Predicting global warming: how water vapor affects the radiative forcing of CO2

Climate sensitivity, the warming of the Earth´s surface due to a doubling of CO₂ concentration, is one of the key quantities in climate research. The thought-experiment of a CO₂ doubling provides a single number to describe Earth´s sensitivity to carbon dioxide. Knowing this number is key to reliable future climate projections. The Intergovernmental Panel on Climate Change (IPCC) estimated the range of climate sensitivity to likely be between 2.5 and 4.5 °C in its last Assessment Report. A particularly intriguing question is how much it depends on the current state of the Earth system. Previous studies have shown that an increase in water vapor feedback at higher temperatures increases climate sensitivity. In many modelling studies, this increase reverses for surface temperatures above 35 °C. This means that after a maximum of 10 °C, climate sensitivity decreases again to a relatively low value of approximately 3 °C.

Earlier this year, a group of researchers from the U.S. were able to explain the decrease in climate sensitivity under massively increased CO₂ concentration. They show that, contrary to common intuition, a massive increase in CO₂ concentration (about 10-100 times today´s concentrations) leads to a stabilizing climate feedback, i.e., less warming. However, not all previous studies that found a decrease in climate sensitivity also used an increased CO₂ concentration.

In this study, the authors therefore investigate another possible source of the decline in climate sensitivity, and that is the radiative transfer scheme used. Radiation schemes are used in climate models to efficiently simulate radiative fluxes from the outermost layer of Earth´s atmosphere down to the ground. Their implementation is based on assumptions about the temperature and humidity distribution as well as the chemical composition of the atmosphere, which resemble the current climate. As a consequence, their accuracy decreases when the simulated climate deviates strongly from the present. For this reason, the authors compare the fast radiation scheme RRTMG with the radiative transfer model ARTS. Models such as ARTS operate at many times the computational cost when calculating radiative fluxes. The massive computational effort leads to more accurate results but prevents their use in operational climate models. However, the results can be used as a reference during the development of the faster radiative schemes.

The authors found that the use of the widely used radiation scheme RRTMG leads to false signals in the climate sensitivity at high temperatures. The reason for this is probably a too narrow range of validity of pre-calculated tables, which RRTMG uses to increase efficiency.

In addition, the authors used the spectral resolution of the ARTS reference model to better understand the subtle interplay of CO₂ and water vapor in the context of atmospheric radiative transfer. They found that the so-called radiative forcing in particular, which is caused by a doubling of CO₂ concentrations, can be strongly masked by water vapor. Here, the scientists poetically described the image of a “sea of water vapor whose varying sea levels can cover mountains of CO₂”. This result highlights that an accurate understanding of the distribution of water vapor in the atmosphere is fundamental to an estimate of future global warming.

Original publication:

Kluft, L., Dacie, S., Brath, M., Buehler, S. A., & Stevens, B. (2021). Temperature-dependence of the clear-sky feedback in radiative-convective equilibrium. Geophysical Research Letters, 48, e2021GL094649. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL094649

Contact:

Dr. Lukas Kluft
Max Planck Institute for Meteorology
Email: lukas.kluft@we dont want spammpimet.mpg.de

Prof. Dr. Bjorn Stevens
Max Planck Institute for Meteorology
Email: bjorn.stevens@we dont want spammpimet.mpg.de

Dr. Manfred Brath
Universität Hamburg
Department of Earth Sciences, Meteorological Institute
Email: manfred.brath@we dont want spamuni-hamburg.de

Prof. Dr. Stefan A. Bühler
Universität Hamburg
Department of Earth Sciences, Meteorological Institute
Email: stefan.buehler@we dont want spamuni-hamburg.de