The consensus view of the majority of scientists, as reflected in the 2001 report of the Intergovernmental Panel on Climate Change (IPCC) and reinforced by recent research, is that there is now an unequivocal linkage between emissions and atmospheric concentrations of greenhouse gases and recent rapid rises in global air and sea temperatures. Measurements using reliable thermometers were first taken systematically at a number of locations throughout the world in the mid 19th century enabling the calculation of mean global temperatures. Graphs of mean combined surface air and sea surface temperatures for the Northern and Southern Hemispheres and averaged for the globe as anomalies on the base period 1961-1990,

Global Temperature Record

produced jointly by the Climatic Research Unit, University of East Anglia link and the UK Met. Office Hadley Centre, show that temperatures since 1856 to the present have increased by about 0.8°C, that positive temperature anomalies have occurred consistently since ~1980 and that nine of the ten warmest years have occurred in the last ten years, with 2004 the fourth warmest year on record. The data in the compilation is averaged for 5º x 5º latitude and longitude grids covering the whole world. Anomalies are calculated around the base period 1961-1990 as this is a well sampled period and ensures that there is no bias from different methodologies used throughout the world in earlier years.

In the centre of England temperatures have been measured since 1659 (Manley time series) giving the longest consistent instrumental record of temperature on the Earth. Recent measurements in this time series confirm for the UK that the 1990s was an extremely warm decade in comparison with an ~350 year record.

Evidence of the increasing temperatures is clearly apparent in the UK with milder winters, less snow and frost and earlier growing seasons. Further afield the thickness and extent of ice in the Arctic is reducing dramatically and glaciers, with the exception of Norway are retreating throughout the world. It is calculated that there will be few northern hemisphere glaciers left in even 40 years time and that the glacier on top of Kilimanjaro in Tanzania will disappear as soon as 2015. Other impacts that may be linked to the rising temperatures include the drought in southern Europe and in the Sahel, increased storminess and the occurrence of the first ever recorded hurricane in the southern Atlantic in 2004.

To determine the extent to which the temperatures of recent decades are anomalous in a longer term time frame information from historical documents and palaeoclimatological proxy sources has been used to estimate historical temperatures. Temperature can be calculated for example from tree rings, corals, ice cores, sediments, borehole temperatures and glacial moraines. As much more information is available for northern lattiudes the reconstructions have primarily been averaged for the Northern Hemisphere. The further back in time the reconstructions for individual years are for the greater the range of statistical error there is in their reliability. A compilation of information produced by Mann and colleagues illustrated in the 2001 IPCC report shows that temperatures declined on average since 1000 AD to the end of the 19th century, since when there has been a sharp increase in two phases to the present. The time series has since been extended back a further millenia, again showing relatively constant average temperatures in the first 1000 years and emphasising that the warming in the 20th century is unprecented in a 2000 year time frame. A number of other reconstructions have been made using different approaches and in some cases different data. For example, this is the millenial temperature record using a number of different temperature proxies.

They show different variability to the Mann data set, but generally confirm the overall conclusion that current global average temperatures are unprecedented. The methodologies used in the reconstructions are however, not without criticism and there is a continuing debate on the reliability of the estimates.