So far for these things, I've just been plucking a topic from the plethora of climate change issues that bother me and ranting about it until I run out of words, at which point I attempt to sum up my rambling and move on with my life. It's an artistic process.
This week is a little different though, since a climate change paper in the Journal Science has given me a perfect chance to talk about temperature reconstructions. This is an important part of the climate change debate, because it deals with answering the question: "What was the average global temperature before we started measuring this kinda stuff?" Really, it's an attempt by scientists to get a reasonable handle on the ultimately unknowable and mysterious deep past; we're like a Koala bear struggling to comprehend a family photo of its grandparents.
Anyway, the paper I'm referring to here is Marcott et al. "A Reconstruction of Regional and Global Temperature for the past 11,300 years". For anyone at a university, it's available at this doi: 10.1126/science.1228026 . If you aren't at a university/research establishment, then it's behind a pay wall and you can't read it (the monetization of scientific research by journals is a scandal, but if I start talking about that, I'll never get to the point). However, if you'd like to read an overview of the paper, you can do that at this sensational titled article. For convenience I've included the key figures below:
To explain these figures a little, the horizontal axis refers to time before present (BP). The vertical axis, the temperature anomaly, is the difference in degrees centigrade from the 1961-1990 average. Anomaly is used to provide a convenient baseline, so we can see how different the global temperature 2000 years ago was from the fairly recent past. The light blue shading indicates statistical uncertainty, which basically says “we're pretty sure the real temperature was somewhere in this range, probably.” It gets bigger the further back in time you go, as we have less and less confidence in our reconstruction. The second figure is really just a close up of the first one, zoomed in on the last 2000 years.
So how did they produce this figure? Well, like the many other reconstructions before it, it was made by combining “Proxy” records. A Proxy in this sense is anything in nature which has been recording temperature for us, before we'd bothered to do the necessary cultural growing up. How thoughtful is that? A warm round of applause please for tree rings (the trunk grows more during warm years), corals (same idea), and ice cores (oxygen ratios indicate how much precipitation was going on), amongst many others. There are lots of these globally, wherever they've been preserved. This paper has averaged 73 of them together using various, fiendishly clever statistical methods to make sure no one proxy is over represented.
The reason this paper is significant is that it made an effort to avoid over reliance on tree ring data, which are almost all in the Northern Hemisphere and have come under criticism for being unreliable. Importantly, there's no significant difference to past reconstructions like the Mann et al. one shown by the gray line. It is a lot longer than the Mann et al. reconstruction though, which provides some perspective on the historical significance of current temperatures.
The take home message from this study is that in the context of the past 10,000 years, current temperatures aren't anything new, as shown by the first figure. The rate at which the temperature is increasing though is unprecedented, with a rate of increase vastly higher than anything seen before. This is why we know global warming is not a natural phenomenon, and why it's a human problem that must be addressed. I realize that makes for a depressing note to end on, but it's all I've got sadly. If you wanted this to be funnier, well, pretend I was making Jazz Hands every time I typed "temperature".
