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Cumulative carbon featured by IPCC

As expected and as had been widely leaked, the IPCC 5th Assessment WG1 Report released last week presented a range of evidence that further underpinned the case for anthropogenic induced warming of the climate system. By contrast with the 4th Assessment Report issued in 2007, the chance of a human link shifted from likely to extremely likely. Pages of supporting evidence were presented. 

But there was another important development since the 2007 report, the concept that cumulative total emissions of CO2 and global mean surface temperature response are approximately linearly related. There was only one reference to cumulative emissions in 2007 and that was simply a means of describing the mitigation challenge we face over this century. The 4th Assessment Report noted that;

Based on current understanding of climate-carbon cycle feedback, model studies suggest that to stabilise at 450 ppm carbon dioxide could require that cumulative emissions over the 21st century be reduced from an average of approximately 670 GtC to approximately 490 GtC.

The 5th Assessment Report takes this much further and devotes considerable attention to the subject. On page 20 of the Summary for Policy Makers, the report states;

The report also featured the chart below.



This is important in that it clearly introduces into the mainstream the notion that the atmospheric CO2 issue is a stock problem, which brings with it a number of implications for both the energy system and the solution set.

For the energy system, the key issue this raises is that the amount of carbon already in the pipeline for consumption is considerably more than the remaining stock equating to a 2°C temperature anomaly goal. This has been picked up by a variety of organizations, both NGO and financial, and is at the core of the recent discussions on a “carbon bubble”.

But it also points to a critical aspect of finding a solution to the CO2 problem, the use of carbon capture and storage (CCS). I have written a great deal about this in previous postings. Sequestration (or removal of atmospheric carbon) is the only reliable mechanism for managing the stock, which means either increasing the permanent bio stock of carbon through forestry and land use or capturing and storing carbon dioxide geologically (CCS). Unfortunately this doesn’t get much of a mention from the carbon bubble proponents, which is a clear shortfall in their analysis. With the mitigation report coming out from the IPCC in the first half of next year, this WG1 finding may be an important placeholder for a more substantial discussion around sequestration.

One area that is left unaddressed, at least for me, is a better discussion on the role of short lived climate pollutants (SLCP) such as methane, particularly in the context of a stock framework for thinking about the climate issue. Although the IPCC say that the effective stock of CO2 must be reduced to account for the warming impact of SLCP, this isn’t the whole story. The difficulty is that while anthropogenic CO2 stays in the atmosphere for a very long time, gases such as methane do not – they break down to CO2. This means that methane isn’t a stock issue, but a flow issue, i.e. the impact of methane released today is to change the rate of current warming, but not really the peak warming that we will likely see at some point late this century or early next century. Methane emissions at that time will impact peak warming. It also means that the current efforts to reduce methane now could be undermined unless CO2 is also reduced.

So that is my take on this first release of the 5th Assessment Report. Of course there is a wealth of data to work through and understand, but this critical concept of cumulative carbon is one that needs to filter through policy circles. Once the penny drops on this story, we might actually see some real progress in policy making that will make a difference.

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