David Wasdell: Climate Sensitivity and the Carbon Budget

[GliderGuider]

Last edited Thu Aug 7, 2014, 11:40 AM - Edit history (1)

There is a new paper by David Wasdell (released this past June) that gives a very accessible and well-supported derivation of the various numbers floating around for climate sensitivity. In addition he looks at the question of the global carbon budget. The results are very interesting, and more than a bit unnerving - even though they are what many here have either known or suspected for a while.

Wasdell supports the long term, slow-feedback model of climate sensitivity, the one that matches the paleo record. He dismisses the short-term Charney sensitivity of 3ºC per doubling of CO2 as being useless as a policy guide. The figure his team has arrived at is instead 7.8ºC per doubling. IMO he does a thorough job of supporting that number.

Of course, this higher value would take hundreds or thousands of years to manifest, but if we are interested in the long-term health of the biosphere it's the one we must use to calculate the damage in the pipeline.

Based on what's already happening to the global climate, Wasdell similarly dismisses the idea of 2ºC as the threshold of dangerous climate change, as do many of us here. He uses threshold values of 1º to 1.5º.

The implication of using a 1-1.5 degree theshold and a sensitivity of 7.8 is that the threshold value for CO2 concentration is 310 to 320 ppmv. So much for 350.org.

He then uses those numbers to calculate our remaining carbon budget. He finds (surprise) that we don't have one. Instead we are in an overdraft situation of about 400 GtC or 1,450 GtCO2:

With the recognition that the 2°C target is set far too high to avoid dangerous climate change comes the realisation that increase in average surface temperature should be kept to no more than 1°C above the pre-industrial benchmark. The equivalent CO2 concentration stands at around 310 ppm, so the overdraft would stand at 88 ppm, i.e. 425 GtC or 1,560 GtCO2. (The required concentration increases to 320 ppm if the temperature constraint is relaxed to 1.5°C. In that case, the overdraft would be reduced to 78 ppm which is equivalent to 377 GtC or 1,380 GtCO2)

Given these numbers, I went to the CDIAC historical carbon emissions database and determined that we have been in a carbon overdraft since about 1900.

In terms of the eventual equilibrium temperature, our current CO2 concentration of 400 ppmv gives a value of +4ºC over the pre-industrial value. If we calculate in all the other minor GHGs, the temperature projection rises another degree, to +5. An eventual concentration of 500 ppmv would give us an equilibrium of about +8ºC, and 700ppmv gives us over +10ºC. For comparison, the temperature difference from the middle of the last glacial period to the beginning of the industrial age was about 5ºC...

One concluding paragraph worth noting reads as follows (emphasis in the original):

It is therefore clear that there is no available carbon budget. In fact the account is already massively overdrawn by a total of 388 GtC. In other words, there is no surplus in the account to be shared out (equitably or otherwise) across the international community. Civilization is deeply in debt to the planetary environment, and every extra tonne of emitted carbon simply adds to that debt. Sadly there are no bankruptcy arrangements in place between human civilisation and its planetary environment.

I recommend the paper to everyone interested in the state of the current, unpoliticized science around climate change.