What's with this 2°C thing? Limiting global warming to just two degrees Celcius has become one of the concensus talking points - not too unrealistic politically, yet not impossible either. WWF, Greenpeace is running a campaign, Time to Lead, to convince politicians to keep warming below 2°C.

The chain of cause and effect of course goes from burning of fossil fuels and related activities to disruption of the carbon cycle and build-up of greenhouse gasses (GHG) in the atmosphere to a increased heating effect on the planet to melting glaciers and drying fields to relocating farmers and forest fires et cetera, et cetera. But the ecosystem isn't simple - each cause-effect link in the chain comes with various degrees of uncertainty, there are many lesser explored minor links and the big picture isn't that precise.

But that picture is sharpening. In one recent "letter" to Nature it was calculated exactly how much the temperature will rise as more and more GHG is emitted. Implicitly how much we can burn before having to expect the temperature rise to exceed 2°C. Rather than a full summary of their methodology, many models used and scenarios explored this is just a brief look a couple of their key results. But first we are warned...

"2°C cannot be regarded as a 'safe level'"

Less than that will still have severe consequences.

We're currently emitting in the range of 35 to 40 gigatonnes (Gt) of CO₂ per year. The scenarios modeled have to take into account not just the peculiarities of ecological feed-back mechanisms but also various patterns of emissions. Their pocket calculators are over-heating too.

The probabilistic reality

(Most of these figures are listed as quite wide intervals in the paper. The above is a simplified excerpt. Please do not take it too literally. The apparent difference between the equivalents- and CO₂-only calculations is in part due to the fact that most GHGs disappear much faster than CO₂ does.)

From these calculations some things can be concluded with high confidence though. Such as...

"Emitting the carbon from all proven fossil fuel reserves would vastly exceed the allowable CO₂ emission budget for staying below 2°C."

...and...

"Given the substantial recent increase in fossil CO₂ emissions [...] policies to reduce global emissions are needed urgently if the 'below 2°C' target is to remain achievable."

So, even given the uncertainties of the carbon cycle, ocean feed-backs, modeling methods and future decisions it is possible to calculate some frame of responsible operation beyond which we by all possible reasoning should expect severe adverse climate incidents.

Further reading

I am not the first to reference to this study. So, even if you don't subscribe to Nature, you can read more about it right away.

RealClimate / Hit the brakes hard

The climatologists' blog talked about this study and one other while the print was still fresh.

"unless humankind puts on the brakes very quickly and aggressively [...] we face a high probability of driving climate beyond a 2°C [..] humankind is already about half-way toward releasing enough carbon to probably reach 2°C, and that most of the fossil fuel carbon (the coal, in particular) will have to remain in the ground."

Potsdam Institute / On the way to phasing out emissions: More than 50% reductions needed by 2050 to respect 2°C climate target

This summary quotes Meinshausen. Ie:

"Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming. We shouldn’t forget that a 2°C global mean warming would take us far beyond the natural temperature variations that life on Earth has experienced since we humans have been around."

Nature Comment / Halfway to Copenhagen, no way to 2 °C

More recently the lead author also co-authored a comment in Nature further linking the emission targets in political debates with consequences to nature.

"Unless there is a major improvement in national commitments to reducing greenhouse gases, we see virtually no chance of staying below 2 or 1.5 °C. Coral reefs, in addition, seem to have certainly no chance"

Meinshausen, M., Meinshausen, N., Hare, W., Raper, S., Frieler, K., Knutti, R., Frame, D., & Allen, M. (2009). Greenhouse-gas emission targets for limiting global warming to 2 °C Nature, 458 (7242), 1158-1162 DOI: 10.1038/nature08017

Rogelj, J., Hare, B., Nabel, J., Macey, K., Schaeffer, M., Markmann, K., & Meinshausen, M. (2009). Halfway to Copenhagen, no way to 2 °C Nature Reports Climate Change (0907), 81-83 DOI: 10.1038/climate.2009.57

Hereby I hope to have inserted a couple more buoys in the TH!NK discourse. Very much related is my first ResearchBlogging post: Predictions of irreversible, unavoidable climate changes, which looked at how slow the biosphere is to readjust to more normal levels of GHGs, CO₂ especially. Consider ResearchBlogging.org for more blogging about climate science!