Simplifying the carbon emissions - temperature link
There’s been widespread agreement that it’s important to keep climate change from exceeding dangerous levels; the European Union has come up with 2 degrees C as a target. What’s less clear is the emissions reductions scenario we will need to keep below this guideline.
And it’s possible that the complex array of different emissions scenarios has delayed agreement. For example, in 2007 the G8 nations said they aimed to halve global emissions of carbon dioxide by 2050, while economist Nicholas Stern in his Stern review of the economics of climate change suggested 25% cuts by 2050 and an eventual reduction target of 80%.
With that in mind, a couple of groups of researchers are aiming to simplify the science involved to aid policymakers come up with decisions, as they will need to do at the climate negotiations in Copenhagen in December.
Chris Jones of the UK’s Met Office and colleagues have developed the concept of the cumulative warming commitment. Jones, who detailed the work at the EGU meeting, sees it as a “framework for keeping policy simple” and a way of “taking the climate uncertainty out of the equation a bit”.
The concept relies on the fact that warming is very sensitive to the cumulative amount of emissions but not to the pathway by which that amount of carbon was introduced into the atmosphere. The cumulative warming commitment provides a measure of the peak amount of warming per unit of cumulative carbon emissions. Reductions scenarios can be compared and analysed by integrating the total amount of emissions produced and converting this to a peak warming.
Of course, the 64 million dollar question is, what is the size of the cumulative warming commitment? Jones says that the value is uncertain, depending on factors such as climate-carbon cycle feedback, climate sensitivity and ocean heat uptake. But following analysis of data from C4MIP (the Coupled Carbon Cycle Climate Model Intercomparison Project), he and his colleagues have agreed on the most likely figure.
Using this most likely value indicates that the maximum cumulative amount of carbon we can emit and still keep to the 2 degrees C target is 1 trillion tonnes. “We are halfway there already,” said Jones.
By focusing on the emissions budget as a whole, rather than the speed and timing of emissions cuts, the concept enables governments to make social and political decisions on how best to achieve their carbon goals. For example, a country might choose to delay action until later and then make faster cuts, although Jones stressed that could have detrimental economic implications.
Further details of the work will appear in a paper in Nature next week.
Meanwhile, Nathan Gillett of Environment Canada, has been developing the carbon-climate response metric, a measure of temperature change (rather than peak temperature) divided by cumulative carbon emissions. Again using C4MIP models, Gillett found that values for his CCR stabilised at 1.0 - 2.1 degrees C per trillion tons of carbon emissions for 2050-2100.
What’s more, Gillett says that the constancy of the value means that it’s possible to estimate CCR from observations by working out the carbon dioxide-induced warming caused by cumulative emissions over a given period. For the 1990s, he and his coworkers’ best estimate of CCR, using a carbon dioxide-attributed warming of 0.48 degrees from a total 0.94 degrees of warming, is 1.5 degrees per trillion tons, which is in good agreement with the models.
More details of Gillett’s work will appear in Nature in a couple of weeks.
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