What is the chance to stay below 2°C?

Two new papers came out in Nature Climate Change today, both with relevance for chances of stay below 2°C and 1.5°C.

The first paper, Less than 2C warming by 2100 unlikely, finds a “likely range of global temperature increase is 2.0–4.9°C, with median 3.2°C and a 5% (1%) chance that it will be less than 2°C (1.5°C)”. They use a statistical approach, trained on historical data. I find the study misleading.

They argue there “model is not a ‘business as usual’ scenario, but rather is based on data which already show the effect of emission mitigation policies”. This is perhaps semantics. Their conclusion states “Our forecasting model does not explicitly incorporate future legislation that could change future emissions. It is based on past emissions, which implicitly account for accumulating legislation and regulation over the past 30 years since climate change became a global issue, and indeed carbon intensity has been improving steadily over that period”.

I would think of this as more like a business as usual, or certainly, continuation of past trends. It seems rather consistent with studies that look at the temperature increase of the INDCs. I did not find the validation that convincing either. Figure 4 shows that the method performed very poorly in the US and EU, two key countries moving forward.

Though, I do agree with their conclusion, but for very different reasons.

The second paper, Committed warming inferred from observations, finds “a committed warming of 1.5°C (0.9–3.6, 5th-95th percentile) at equilibrium, and of 1.3°C (0.9–2.3) within this century” if atmospheric concentrations are kept constant at current levels. They use an observational approach, which seems broadly consistent with the numerous modelling studies of committed warming.

If emissions are put to zero instead, the “committed warming is reduced to 1.1°C (0.7–1.8)”. Their methodology for zeroed emissions is rather crude, and other studies may be better for that type of analysis.

The study basically means that if we stop emitting today, we already have at least 1.1°C backed into the system in 2100. In the short term, this is even higher because of the current cooling effect of aerosols like sulphur dioxide.

The way I think of this, is that we are really starting at at least 1.1°C today. Any new emissions simply add to the warming we get at the end of the century. If you also include the committed warming from existing infrastructure (and update to 2014 values), we will fly over 2°C. This is a key reason why I find keeping below 2°C a challenge (the problem is not building wind turbines and solar panels, but closing existing infrastructure). It is also a key reason we need carbon dioxide removal, despite the challenges.

Chris Mooney has a good article over at The Washington Post

A modified version was originally published at LinkedIn on August 1, 2017.