Part of the turmoil surrounding the issue of anthropogenic global warming hinges on whether or not observed changes in annual mean global temperature since the Industrial Revolution may be due to natural climatic cycles similar to those that operated previously during the Holocene Epoch. Actual measurements of temperatures of the air, sea surface and so on date only as far back at the early 18th century when thermometers were invented. Getting an idea of natural climate change through the 11.65 thousand years since the end of the last period of extensive glaciation depends on a variety of indirect measurements or proxies for temperature. For sea-surface temperature (SST) the proxy of choice is based on the way that surface-dwelling organisms, specifically planktic foraminifera, extract magnesium and calcium from sea water to construct their tests (shells). The warmer the sea surface the more magnesium is incorporated as a trace element into the calcium carbonate that forms their tests. The Mg/Ca ratio in planktic foram tests recovered from sea-floor sediment layers changes in a reliably precise fashion with warming and cooling. Following the Younger Dryas frigid millennium this proxy suggests that the average sea-surface temperature at mid-latitudes in the North Atlantic rose to a maximum of 0.5°C above the present value between 10 to 6 thousand years ago. After this Holocene Climate Optimum the sea surface seems to have cooled until very recently. Much the same pattern has been recorded in sediment cores from many parts of the world. Another approach is based on the varying amount of solar heating modelled by the Milankovich theory of astronomical climatic forcing and a variety of other forcing factors, such as albedo changes and the greenhouse effect. The two sets of data, one measured the other based on well-accepted simulations, do not agree; the modelling suggests a steady rise in SST throughout the Holocene and no climatic optimum. This conundrum either casts doubt on computer modelling of climate forcing, otherwise reliable on the broader time scale, or on some unsuspected aspect of the Mg/Ca palaeothermometer. The second could involve some kind of bias.
Samatha Bova of Rutgers University, USA, and colleagues from the US and China have examined the possibility of seasonal bias in estimates of SSTs from West Pacific ocean floor sediment cores off New Guinea (Bova, S. et al. 2021. Seasonal origin of the thermal maxima at the Holocene and the last interglacial. Nature, v. 589, p. 548–553; DOI: 10.1038/s41586-020-03155-x). First they examined the Mg/Ca proxy record from the last, Eemian interglacial episode (128-115 ka), on the grounds that astronomical modelling indicated much stronger seasonal contrasts in solar warming during that period, whereas other forcing factors were comparatively weak. By calculating the varying sensitivity of the older Mg/Ca record to seasonal factors they were able to devise a method of correcting such records for seasonal bias and apply it to the Holocene data from northeast New Guinea. The corrected Holocene SST record lacks the previously suspected climate optimum and its peak at ~8000 years ago. Instead, it reveals a continuous warming trend throughout the Holocene. The early part is far cooler than previously indicated by uncorrected SST thermometry. That may have resulted from the increased reflection of solar radiation – albedo forcing – from a larger area of remnant ice sheets on high-latitude parts of continents than was present during the warmer early-Eemian interglacial. Final melting of the great ice sheets of the Northern Hemisphere took until about 6500 years ago, when albedo effects would be roughly the same as at present. Thereafter, rising levels of atmospheric greenhouse gases warmed the planet towards modern levels.
Bova et al’s findings fundamentally change the context for modelling future climate change, and also for the interpretation of all previous interglacials, palaeotemperature records from which remain uncorrected. It seems likely that none of them had an early warm episode. As regards the future; climate modelling will have to change its parameters. For climate-change sceptics; two of their favourite arguments have been questioned. There are no longer signs of major, natural ups and downs in the early Holocene that might suggest that current warming is simply repeating such fluctuations. The other aspect of the Holocene climate conundrum, that greenhouse gases increased naturally since 6000 years ago while global mean SSTs declined, has been removed from the sceptics’ arguments
See also: Hertzberg, J. 2021. Palaeoclimate puzzle explained by seasonal variation. Nature, v. 589, p. 521-522; DOI: 10.1038/d41586-021-00115-x. Kiefer, P. 2021. Earth used to be cooler than we thought, which changes our math on global warming, Popular Science, 28 January 2021
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