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The low value of climate sensitivity is revised upwards

According to an international collaboration involving the LSCE (CEA-CNRS-UVSQ), the doubling of atmospheric CO2 content compared to the pre-industrial era would "probably" warm the earth between 2.6°C to 3.9°C. This assessment – the best to date – is based on three independent data sources that do not use climate models.
Published on 30 July 2020

The atmospheric concentration of CO2 has risen from 280 parts per million (ppm) in pre-industrial times to about 416 ppm today. If no action is taken to reduce anthropogenic CO2 emissions, they could reach 560 ppm by 2060.

What will global warming be like when the atmospheric CO2 content has doubled relative to the pre-industrial era? The earth's "Equilibrium Climate Sensitivity" (ECS) addresses this question, which is fundamental for predicting the magnitude of future climate change and mitigating it.

The climate sensitivity assessment requires a good estimate of the external perturbations that modify the planet's energetics, global temperatures, and the various feedbacks that amplify or buffer the initial perturbations. The last IPCC report proposed a probable value between 1.5°C and 4.5°C.

Since 2013, researchers have made an effort to overcome these discrepancies and combine different independent data sources by applying a series of statistical tests to them:

  • historical climate data,
  • a number of paleo-climatic data sets,
  • the study of climate processes (positive or negative feedbacks on warming).

The results converge on the fact that a value below 2°C is barely compatible with the three types of study. The last glacial maximum indicates that values above 4.5°C are not very credible. The results reduce the range of values at 66% chance (the probable range given in the IPCC reports) to 2.6°C - 3.9°C, and the 5% - 95% range of chance to 2.3°C - 4.7°C. The estimates indicate that this range is restricted to an interval of 2°C - 5.7°C.

These results are important in order to realistically target the objectives set by the Paris Agreement. Indeed, this study provides a strong constraint on the lower limit of climate sensitivity. Considerations regarding how to keep the temperature below 2°C or 1.5°C of the Paris Agreement must seriously take into account these results in order to meet the objectives of sustainable development.

It is possible to compare the results from climate models with this new climate sensitivity assessment since it does not use any of their data.

Some of the CMIP6 (Coupled Model Intercomparison Project Phase 6) models that will contribute to the 6th IPCC report producing sensitivities below 2°C appear to underestimate warming. Several new-generation models produce values around 5°C or more. Although they are outside the most likely range, they produce high values that remain possible.

These results were obtained within the framework of the World Climate Research Programme, which was established in 1980 under the joint sponsorship of the World Meteorological Organization and the International Council for Science (ICSU) and, since 1993, the Intergovernmental Oceanographic Commission (IOC) of UNESCO. The paleoclimatic contribution was organized by a group within the Paleoclimate Modelling Intercomparison Project (PMIP) coordinated at the LSCE.

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