One term that is highly unlikely to be discussed at COP21 is “carbon budget”, a term that identifies one of the simplest and most straightforward methods for apportioning and tracking carbon emissions. Budgets can be used determine a total amount of resource by identifying and quantifying the amount in various compartments. Such budgets as those produced by the U.S Department of Energy as part of the collaborative, international Global Climate Project can be used to trace the flow of carbon through the various components the global ecosystem. However a budget can also be used to allocate a resource total to various compartments, and it is this “allocation” type of budget that will not be discussed at COP21.
COP15, Copenhagen, 2009, adopted the goal of limiting the maximum global temperature increase to 2oC above pre-industrial levels to avoid “dangerous” impacts of climate change. Due to a number of scientific claims that a 2oC might be too high, COP15 also recognized that a 2oC limit might not be adequate and called for a review in 2015 to determine if there was a need to lower the goal to 1.5oC, or 350 ppm of C02. The 2oC (3.6oF) number was based more on political reality than science, but has now become universally established as the maximum temperature increase that would avoid “dangerous” or catastrophic impacts.
While there is no widely accepted value for the base pre-industrial temperature for this target, it is generally agreed that we have already used up between 0.8o and 1.2o of the permissible 2oC increase target.
The UN Intergovernmental Panel on Climate Change (IPCC) 2013 Fifth Assessment Report(1) for the first time endorsed the view that there are absolute limits on the amount of carbon that can be added to the atmosphere. Based on a number of scientific studies the IPCC calculated that in order to have a 66% chance of staying below the 2oC target total cumulative carbon emissions cannot exceed 1000 gigatonnes(2) of carbon. Note that the “66% chance” means a 33% chance of over-shooting the carbon target which would rapidly reduce the chances of meeting the temperature goal.
In contrast to other greenhouse gases (GHGs) such as methane, nitrous oxide and the fluorocarbons, once in the atmosphere CO2 stays there for thousands of years. Since these non-persistent GHGs constitute 20% of the total emissions, the IPCC adjusts the total accumulated carbon limit of the 1000 gigatonne number to 800 gigatonnes of carbon from CO2 emissions.
The 2013 IPCC Fifth Assessment Report also states that by 2011 an estimated 531 gigatonnes of carbon had been accumulated in the atmosphere. Since CO2 emissions are adding 10.1 gigatonnes of carbon a year about 40 gigatonnes have been added since the beginning of 2011. Thus out of the “allowable” 800 gigatonnes 571 gigatonnes have been utilized, leaving only 229 gigatonnes to be allocated, or budgeted, before global net CO2 emissions must be reduced to zero – if we are to have a 66% of reaching the 2oC target established by the COP.
At the current annual emission rate of 10 gigatonnes of carbon (37 gigatonnes of CO2) emission, the remaining 229 gigatonnes of carbon will be emitted in 23 years. Net carbon emissions must then be zero if we are to have a 66% chance of meeting the 2oC target established by the COP.
If COP21, were to follow the guidance of the IPCC (essentially the scientific advisers to the COP) and adopt a carbon budget with 229 gigatonnes of carbon remaining, any method of allocation would be politically unacceptable to the majority of members due to the the drastic emission reductions required. Therefore COP21 will not discuss a carbon budget.
At this point, the introduction of a carbon budget into the COP21 discussions would do little more than demonstrate that the country pledges (commendable as they are) to be far too little, and far too late.
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1.) The complete Fifth Assessment Reports can viewed and downloaded at: http://www.ipcc.ch/report/ar5/index.shtml
2.) A gigatonne is one billion metric tonnes (2240 pounds). If we add the mass of oxygen molecules to our calculation, the 1000 gigatonnes of carbon can also be expressed as the more awkward number of 3670 gigatonnes of carbon dioxide. Or, the other way around, 3670 gigatonnes of carbon dioxide contains 1000 gigatonnes of carbon. Since we are calculating a carbon budget 1000 gigatonnes is the most appropriate number to use.
