In this post I will look at measuring global temperature; how it is done and the global record up to the year 2000
To know whether the world is warming you have to have a means of measuring the temperature of the world, and to have a meaningful record that goes back a sufficient period of time to be able to make convincing confusions from the data.
There are a range of useful measurements; sea temperatures, land temperatures, air temperatures, and radiated temperature detected by satellite.
Sea Surface temperature
Measuring ocean temperature would seem to be a straightforward thing to do, but it varies over latitude and depth. Surface temperature is measured by satellite detecting a variety of infra-red and other electromagnetic radiation . This has only been available recently in a consistent form since 1980. Historic measurements were taken from buckets dipped in the sea or from water around engine intakes.
The UK met office has been very active in measuring temperature, and there is a much quoted paper from 2002 which gives 22 pages of analysis, charts and graphs on sea measurement . It states “Our objective is a spatially complete, monthly SST analysis for 1871 to date, preserving real climate signals on global, ocean-basin and subregional scales, while minimiz- ing random errors, sampling noise, and systematic biases.” And has 22 pages of detail on how data has been corrected, etc and then produces graphs from 1880.
The data is presented as the difference (referred to as the anomaly) between the measured temperature and the average of a given period (generally 1961-1990). The conclusion is generally of slow increase up to around 1980 and then a marked increase from 1980 to 2000. The graphs look alarming but the full increase from 1880 to 200 varies from 0 to 2C depending on region. Whether 2C is a significant increase is something I will return to, and given the amount of massaging that has been done I think it is reasonable to have some scepticism about the results.
Land temperature is derived from a large network of ground stations which have technical and positioning specifications. Again this is problematic because over time the world has become more urbanised and urbanisation generally increases temperature. There has been some discussion on how reliable these measurements are, but the results need to be viewed in the light of other measurements .
The temperature of the atmosphere at height is measured most consistently through satellite sensing . This was the subject of much debate in the late 20th century as results did not support warming seen from land or sea. Some work has been done in explaining this (“atmospheric drag” for instance) but at the end of the 20th century this reading still lagged causing some to dispute the other readings. It is not clear why, given a dispute, we would choose to believe the atmospheric ones, particularly as these measurements relate to the higher atmosphere and the natural world largely occurs at the surface of the world or in the oceans.
One of the most widely used data sets for temperature data is the HadCRUT series . This comes from the UK Met Office at Hadley. Their web site has a number of data sets including marine, upper air, and land surface measurement that are combined in the HadCRUT series of data.
HadCRUT Results up to 2000
There was considerable debate around the turn of this century about global warming with some predicting imminent disaster and some denying the world was warming at all. So here is some data from the combined data set HadCRUT 4 
The LH scale is the difference between the measured temperature and the average for 1961-1990.
The total temperature increase recorded here from 1850 is around 1C. By eye the temperature is stable up to 1910, there is a significant increase from 1910 to 1945ish, then flat until 1975, then strong increase to a record year in 1998.
1998 in particular was a watershed year. The rapid increase led to some predicting runaway temperature increases and significant global warming with catastrophic major changes in climate occurring shortly. Others disagreed. Some thought the overall temperature changes were low and the data questionable, and that we weren’t warming at all. Another group argued that the earth’s temperature changes over time anyway, and this was just part of the natural variation in climate. The century ended with disagreement and debate. In the next post I’ll look at how this debate looks nearly 20 years later.
 “The Rough Guide to Climate Change” by Robert Henson, Part Three – keeping track.