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The science of climate change

Although the Earth’s climate changes continually, and often dramatically, as a result of natural processes, there is a strong scientific consensus that some significant and very rapid changes are now being caused by greenhouse gas emissions generated by human activity.  Many of these changes could have enormous and adverse impacts on the global economy and environments and societies worldwide.

During 2007 Inter-Governmental Panel on Climate Change (IPPC) published its Fourth Assessment Report..  IPPC Working Group 1, charged with examining the physical science basis for climate change, concluded that global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years (IPCC 2007). Working Group 1 also noted that due to an improved understanding of anthropogenic warming and cooling influences on climate they were very highly confident (90 per cent certain) that the globally average net effect of human activities since 1750 has been one of warming.

Amongst the climate changes the Fourth Assessment Report draws attention to are:

• Warming
  Eleven of the 12 years between 1995 and 2006 are among the 12 warmest years since the recording of global surface temperature began in1850 (the average global surface temperature rose by 0.74 °C between 1906 and 2005). Over the past 50 years the rate of warming has been nearly twice as fast as it was over the past 100.

  Over the past 50 years the patterns of extreme temperatures have changed markedly. Cold days, cold nights and frost have become less common, and there are more hot days, hot nights, and heat waves.
   

• Sea level rise
  Sea water expands as it warms, and this – together with melt-water from glaciers and the ice caps – has caused the global average sea level to rise at some 1.8 mm per year between 1961 and 2003. From 1993 to 2003, the rate increased to 3.1 mm per year and the total rise during the 20th century is estimated to be between 0.12 and 0.22 m.
   
• Drought
  Since the 1970s, higher temperatures and decreased rainfall have led to more intense, longer and more widespread droughts, particularly in the tropics and subtropics.
   
• Extreme weather
  Heavy precipitation (especially rainfall) has become more common over most land areas. This is consistent with warming and with the observed increases in atmospheric water vapour. There is some evidence that intense tropical cyclone activity in the North Atlantic has been increasing since about 1970, in correlation with increases in tropical sea-surface temperatures.

The report makes a number of projections of the how the climate might change over the course of the next century, depending on the rate of greenhouse-gas emissions.  These suggest that global surface temperatures are likely to increase by about 0.2°C per decade over the next two decades (any further emissions at current rates, or increased rates, will make this higher). The best estimates suggest that by the end of the 21st century temperatures could have risen by between 1.8 and 4°C or (if all uncertainties are taken into account) by 1.1 to 6.4°C. The IPCC believes this will make heat waves and extreme rainfall more frequent, and although the number of storms may actually decrease, they are likely to become more intense. 

The average rise in sea levels is likely to be from 28 to 43 cm by the end of the century. In reaching this figure, the IPCC chose not to include the full effects of ice-sheet flow, the mechanics of which are still uncertain. This means that these figures may be underestimates. 

In October 2006 Sir Nicholas Stern (Head of the Government Economic Service and former World Bank Chief Economist) published his independent review on the Economics of Climate Change (Stern 2007).  He concluded that there is still time to avoid the worst impacts of climate change, if concerted action is taken now.  The review estimtes that the impact of unmitigated climate change will be equivalent to a permanent loss in world consumption per head of at least 5 per cent and, if a wider range of risks and impacts is taken into account, potentially as much as 20 per cent of global GDP each year. By contrast, action to reduce greenhouse-gas emissions and thus avoid the worst impacts of climate change was calculated to cost a maximum of around 1 per cent of global GDP each year.
 

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