The hockey stick graph of global temperature is one of the most iconic images in climate change. It shows a sharp rise in temperatures from around 1900 to present, with the 1990s showing the steepest climb. In 2009, it was revealed that Michael Mann had created this graph by combining two sets of data – one covering land and another sea surface temperatures – to create an integrated record for both land and oceans. However, recent research has shown that this analysis may have been biased because there are many more long-term records for land than for sea surface temperatures.
Scientists extend and straighten iconic climate hockey stick graph, by eliminating unreliable sea surface temperatures. Figure 1: The original hockey stick graph (left) compared to the improved version (right), when the lower quality data for earlier periods are discarded.
Figure 2: Temperature variations in Europe and North America over the past 2000 years based on proxy records (tree rings, corals, ice cores). The red line shows the average of 14 proxy records, compared to the blue line which has been “smoothed” using a computer algorithm. The black data points are more recent thermometer-based temperature measurements.
The study provides an updated and more accurate assessment (red) of global temperatures over the past 2000 years
Two new scientific studies have highlighted that using unreliable data for earlier periods in the instrumental record, combined with better data for recent years, is likely to have biased the original hockey stick temperature graph. The use of more accurate information has led researchers to re-assess past temperatures and the result is a graph that shows less pronounced warming over the twentieth century.
The first study, by Dr Tim Osborn and colleagues at the University of East Anglia and published today in Nature Geoscience (and available as an early online release), re-assesses global temperatures from 1901 to 2008 with more accurate land surface data. The results show that the rate of warming in the early twentieth century has been underestimated in previous studies and was similar to that in recent decades.
The second study, by Dr Keith Briffa at the University of East Anglia and colleagues in Switzerland and Russia, reconstructs temperatures from 1897-2009 using tree ring data. It shows reduced variability since 190. The research is published in Climate Dynamics , and suggests that previous studies using tree rings to estimate past temperatures may also have underestimated the rate of warming.
Dr Tim Osborn, lead author of the first study, said: Our work uses new data on land temperature observations over the twentieth century, improving our estimates by incorporating new data, correcting for some known biases and using an up-to-date statistical analysis. The land temperature data are much less noisy than the marine temperature data we used in the 1990s, which gives us more confidence in the results.
The new analysis suggests that global temperatures around the millennium were slightly lower than previously thought but that by 2008 they had returned to almost exactly the same level as in 1975. In addition, the results confirm that a steep rise in global temperatures took place from 1970-1998 which has been attributed to increased levels of greenhouse gases.