What do IPCC emissions budgets mean?

The new IPCC synthesis report recommends we use total greenhouse gas (GHG) budgets instead of emission targets. This is better because it turns out that the global warming we can expect is more or less a linear function of the total GHG emissions we produce, regardless of when it happens. For a 66% chance of keeping below 2C warming we get a total emissions budget (since 1870) of 2900 Gt CO2e. We already emitted 1900 Gt up to 2011 so we have 1000 Gt left in the budget. The current rate of emissions is about 45 Gt/year. At this rate we would use our whole budget by 2033 - but it is supposed to last us forever. At the moment we are increasing emissions not decreasing them but at some point we have to reduce our net carbon emissions to zero. The later we leave it, the faster we will have to reduce. [1]

If emissions exceed the budget we have to take the excess carbon out of the atmosphere. The IPCC discusses doing this by growing biomass to extract the carbon and then burning it for energy while capturing and storing the CO2 produced. They call this BECCS (Bio Energy Carbon Capture and Storage). The IPCC projections do this on a very large scale - for example at 20 Gt CO2/year i.e. half current global emissions. The UK emissions were 573 Mt CO2 in 2012. Half that would be 286 Mt. If this was produced from crops such as miscanthus, a type of fast growing grass, it would require more than twice the area of Wales [3]. That is a lot of land that we currently use for other purposes. Plus we have to work out where to store the CO2. We don't need to do this if we reduce our emissions fast enough, but the more GHG we emit the more we have to store.

I have produced a graph showing trends in carbon emissions similar to one in my book which showed energy use (my favourite graph). The axes are population and per-person emissions so the total emissions increases from bottom left to top right. Each region is represented by a wiggly line tracking from 2000 to 2011. However Asia/Oceania has been split into Japan, China, India, South Korea and the rest (which includes Australia) - partly because it is very big and partly because those countries show interesting trends.

Trends in carbon emissions due to energy production- total emissions are population (bottom axis) times emissions per person (left axis) so the total increases towards the top right. Blue lines show regions that decreased emissions (2000 to 2011). Orange lines show regions that increased emissions. Data from [IEA]

For example South Korea shows nearly 50% growth in emissions with hardly any growth in population (straight up) but Africa's increase of 30% is entirely due to population growth (horizontal). Both North America and Europe have decreased emissions a little while increasing population slightly (going right and down). China has grown strongly mainly due to per person emissions but with some increase in population (right and up). Per person, emissions in China are now similar to Europe.

It is unfortunate that whereas the western world is mainly responsible for historic emissions the bigger impact over the coming decades will come from rapidly developing countries such as China. China has already overtaken North America in total emissions. China emitted 8.1 Gt/year CO2e in 2011 compared to only 6.5 Gt/year CO2e for North America (emissions due to energy production) [2].

Some of the growth in emissions in China is due to production of goods for the west but most is due to increasing domestic consumption, as shown in this graph which I have lifted from the IPCC report. China belongs to the upper middle income countries group.

Most of the growth in emissions is in middle income countries such as China even when emissions are attributed by consumption (for goods used in the region) rather than production (where goods are produced) [IEA] Working group III figure TS.5

Based on historic emissions, high income countries should have a smaller share of the budget. But it is actually easier for developing countries to develop clean energy because they don't have a lot of existing infrastructure that wasn't designed for it. It doesn't really matter though because in the end we need to get to zero net emissions everywhere. If we take longer to do this and exceed our budget then global warming will be worse - but we still need to get to zero eventually. The question is when, not if.

The IPCC is also keen to point out that the things we need to do to reduce the impacts of climate change have other benefits - such as reducing air pollution, improving energy and water security and maintaining important ecosystems such as forest and wetlands. In fact it is difficult to see how we can achieve energy and water security without cutting fossil fuel use and making our cities more efficient. The best thing about these benefits is that they come quickly, whereas the dangers of climate change we need to avoid aren't here just yet. So we can have jam today and jam tomorrow too once we sort ourselves out.

[1]  IPCC 5th Assessment Report (http://www.ipcc.ch) November 2014
[2] International Energy Statistics (www.iea.gov)

[3] How much land is needed to grow Miscanthus to sequester half our carbon emissions?
Miscanthus produces up to 14 tonnes/acre biomass. (Biomass energy centre)
Half the mass is carbon so in terms of CO2 we have 14*3.7/2 tonnes/acres = 26 tonnes/acre
We need to store 286 Mt/year so we need 286/26= 11 million acres.
The area of Wales is 5 million acres so we need 11/5 = 2.2 x Wales.