Greenland ice-bore climate doubts
This one has its limitations ...
On the eve of the all important COP26 Climate Conference in Glasgow, my slow motion tennis buddy Stan Randle passed on a link to a video clip posted at YouTube a few months back that seems to question the validity of human-induced climate change. He asked for my comments. Here they are, in case others are interested.
The YouTube video presenter, J. P. Steffensen, is indeed a scientist employed by a reputable science organisation. His video shows that temperatures over the last 8 thousand years generally decreased and that the beginning of the instrumented temperature record coincided with a minimum. He makes the point that at the start of temperature-recording in the mid-19th century, temperatures were lower than that ever been in the last 8 thousand years (i.e., prior to the last glacial), and were about 1.5°C cooler than in the medieval warm period a thousand years ago. But is the picture it paints correct?
The plot’s a bit misleading. The time axis is labelled ‘thousands of years before present’, but it doesn’t include the recent past when temperatures have increased by around 1°C. Temperatures are now already greater than the medieval warm period, and are increasing faster than ever before. The ice-bore he examined is from the “Northgrip” core, one of several in Greenland. The main (funded) purpose of the Northgrip project was to analyse the historical composition of air bubbles trapped in the ice, and derive temperatures from the oxygen signatures in them. Higher concentrations of a heavy isotope of oxygen are deposited in the ice during warmer periods. It’s not perfect. Those isotopic concentrations in the precipitation that leads to these ice deposits are also affected by weather patterns that may of course be influenced by terrain and long term changes ocean circulation.
According to Wikipedia, this particular ice core is unusual because there is ice-melt at the bedrock level, which is thought to be due to geothermal heating. To me that raises questions about the validity of the much simpler method described in the video clip to derive past temperatures. He says their changes in this case are inferred opportunistically from direct measurements of tiny residual variations in temperatures down through the 3 km deep 11 cm diameter borehole in the ice that was left after extraction of the ice core. Given that (a) ice is a reasonable thermal conductor, (b) that air motions would occur within the column, and (c) that geological heating is also at play in this particular ice core, I’m a little skeptical. Especially since I can’t find any peer-reviewed papers in the literature describing his method. I’d need to critically review one to be convinced. I also wonder why his temperature record shown stops 8 thousand years ago, when the deepest part represents the climate more than 100,000 years ago.
See here for a more detailed discussion about these limitations.
A bit more context is required. The currently accepted long term temperature changes - as inferred from air bubbles in the ice cores - can be seen in the plot below. It shows the mean changes from several Greenland ice cores.
Although the period covered is similar to that shown in the video clip image, the picture is very different. And not just because of its obvious higher resolution in time. It shows that current temperatures (shown in black) already exceed those in the medieval warm period, and - in fact - any time in the last 2000 years. According to this record, temperatures were slightly higher 3 to 8 thousand years ago, but those will soon be overtaken. The orange and red lines show how temperature is projected to change over the remainder of the century for two future scenarios (both rather scary).
But even these data may not be correct. At these Greenland sites, there have been large changes in altitude over time - related to changes in the thickness of the ice cover - which muddy the picture. In some cases those changes have exceeded hundreds of metres. It’s also just one region of the globe, a region that may be affected atypically by changes in the Gulf Stream ocean current that transports warm water from the tropics to the arctic.
For full global coverage, we need to include estimates from a wider geographic region. Proxy records for these are available from analyses of tree-ring data, peat-bog deposits, lake-sediments and ice-cores in Antarctica. The global picture is indeed slightly different, indicating that the medieval warm period was only regional in extent.
The figure below from the most recent IPCC report summarises our current understanding of the global situation.
The points about the picture above are worth noting:
The left panel shows that current warming is unprecedented in the last 2000 years, and that the current temperature already exceeds the warmest multi-century period in the last 100,000 years.
The right panel shows that the observed warming since 1850 is due to increased greenhouse gases from human activities. Without those increases any temperature changes since 1850 would have been small.
What matters is the recent temperature changes, and the rapid changes projected in the next decades. Although temperatures were higher at some stages of Earth’s five billion year history, that was long before the emergence of mankind. Quite apart from the forecast weather and flood ravages we’re already starting to experience, a stable climate will be needed to satisfy mankind’s insatiably growing appetite as our population continues to soar.
Or should that “as” be an “if”? Even if we successfully stave off COVID-19, inevitable future pandemics and decreasing biodiversity already being experienced make our future growth less assured.
Bad for Mankind, but good for Planet Earth …
Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to subscribe for occasional free updates.