1. Climate change: a primer

Our planet’s climate is a highly complex system driven ultimately by solar activity but also involving highly convoluted interactions between the atmosphere, the oceans and the land surface. Before the industrial revolution the climate can reasonably be regarded as an almost entirely natural system. Since the late 17^th century, however, the influence of human activities on the composition of the atmosphere and the utilization of the land has meant that the climate system and its behaviour has become intimately linked to the growth and development of our industry and commerce based society.

The climate is naturally variable at all time scales, as demonstrated by significant changes in temperature and atmospheric composition throughout Earth history. 50 million years ago, the concentration of carbon dioxide (the most common greenhouse gas) was probably over 1000 parts per million (ppm), compared to pre-industrial levels of around 279 ppm and today’s (2005) level of 381 ppm. These very high concentrations resulted in a much warmer world, with little ice locked up at poles and sea-level around 50 m higher than it is today. Following this peak, carbon dioxide levels and temperatures fell progressively until, around 30 – 40 million years ago a major icecap began to develop in Antarctica. 3 to 4 million years ago, the concentration of atmospheric carbon dioxide fell to levels below those of pre-industrial times, triggering the development of great ice sheets in the northern hemisphere. Over this period ice sheets spread towards the equator on many occasions, including four times during the past 680,000 years, covering much of Europe, North America and Asia in ice up to 3 km thick. Each glacial episode was characterised by very low carbon dioxide levels and sea-levels up to 130 m lower than they are today. During the intervening interglacials, carbon dioxide concentrations in the atmosphere rose along with sea-levels, as the ice sheets retreated towards the poles.

The last glacial period ended around 11,500 years ago, and we are now in an interglacial known as the Holocene. Normally, we could expect the ice to make a return within around 15,000 years, although the effect of global warming due to human activities on the timing of this return is not known. While the transitions from interglacial to glacial period occur slowly, there is evidence for very rapid changes in climate that occurred in just a few decades. The best example is the Younger Dryas, a short, sharp period of cold that saw a return to glacial conditions just as the world was warming up. The Younger Dryas lasted from about 12,900 – 11,500 years ago and was characterised by a dramatic fall in temperatures within just a decade or so. Temperatures in Greenland fell 15º C lower than today, while UK mean annual temperatures averaged around - 5º C. As quickly as it arrived, the Younger Dryas ended, with temperature rises as great as 7º C in perhaps just a few years, launching the beginning of the current Holocene interglacial.

Both the timing of ice ages and the patterns of glacial advance and retreat remain controversial, although it is broadly agreed that the major controls are a combination of changing greenhouse-gas (primarily carbon dioxide and methane) concentrations in the atmosphere, variations in the Earth’s orbit about the sun, and changes in the disposition of the continents. Smaller-scale natural variations in our planet’s climate also occur, and are due to different factors, including small changes in the output of the Sun and volcanic activity. Two examples over the last 2,000 years (Figure 1) include the Medieval Warm Period (MWP), from around 1100 – 1250 AD, and the so-called ‘Little Ice Age’ (LIA), that lasted from the 14^th to the mid-19^th centuries. The MWP coincided with a peak in solar activity, and saw extensive wine production in England and the colonisation by the Vikings of parts of Greenland. The succeeding LIA brought bitterly cold winters to Europe and North America, and is characterised by the advance of Alpine glaciers. Iceland was locked in by pack-ice in winter and the population fell by half due to famine. The Viking communities in Greenland died out. Neither the MWP nor the LIA appear to be of global extent. The cause of the LIA is probably a combination of reduced solar activity and a number of large volcanic eruptions that cut still further the level of solar radiation reaching the surface.

Figure 1: Comparison of 10 different published reconstructions of mean temperature changes during the last 2000 years. More recent reconstructions are plotted towards the front and in shades of red, older reconstructions appear towards the back and in shades of blue. The Medieval Warm Period and Little Ice Age are labelled at roughly the times when they are historically believed to have occurred, although it is still disputed whether or not either were global events. The single, unsmoothed annual value for 2004 is also shown for comparison. Courtesy: Dragons Flight.

Since the end of the Little Ice Age, global temperatures have climbed inexorably. This climb has coincided with a steady rise in carbon dioxide levels (Figure 2) and against a background of steady solar activity. The link between warming and the concentrations of carbon dioxide and other greenhouse gases in the atmosphere is now firmly established and beyond reasonable debate. During the 20^th century, global temperatures rose by 0.6º C, with a sharp acceleration starting in the 1990s (Figures 3 and 4). Nineteen of the hottest twenty years on record have occurred since 1980, with the eleven hottest years in the last twelve. Even if greenhouse gas concentrations could be frozen at today’s values (381 ppm), it is highly likely that temperatures would continue to rise by another 0.6º C or so before stabilising at a new level. Assuming that emissions will continue to rise, however, the Intergovernmental Panel on Climate Change (IPCC) has produced a series of emissions scenarios in order to estimate the range of temperature rises we might expect this century and beyond. In its 2001 Third Assessment Report (TAR), the IPCC estimated that global average surface temperatures would rise by 1.4 – 5.8º C above 1990 values by 2100. Warming at the high end of this range would have catastrophic consequences, while even mid-range rises would cause serious problems. Current consensus envisages average rises of perhaps 2 - 3º C, although larger rises remain perfectly possible. Of great concern is the fact that average temperature rises to date, and those predicted for the future, hide important regional variations, particularly in relation to greater warming of the Polar Regions. The Antarctic Peninsula, for example, has seen temperatures rise by up to 3º C in the last 50 years, several times the global average. Temperatures in Alaska and Siberia have risen by close to 2º C over the same period, and in southern Greenland, air temperatures are now 3º C higher than just 20 years ago. With Arctic temperatures forecast to rise by 4 - 7º C by 2100, there is great concern that continued rapid warming at the poles may bring forward the possibility of dangerous climate change.

Figure 2: Levels of atmospheric carbon dioxide have climbed dramatically since the late 1600s and reached 381 ppm in 2005, compared with 279 ppm prior to the industrial revolution.. Courtesy: Dragons Flight.

Figure 3: The instrumental record of global average temperatures as compiled by the Climatic Research Unit (University of East Anglia) and the Hadley Centre of the UK Met Office. Courtesy: Dragons Flight.

Figure 4: Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980. Courtesy: Dragons Flight

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