What is the Global Carbon Budget for the 2-Degree Limit?

We just defined a very important concept, the notion of a global carbon budget. I admit this is a pretty complex and sophisticated notion, but I promise we’re going to make good use of it. And we’re going to do that right away.

We’re going to apply the concepts to what is of interest to us of course and that is the global carbon budget for the 2-degree limit, because this is our objective. This is what we need to do to avoid dangerous climate change.

So how do we calculate this budget? Well we need to look at the greenhouse gases concentration that is consistent with this objective.

So we said it was in between 430 and 480 parts per million of CO2 equivalent. And what is especially important is to calculate the global budget for CO2 emissions only, because as we’ve said repeatedly, CO2 is the single largest source of total greenhouse gases emissions. And it…also because it remains in the atmosphere much longer than most of the other greenhouse gases and in particular methane.

So we’re trying to narrow down the issue a little bit and to calculate a global carbon budget for CO2 only as opposed to all greenhouse gases. So in order to do that, to limit the analysis to CO2, we need to be making a number of assumptions. First, regarding the known CO2 greenhouse gases emissions like methane, nitrous oxide, or fluorinated gases. Second, we need to make assumptions regarding the contribution from other climate changing factors such as the aerosols and land use albedo.

Jeffrey Sachs explained in detail in some of the previous lectures on where the impacts of these factors. Third, we need to make an assumption regarding the timing of CO2 emission reductions and therefore the time the carbon cycle has to absorb the CO2 that is emitted. And fourth and finally, we have to make assumptions regarding the sensitivity of the climate to CO2 emissions and these other climate forcing.

So it’s, it’s a comprehensive set of assumptions we need to be making. There are uncertainties in climate science at each and every step of these assumptions. But taking into account all these factors, the global budget for CO2 emissions to the end of the century to stay within the 2-degree limit is within the range of 630 and 1180 billion tons of CO2, or gigatons.

That’s the budget for CO2 to the end of the century. What is also very important is to calculate this global budget for CO2 emissions to stay within the 2-degree limit, not only to the period to the end of the century, but only to mid-century, to the year 2050, because I guess you will agree with me that the end of the century, although I hope we’ve shown you that it is very relevant from a climate science perspective is of course very far from an economic perspective or even worse, from a political standpoint.

Countries’ long-term emission reduction objectives, when they have one and the problem in fact is that not that many of them have one so far, but we’re going to be discussing in the, in the next lectures why they should have one and what it should be. Countries’ emission reduction objectives are to the best to the year 2050 and not to the end of the century. So it is very important that we’re able to calculate a global budget to 2050 because we need to be able to calculate, to assess if countries’ projected cumulative emissions to 2050 collectively fit within the global budget to stay within the 2-degree limit, because we need to be able to assess if the emission reduction targets that are taken by the different countries add up and are sufficient to reach the global goal.

So if we’re trying to define a budget to mid-century as opposed to, to the end of the century, we need to very simply take the century long CO2 emissions and divide them into two time periods, to mid-century first and then from 2050 to the end of the century. And of course the bulk of emissions will occur during the first period to the year 2050 because to stabilize greenhouse gases concentrations the net emissions should decline to zero during the second period.

But here I want to mention something very important and that is that some scenarios, not all of them but a significant fraction of them are based on the idea of net negative emissions during the second half of the century. What is it? What, what does net negative emissions mean?

How could emissions be net negative, so below zero? Well there is a number of different potential technologies that could be used to achieve net negative emissions. In particular, it could be achieved if the use of biomass for energy production was deployed in combination with carbon capture and sequestration. So it means that the biomass would be burned in the power plants and then the power plants would in turn capture and sequester the CO2 that is emitted.

This is what we call bio-energy plus carbon capture and sequestration. And this is only one out of several potential net negative emissions technology, including the direct air capture of CO2. So to the extents that negative emissions are available on a large-scale in the second half of the century, the budget for CO2 emissions for the first half of the century would be of course correspondingly higher.

But there is an important but here. The feasibility and the sustainability of the large-scale deployment of net negative emissions technology is still very much under debate. I mean not impossible and we should do everything that’s possible to make it a future reality, but still unproven at this stage. In particular, what I just mentioned bio-energy plus carbon capture and sequestration raises very serious issues, because it combines the twin challenges of large-scale biomass production on the one side and the large-scale storage of CO2 on the other hand.

And at the global level, the large-scale use of biomass for energy production could cause deforestation or compete with land use for other purposes such as food production for example, which would have a very negative effect on food security. This is not to say that it cannot be done. In particular in some countries and under some specific circumstances with new generations of biofuels it could be done in a sustainable manner, but we’ve got to be very careful about the way we do it. And it isn’t clear what is the scale of the potential for doing it in a sustainable manner without contributing to deforestation or without competing with land use for food purposes.

That’s one. On the other end the scale of the geological potential for a CO2 sequestration is also very much under debate.

We’re going to be discussing in some of the next lectures that CCS is a well-known technology. I mean it’s not as if we had no clue how to do it. Each and every element of CCS capture and sequestration and transport is a known technology, but it’s unclear what is the geological potential to sequestrate in a safe manner this CO2 under the ground. And what’s more, even if CCS became a reality, it would have to be deployed first on fossil fuel power plants and industry and not first on bio-energy plants. So that being said, if we exclude at least for the time being the option for a net negative emissions, again that seems to be prudent at this stage of research and development given the uncertainty regarding their feasibility and their sustainability.

If we exclude the option for a net negative emissions, then the global budget for CO2 emissions to mid-century, to the year 2050 is of 825 gigaton, or billion tons. And it is of 950 gigaton to the end of the century, which simple calculation here would imply a 125 gigaton of cumulative emissions during the second half of the century, starting the year 2050 to the end of the century.

That being said, it is very important that we scale up the support for research programs that would explore the feasibility and the sustainability of net negative emission technology options such as bio-energy plus carbon capture and sequestration, or direct air capture of CO2 emissions, because it would increase the size of the carbon budget that we can use when the carbon budget is very tough and very strict. So, so far we have calculated a global budget to stay within the 2 degree limit for CO2 emissions, but we’ve combined CO2 emissions from very different sources in fact. I mean from the burning of fossil fuels and industrial processes, but also from land use. So the emissions coming from deforestation or the emissions coming from agriculture.

If we want to define a global budget for CO2 coming only from the burning of fossil fuels and industrial processes, then we need to make yet another assumption. We need to make an assumption regarding the potential for CO2 emission reductions in the land use sector, but also for the potential net biological sequestration of CO2, because it’s possible to achieve net biological sequestration through different types of practices such as reforestation or peat production or wetland restoration, or even improved agricultural practices.

The assumption that is the most often made in the scientific literature is the assumption of net zero emissions from land use over the century. So if that is the case, it means that the 950 gigaton global budget for total CO2 emissions we defined by the end of this century to stay within the 2-degree limit can be considered as a budget for CO2 energy only. But here again I want to stress that there is great uncertainty regarding the precise potential but also the timing for possible CO2 emission reductions and net biological sequestration of CO2 in land use.

I mean it might well prove impossible to achieve net zero emissions in the land use sector which would be bad news because it would further reduce the size of the budget for CO2 energy. So it would reduce the emissions that can come from energy production, transformation and consumption. But on the other hand it, it may well prove possible to achieve net negative emissions in advance of the end of the century in which case, the global budget for CO2 energy would be higher which would be a, a good news. S

o far, the answer is, given the state of the research that we don’t really know, so it’s really urgent that we do more research to see and define more clearly the budget for CO2 energy. But as a first approximation, I mean we can use what we calculated and that is 950 gigaton of CO2 energy emissions by the end of the century in total to stay within the 2-degree limit.

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