Is 5 the New 3?

Like it or not, climate science depends heavily on computer modelling.  It's how scientists can try to predict future outcomes of changes that have not yet occurred. For example, "if CO2 levels reach X by 20X0, then surface temperatures will reach Y degrees or sea levels will rise by this amount or that."

It's a very intricate system that entails processing mountains of data with known physical principles to produce a likely range of results.

Experience shows that these predictions often are proven inaccurate. That's the good news. The bad news is that when they're shown to be wrong it is quite often because the predictions are understated.  This is one of those moments.

Recent modelling data suggests the climate is considerably more sensitive to carbon emissions than previously believed, and experts said the projections had the potential to be “incredibly alarming”, though they stressed further research would be needed to validate the new numbers. 

Modelling results from more than 20 institutions are being compiled for the sixth assessment by the United Nations Intergovernmental Panel on Climate Change, which is due to be released next year. 

Compared with the last assessment in 2014, 25% of them show a sharp upward shift from 3C to 5C in climate sensitivity – the amount of warming projected from a doubling of atmospheric carbon dioxide from the preindustrial level of 280 parts per million. This has shocked many veteran observers, because assumptions about climate sensitivity have been relatively unchanged since the 1980s. 

“That is a very deep concern,” Johan Rockström, the director of the Potsdam Institute for Climate Impact Research, said. “Climate sensitivity is the holy grail of climate science. It is the prime indicator of climate risk. For 40 years, it has been around 3C. Now, we are suddenly starting to see big climate models on the best supercomputers showing things could be worse than we thought.”

The new models are coming from 'gold standard' outfits such as the British Met Office's Hadley Centre. So what were we getting wrong? It turns out to be clouds and the role they play in atmospheric temperatures.  Science had treated them as neutral, a non-factor. Now it seems they actually contribute to heating. That has sparked considerable debate.

While acknowledging the continued uncertainty, Rockström said climate models might still be underestimating the problem because they did not fully take into account tipping points in the biosphere. 

“The more we learn, the more fragile the Earth system seems to be and the faster we need to move,” he said. “It gives even stronger argument to step out of this Covid-19 crisis and move full speed towards decarbonising the economy.”