The last 12,000 years show a more complex climate history than previously assumed

We rely on climate models to predict the future, but models cannot be fully tested because climate observations rarely go back more than 150 years. Understanding Earth’s past climate history over a longer period of time gives us an invaluable opportunity to test climate models on longer timescales and reduce uncertainties in climate predictions.

In this context, changes in the Earth’s mean surface temperature during the current interglacial period, the Holocene (roughly the last 12,000 years), have been the subject of intense debate over the last few decades. Reconstructions of past temperatures seem to indicate that the global mean temperature peaked about 6,000 years ago and cooled by the onset of the current climate crisis during the Industrial Revolution. Climate model simulations, on the other hand, indicate continuous warming since the beginning of the Holocene. In 2014, researchers dubbed this large discrepancy between models and previous climate observations the “Holocene Temperature Conundrum.”

In this new study, published in nature communicationScientists used the largest available database of past temperature reconstructions, going back 12,000 years, to carefully examine the geographic pattern of temperature change during the Holocene. Olivier Cartapanis and colleagues state that, contrary to previous assumptions, there is no globally synchronous interglacial period during the Holocene. Instead, the warmest temperatures are found at different times not only in different regions, but also between the ocean and land. This raises the question of how meaningful comparisons of global mean temperature between reconstructions and models actually are.

According to lead author Olivier Cartapanis, “the results challenge the paradigm of a globally concurrent Holocene thermal maximum.” And while the warmest temperatures were reached in western Europe and North America 4,000 to 8,000 years ago, ocean surface temperatures have been cooling at mid- and high latitudes for about 10,000 years and have remained stable in the tropics. Regional variability in the timing of maximum temperature suggests that high-latitude insolation and ice extent played important roles in climate changes throughout the Holocene.

Lukas Jonkers, co-author of the study and researcher at MARUM – Center for Marine Environmental Sciences in Bremen, says: “Because ecosystems and people do not experience the average temperature of the earth, but are affected by regional and local changes and timing patterns of climate change to guide policy makers.”

Therefore, the new work by Cartapanis and colleagues presents a clear goal for climate models, as the ability to reproduce Holocene climate variability in space and time will increase confidence in their regional projections of future climate change.

Provided by MARUM