How sensitive is our climate to an increase of CO2 ?

How can we characterize the climate response to an increase of CO₂ concentration ? Climate scientists have chosen simple definitions which allow to grasp the magnitude of the forthcoming climate issue.

Equilibrium climate sensitivity

The first definition, by far the simplest, consists in giving the final mean atmospheric temperature long after the CO₂ concentration has been doubled. The name given to this final mean temperature is Equilibrium Climate Sensitivity (ECS). It is the long term response of the Earth system (a few centuries at least) since it takes a very long time to heat the deep ocean.

There are several methods to determine this climate sensitivity. First, we can use climate models. We then look at the future evolution predicted by these models for a sudden doubling of CO₂. This is an emergent property of the models. Furthermore, we can also use the climate models for their understanding of the past temperature variations. Indeed, we have now a good knowledge of the various factors which have modified the radiative forcing, that is the extra heating. Hence we can identify the past response of the Earth to volcanic eruptions or to solar cycles, and this method is called instrumental.

In the same spirit, we can look at the paleoclimate, instead of the recent past, by comparing the variations of temperatures with variations of radiative forcing (the extra heating power due to greenhouse effect). These two quantities must however be estimated indirectly.

The results obtained with these different methods are gathered in the following graph. The IPCC collects and summarizes these evaluations by reporting an equilibrium climate sensitivity between 1.5C and 4.5C.

We can also see on figure 2 below that these estimates have not varied much over the recent years. Hence a number we can remember for simplicity is that on the long term, there will be an increase of 3 degrees if the CO₂ concentration is doubled. This will happen if we still add 2400 GtCO2, that is in 55 years if maintaining the current pace of emissions (43 GtCO2 per year). We could even reach the 4 degrees if we go beyond !

We can even continue this exercice of predicting the future by adding the effects induced on vegetation and ice cap, since they both modify the albedo and thus the climate. By doing this, we estimate the Earth System Sensitivity (ESS) on a time scale of thousands of years. However, uncertainties are rather large because these factors (vegetation and ice cap) are not simply proportional to the climate change, and are thus harder to predict.

Transient climate response

The major shortcoming of the equilibrium climate sensitivity, is that it gives an information about a future which is too distant, and this can be perceived as a simple academic exercice, namely showing that the various methods give compatible results. Therefore, we need an estimation of the climate sensitivity for the near future, and this is the purpose of the Transient Climate Response (TCR).

Its definition is also based on a doubling of CO₂ concentration, but we arbitrarily specify the pace of greenhouse gas emissions, and the moment when the induced temperature change is estimated. Climate scientists came up with a scenario of CO₂ concentration increasing by 1% per year, that is a doubling in 70 years. In addition, they have chosen to estimate the temperature variation at the end of these catastrophic 70 years, instead of waiting much longer that the Earth system settles in an equilibrium. It is a very simple but useful definition for policy makers, since it is based on a time scale which is commensurate with life expectancy.

According to the IPCC’s last report, the value of the transient climate response is between 1C and 2.5C, and the range of values found from various scientific studies are reported on the following figure :

As expected, the transient climate response is smaller than the equilibrium climate sensitivity, given that in 70 years the deep ocean does not have enough time for warming up significantly.

Even though these various definitions of climate sensitivities are very useful for climate scientists, the most practical definition to guide our decisions on energy transition remains the variation of the mean temperature at the end of the century, for a given total amount of CO₂ emissions. This quantity allows to determine the amount of greenhouse gases that we can still release in the atmosphere, given all what we have emitted in the past, and given a target of temperature increase that we do not want to overshoot.

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Image by Martin Eklund (Pixabay)