The BBC today reported a new US China deal to phase out HFCs, and claimed that it ‘could, by itself, curb global temperatures by 0.5 degrees Celsius. No small achievement.’
It is frustrating that neither the BBC nor the White House chose to say how many actual tonnes of HFCs this is.
But no matter. It so happens that I can use my PAGE09 integrated assessment model to work out roughly the drop in temperature that not emitting 90 gigatons of CO2 would produce. At its peak, the drop in global mean temperature would be 0.02 – 0.07 degrees C (5-95% confidence interval), with a mean value of 0.04 degrees C. The range is so wide because we don’t know exactly what the climate sensitivity is. But even allowing for this, the effect of the deal will be nowhere near a 0.5 degree C drop.
So where did the BBC get 0.5 degrees C from? Could it be as simple as a misplaced decimal point?
Update added 12:15 BST 13 June 2013
No it’s not that simple. Matt McGrath, author of the BBC report, has told me that the 0.5 degrees C figure comes from Bill Hare, one of the authors of the original report.
Update added 17:00 BST 13 June 2013
It appears that what Bill Hare actually said was: ‘If HFCs were scrapped globally, “it… could result in a reduction of between 0.1 and 0.5 degrees”.
In other words, the 0.5 degrees C reduction is the top end of a range for the benefits of releasing no more HFCs anywhere ever, not a central expectation for the results of the US China deal. This is much more believable.