Overall, the evidence for climate warming now appears overwhelming, with evidence not only from climate observations, but also from the physical and biological indicators of environmental change, such as retreating glaciers, thinning of Arctic sea-ice and longer growing seasons (IPCC 2001a). During the latter part of the 20th century, there have been increasing concerns that human activity, in particular the emission of carbon dioxide and other greenhouse gases of fossil origin, is responsible for the observed increases in mean temperature and other aspects of climate change (IPCC 2001a). Global climate can obviously vary naturally, due both to what is called ‘internal variability’ within the climate system and to changes in external forcing unrelated to human activities; the latter would include changes in the sun's radiation and volcanic activity, for example. Recent model simulations of global climate show, however, that these natural causes of global temperature variability cannot alone explain the observed surface warming (IPCC 2001a). Only when model simulations incorporate the rising historical concentrations of greenhouse gases and shifting distributions of sulphate aerosols can a much better agreement between the observed and modelled global patterns of temperature change be achieved. It is of course difficult to be precise about the exact contribution of human activities to global warming, but the clear consensus is that the planet would not be warming as rapidly if humans were not currently emitting about 6.5 billion tonnes of carbon into the global atmosphere each year. The IPCC concluded in its recent Third Assessment Report (IPCC 2001a) that, ‘… most of the warming observed over the last 50 years is likely to be attributable to human influence.’ What of the future?
Fundamental to the prediction of future climates are estimates of future greenhouse gas emissions, and in particular those of carbon dioxide, the greenhouse gas that causes about 60–65% of the human-induced greenhouse effect. It is predicted that the 2001 CO2 concentrations of about 370 p.p.m.v. (parts per million by volume) may rise to 540–970 p.p.m.v. by 2100 (IPCC 2001a), compared with the pre-industrial concentrations of 280 p.p.m.v. Concentrations of other important greenhouse gases are also expected to continue rising, with changes in methane (−11 to +112%), nitrous oxide (12–46%) and tropospheric ozone (−12 to +62%) predicted in the same set of emissions scenarios (IPCC 2000).
There are considerable uncertainties in the impact that these changes may have on global climate as a result of uncertainties of how sensitive the Earth's climate is to rising greenhouse gas concentrations. In order to take account of these uncertainties, a range of climate sensitivities have been combined with a range of possible future emissions to calculate a range of future changes in global temperature and sea-level (IPCC 2001a). The projections are that the annual global mean surface air temperature will rise from 14.0 °C (1961–1990 average) to 15.4–19.8 °C by 2100. By comparison, maximum temperatures during the last interglacial about 125 000 years ago are estimated to have reached 15.0–15.5 °C. The latest projections represent rates of change of about 0.1–0.5 °C per decade, which compare with 0.15 °C per decade since the 1970s and with a warming rate of about 0.05 °C per decade since the late 19th century.
Accompanying the predicted warming is a rise in global mean sea-level. Observed sea-level has risen by 10–25 cm over the last century, reaching its highest level during the 1997/98 El Niño event. The recent IPCC calculations (IPCC 2001a) suggest a future rise of 9–88 cm by 2100 compared with the average 1990 level, with the largest contribution to this increase coming from the expansion of warmer ocean waters and up to 20% from the melting of land glaciers.
Mean temperature and sea-level rise are only two aspects of climate change; there are also predicted changes in seasonal and diurnal temperatures together with precipitation and the frequency of extreme events (IPCC 2001a). These will undoubtedly have important consequences for the dispersal and distribution of species. In looking to the future as to the past we are, however, much less certain about such aspects of climate change than those of global mean temperature and sea-level.
Regional climate change scenarios
For policy makers, managers and the general public at large there is obviously considerable interest in translating the global climate change projections to those at a more regional level (e.g. NAST 2000, Hulme et al. 2002). The approach taken in the generation of future climates for the UK (Hulme et al. 2002) was to adopt a single model that performs well in simulating the observed recent average climate in the UK (the Hadley Centre global climate model HadCM3) with four different emissions scenarios: low emissions, medium–low emissions, medium–high emissions and high emissions. These span almost the full range of emissions described in the IPCC Special Report on Emissions Scenarios (IPCC 2000) and represent changes in global temperature and atmospheric carbon dioxide concentrations for the 2080s of 2.0 °C, 525 p.p.m. (low), 2.3 °C, 562 p.p.m. (medium–low), 3.3 °C, 715 p.p.m. (medium–high) and 3.9 °C, 810 p.p.m. (high).
The analysis indicates that all aspects of the climate of the UK will be affected by changes in global climate. Natural climate variability (i.e. the noise of the system) will in reality modify these magnitudes and patterns of change, whether this variability is internally generated or whether it results from external factors such as solar variability or volcanic eruptions. However, it is generally expected that the UK climate will become warmer by 2–3.5 °C by the 2080s, with greater warming in the south and east than in the north and west, and with greater warming in the summer and autumn than in the winter and spring. It is also expected that high summer temperatures will become more frequent and very cold winters increasingly rare, continuing the trend that is already seen in the observed climate (see above). Consequently, snowfall amounts are expected to decrease throughout the UK. Winters are, however, expected to become wetter with heavier winter precipitation whereas summers may become drier. Given the importance of severe winter weather on mortality of many coastal bird species (described above), reductions in the frequency of severe weather events in winter may therefore reduce the probability of such cold-weather-induced mortality. In addition, however, the decreases in recruitment of intertidal invertebrates following mild winters (Beukema et al. 2001) suggest that although reduced frequency of cold winters may reduce the probability of overwinter mortality, it may also decrease the availability of prey to these species. The consequences of temperature changes for coastal birds in winter can therefore be extremely difficult to predict.
In addition to temperature changes, future changes in the seasonality of precipitation may have profound effects on the structure and quality of important habitats for coastal species. For example, both extensive winter and spring flooding and drought during summer could alter the suitability of many grasslands as wintering and breeding sites for the large number of species which use them (Ausden et al. 2001, Milsom et al. 2002).
Relative sea-level is predicted to continue increasing around most of the UK coast, but the rate of increase will depend on the natural vertical land movements in each region; much of southern Britain is sinking at 1–1.5 mm/yr whereas much of northern Britain is rising at 0.5–1 mm/yr relative to the sea. Under the lower scenarios of climate-induced sea-level rise, these natural land movements can be very significant in exacerbating or reducing the estimated climate-induced change in mean sea-level around the British coast.
A second factor to consider in relation to sea-level rise and coastal flooding risk is the changing nature of storm surges caused by low atmospheric pressure and strong winds. A rise in mean sea-level will result in a lower surge height being necessary to cause a given flood event, leading to an increase in the frequency of coastal flooding. Using the atmospheric winds and pressure that are generated by the climate change model to drive the Proudman Oceanographic Laboratory model of the shelf seas around the UK indicates that the largest increases in surge height (up to 1.4 m) may occur off the southeast coast. It is important to note here though that the modelling uncertainties are very large and hence there is relatively low confidence in the patterns and magnitudes of storm surge height.
A major consequence of future sea-level rise for coastal birds seems likely to be changes to habitat structure and quality (Austin & Rehfisch 2003). For example, the extent to which the invertebrate populations of coastal mudflats will be influenced by sea-level rise is likely to depend on whether rates of sedimentation can compensate for sea-level rise (Beukema 1992). Similarly, the structure of habitats such as saltmarshes and beaches may change significantly as a result of sea-level rise, which is likely to influence the important breeding and wintering populations of wildfowl (Vickery et al. 1995), waders (Liley 1999, Norris et al. 2004) and passerines (Brown & Atkinson 1996) which use these habitats. Many brackish and coastal freshwater sites also hold internationally important bird populations and sea-level rise may threaten these sites through tidal inundation following breaches of any sea defences. Thus, difficult decisions regarding the protection of coastal habitats are likely to be necessary in the near future.
For coastal birds within the British Isles the projected changes in climate and sea-level might be expected to have considerable impacts on the availability of habitat and resources. Climate change threatens both increased levels of flooding in the coastal and riverine floodplain, as well as the degradation of nationally and internationally important coastal ecosystems such as saltmarsh and coastal grazing marsh (Nicholls & Wilson 2002). However, analysis by Nicholls and Wilson (2002) of a range of socio-economic storylines within the RegIS project to examine the impact of climate change on the soft coasts of eastern England indicates present trends in coastal management are likely to have as profound an effect on the stock of different habitats over the current century as climate change itself. For example, because of sea defenses, sea-level rise might be expected to produce loss of saltmarsh, whereas planned and unplanned coastal realignment will produce gains in saltmarsh and associated intertidal habitats (Atkinson et al. 2004). Managed realignment, in turn, will often produce losses of coastal grazing marsh, a habitat which is important to both breeding and wintering waterfowl and waders. The stock of available habitats therefore depends critically not only on the question of climate change and sea-level rise but also on human management of the coast, which is an important determinant of habitat loss and creation.
For migrant birds that overwinter on temperate coasts, changes in climate and sea-level within the breeding grounds in the Arctic, for example, are likely to have as large an impact on populations as changes in the wintering grounds. In considering what may happen to coastal birds as a result of climate change it is therefore important to consider climate change across the breeding, passage and wintering grounds.