Jolly Hockey Sticks…

This entry is part 2 of 2 in the series Global Warming

Science involves a process called peer review. When a scientist wants to publish his experiment report in a journal for others to read, it has to be submitted to a few other experts in the field who check that the basic science is sound. If they feel that a good job has been done with the design and interpretation of the experiment then they recommend that it is suitable for publication. Otherwise they will either recommend that the report be rejected or returned to the authors for revision. It is this system of review that, although not perfect, attempts to keep as much nonsense out of science publishing as possible. The more prestigious the journal, the more rigorous is the review process. I mention this because it exemplifies the way science is willing to listen to criticism and change as new evidence becomes available.

In 1998, Michael Mann and his colleagues were amongst several groups attempting to reconstruct historical temperature records for the last several centuries. Because no direct records exist, the idea was to use ‘proxy’ indicators. This means to take information that is datable and relate it to the atmospheric conditions of the time. An example of a proxy would be to look at stalagmites.

These have bands that correspond to each year’s growth and the thickness of the bands can be used to estimate average temperatures for each year.

Mann and his team used tree ring data as their proxy. Tree rings can give us temperature data reaching back over 1000 years so they are potentially very useful. The width of tree rings can be affected by many different things – any of the factors that affect the speed that plants grow.

Temperature is only one of them so the margins for error are large. Mann and his team included proxy data from corals and other sources and combined them using various mathematical techniques to try to make sense of the data. Eventually they produced a graph that featured in the IPCC’s (Intergovernmental Panel on Climate Change) 2001 report to the United Nations. The blue lines represent the data that Mann and his team calculated and the red line is from thermometer readings. The timeframe on the x-axis runs over the last 1000 years.

The grey area is the really interesting bit because it represents Michael Mann’s estimated possible error. What it is saying is that although he thinks the blue line is most likely to be right but he would not be utterly surprised if it had actually fallen anywhere inside the grey shaded area. Awareness of potential error is always vital in good scientific reporting. Despite the error, the graph appears to show that temperatures have been cooling for a thousand years with a sharp increase in the twentieth century. This graph was heavily used as evidence for man-made climate change.

A few years later, one or two papers were published that criticised the way that Mann and his team did their statistical analysis. If the maths was done incorrectly then the graph could not fairly show what it purported to show. This criticism was very technical in nature. There was disagreement about the use of particular mathematical techniques and concerns over the inclusion of data from a particular bristle cone pine tree. Michael Mann responded strongly to the criticisms but doubt remained over whether the graph was valid or not. Al Gore used the graph in his film, “An Inconvenient Truth”, and the questions about the hockey stick shaped graph have since been used by many to suggest that climate change is either not happening or is not man-made.

Clearly, a definitive answer was needed about the Mann graph, so various independent assessments of the hockey stick graph have been conducted. The National Research Council and the National Academy of Sciences have both reported and although they did express doubts over some of the statistical methods used, they supported the findings that Mann and his fellow researchers reported. When the graph is recalculated, without the controversial data and techniques, a similar trend is shown. Indeed, in the last 10 years over a dozen independent reports have been published using a wide range of different proxy data. Almost without exception, these different methods of addressing the same problem have supported Michael Mann’s original assertion – that the increase in temperature over the last 50 years or so is historically unprecedented.