Prof Richard Betts, University of Exeter and Met Office Hadley Centre
On 6th December, the Sunday Times ran an article headlined “Revealed – greenhouse gases to fall“ In my view this is potentially confusing, as it gives the impression that the amount of greenhouse gases in the atmosphere is going to reduce. It isn’t – at least, not for a long time, and only if global emissions of these gases reduce substantially.
The article beneath the headline is more accurate. It covers a new set of scientific paper published this week on the 2015 global carbon budget, which found that global CO2 emissions in 2015 were about the same as in 2014, or possibly slightly less. So, emissions have stalled, or possibly fallen slightly.
This, however, is very different to what the headline seems to suggest. Even if emissions do not rise any further, they are still occurring, which means that the amount of CO2 in the atmosphere is still increasing. CO2 concentrations will still continue to rise.
This figure from the IPCC 5th Assessment Report shows this clearly – the main panel shows scenarios of emissions, and the inset shows what this means for concentrations. In the scenario labelled “RCP2.6”, in dark blue, emissions are assumed to peak around now and then decline in future – but the concentrations still continue to rise, and only stabilise mid-century when emissions become low. Concentrations only actually fall when emissions have become very low.
It’s rather like filling a bathtub. The water flowing from the taps represents CO2 emissions – the amount being released to the atmosphere. The water that is actually in the bath represent the CO2 concentrations – the amount of CO2 that is in the atmosphere. So far, it’s as if we’ve been turning the taps on more and more, letting the water flow into the bath faster and faster. Now, this year, we’ve left the taps alone – we’re not opening them up more, but neither are we shutting them off. The water into the bath is still flowing in, but at the same rate as last year.
This means that the amount of water in the bath is still increasing. If we want to stop the bath from overflowing, we will need to actually turn the taps back the other way. Indeed, sooner or later, the taps need to be turned off completely* to stop the bathwater overflowing.
*Or at least, turned back enough so that the flow is less than the water draining out of the plughole if it is open – the analogy to include carbon sinks, but that’s not the point here.
