Thread by @Peters_Glen

THREAD on historical trends & emission scenarios

"Current emissions are tracking a 5°C scenario" & "Current policies are taking the world towards ~3°C of warming by 2100" are very different things & they change over time.

Let’s work through these…

nature.com/articles/d4158…

1/

Back in 2012 we wrote “The latest CO₂ emissions continue to track the high end of emission scenarios”.

This is often used as justification for continued use of RCP8.5. That extrapolation is too simplistic & more thought is needed.

Figure from 2012: rdcu.be/brDGx

2/

First, it is possible to update the figure!

Across a range of scenarios, starting in 1990, the picture is more nuanced today, different with & without LUC.

I would say "current emissions track the mid- to high-end of a range of long-term scenarios”.

But, interpret yourself.
3/

Comparing fossil emissions only or including land-use change emissions is a little tricky. Land-use emissions are quite uncertain, often updated, but 10% of the total (decilning).

A comparison with fossil emissions is more robust, but with LUC is preferable. Compare both!

4/

Second, it is possible to include the latest set of scenarios, the SSPs, but the comparison becomes moot.

Basically, current trends will almost always be in the middle of the latest scenarios, almost by definition, as scenarios are updated with current trends…

5/

Third, in another paper we discussed that you should track over a range of elements in the energy system, not just aggregated emissions. rdcu.be/bPgJr. This was based on AR5 scenarios, and I have not compiled coal, oil, gas etc from historical scenarios (one day…).
6/

In summary, I would say current emissions are tracking mid- to high-end scenarios. But, does that mean therefore we end up at 4-5°C? No.

RCP85 emissions require huge coal use. It is possible to track RCP85 now, but deviate substantially in 2040.

7/

You shouldn’t extrapolate from "current emissions tracking X" to "therefore current emissions will always track X".

Even in a baseline no climate policy scenarios, RCP85 is one of many baselines, & RCP85 is the highest in many elements of the energy system (by design).

8/

There are some studies that try to estimate where emissions are actually going, based on policy, technology, & society.

Most point in the direction of 3°C by 2100 (yes, there is uncertainty, yes, temperatures will rise after 2100 unless emissions are zero).

9/

The UNEP Emissions Gap Report tracks no policy, current policy, and emission pledges, from a range of studies.

In the 2019 report, they say NDCs are consistent with 3.2°C for a 66% chance (50% would be more like 2.9-3.0°C).

10/

Rogelj et al (2016) give a range of numbers around 3°C, for the median, but only show climate uncertainties on the high side (not sure why).

The temperature for emission uncertainty is shown.

nature.com/articles/natur…

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The IEA scenarios forward give less than 3°C, % the IEA is often critiqued for being overly conservative on its baseline scenarios.

thebreakthrough.org/issues/energy/…

12/

The climate action tracker gives around 3°C for current policies and 2.8°C for the emission pledges.

climateactiontracker.org

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One could go on…

There are a range of studies which suggest current policies are in the range of 3°C (with uncertainty), NDCs go a little lower.

3°C is far from 1.5°C & <2°C, so it should be completely obvious we need to do far, far more than we are currently doing.

11/

Overall, current emissions are tracking the mid- to high-end of a range of historical scenarios, but based on the underlying energy system dynamics & studies of current policies, the world is most likely heading to ~3°C in 2100 (median).

nature.com/articles/d4158…

12/

Of course, we published a comment to stimulate discussion.

You can disagree, put forward other studies or interpretations. You can also dig into the scenario data (most freely accessible) & literature. Most that do that actually come back agreeing...

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