We have a big challenge ahead of us if we are to hold global warming to under 2°C. This is largely because we have already used up most of our collective 'carbon budget'.
The amount that global temperatures rise under global warming is very closely related to how much carbon dioxide (CO2) and other greenhouse gases we emit to the atmosphere, and this defines our 'budget', or the limit of much we can emit.
In the animation above, our CO2 emissions are shown in red, while the accumulated total emissions since 1850 are shown in grey. While reducing our annual emissions is critical, it is insufficient: we must in fact reduce annual emissions to zero so that the accumulated emissions stop increasing.
Glass filling fast
You might think of annual emissions as drops of water (or your favourite liquid fossil fuel) falling into a glass, and accumulated emissions as the amount that's in the glass. What we've been doing for most of history is increasing the rate at which those drops fall into the glass, filling it up faster: accelerating.
If we simply stop accelerating, and add drops at a steady rate, then we're still going to fill up that glass. We actually have to stop adding water for the water level to stabilise.
Likewise, stabilising global emissions doesn't solve the climate problem. We actually have to stop emitting.
Zero emissions in 2069
In the future I have represented above – one that might hold temperature rise below 2°C – our annual emissions drop to zero in the year 2069, and at that point we finally see a peak in the accumulated emissions, which thereafter decline as we actually remove more CO2 from the atmosphere than we add to it.
But of course our annual emissions don't drop to zero in this future: they drop substantially, but not to zero, and the remaining, intransigent emissions must be offset by so-called 'negative' emissions. This recognises the likelihood that there will be many sources of emissions that we will not be able to turn off before the end of the century, especially in developing countries and in agriculture.
Because of these residual emissions, we will need to introduce substantial negative emissions: removing CO2 from the atmosphere.
When we reach net-zero in this scenario, there will be emissions of CO2 in some parts of the world and removals of CO2 in others, and at some point the two will balance out so that our 'net' emissions across the globe are zero.
At that point of balance between positive and negative emissions, the accumulated emissions will stop rising. If we continue to mitigate positive emissions and to increase negative emissions then we will turn the accumulation curve around and go into reverse: we may actually start to reduce global warming rather than 'merely' putting a stop to its rise.
Why would we want to go into reverse? One reason is that we may not be able to reduce our emissions fast enough, and we may overshoot our target of 2°C, in which case we have to back up.
Another reason is that we may decide that 2°C, while a useful target through the 21st century, isn't necessary where we want to remain, and that less warming is better.