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Climate change

The Third Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change (IPCC, 2001) synthesises the current state of knowledge on climate change, and the following summary points can be made:

The global average surface temperature has increased over the 20th century by about 0.6°C.

• The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has increased since 1861 (see Figure below). These numbers take into account various adjustments, including urban heat island effects. The record shows a great deal of variability; for example, most of the warming occurred during the 20th century, during two periods, 1910 to 1945 and 1976 to 2000.
• Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861.
• New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest. Because less data are available, less is known about annual averages prior to 1,000 years before present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.

Snow cover and ice extent have decreased.

• Satellite data show that there are very likely to have been decreases of about 10% in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid- and high latitudes of the Northern Hemisphere, over the 20th century.
• There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century.
  

Global average sea level has risen and ocean heat content has increased.

• Tide gauge data show that global average sea level rose between 0.1 and 0.2 metres during the 20th century.
• Global ocean heat content has increased since the late 1950s, the period for which adequate observations of sub-surface ocean temperatures have been available.

Climate change is mainly caused by the emission of greenhouse gases (see Table 1). The impacts of greenhouse gas emissions are a global issue. It is thus important that measures to reduce greenhouse gas emissions are implemented globally, as has been agreed internationally at the Kyoto Conference in 1997 ( Kyoto Protocol to the United Nations Framework Convention on Climate Change).

Table 1: Greenhouse gases and % contributions to 1990 UK emissions
(source: DETR (1998): UK Climate Change Programme: Consultation Paper, London, HMSO)

The main sources of greenhouse gas emissions are:

• Energy (fuel combustion, fugitive emissions from fuels)
• Industrial processes (mineral products, chemical industry, metal production, production and consumption of halocarbons and SF6
• Solvent and other product use
• Agriculture (enteric fermentation, manure management, rice cultivation, agricultural soils, biomass burning)
• Waste (solid waste disposal on land, wastewater handling, waste incineration)

The expected effects of climate on the climate space of species and the properties of habitats have been modelled in the first phase of the MONARCH project, commissioned by the conservation agencies in the UK and Ireland. The report of this study, Climate Change and Nature Conservation in Britain and Ireland: Modelling natural resource responses to climate change was on 14^th November 2001.

The UK Climate Impacts Programme (UKCIP) 2002 briefing report "Climate Change Scenarios for the United Kingdom" examined four alternative scenarios of how climate change may affect the UK climate over the next one hundred years. These scenarios are labelled Low Emissions, Medium-Low Emissions, Medium-High Emissions and High Emissions and relate to four scenarios of future global emissions of greenhouse gases. Key results from the 2002 report are:

• UK climate will¹ become warmer. By the 2080s, annual temperature averaged across the UK may² rise by between 2ºC for the Low Emissions scenario and by 3.5ºC for the High Emissions scenario. There will be greater warming in the south and east than in the north and west, and there may be greater warming in summer and autumn than in winter and spring. By the 2080s for the High Emissions scenario, parts of the southeast may be up to 5ºC warmer in summer. The temperature of UK coastal waters will also increase, although not as rapidly as over land.
• High summer temperatures will become more frequent and very cold winters will become increasingly rare. A very hot August, such as experienced in 1995 when temperatures over central England averaged 3.4ºC above normal, may occur one year in five by the 2050s for the Medium-High Emissions scenario, and as often as three years in five by the 2080s. Even for the Low Emissions scenario, by the 2080s about two summers in three may be as hot as, or hotter than, the exceptionally warm summer of 1995.
• Winters will become wetter and summers may become drier everywhere. The relative changes will be largest for the High Emissions scenario and in the south and east of the UK, where summer precipitation may decrease by 50 per cent or more by the 2080s and winter precipitation may increase by up to 30 per cent. Summer soil moisture by the 2080s may be reduced by 40 per cent or more over large parts of England for the High Emissions scenario.
• Snowfall amounts will decrease throughout the UK. The reductions in average snowfall over Scotland might be
  between 60 and 90 per cent (depending on the region) by the 2080s for the High Emissions scenario.
• Heavy winter precipitation (rain and snow) will become more frequent. By the 2080s, winter daily precipitation
  intensities that are experienced once every two years on average may become between 5 per cent (Low Emissions) and 20 per cent (High Emissions) heavier.
• Relative sea level will continue to rise around most of the UK’s shoreline. The rate of increase will depend on the
  natural vertical land movements in each region and on the scenario. By the 2080s, sea level may be between 2 cm below (Low Emissions) and 58 cm (High Emissions) above the current level in western Scotland, but between 26 and 86 cm above the current level in southeast England.
• Extreme sea levels will be experienced more frequently. For some east coast locations, extreme sea levels could occur between 10 and 20 times more frequently by the 2080s for the Medium-High Emissions scenario than they do now.
• The Gulf Stream may weaken in future and the changes in climate described in this report reflect this. It is unlikely that this weakening would lead to a cooling of UK climate within the next 100 years. We do not understand enough about the factors that control this ocean circulation, however, to be completely confident about this prediction, especially in the longer term.

¹The word ‘will’ is used where there is High Confidence about an outcome.
²The word ‘may’ is used where there is less then High Confidence about an outcome.

References:

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