A comment on another blog made me think that I should write again about climate change. The basic comment was that we should all be campaigning to get governments to take climate change seriously and to act to do something about it. It is, however, an extremely controversial topic with many feeling that even if global warming is taking place, it is not yet clear that it is due to the action of man and that it is probably just part of a natural cycle. The controversy seems to often focus on the Hockey Stick graph, shown below. This is taken (I believe) from a paper published in 1999 by Mann, Bradley and Hughes (GeoRL, 26, 759-762). I say, I believe, because I couldn’t actually access the paper, even when running my university’s VPN.
The graph covers the period 1000AD-2000AD and shows the variation in temperature relative to some mean. There’s a gradual cooling from 1000 AD till about 1800 AD and then quite a sudden rise starting just before 1900 AD. The interpretation is that much of this rise in temperature (especially from the mid 1900s onwards) is driven by the large amounts of CO2 we have been pumping into the atmosphere through the burning of fossil fuels.
The criticism is, however, that variations in temperature are natural and also, that there were problems with the analysis carried out by Mann, Bradley and Hughes (1999). It turns out that there were indeed some issues with their statistical technique. Many climate change sceptics use this to then claim that it shows that the hockey stick graph is not suitable for determining if climate change is being driven by man-made activities. What they fail to mention is that even though there are some issues with the analysis (and this if often true when very complicated techniques are used for analysing complicated data sets) it doesn’t appear to make any substantive difference. Furthermore, other methods for determining temperature variations over the last millennium produces results that are largely consistent with that of Mann, Bradley and Hughes (1999).
What I thought I would then do is look through some of the papers that have cited Mann, Bradley and Hughes (1999) to see if there was any evidence that serious scientists were questioning their work. It wasn’t an exhaustive or particularly systematic search, but I did find a number of recent papers that produced long-term temperature variation trends that largely matched that found by Mann, Bradley and Hughes (1999). I did find one particular result from a 2012 paper (Christiansen, B. and Ljungqvist, F.C., 2012, Climate of the Past Discussions, 7, 3991-4035) which is shown in the figure below.
It is clearly consistent with Mann, Bradley and Hughes (1999), showing quite a rapid rise in temperature starting at about 1900. It also, however, shows an additional peak at about 1000 AD. This is known as the medieval warm period and some also use this to argue that today’s climate change is simply part of natural variations. Some analysis does indeed show that it was as warm in about 1000 AD as it is today. Others show less of a peak at about 1000 AD and that the warmest period in the last 2000 years has been since about 1950.
Basically, it appears as though most serious scientific studies do produce temperature variations over the last 1000 – 2000 years that are consistent with the hockey stick graph produced by Mann, Bradley and Hughes (1999). The last 50 years has seen a substantial rise in temperature and the general consensus is that it is driven by the action of man. The only real issue I can find is the existence of the medieval warm period, when it may have been as warm as it is today. I don’t, however, think that is necessarily particularly relevant. Given that this post has got a little long, I thought I would carry on in a second post where I will try to explain why I think the current warming trend is driven by man and why it is crucial that we invest in developing alternative energy technologies that reduce how much CO2 we put into the Earth’s atmosphere. Having said that, I am not an expert on climate change and am simply expressing my views based on my understanding of what is happening. As always, happy to take comments and corrections. Given the type of rhetoric typically surrounding the climate change debate, I may regret that offer.