Climate change in a global perspective

Media reports often give the impression that the jury is still out on whether climate change is a real threat. However as Stern (2006a, p. i) notes, the scientific evidence 'is now overwhelming: climate change presents very serious global risks, and it demands an urgent global response'. The Intergovernmental Panel on Climate Change (IPCC) (IPCC, 2007a) reports that global average temperatures are already 0.7 °C above the pre-industrial level, with another 0.5 °C of warming 'in the pipeline' as a consequence of the thermal inertia of the earth's system. The 2007 report concluded with 90 per cent confidence that the 'globally averaged net effect of human activities since 1750 has been one of warming', and that this has occurred 'due to the observed increase in anthropogenic greenhouse gas concentrations'.

Figure 1.1 Historic global average temperatures and atmospheric carbon dioxide concentrations

Source: The Woods Hole Research Centre.

Figure 1.1 Historic global average temperatures and atmospheric carbon dioxide concentrations

Source: The Woods Hole Research Centre.

Carbon dioxide (CO2), mainly emitted from fossil fuel use and land-use change, is the most important of the anthropogenic greenhouse gases (GHGs) and is expected to account for two-thirds of the warming over the next 100 years (Hadley Centre, 2005). An illustration of the close relationship between CO2 in the atmosphere and temperature variations over the last 450,000 years is presented in Figure 1.1. This shows how pre-industrial concentrations of CO2 rose and fell between approximately 190 and 270 parts per million (ppm), as the ice sheets expanded and contracted. But current CO2 concentrations now exceed by far the natural variation over the last half a million years and they are rising sharply. Currently the warming effect from non-CO2 GHGs is approximately balanced by the cooling effect from aerosols, leaving the current 'net radiative forcing' broadly equivalent to that from CO2 alone, at around 380 ppm 'carbon dioxide equivalent' (CO2-eq). However, if the concentration of aerosols declines in the future, the radiative forcing and hence overall warming will increase.

The IPCC (2007a) estimates that with a stabilisation level of 550 ppm CO2-eq there would be a greater than 90 per cent probability of global average temperatures increasing by 1.5 °C or more this century (relative to pre-industrial levels), and a greater than 60 per cent probability of increasing by 2 °C or more. Two degrees is the limit beyond which many scientists and NGOs consider that warming would become 'dangerous'. Avoiding this has been adopted as a target by both the UK government and the European Union (EU) (EC, 2007b). The likely impacts of a 2 °C increase in global mean temperatures include: an additional two billion people affected by water scarcity, the loss of many of the world's most bio-diverse ecosystems including coral reefs, significant reductions in agricultural productivity and food availability in developing countries and increased risk of extinction for 20-30 per cent of the world's species (IPCC, 2007b).

The potential for triggering the irreversible disintegration of the Greenland and West Antarctica ice sheets, leading ultimately to a global sea level rise of around 12 metres, is of particular concern. The mechanisms of ice sheet disintegration are not well captured by current models, but an increasing number of studies suggest that the process could occur more rapidly than the IPCC assumes (Lowe et al., 2006; Overpeck et al., 2006). For example, James Hanson, Director of the NASA Goddard Institute for Space Studies, considers that 'it is inconceivable that BAU ['business as usual'] climate change would not yield a sea level change of the order of metres on the century timescale' (Hansen, 2007). If such rapid sea level rise occurs, it will have profound consequences for humanity, including large-scale displacement of populations (more than ten per cent of the world's population live less than ten metres above sea level), destruction of many of the world's major cities and widespread contamination of coastal water supplies.

To have a greater than 50 per cent chance of keeping global temperature increases from exceeding 2 °C, the concentration of GHGs in the atmosphere would need to be stabilised below 450 ppm (Meinshausen, 2005; Baer and Mastrandrea, 2006). This suggests that global emissions of GHGs will need to peak sometime within the next 10-15 years and then be reduced by at least 50 per cent by mid-century, with further reductions beyond (by which time it is anticipated that the global population will exceed nine billion). However, there is a significant risk that the terrestrial biosphere will become a less effective carbon sink over the next century, and it may even become a net source (Cox et al., 2000). If this is the case, the permissible emissions to achieve CO2 stabilisation may be lower.

Hence, the scientific evidence of the human contribution to climate change is close to incontrovertible, the risks are enormous and the primary policy response must be major cuts in emissions - particularly in the industrialised world. While the role of developing countries is contested, the need for urgent action suggests that their participation in mitigation efforts cannot be long delayed. To put this challenge in context, the following section reviews some basic data on the pattern of energy use and carbon emissions throughout the world.

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