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.

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