Revisiting the climate budget maths

As COP27 gets underway this week, world leaders will be descending on Egypt with a further round of speeches, promises and pledges to keep 1.5°C alive as a goal to strive for. But as each day passes and global emissions continue at around the rate of 40 gigatonne per year (Gt/y), the simple maths behind the carbon budget gets more and more difficult.

In the IPCC 6th Assessment Report WGI summary the climate scientist authors developed Table SPM.2, shown below, which detailed the remaining carbon budget from 1st January 2020 for a given eventual global warming, relative to 1850-1900. In the case of 1.5°C this is 500 Gt. The permitted budget changes depending on the level of warming and the likelihood of the outcome. So a 2°C outcome with a 67% likelihood offers a budget of 1150 Gt, or over twice that for 1.5°C at 50% likelihood.

Source: IPCC 6th Assessment Report WGI Summary for Policy Makers

While a number measured in hundreds of billions of tonnes (or half a trillion in the case of 1.5°C with 50% likelihood) may seem very large, when set against current annual global emissions of over 40 Gt/y, it is around a dozen years. This means we are knocking on the door of 1.5°C right now. In fact, the 1.5°C carbon budget is being consumed by society at such a pace, that just during the time COP 27 is held, another 1 Gt of the 500 Gt will have been used.

Over time, the consumption of the carbon budget is measured in terms of cumulative emissions, or in a chart of annual emissions vs. time in years it is the area under the line. This then gives us a simple way of looking at the carbon budget and establishing what different trajectories might mean in terms of outcome.

The chart above starts at 2020 and goes through to 2070. In each of 2020, 2021 and 2022 the emissions are either known or almost known, so they are represented in grey as budget consumed. The total is about 120 Gt, with 2020 being the lowest due to the sharp COVID related downturn in March, April and May of that year. With the simplified assumption that emissions won’t be negative at some future time (but of course they may be and undoubtedly will need to be), the linear trajectory for 1.5°C now requires a step down in emissions in 2023 to 40 Gt, then a rapid reduction to 20 Gt in 2030 followed by net-zero emissions in 2047. By contrast, a direct linear reduction from current levels to net-zero emissions in 2050 means a 1.6°C outcome.

Three other combinations are also shown:

  • A plateau in emissions to 2030 then a quick linear descent to net-zero in 2050 results in 1.7°C of warming.
  • A further rise in emissions to 2030, then a fall to net-zero in 2060 gives 1.9°C of warming.
  • If a further rise is followed by net-zero emissions in 2070, then we might expect 2°C of warming.

Given the acceleration of the energy transition in recent years and the new pressures now being placed on fossil fuel use through both price and supply concerns, there are reasons to believe that the transition could accelerate further. It will certainly need to. In the last 15 years the share of coal, oil and natural gas in the primary energy mix has dropped by just 2 percentage points, a trend that would require 600 years to get to zero. By contrast, a 1.6°C outcome requires that rate of change to rise by a factor of 20 to nearly 3 percentage points per year. This is the challenge that leaders at COP 27 need to focus on.