Introduction
The aim of this overview is to present a balanced summary of the science behind climate change.
Temperature
One of the most important indicators of climate is temperature. There are 5 main records. Three of them are based on observations from climate stations and two of them of on measurements from satellites. One of the observation based climate records is produced in the United Kingdom and all the others in the USA. The records based on observations are direct measures of temperature starting in the 19th century but global coverage is patchy with most observations being in advanced industrialised countries and over land. Satellite measurements started in 1979 and use indirect indicators of temperature but have the advantage of almost uniform global coverage.Whilst there are concerns that the climate observation network has been influenced by spreading urbanisation, the ‘urban heat island’ effect, the agreement between climate observations and satellites suggests that temperatures at the end of the 20th century were higher than in the previous 150 years (Figure 1). Climate models are used as tool to distinguish between natural and anthropogenic (human caused) climate change. The IPCC reports only present a very small diagram of how well climate models represented the temperature of the 20th century. We have examined this aspect in much more detail. This shows that in terms of modelling temperature changes the models had good agreement for the late 20th century but were not as good for earlier periods (Figure 2).
Precipitation
Most of the discussion on climate change has focussed on temperature however changes in precipitation are also important. If temperature and precipitation both increase the extra rain will, to a greater or lesser extent, compensate for increased evaporation. If, on the other hand, precipitation reduces it will exacerbate temperature rises. As with temperature the IPCC only present a small figure showing how well climate model represented 20th century precipitation. We have also examined this aspect in more detail. In general the representation of changes in precipitation is not as good as for temperature. (Figure 3).Gases
If climate is being changed by humans then the main, but not only, mechanism by which this is done is by changes in gases in the atmosphere. These come about by burning fossil fuels and by deforestation. Land use changes (which alter the amount of heat reflected back to the atmosphere) or fine particles in the atmosphere can also change the climate. Whilst the phrase “greenhouse effect” is convenient shorthand it does not describe how gases actually change temperatures. The heat which arrives from the sun is at one wavelength and the heat emitted back to space by earth, after being warmed by the sun, is at a different, longer, wavelength. Different gases in the atmosphere block radiation at different wavelengths. The long wave radiation from the earth is mainly in the 5 to 50 microns range with a peak at around 12 microns. Water vapour blocks outgoing radiation over most of this range, other gases (carbon dioxide, methane, nitrous oxide and ozone) at different but narrower bands. The amount of radiation blocked depends on the concentration of the gas in the atmosphere but as all the gases, except water vapour, operate over a limited range of wavelengths their maximum potential effect is limited (Figure 4).The situation is complicated by the fact that carbon dioxide is removed only slowly from the atmosphere and the duration of its effects is measured in centuries rather than decades. Climate models are based on the assumption the increased carbon dioxide increases temperature, the increased temperature leads to more evaporation and hence more water vapour in the atmosphere: the extra water vapour increases temperatures more than would be possible by carbon dioxide acting on its own. The situation is complicated by the fact the warming effect is different if the water vapour forms clouds and also on how high in the atmosphere the clouds are formed. This is one of the main areas of uncertainty.
Proxies
In the section on temperature we mentioned that measurements have been made since the 19th century. To know what temperatures were in earlier times we have to use proxies. Proxies are something natural which can be measured and which responds to variations in temperature. The two main proxies are tree rings and ice cores. In the case of tree rings, it is believed that, provided trees have enough water all the year round, their rate of growth will increase with increasing temperature. In the main section on proxies we identify some of the limitations of this approach. Temperature records from tree rings have been produced going back hundreds, or even thousands of years. The evidence from tree rings suggests that climate features reported in the historical records, such as the Little Ice-age or the Medieval Warm Period, did occur that their severity and duration was not uniform over the globe. This remains a contentious area.Ice cores drilled deep in the Arctic, Antarctic and glaciers go back in some cases for 100s of thousands of years. Careful analysis of cores at different depths enables the contents of carbon dioxide and other gases at different times in the past to be determined. By examining the isotopes of oxygen in the ice it is also possible, from the relative amount of different isotopes, to estimate temperature for close to a million years ago. One of the main outcomes of this analysis is that it has enabled us to see the pattern of ice ages and relatively warm interglacial periods. Another finding is that temperature increase slightly sooner than carbon dioxide and declines much more rapidly than the gas. This suggests that carbon dioxide responds to the increase in temperature though it may act as a multiplier to increase temperatures more than would otherwise be the case (Figure 5).
Although not exactly a proxy for temperature, phenomenological evidence is also important. Such evidence tends to confirm recent warming (extended ranges of insects, melting glaciers, northern extent of vineyards in England), the Medieval Warm Period (farming in Greenland, northern extent of vineyards England(again)) and the Little Ice Age (Ice fairs on the Thames and advancing glaciers).
Forcing
Forcing refers to those factors which result in a change in temperature. One factor which operates over time scales around 100,000 years is the variation of the orbit of the earth around the sun, the so called Milankovitch cycles. Comparison of these cycles, calculated from astronomical measurements, with temperature variations, determined from ice cores, shows that they are the main causative factor in ice ages (Figure 6). How the comparatively small changes to solar radiation reaching the earth lead to ice ages has not yet been satisfactorily determined. The earth’s temperature also responds to changes in pressure, wind direction and ocean currents in the Pacific and Atlantic oceans, an example of this is the so-called El Nino effect. The changes operate on irregular cycles of a few years to a few decades. Whilst their effect on the earth’s weather is reasonably well understood there is as yet no firm explanation of why they occur.Impacts
The reason that the study of climate change has recently become such an important topic is fear that climate change could have severe negative impacts on earth. Analysis of long-term data suggests that there is little evidence for some of the more extreme forecasts. For example, the energy of tropical storms increased as temperatures increased in the early years of the 20th century, declined as temperature fell in mid-century and rose again toward the end of the 20th century. There is no evidence that they are more prevalent now than in the past. The projections of the models do not support the apocalyptic future than some predict. For example in most parts of the world precipitation will increase by an amount that more than offsets the increased evaporation due to higher temperatures.Where we stand
In recent years climate change has come to be considered the predominant environmental threat. A corollary of this is that to the question of any of the related science is seen as proof of a lack of concern for the environment. We therefore feel it is important to state clearly where we stand. We believe that temperatures at the end of the 20th century were at the highest level since regular instrumental measures started around 150 years ago. We do not believe that the evidence from proxies is robust enough to determine whether temperatures in the medieval warm period were higher or lower than they are today. The phenomenological evidence suggests that current climate and the medieval warm period had similar temperatures. We believe that due to anthropogenic effects temperatures at present are higher than they otherwise would have been. We recognise that increasing temperatures have impacts on a range of variables including sea levels and tropical storms but the data suggest that the current rates of level increase and frequency of storms are not vastly different from recent centuries.We believe it is important to resolve the following areas of uncertainty in climate science by further research and/or investment in measuring networks or satellite sensors:
- How do the changes in orbit of the Milankovitch cycles cause ice ages and warm interglacial periods.
- What causes cycles such as the ENSO (El Nino/ Southern Oscillation) and the NAO (North Atlantic Oscillation).
- To what extent does an increase in Carbon Dioxide lead to an increase in water vapour and hence to an increase in global temperatures.
