What is the Global Emissions Reduction Pathway for the 2-Degree Limit?

So we just defined the global budget for C02 emissions that is consistent with what is our objective, the objective of limiting the temperature increase to less than 2-degree Celsius.

What we’re going to do in this chapter is we’re going to look at the shape of the emission reduction trajectory to reach what we said was necessary and that is to reach net zero emissions by the second half of the century.

So what are the emission reductions that are necessary over time and what are the implications of doing that?

In 2010 the global greenhouse gases annual emissions reached 50 gigaton of C02 equivalent. So if we are to reach net zero emissions by the second half of the century, it means that global greenhouse gases emissions need to decrease not only very significantly, but very rapidly.

Given all the uncertainties we discussed at each and every step of the calculation of the global carbon budget, there are several possible global emission reduction trajectories that give a likely chance of staying within the 2-degree limit.

Again, we’re defining likely here as a probability higher than two-thirds, higher than 66%. But in spite of these uncertainties we can come up with pretty good estimates and we can define the range of a portfolio of scenarios staying within this 2-degree limit. In these scenarios the global greenhouse gases emissions need to get down to 22 gigaton of C02 equivalent by 2050.

The full range is in between 18 to 25 gigaton. That’s the level we need to reach by 2050. Again, you need to compare that to where we are today, it’s 22 in 2050 compared to 50 today. By the year 2020, the global greenhouse gases emissions already need to be lower than today’s level. It means that we need to pick the global emissions even before 2020, because by 2020 they need to reach 44 gigaton of C02 equivalent.

The full range again is in between 38 to 47 gigaton. And by the year 2030, it’s important to look at that because it’s also the focus of the current negotiations on climate change. So on the run-up to Paris in December, 2015, the countries are preparing contributions emission reduction pledges to the year 2030, so it’s important that we have this metric to compare it to what they will pledge in the international negotiations. And so to state within the 2-degree limit the global greenhouse gases emissions in 2030 need to reach 35 gigaton of C02 equivalent, the full range being in between 32 to 42 gigaton. Here I was talking about greenhouse gases emissions overall. So all the greenhouse gases.

But the focus here is going to be on C02 emissions from the burning of fossil fuels in industry. So what does it mean for these emissions in particular as we’ve said, I mean this is the largest source of emissions. And here too, of course given only uncertainties there are many possible C02 energy emission reduction pathways that give a likely chance of staying within the 2-degree limit.

But let’s pick two examples. The first is the representative concentration pathway, 2.6. It’s a complicated term but it’s a scenario that was developed by the Netherlands’ Environmental Agency and it’s the scenario that was discussed in the IPCC Fifth Assessment Report, Working Group One. And that’s the scenario among others, but that was the central scenario discussed that gives a likely chance of staying within the 2-degree limit. And it reaches approximately 12 gigaton of C02 energy only in 2050. So remember, that was 22 gigaton of C02 equivalent by 2050 for all greenhouse gases and here we’re talking about 12 gigaton of C02 energy only in 2050.

If we want to compare the result of that particular scenario with another one, we can use the scenario from the International Energy Agency, its 2-degree scenario. Important precision here, the International Energy Agency scenario gives not a likely chance of staying within the 2-degree limit, but only a 50% chance of staying within the 2-degree limit. So the level of C02 energy emissions in 2050 is a bit higher than in the previous scenario. And in fact it’s 15 gigaton of C02 energy by 2050. This is what you can see on the graph with the two trajectories from 2010 to 2050 and you can see that the scenario from the International Energy Agency is a bit higher than the scenario, RCP2.6 that was discussed in the IPCC Fifth Assessment Report, in part because they do not give the same probability of staying within the 2-degree limit.

So that’s what we should do if we were serious with the commitment we took, the commitment to limit the temperature increase below 2-degrees Celsius. So are we serious?

Are we doing what it takes to reach the objective? Unfortunately the answer is pretty simple and this is, no. We’re not on track and we’re not even close to being on track with this objective.

In Copenhagen in 2009 and then in Cancún in 2010, all the large greenhouse gases emitting countries, so that is the U.S., China, the European Union, India, Brazil, South Africa, Mexico, Canada, Russia, all of them, they took some quantitative emission reductions or limitations in the case of some countries, targets to 2020.

That was of course a major breakthrough in the history of international climate negotiations because before that, only the developed countries and not even all of them had quantified targets to reduce their emissions. And for the first time in 2009 and then in 2010, all the large emitters of greenhouse gases, whether they be developed countries or middle-income countries took a target to the year 2020.

The problem though is that these targets are collectively, widely insufficient to put the world on the 2-degree path. Though there is an internal inconsistency in our climate change agreement because we have a global goal, the 2-degree limit, but then the countries emission reduction targets do not add up to this goal. By how much is the question?

Well if countries emissions reduction pledges were fully implemented, then the 2020 level of global greenhouse gases emissions would be in between 52 and 56 gigaton of C02 equivalent.

And you know that in order to stay within the 2-degree limit, it’s not 52 or 56 gigaton of C02 equivalent that we need, but 44. So there is a big difference in between these two numbers. There is by the way an uncertainty. I’m saying in between 52 and 56, because it was not always very clear what was the target pledged by the different countries. It was not always clear mainly for two reasons. The first is that some pledges were expressed, framed as deviation from business-as-usual emissions or as improvements in the carbon intensity of GDP. So the ratio in between the carbon emissions and the GDP.

But there is of course an uncertainty regarding business-as-usual emissions, or what Is the rate of future GDP growth. So it’s not easy to translate into an absolute level of emissions, pledges when they are made in such a way. The other reason is that some countries put forward in the Copenhagen and then Cancun agreements some emission reduction ranges. And not only a single number.

The European Union for example pledged a 20% to 30% emission reduction targets in 2020 compared to their level in 1990. And it said, well I’m only going to move to the high end of the range, to the 30% if other countries do their fair share of the effort. In the meantime, China pledged a 40% to 45% improvement in the carbon intensity of GDP. And India, a 20% to 25% improvement, also in the carbon intensity of its GDP.

And it said, both countries said, well we’re only going to move to the high end of the range if the developed countries provide us with the means we need, the international financial and technological support we need to implement the intended climate change mitigation actions and measures. So that’s why there is an uncertainty and that’s why I’m not able to tell you, well as a result of the agreement we have, here is where would be the emissions in 2050…in 2020.

It’s a bit more complicated than that. We have to play with ranges. But the point is, start anyway. And the point is that there is a very significant gap in emission reductions to have a likely chance of staying within the 2-degree limit. The gap in 2020 is in fact as high as 8 to 12 gigaton of C02 equivalent.

This is a huge number, this is not only a margin of error, this is a very significant gap in emission reductions to again, just stick to the commitment we made. In particular, when you compare this gap with what the emission reduction pledges that have been made by countries would deliver, because they would in fact only deliver is 3 to 7 gigaton reduction compared to business-as-usual.

So the gap is even higher than what the pledges achieve compared to the continuation of the current trends. There is a very simple calculation that we can do and let’s do it together to give us an idea of the order of magnitude that is necessary to stay within the 2-degree limit. As we’ve said, today’s level of C02 energy emissions globally is approximately 32 gigaton of emissions.

If we use the International Energy Agency 2-degree scenario as a reference and again, that gives only a 50% chance of staying within the 2-degree limit, it means that the global C02 energy emissions need to be divided by a bit more than 2, by 2050, since they need to reach 15 gigaton of C02 energy by 2050. But in the meantime, according to the U.N. Population Division, Medium Fertility Forecasts the world population is expected to grow by 2050. In fact it’s expected to grow by 35% in the next 40 years, from approximately 7 billion people on the planet today to a bit more than 9.5 billion in 2050.

So if we divide the two numbers, it means that the global average per capita emissions need to be divided by 65% by 2050. Needs to go from 4.6 today to 1.6 tons of C02 energy emissions per person and per year in 2050. Why is it important to make this calculation? It’s important because it’s a pretty uncomfortable truth, but all countries will need to converge close to this global average of 1.6 tons of C02 energy per capita by 2050. Not that many countries will be able to have emissions higher than this level.

And this is the result of a simple mathematical calculation, because on the one hand, very few countries with C02 energy per capita higher than 1.6 tons today will simply technically be able to go far below this level by 2050, because that would push the boundaries of what is technically feasible. But on the other hand, the catch-up economic growth in the low income countries that currently emit less than 1.6 tons per head will increase their per capita emissions by 2050, or at least one should hope that growth in these countries has this effect, because it would be very good news for them.

And that would be the case even if they decrease the carbon intensity of their economic growth. So the truth is that if nobody can be far below or above the global average, then all countries should converge close to this average by 2050. It doesn’t mean that the convergence of per capita emissions is a way to allocate equitably the global carbon budget in between different countries.

And we’re going to come back to that in the next lectures, because it doesn’t take into account very important elements that are central to the equity of the global effort to reduce greenhouse gases emissions. In particular, it doesn’t account for the fact as we discussed previously that the historic contribution of the different countries is not the same.

So the developed countries have emitted much more so far than the developing countries. But it’s a simple mathematical conclusion that almost all countries will need to converge at least close to this level of 1.6 tons of C02 energy per capita by 2050.

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.

What is a Carbon Budget?

In the previous lectures we have discussed why limiting the temperature increase below 2-degrees Celsius compared to the pre-industrial levels was so important. We’ve seen of course why it is so important from an environmental point of view to protect the planet, but we’ve also seen why it was so important from an economic and and social perspective, because the impacts of climate change, of uncontrolled climate change, would really be catastrophic and would even threaten our basic ability for example to eradicate poverty.

Today we’re going to continue in our journey to see how we can avoid these catastrophic impacts and how the world can meet the challenge of what we’ve called the deep decarbonization in the sense removing the carbon from our economy and in particular from our energy systems. We’re going to see in particular, what is the level of emission that is consistent with this 2-degree limit over time.

So this first chapter really is about a key concept, is answering this question. The question is how much anthropogenic greenhouse gases emissions, so again, the emissions resulting from our human activities, our energy production transformation and consumption processes, the emissions from our industrial activities, cement, steel, aluminum production, et cetera.

And the emissions also from land: land use change, from agriculture or from deforestation. How much of these anthropogenic greenhouse gases emissions can we still emit if we want to have a chance to live within the limit we’ve set, the 2-degree limit?

The key concept we’re going to be using to answer this essential question is the concept of the carbon budget. So I’ll explain in further detail what I mean by that, but in, in simple terms a carbon budget is defined as the maximum level of cumulative that is over time as opposed to just emissions a given year.

So a carbon budget is defined as the maximum level of cumulative emissions to stay within a degree of temperature increase. So how do we calculate this budget? Well as we’ve said repeatedly now, there are four main greenhouse gases: carbon dioxide, methane, nitrous oxide, and the fluorinated gases.

What is important when trying to estimate a carbon budget is to know that these different greenhouse gases remain in the atmosphere for different amounts of times. Very different in fact, amounts of time, from several months for some to millennium for some others. So they affect the climate on very different time scales. The lifetime in the atmosphere of CO2 in particular, the single largest source of greenhouse gases is unfortunately for us the most difficult to determine because there are several different processes that remove C02 from the atmosphere.

First, in between 65% and 80% of CO2 that is released in the atmosphere dissolves into the ocean over a period of somewhere in between 20 to 200 years. The rest of CO2 that’s not dissolved into the ocean is removed by other processes, but by processes that take up to several hundreds of thousands of years to happen.

That means that once in the atmosphere, the CO2 can continue to affect the climate for hundreds of thousands of years. Methane, by contrast, CH4, is mostly removed from the atmosphere by chemical reaction. And this is a much faster process which takes about 12 years to complete. So overall, since greenhouse gases emissions and in particular, CO2 stay in the atmosphere for a very long period of time after they have been emitted, it is the cumulative level of greenhouse gases emissions that has an impact on the climate and not the emissions at a particular point in time.

That’s why we’re going to be using the notion, the concept of a carbon budget to look at the cumulative level of emissions over time. So what is the relation here in between greenhouse gases emissions and the temperature increase?

Well there is a meaningful correlation in between three things. The first is the level of cumulative greenhouse gases emissions, which we measure in tons of CO2 equivalent. Again, CO2 equivalent to measure with one unit and all the greenhouse gases are not only CO2, because they will have a warming potential.

Second, their long-term concentration and their radiative forcings which we measure in parts per million of CO2 equivalent and in watts per square meter, so watts per surface, respectively.

And third, the resulting mean surface temperature response which we measure very simply in increases in global average and temperature. The relation in between the cumulative greenhouse gases emissions and the global temperature response is approximately linear.

So for any degree of temperature increase, and let me remind you that we’re interested in one in particular, the 2-degree limit, it is possible to determine the corresponding maximum level of cumulative greenhouse gases emissions. And this is what we call the global carbon budget.

That being said, there is an uncertainty surrounding the relationship of cumulative greenhouse gases emissions and the resulting global temperature increase. So since there is such an uncertainty, we must speak in terms of probability. A given cumulative path of greenhouse gases emissions will only yield for a given probability of staying below an increase of let’s say 2-degree Celsius of global warming.

In general we are interested in greenhouse gases emission pathways that give a likely chance of staying within the 2-degree limit and not only a very small probability because that wouldn’t be that interesting to know that it offers only a 5% chance of reaching the objective. Usually, we define likely to try to give it a quantitative meaning, we define likely as a probability of two-thirds or higher. So higher than 66% chances of staying within a certain degree of warming.

Science tells us that to have a likely chance of staying within the 2 degree limit, again, defined as a probability higher than two-thirds, the greenhouse gases concentration must stabilize at somewhere in between 430 and 480 parts per million of CO2 equivalent. This is an important number for you to remember because it gives the equivalent concentration of greenhouse gases to the 2-degree limit.