New model reconciles climate history with forecasts

How much will burning fossil fuels warm the planet? It depends on two basic factors: the direct contribution the emissions make to higher temperatures through the greenhouse effect – known as radiative forcing – and the response of the complex interactions of the global climate system to that forcing, known as climate sensitivity.

Climate sensitivity, which predicts the total change in temperature in response to a given change in carbon dioxide levels, is the crucial and more controversial one. This week Harvard University scientists Cristian Proistosescu and Peter Huybers have made an important contribution to our understanding of it. Their new modelling helps to reconcile apparent discrepancies between measurements of climate sensitivity over the past century and predictions for the next.

Over the past decade, climate sensitivity has been the hot-button issue for the more seriously minded dissenters from the mainstream scientific view that human-driven climate change is a serious and pressing issue. There is generally little disagreement about the numbers projecting temperature rise from radiative forcing. 

Climate sensitivity is another matter. Disputing the projections advanced in the reports of the Intergovernmental Panel on Climate Change is what the so-called “high priest” of climate sceptics, Christopher Monckton, built his reputation on. Even when his contra calculations were shown to contain some remedial errors, there remained the fact that even the consensus figures for climate sensitivity were hardly precise. 

The IPCC’s Fifth Assessment Report assessed “equilibrium climate sensitivity” – ECS, based on the global annual mean surface air temperature change experienced by the climate system after it has attained a new equilibrium in response to a doubling of atmospheric CO₂ concentration – as being between 1.5°C and 4.5°C, a large range that might make all the difference in the world depending on where you live. {%recommended 5179%}

The difficulty of calculating climate sensitivity is due to the sheer number and complexity of factors that must be considered, weighing potential positive feedbacks amplifying global warming, such as the release of methane and decrease of the albedo effect due the thaw of Siberian thermafrost, against negative feedbacks, such as higher carbon-dioxide levels resulting in enhanced leaf growth in tropical regions, leading to more transpiration and therefore a cooling effect.

The most recent IPCC reports, as Proistosescu and Huybers note in their paper, published in Science Advances, actually increased the range of uncertainty about predicted climate sensitivity (the range in the previous assessment report was 2° to 4.5°C) in order “to account for the lack of consensus between estimates based on models and historical observations”. A recent review of estimates based on historical observations, for instance, indicated a median climate sensitivity calculation of 2°C, with 80% of the estimates between 1.6° and 3°C.

These numbers, however, provide no basis for complacency about the future, the authors suggest.  Slow feedback contributions to warming are “only weakly manifest” in the current climate system, they say, because increases in greenhouse gas concentrations have occurred primarily in the past 50 years – “a short period” that makes it “difficult, if not impossible to accurately estimate ECS from historical observations”. 

Their conclusion: longer, centennial modes of radiative forcing “display stronger amplifying feedbacks” and ultimately contribute 28 to 68% of equilibrium warming, yet comprise only 1 to 7% of current warming. “Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates.”

Which is not good news for climate sceptics, or for the planet.