When first published the ‘hockey stick’ curve was controversial. However, other scientists have since independently confirmed that the Earth’s surface has warmed significantly and very rapidly since 1900.

In 1998, three American scientists named Mann, Bradley and Hughes used data from ice cores and historic records, yearly growth lines in trees and corals plus thermometer records from the 20th century, to estimate the Northern Hemisphere’s average surface temperature for the last 1000 years. When the resulting curve was smoothed (averaged over a number of years) it showed a trend of declining temperatures until the early 20th century followed by a precipitous rise of about 1°C, with only one temporary dip, until the present day. Because of its shape this curve has become known as the ‘hockey stick’ curve. Sceptics hailed this curve as false or even a fraud. It is true that there were many possible problems with the curve; data from different sources had been merged and the older data probably have larger uncertainties. It was also unclear how much increasing atmospheric carbon dioxide may have changed tree ring growth and affected the authors’ ability to infer temperatures from these rings. However, in 2008 the same authors with others showed, with even more data, that even if the criticised tree ring data were excluded essentially the same interpretation of a rapid 20th century rise in temperature can be discerned. Other scientists carried out similar but independent studies and came up with their own curves not much different from the curve of Mann and his colleagues. Thus to a very large degree all the curves tell the same story - the Earth’s surface has warmed significantly and very rapidly since 1900.