The Palaeoclimate Problem
A new paper claims that global temperature hasn't changed much in the last 10,000 years - until recently.
Modern warming differs from the gradual rise in temperature seen in the past 10,000 years. That’s the conclusion of a paper just published in the journal Nature. Reconstructing the temperature timeline back to 24,000 years ago - the so-called Last Glacial Maximum – a team of researchers show that recent warming is unusual.
Knowledge of past climate is important to put our present climate into context, allowing us to see what climatic variations can take place in the absence of contemporary amounts of greenhouse gasses. Because there were no thermometers back then scientists have become adept at looking for proxies – things that are affected by temperature - such as tree rings, stalactites and isotope ratios in sea shells to give a few examples.
The temperature of the current geological epoch – the Holocene - has in this new work been reconstructed in remarkable detail by melding proxy data (plankton and microbes found in sediments) with a computer model of the climate. It is said that the benefits of this hybrid approach is that it brings the model closer to reality allowing confidence when the model fills in the gaps in the timeline when no data is available. The work by Matthew Osman of the University of Arizona and colleagues has been hailed as a triumph by two of the pioneers of paleoclimatic reconstruction, Shaun Marcott of the University of Wisconsin-Maddison and Jeremy Shakun of Boston College.
Looked at in more detail you could say this work has straightened and lengthened the infamous hockey stick and removed the Medieval and Roman Warm Period, or at least showed they were too local to have a global effect. They found a gradual warming from 11,650 BP that is somewhat different from that seen in other reconstructions although another recent study by Samantha Bova of the University of San Diego draws similar conclusions to Osman whilst using a different technique. As the authors admit this will require verification as the technique is expanded. This time it incorporates no terrestrial data and had big data gaps in the major oceans. What’s more, it uses only one climate model which is clearly a direction to enlarge.
Osman et al find that the Ice Age was about 7 degrees C cooler than preindustrial temperatures, which is about a degree colder than previous estimates. In addition, the new findings contradict the Milankovich theory that links changes in the Earth’s axial inclination to climate by finding that increases in summer solar radiation as a result of axial changes coincide with cooler global temperatures. However last year a temperature reconstruction using several hundred terrestrial proxies showed the opposite to Osman et al.
Taking a wider view of the field it is difficult to reconcile this latest research with many other lines of enquiry to determine past temperatures.
Scientists from the University of Puebla (Mexico) find that a cave in Argentina was 4.5 degrees C warmer in 1860. From Brown University (USA) and the Woods Hole Marine Biology Lab (USA) comes news that the temperature of the North Atlantic was higher a thousand years ago than it currently is, as was Svalbard somewhat earlier according to work performed at the Arctic University of Norway. The North East Federal University in Russia along with the Alfred Wegener Institute (Germany) shows Far East Russia was warm a millennia ago. Tree rings analysed by scientists at Nanjing University (China) show that it was warmer a century ago and researchers at the Geological Survey of Canada and the Pacific along with the University of Minnesota show that sea surface temperatures in the South China Sea was warmer in the past.
The Early Holocene was 2-3 degrees C warmer according to scientists working at the Korean Institute for Geosciences in cooperation with the University of Stockholm and Oregon State University. Five thousand years ago Sumatra was considerably warmer than today according to work carried out by the Universities of Göttingen and Stockholm whilst the University of Cordoba (Argentina) with the Alfred Wegener Institute conclude that Southern South America was warmer in 1500 BC. Ottawa University researchers conclude that 9,000 years ago the Canadian Arctic was warmer than today, a similar conclusion reached by scientists working at the Australian National University working alongside the Chinese Academy of Science.
The recent Nature paper is undoubtedly impressive and significant and it would have received wider publicity had it not been for COP26. But perhaps deciphering past climates is more complicated and messy that it envisages, at first sight at least. Clearly there is still a long way to go.
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