On a recent ABC interview, a well known scientist made the following statement about global temperatures.
Actually, there has been cooling, if you take 1998 as your point of reference. If you take 2002 as your point of reference, then temperatures have plateaued. This is certainly not what you’d expect if carbon dioxide is driving temperature because carbon dioxide levels have been increasing but temperatures have actually been coming down over the last 10 years.
Is she right?
The assertion led denialist commentators such as Andrew Bolt to claim temperatures are reducing and that there was a conspiracy of silence about the dropping temperatures. A graph of global temperatures (from two sources) is displayed below. She is correct on both counts! Temperatures are indeed lower than in 1998. She is also ostensibly correct that temperatures have plateaued since 2002.
It’s all a matter of your point of comparison. The baseline of 1998 she chose was a big peak, which was apparently associated with a large El Nino event. There are lots of these events that need to be taken out before you can see “the trend”. How about the plateau idea? This assumes that 2002 and 2007 are somehow similar in all respects. But if you take into account the 11 year sun cycle (which you can just about see) there should have been lower temperatures in 2007. So a plateau is actually a rise compared to the expected fall. See here for a robust discussion of whether the public comments were consistent with the data.
Even the graph above, which begins in 1978, gives a very different picture to the one below, which starts in 1850. (The trend lines were not added by me, but they don’t look unreasonable).
Or how about going back to the time of Christ? This is mainly based on tree ring data. It gives quite a different perspective.
This all makes me start to question the wisdom of the old adage a picture is worth a thousand words. After removing estimated effects of El Nino and El Nina events and the solar cycle, we might have a picture that would tell us something. (Can anyone point me to such a graph?) But the issues of time period and point of comparison still remain. The problem with graphs is that they can be easily selected to tell just about any story.
OK. I am on a Google inspired roll. Let’s go back 400,000 years! When I see data like this I am filled with a combination of awe and skepticism. It is based on analysing the gas bubbles in ice cores. Let’s accept the science behind this and suppose that it is a reasonable measurement of historical temperatures.
Have a look at the right hand axis. We are talking variations of the order of 10 degrees. The data from the time of Christ that we saw in the third graph above is the uninteresting little stable patch at the right hand extreme.
The graph also shows a time series of CO2 concentration on the same graph. There is a pretty clear association between temperature and CO2. Association does not mean causation but climatologists have theoretical reasons to thing that CO2 drives temperature. And it there is any reverse causality it means a feedback which makes the causality stronger. There is a very obvious cycle in CO2 which roughly predicts high levels of CO2 are to be expected during this millennium, even if human kind were still living in trees. But look at the very end of the red curve. The vertically increasing bit at the end is not a plotting error! If you accept that the association is causal, and the accuracy of the above graphh, you would have to say we are in serious trouble if CO2 levels out at around 370ppmv.
The only argument against is that either the association is not causal (which not many people accept) or that the increasing in CO2 is not as large as it appears in the above plot. This is certianly possible because the time series of CO2 combined ancient ice-core data with recent direct measurements. So a systematic bias in the ice-core CO2 estimates could mean that recent CO2 levels are not historically high.