Friday, January 03, 2014

Sherwood shows that Earth's climate is sensitive to being messed with

Almost unnoticed, a recent paper in Nature by Sherwood et al could transform the debate on climate change.

To understand what our greenhouse gases may do to our home planet, we need computer models that can make relatively accurate representations of what will happen.

Models are not perfect, but they are undeniably a damn sight better than amateur armchair climatologists who wave envelopes on the back of which they have developed their own personal pet theory of how hot or cold the planet will be in 2100 AD. Models are vastly complex arrays of interrelated calculations of how the physical processes of the planet will interact with each other.

Mirabile dictu, models are evolving. Modern models are more accurate than old ones. Who'd a thunk it?

All scientists, including the so-called sceptic climatologists such as Richard Lindzen, agree that doubling the CO2 in the atmosphere since pre-industrial times, will raise the surface temperature by about 1.2C.

The present debate concerns the amount of positive feedback in the system. An initial warming will affect water vapour, albedo, cloud, release of CO2 and Methane from soil and water, etc. Will the net effect of these feedbacks lead to an eventual elevation of only 1.5C? Or will it be 3C? Or 5C?

The lukewarmers (sceptics like Environment Secretary Owen Paterson, who accept that climate change is real, but believe that its effects will be minor) argue that the earth system is insensitive to change*, and say that doubling will only produce an eventual warming of 1.5C. Mainstream climatologists think it may go as high as 5C, but until last week there was doubt, since the model outputs varied somewhere between 1.5 and 5C.

The cause for this variance was the cloud effect. It is very complex. Lindzen and other sceptical scientists even believe passionately  that clouds have a net cooling effect. They struggle to substantiate this since Dessler's work, which shows that cloud has a mainly warming effect. But until last week, our understanding of the cloud effect was still fuzzy.

Now Sherwood's paper has brought things into focus. He looked at data from radiosondes, air balloons that measure temperature and humidity. He focused on exactly what happened when air was mixed by convection. He showed that the data matched the computations of models that had high climate sensitivity, but did not match the models that had lower sensitivity.

In a classical application of scientific method he has refuted the low sensitivity hypothesis. We can now say that the lukewarmer hypothesis, which requires low levels of climate sensitivity, is no longer tenable. Environment Secretary Owen Paterson, Nigel Lawson and other lukewarmers have no leg to stand on.

The debate has shifted. we can say with confidence that continuing to increase the concentration of CO2 and other greenhouse gases will do serious harm to the planet's energy budget. We must now make the transition from carbon to renewable energy technologies.


*When it suits their case. Low climate sensitivity is incompatible with their other argument, that the changes that we see are due to natural variation.

Want more detail on the science? Go here.

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