There is an interesting article in a recent issue of the American Geophysical Union newspaper Eos concerning the climate change record, future projections of global temperature, and the need to reinforce the data record so as to convince an increasingly sceptical public that man-made global warming is real.
The article by Utah geophysicists David Chapman and Michael Davis begins with an historical overview of what is popularly known as the “hockey stick” graph of global temperature change over the past millennium, and projections up to the end of this century. This much-discussed figure was published by the Intergovernmental Panel on Climate Change (IPCC).
From around the middle of the nineteenth century we have an accurate instrumental record of surface temperatures at various locations. From these data it is a straightforward statistical exercise to extract a time series of global mean temperature change.
Prior to the age of routine geophysical data collection we have to rely on so-called proxy data, or indirect indicators of air temperature such as tree ring growth, corals, sediments and glaciers. But there it can get messy, and it is testament to the skill of Earth scientists that from various proxy measurements they can derive temperature estimates which are broadly consistent.
Well-written and substantive overviews of climate science are always welcome and valuable, and I wish more of them would find their way into the mainstream press. Communicative strength alone would not justify the publication of such articles in specialist journals such as Eos; what distinguishes the piece by Chapman and Davis is its discussion of subsurface temperatures, and how they confirm our understanding of climate change as derived from proxy data.
Now you might think that temperatures measured underground in boreholes would be influenced exclusively by heat flowing outwards from the Earth’s molten interior. But that isn’t the case. In borehole data we also see transient departures from the steady state heat flow from the interior, and these can be attributed to past surface temperature changes that diffuse downwards into the Earth’s crust, with deeper layers representing earlier times, as is the case with ice cores and atmospheric chemistry.
As a result of the thermal diffusion process, short-term fluctuations in surface temperature are filtered out of the record, revealing century-long trends and averages. And borehole-derived temperatures are a direct measure of past temperatures, unlike those estimated from tree rings and other proxy methods.
The borehole temperature record is fairly sparse, but the available data appear to confirm the millennium-scale surface temperature reconstructions derived from proxy data. Taking the hockey stick graph, for example, Chapman and Davis show that the proxy temperature change estimates are all within 0.05 degrees of their northern hemisphere borehole data.
“It is possible that borehole temperature profiles could be used to pick winners in the temperature reconstruction stakes, but it would require a greater number of deep boreholes collocated with proxy sites.”
The agreement is impressive, and, political dynamics aside, it should strengthen our confidence in temperature changes derived from proxy data, without which we would have no historical record of global climate change.
Further reading: David S Chapman & Michael G Davis, “Climate change: past, present, and future”, Eos 91, 325 (2010)