New study suggests climate has been hottest for at least 115,000 years

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Researchers at Rutgers University (New Jersey, United States) found that paleoclimatic reconstructions significantly overestimated temperatures in the first half of the Holocene. In fact, it appears that the Earth has actually been at its hottest for at least 115,000 years. A horizon that brings us back to the previous interglacial. The results were published on January 27 in the scientific journal Nature.

Since the start of the industrial revolution, the global average temperature has risen by just over 1 ° C. An increase that was mainly articulated from the 1970s. Note that this value may seem low at first glance. However, it must be realized thatit is the result of an extended spatial and temporal integration. To observe changes of the order of degree on this scale of integration is very significant. For example, glacial-interglacial fluctuations represent a variation of about 5 ° C only overall average.

The temperature riddle of the Holocene

Until now, scientists believed that current temperatures had not yet passed the hot peak of the Holocene – the present interglacial – reached between 6,000 and 10,000 years ago. We are talking about Holocene thermal maximum. However, new results challenge this vision. Indeed, the work of researchers has shown that previous reconstructions significantly overestimated the level of temperatures between -12,000 and -5,000 years.

Our reconstruction shows that the first half of the Holocene was cooler than industrial times due to the cooling effects due to the remains of ice caps from the previous cold period Says Samantha Bova, lead author of the study. “ The warming at the end of the Holocene was indeed caused by the increase in greenhouse gases, as predicted by climate models, and this eliminates any doubt about the key role of carbon dioxide in global warming. »She adds.

Temperature during the Holocene as an annual average. The new reconstructions – blue and red curve – no longer show a thermal maximum compared to the old reconstructions (in yellow). Indeed, the latter indicated the summer temperature. In addition, the black curve indicates the atmospheric concentration of CO2. Finally, some key episodes such as the start of the industrial revolution are indicated. Credits: Samantha Bova et al. 2021.

Reconcile observations and numerical models

These results make it possible to resolve the disagreement that persisted until now between reconstructions based on field samples and numerical simulations. Indeed, when the models simulate the climate of the Holocene, they tend to produce a less hot Earth than what the paleoclimatic series show. And for good reason, the proxies used to produce these mainly give an indication of the summer temperature. In other words, the values ​​are not representative of the annual average contrary to what was assumed.

The method developed by the scientists makes it possible to readjust these seasonal values ​​towards annual values. It is based on new cores of marine sediments carried out in the north of Papua New Guinea. The limestone shells of foraminifera contained in the samples made it possible to go back to the temperatures of the Holocene but also of the previous interglacial from 128,000 to 115,000 years ago. When this readjustment is made, the Holocene thermal maximum no longer appears.

The apparent discrepancy between climate models and data has cast doubts among skeptics about the role of greenhouse gases in climate change during the Holocene and possibly into the future. ”Notes Yair Rosenthal, co-author of the study. ” Our reconstructions show that the current annual global temperature has exceeded levels of the past 12,000 years and is probably approaching the heat of the last interglacial period – 128,000 to 115,000 years ago »Reports, not without a certain gravity, the paper.

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