Monthly global-average temperatures are calculated independently by three centres – one in the UK, the others in the USA:

• UK Meteorological Office in collaboration with the Climatic research Unit (CRU) at the University of East Anglia
• National Climatic Data Center (NCDC) – part of the National Oceanic and Atmospheric Administration (NOAA)
• Goddard Institute for Space Studies (GISS) – part of NASA

Although these centres operate independently, the results from all three show the same general trend of increasing global temperatures.

Aside from measurements taken on land and sea, use is made of satellites with microwave instruments to measure the temperature in the troposphere. The satellite measurements are made by the University of Alabama in Huntsville (UAH ) and by Remote Sensing Systems (RSS ) based in California. The results from both earth-bound and satellite data indicate consistent warming.

In climate change studies, temperature anomalies are used rather than absolute temperature. A temperature anomaly is the difference from an average, or baseline, temperature typically computed by averaging 30 or more years of temperature data. For The Met Office HadCRUT4 record the base period is 1961-1990. A positive anomaly indicates the observed temperature was warmer than the baseline, while a negative anomaly indicates the observed temperature was cooler than the baseline.

There are estimates of global temperatures going back 2000 years. These rely on palaeoclimate records which serve as proxy methods for temperature measurement using data previously preserved in, for example rocks, ice sheets, tree rings, corals, shells, and microfossils. Also historical references such as the presence of the Vikings in Greenland have been used.

A substantial portion of pre-industrial (1300–1800 CE) variability at multidecadal timescales is attributed to volcanic aerosol forcing – the volcanic aerosols scatter incoming solar radiation to space thereby reducing the amount of solar energy reaching the earth’s surface, hence reducing daytime maximum temperatures.

The Medieval Warm Period (MWP) and the Little Ice Age (LIA)

These refer to times when temperature was understood to be elevated or depressed. Extensive debate followed since their existence was confirmed in a paper by Hubert Lamb published in 1965. MWP and the LIA made an appearance in the 1990 IPCC report ‘Climate Change – The IPCC Scientific Assessment’ in a paper by Folland, Karl and Vinnikov entitled ‘Observed Climate Variations and Change’ – an extract from which is shown below. Variations in temperature in the northern hemisphere were analysed again in a paper by Mann and Bradley published in 1994.

The MWP has been used by some to argue that natural climate variation plays a larger part in driving current elevated temperatures than does that from human intervention. Furthermore, it is felt that the presence of the MWP has been obscured or ignored in later presentations and publications to give more prominence to the temperature ‘upticks’ since the 1900s. Those wishing to follow the debate may refer, for example, to ‘The Hockey Stick Illusion’ by Andrew Montford, published by Stacey International in 2011 and ‘The Hockey Stick and the Climate Wars’ by Michael Mann, published by Columbia University Press in 2012.

Climategate

In November 2009 a server at the Climatic Research Unit (CRU) at the University of East Anglia was hacked and thousands of emails and documents found themselves copied to other internet locations. Amongst all this material the following sentence in one email from Phil Jones, head of the CRU was the focus of attention as it seemed to indicate deception and fraudulence in the presentation of climate data.

‘I’ve just completed Mike’s Nature trick of adding in the real temps to each series for the last 20 years (i.e. from 1981 onwards) and from 1961 for Keith’s to hide the decline’

This proved a gift to climate sceptics and deniers who conflated ‘trick’ and ‘decline’ to mean the ‘trick to hide declining temperatures’. The resulting furore caused numerous investigations to be conducted into the allegations by specialist agencies and government bodies. These all concluded that there was no evidence of fraud or scientific misconduct.

The two controversial elements on the sentence related to the ‘trick’ and ‘hiding the decline’.

It was practice at the CRU (and elsewhere) to take tree ring measurements and use them as proxy data to estimate temperatures for periods when instrument data was not available. Since around 1960, however, the measurements from tree rings diverged – were lower than – those from instrument data and so could not be relied upon.

So, the ‘decline’ mentioned in the email was not in actual measured temperatures but in the tree ring-derived data. The ‘trick’ that Jones did was to add the actual instrument data from 1981 onwards.

Much was written to foster the conspiracy belief amongst climate sceptics who on a broader front often question the direction of travel concerning global warming and methods to deal with it. See for example ‘The Global Warming Policy Foundation’ in the UK who state that they ‘are deeply concerned about the impact of climate change policies: that they may be doing more harm than good, both to the world’s poorest people and the environment’, and deSmog who maintain a database of individuals and organisations who ‘have helped to delay and distract the public and our elected leaders from taking needed action to reduce greenhouse gas pollution and fight global warming’.

The story of Climategate was dramatized in the BBC drama ‘The Trick’ broadcast in October 2021 and there has been time for some sceptics to revisit the CRU data and come to the same conclusion as the University. One was Steve Mosher who stated ‘If I get to make an apology, even if it’s remotely via (the) BBC, to Phil Jones and Tim Osborn, then I’m happy’.

The next page considers what factors are contributing to the temperature increase