Understanding climate change in the past is crucial to putting modern global warming into context. Climate reconstructions during the Holocene – the current interglacial era, which began 11,700 years ago – based on geological evidence, suggest that a peak in world average annual temperatures between 10,000 and 6,000 years ago was followed by a cooling trend. , which then reversed in the report industrial era1,2. However, computational simulations of the Holocene climate show only a long-term warming trend3. Subscribe Nature, Bova et al.4 report an analysis that effectively brings Holocene climate reconstructions into line with computational simulations This result has important implications for our understanding of the drivers of climate change during the Holocene and for the context of post-industrial warming.
To reconstruct the climate in the past, scientists rely on proxies: geological materials that have properties that can be measured and correlated with modern climate parameters. The apparent temperature peak during the early Holocene (known as the thermal maximum of the Holocene) is a prominent feature in worldwide syntheses of proxy-based climate reconstructions1,2 (Fig. 1). The striking absence of computational modeling is called the Holocene temperature frame and has amazed climate scientists for years.3. The difference of opinion is attributed to seasonal biases in proxy reconstructions5 – that is, the proxy reflects the evolution of seasonal temperatures, rather than the average annual, and with shortcomings in modeling6. Global proxy synthesis is mainly dominated by records for sea surface temperature (SST) (see for example ref. 2)2, who are known to be seasonally biased5.
Figure 1 | Correction of seasonal bias in climate reconstructions. Average world temperatures during the Holocene (the current interglacial era, which began 11,700 years ago) can be reconstructed from geological records. These reconstructions (blue line) indicate that surface temperatures (SSTs) peaked between 10,000 and 6,000 years ago, and then decreased until the post-industrial period, when temperatures began to rise. However, computational simulations of Holocene SOSs do not agree with those reconstructions, probably because the geological records are seasonally biased – the records reflect the evolution of seasonal temperatures, rather than the average annual temperatures. Bova et al.4 report a method that quantifies and corrects for seasonal bias in marine geological records, and uses it to estimate the average annual SSTs (red line). The corrected data is consistent with the computational simulations, which solve a long-standing problem in this field. SSTs are shown as deviations: the difference between the SST of a reference time interval and the average SST between 0 and 1,000 years ago. Shadow areas represent the limits of one standard error.
The new method of Bova and colleagues identifies seasonal biases in SST records and makes it possible to calculate the average annual SST from seasonal SST. It uses the features of the last interglacial period (128,000–115,000 years ago), which were characterized by mild world temperatures, smaller ice sheets, and higher sea levels than today.7. This period is beneficial for the authors purposes in that the seasonal difference of incoming solar radiation (insolation) was greater than during the Holocene, while the effects of other climate change factors, such as greenhouse gases and ice, were weaker, making it easier to season identify prejudices.
More specifically, the authors’ method involves the identification of seasonal bias in the section of an SST record that corresponds to the last interglacial, and in which there was a stronger correlation between SST and seasonal insolation than with average annual insolation. The sensitivity of the SST record for seasonal insolation during this period is then calculated and used as a measure to remove seasonal bias from the entire record, whereby the average annual SST can be determined from the record. The authors first applied their method to an SST reconstruction based on a proxy taken from a marine site off the northeast coast of Papua New Guinea. The transformed average annual SST record was independently validated by applying the new method to SST data for the same geographical region produced in computational simulations of the last 300,000 years – the transformed data corresponds to the average annual SST output of the simulations.
Bova et al. made a synthesis of previously published SST records spanning the last interglacial and the Holocene period. These records are based on two general conclusions used for the reconstruction of SST: the chemical composition of the fossilized calcium carbonate shells of superficial single-celled marine organisms, known as foraminifera; and organic biomarkers known as alkenones, which are synthesized by marine algae and settle in marine sediments. The authors found that the majority of the SST records examined were indeed seasonally biased.
After converting the seasonally biased SST records into average annual SST records, Bova and colleagues conclude that the climate has warmed since the early Holocene – that is, there is no evidence of a thermal maximum of the Holocene in average annual temperatures (Fig. 1). They suggest that the thermal maximum of the Holocene is a seasonal feature driven by a peak in the summer isolation in the Northern Hemisphere that occurred during the early Holocene.
The reconstruction of average annual temperatures produced by authors’ synthesis of proxy records strongly resembles a computational simulation3 of Holocene climate which also reflects average annual temperatures – to solve the Holocene temperature mystery. This has enabled Bova and colleagues to shed new light on the drivers of climate change by Holocene. They find that the increase in world average annual temperatures that occurred during the early Holocene (12,000-6,500 years ago) was a response to the retreat of ice sheets, while the continuous increase in temperatures over the past 6,500 years is due to rising concentrations of greenhouse gases. .
The authors also show that the average annual temperatures during the last interglacial period were more stable and higher than their estimates of Holocene temperatures. They attribute this to the almost constant greenhouse gas concentrations and the reduced extent of ice sheets during the last interglacial. Most importantly, the researchers find that the average annual temperature exceeds the temperature of the past 12,000 years, and that it is likely to approach the heat of the last interglacial period.
The method of Bova and colleagues to identify and correct seasonal biases in proxy SST reconstructions can now be applied to other temperature records at different time scales. This is an important advantage of their study, because paleoclimatic scientists have long known that temperature reconstructions are likely to be seasonally biased, but do not have a method to address the problem.
One limitation of the findings is that the new synthesis of proxy SST records is limited to the global region between 40 ° N and 40 ° S. Proxy records of higher latitudes were deliberately excluded due to the scarcity of such records for the last interglacial, and due to the proximity of these regions to seafronts, where the dynamics of the ocean can affect the SST. However, the inclusion of these regions will be necessary in the future, as processes at high latitudes play a significant role in many climate processes. The new synthesis also examines records based on only two SST proxies. Future work should include more records based on other temperature proxies. Nevertheless, the study by Bova and colleagues is an important step forward for the field by solving a mystery that has amazed climate scientists for years.