Climate sensitivity and the myth of ‘burnable carbon’

The ink is scarcely dry in the IPCC’s fifth report when it has become clear that they have badly underestimated the risks in two key areas – sea level rise and climate sensitivity. With respect to the latter, David Spratt at Climate Code Red comes to the conclusion that there is no carbon budget left, no ‘burnable carbon’ if we are looking for a safe climate.

As I said in Climate clippings 97 the fourth IPCC report in 2007 estimated that the planet will warm between 2 and 4.5°C warming in response to a doubling of the amount of CO2 in the atmosphere, with a best estimate of 3°C. This estimate was followed by a number of studies suggesting a lower sensitivity, leading the IPCC’s fifth report to extend the range to 1.5°C at the lower end and omit a best estimate entirely.

Back in May Dana Nuccitelli reported on a study by Kummer & Dessler which showed that recent studies suggesting an insensitive climate are flawed. They eliminate the lower part of the range but still converge on a value around 3°C.

Spratt now reports:

a recent paper by Sherwood, Bony et al looking at clouds and atmospheric convective mixing finds that on “the basis of the available data… the new understanding presented here pushes the likely long-term global warming towards the upper end of model ranges.” Taking “the available observations at face value,” they write, “implies a most likely climate sensitivity of about 4°C, with a lower limit of about 3°C.”

Problem is that these estimates are based on short-term feedbacks only, or what is known as Equilibrium Climate Sensitivity (ECS). Spratt says:

Paleoclimatology (study of past climates) suggests that if longer-term feedbacks of “slow” factors are taken into account, such as the decay of large ice sheets, changes in the carbon cycle (changed efficiency of carbon sinks such as permafrost and methane clathrate stores, as well as biosphere stores such as peatlands and forests), and changes in vegetation coverage and reflectivity (albedo), then the Earth’s sensitivity to a doubling of CO2 could itself be double that of the “fast” climate sensitivity predicted by most climate models, or around 6°C.

A measure of these effects for a doubling of CO2 is known as Earth System Sensitivity (ESS).

He says that “ESS is generally considered to come into play over periods from centuries to several millennia.”

If that’s how the earth system operates, that’s how we must operate.

Now in February 2013, new research on Russian cave formations measuring historic melting rates gives rise to a warning that a +1.5°C global rise in temperature compared to pre-industrial is enough to start a general permafrost melt.

Other research indicates that thaw and decay of permafrost carbon, once seriously started, is irreversible.

Rather than 2°C as a guardrail to avoid dangerous climate change, we must expect danger to occur at 1.5°C.

We are pushing the climate harder than it has been pushed in the last 65 million years. In this post I asked what does 4°C mean?

Professor John Schellnhuber, Director of the Potsdam Institute for Climate Impact Research, provides a stark assessment of the difference between a rise of two and four degrees. ‘The difference,’ he says, ‘is human civilisation. A 4°C temperature increase probably means a global [population] carrying capacity below 1 billion people’.

For a safe climate as we saw in The game is up, again quoting David Spratt:

We have to come to terms with two key facts: practically speaking, there is no longer a “carbon budget” for burning fossil fuels while still achieving a two-degree Celsius (2°C) future; and the 2°C cap is now known to be dangerously too high.

I hate to say this, but at the leading edge people are catching up with what James Hansen was saying back in 2008. Here’s a table from page 17 of the 2011 paper Earth’s Energy Imbalance and Implications where he gives four categories of climate sensitivity:

Climate sensitivity_cropped_600

The fourth doesn’t bear thinking about. Clearly he’s a dangerous man and needs to be locked up.