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natural variability, solution to a long-standing controversy

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Researchers have recently shown that natural climate variability can resolve an apparent disagreement between models and observations. Results published in the scientific journal PNAS this March 30.

The atmospheric concentration of greenhouse gases (GHG) has increased sharply over the past 50 years. Driven in large part by the combustion of coal, oil and natural gas, this change in the composition of our atmosphere has a strong impact on the climate. Indeed, as the GHG intercept the heat radiated towards space by the Earth, any increase in their concentration causes excess energy in the climate system. Put in very simple terms, the insulation of the planet increases and therefore the planet retains heat better.

Controversy over the “tropical hotspot”

However, the excess energy is not distributed evenly over the globe. Whether in latitude, longitude or altitude. Thus, in the polar regions, the presence of ice and the absence of convection tend to amplify and concentrate the warming near the ground. On the contrary, in the tropics, the large vertical movements associated with thunderstorms concentrate the warming at altitude – see figure below. We are talking about tropical hot spot. However, the latter has been the source of long-standing controversy in the scientific community.

Structure of simulated warming according to latitude and altitude. Note the maxima at the surface in the Arctic and at altitude in the tropics. Here, only the winter season appears. Credits: Y. Peings et al. 2019.

And for good reason, there is a divergence between what the climate models project and what the observations show. While the former predict a significant warming of the upper tropical troposphere – around 10 kilometers altitude – the latter are struggling to account for it. More precisely, the tropical hot spot appears half as important on the measurements. Also, some have been able to argue that models were too sensitive to forcing by GHG. A suspicion that would imply that the simulations exaggerate the intensity of the changes to come.

In a recent study, however, a group of researchers showed that this was not the case. By performing more than 400 simulations with a latest generation climate model, scientists have found that 13% of these agreed with the observations. Also, the exercise indicates that for the same forcing by the GHG, the rate of elevation warming can vary widely on a multi-decadal scale. The cause ? The natural variability of the climate system which changes the distribution of heat at different time scales – Going through theENSO or the PDO especially.

Natural variability or respect for complexity

It is therefore not wise to directly compare projected warmings and measurements as has been done in the past. Indeed, observations are strongly influenced by internal variability phenomena that complicate the real climate signal linked to GHG. The one that model averages seek to isolate by filtering out natural noise. This is all the more significant as the observations come from satellites and therefore only available since 1978.

atlantic indian ocean
Credits: NASA Earth Observatory.

“The natural variability of the climate probably reduced the warming observed during the satellite era”, summarizes Stephen Po-Chedley, lead author of the study. “While it is well known that natural variability can produce periods of attenuated warming over ten years, this study shows that it can also play a role. an important role on the relatively long time scales of 40 years specific to satellite recordings’.

In addition, the 13% of simulations in agreement with the measurements are associated with a recurring pattern of phenomena similar to the girl. Like what has actually happened in the real world over the past few decades. An agreement which strongly reinforces the relevance of the conclusions stated by the researchers. Finally, the authors show that the presence of a tropical hot spot limited over the recent period n‘does not necessarily imply models that are too sensitive to forcing by GHG.

“Models with both high and low sensitivity to GHG increases can produce simulations consistent with the warming estimated from satellites”, details Po-Chedley. “By reconciling the modeled and observed warming rates, it is quite clear from our work that climate sensitivity is not the only determinant of this atmospheric warming. Natural variability is an important piece of the puzzle ”.

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