Current climate models may underestimate long-term climate change

An analysis of past climate periods has been published in Nature Geoscience by 59 researchers from 17 countries, including several French experts from the CNRS, the University of Bordeaux, PSL University, the CEA and the UVSQ (*). These past climate variations can help us understand the implications for global warming of a 2°C increase in temperature, and test our ability to simulate climate functioning.

  • 06/07/2018

Recul du au réchauffement climatique du glacier Athabasca, Parc National de Jasper, Alberta, Ouest Canada © Alain Mazaud Recul du au réchauffement climatique du glacier Athabasca, Parc National de Jasper, Alberta, Ouest Canada © Alain Mazaud

Some periods in the past were at least as warm as the present day. From their analysis, researchers have gained some interesting insight into global warming’s development, allowing them to build global warming models. 

On June 25th 2018, an international team from 17 countries published an analysis of previous periods, in Nature Geoscience. It concludes that global warming, even if limited to within 2°C above pre-industrial levels, as recommended by the Paris Agreement, will lead to rapid displacements of climate zones and their associated ecosystems. The polar ice caps will reduce significantly for periods of several thousands of years. Rapid warming of the poles will release a surplus of greenhouse gases, and sea levels will rise by several meters over the next millennia. Finally, these observations also show that many current climate models used to simulate changes in the 21st century may underestimate long-term changes.

The French laboratories involved in the study are the Laboratory of Oceanic and Continental Environments and Paleo-environments, EPOC (CNRS unit/University of Bordeaux), in partnership with the Ecole Pratique des Hautes Etudes/PSL University (Paris) – Laboratory of Climate and Environmental Sciences (LSCE–CEA/CNRS/UVSQ, Paris Saclay)

Several time periods over the past 3.5 million years have been identified as being 0.5 to 2°C warmer than the pre-industrial age. They reveal that warming was stronger at higher latitudes than in tropical regions, which is similar to results from climate model simulations for 2°C global warming by 2100. Although these previous warm periods were not all caused by an increase in atmospheric CO2, by studying them, the effects of warming comparable to the limit recommended by the Paris Agreement can be evaluated

Ecosystems and climate zones will migrate

The study confirms that ecosystems and climate zones will generally migrate towards the poles, or towards higher altitude zones. It also confirms that the permafrost thawing will release additional carbon dioxide and methane, causing additional warming. Observations of the earlier warmer periods suggest that if warming can be limited to 2°C, as recommended by the Paris Agreement, the risk of catastrophic runaway warming linked with strong emissions of greenhouse gas is relatively low. Nevertheless, even in this case, the additional CO2 released from permafrost and soils must be taken into account.

A long-term rise in sea level of more than 6 meters

Even limited warming of 1.5 to 2°C above pre-industrial levels will be sufficient to cause substantial melting in Greenland and Antarctica in the long term, and generate a sea level rise of over 6 meters that will persist for thousands of years. It is likely that the rate at which sea levels rise will then be higher than in the last decades. 

Climate warming in the past was greater than simulations obtained with climate models

Comparisons between data from the past and numerical simulations suggest that climate models may underestimate long-term warming and its increase in the polar regions. While climate model projections seem reliable for moderate-sized changes over the next decades, these models probably underestimate future climate change, particularly for long-term response projections for scenarios where greenhouse gas emissions continue to rise.