ⁷⁸ of the US National Center for Atmospheric Research, reports that simulations of the 21st century climate reveal a consistent poleward and upward shift and intensification of the tracks of extra-tropical storms. This is in line with recent findings that the latter half of the 20th century saw a poleward shift in the mean latitude of extra-tropical cyclones, and that such cyclones have become fewer and more intense. Yin also notes that the poleward shift of storm tracks is accompanied by similar shifts in mid-latitude precipitation and surface wind stress (the force acting on a surface due to the flow of air above it).

From a hazard point of view, both wind speeds and precipitation levels are important, in terms of the future behaviour of extra-tropical storms. The impact of climate change on near-surface wind speeds is examined in another paper by Sara Pryor ⁵⁴, of Indiana University, and colleagues. Pryor and her team note that Global Climate Models (GCMs) are unable to replicate the historically-observed magnitude and spatial variability of wind speeds. To solve this problem, the authors apply a ‘downscaling’ technique to generate probability distributions of wind speeds at sites in northern Europe for historical periods (1961 - 1990 and 1982 - 2000), and for two future periods (2046 - 2065 and 2081 - 2100). Pryor and her co-workers conclude that both the mean and 90th percentile wind speeds over northern Europe during the 21st century are likely to differ from those at the end of the 20th century by less than ± 15 percent, with no consistent signal - with regard to an increase or decrease - being discernable.

In the third paper, Katja Woth ⁷⁶, of Germany’s Institute for Coastal Research, looks at how higher wind speeds over Northwest Europe, predicted in GCMs, may affect storm surges in the North Sea. Four different storm surge height projections are derived from a storm surge model, driven by a Regional Climate Model, which is itself driven by two different GCMs. The four projections come from two different scenarios using two different GCMs, and are developed for the last three decades of the 21st century. All four projections predict a significant increase in storm surge elevations for the continental North Sea coast, up to a maximum of 22cm. Clearly, in combination with rising sea levels, such a finding has significant implications for future coastal flooding and flood defence policy (figure 16).


Figure 16. The 1953 North Sea storm surge topped 2.5 m and took more than 2,000 lives in the UK and the Netherlands. For the last three decades of the 21st century, Woth (2005) predicts that higher wind speeds will result in storm surge elevations along the continental North Sea coast being between 15 and 22 cm higher.

Courtesy: Environment Agency.

Changes in precipitation extremes in Europe

Growing evidence suggests that as the world warms up, so the frequency of heavy precipitation will increase across many parts of the world. In the Journal of Geophysical Research, Christoph Frei ²⁴ of the ETH in Zurich, and his colleagues, zero in on Europe to examine how the continent will be affected, and how this is predicted by different models. Frei and his group undertook an inter-comparison of precipitation scenarios as simulated by six different European regional climate models (RCMs), all with comparable model settings and driven with boundary data from the same GCM. The results show that RCMs are capable of representing mesoscale spatial patterns in precipitation extremes that are not resolved by today’s GCMs. The authors note that the simulated future changes in European precipitation extremes show a seasonally very distinct pattern, with Europe north of about 45ºN experiencing, in Winter, an increase in precipitation extremes, and the region to the south showing little change, but with a small tendency towards a decrease. Five-year return values of 1-day precipitation extremes rise by up to 11 percent in central Europe, and by between 10 and 25 percent in southern Scandinavia. In Summer, the authors report a gradient from north to south, with increases in northern Europe and decreases in the Mediterranean region. There are, however, more differences between the RCMs used, giving a much greater spread for the 5-year return value, than in Winter. This varies from - 13 to + 21 percent for central Europe, and from + 2 to +34 percent for southern Scandinavia.

Global runoff trends in a warmer world

Increases and decreases in precipitation, as the climate warms, will lead to significant changes in runoff, which is essentially the difference between precipitation across a river basin and evaporation + transpiration (by plants). Runoff is critical for a number of reasons, the most relevant here being with respect to flood, drought and water availability. In a paper in Nature, Chris Milly ⁴² of the United States Geological Survey’s Geophysical Fluid Dynamics Laboratory, and his team, summarise expected global patterns of run-off and water availability in a changing climate. By 2050, the authors predict that major changes in runoff will be apparent across the world. These include increases of between 10 and 40 percent in eastern equatorial Africa, southern South America and high latitude North America and Eurasia, with decreases of between 10 and 30 percent expected in southern Africa, southern Europe, the Middle East and mid-latitude western North America. The big picture points to areas of increased runoff shrinking over time, while areas of decreased runoff grow, with - for example - increased runoff in the 20th century in the western central plains of North America showing a decreasing trend into the 21st century, and the drying of the Mediterranean region extending further northwards.

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