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Dr. Zeke Hausfather Profile picture
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3 Dec, 4 tweets, 2 min read
This sort of language is deeply problematic. A flatting of emissions does not result global warming stopping.

Rather, if emissions are flat global warming continues at its current rate, resulting in a significant rise in global temperature of around 3C by 2100 vs preindustrial.
The only way to stop the world from warming is to get all emissions to (net) zero. The only way to meaningfully cool the planet back down during the next few centuries is to remove more carbon from the atmosphere than we add to it. Image
So while its good news that high-end scenarios where global emissions double or triple are much less likely in a world of falling clean energy prices and a global coal industry in structural decline, emissions still need to fall dramatically to avoid significant future warming.
And, of course, climate system uncertainties are such that a current policy world of < 3C warming does not preclude outcomes of 4C if we end up with high sensitivity or carbon cycle feedbacks. Greater emissions reductions are needed to preclude these risks thebreakthrough.org/issues/energy/… Image

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Dr. Zeke Hausfather Profile picture
29 Nov
The recent IPCC AR6 updated remaining carbon budgets for 1.5C and 2C scenarios.

These budgets let us make simple estimates of when emissions need to reach zero if we allow no net-negative emissions.

1.5C (66%): 2039
1.5C (50%): 2044
2C (66%): 2077
2C (50%): 2088

A thread: 1/8
These are subject to a number of assumptions (and uncertainties), of course. Allowing net-negative emissions expands remaining budgets, while more convex (or concave) emissions pathways would change the date at which zero emissions needs to be reached: 2/
Assumptions around non-CO2 GHG emissions and aerosols also matter. The IPCC provides a best estimate (and uncertainties), but more pessimistic or optimistic assumptions for non-CO2 forcings would reduce or expand the remaining carbon budgets accordingly. 3/
Read 9 tweets
Dr. Zeke Hausfather Profile picture
28 Nov
This is thankfully not true. As we reported in the recent IPCC 6th Assessment Report, scenarios that limit us to around 1.8C (with a 66% chance of avoiding 2C) require getting to net-zero emissions by around 2070, not 2030 as this tweet and article inaccurately imply.
Climate change is a big enough problem to solve that we really don't need disempowering hyperbole like this. Yes, the window to limit warming to 1.5C is rapidly closing, but at the same time below 2C is increasingly within reach.
For reference, here is the SSP1-2.6 scenario that limits warming to ~1.8C by 2100. If you were to exclude the net-negative portion of emissions you would end up closer to 2C by 2100 (or would have to get to net-zero closer to 2060). Either way its a far cry from 2030!
Read 5 tweets
Dr. Zeke Hausfather Profile picture
27 Nov
When it comes to zero emissions, both journey and destination matter.

Warming depends on cumulative emissions, and delaying emissions reductions could result in ~0.15C more warming compared to a linear decline to zero by 2050. Image
If we look at what might be a more plausible pathway given commitments by governments today – zero emissions by 2070 – a slower reduction pathway would result in ~0.25C more warming. Image
These are based on the latest median TCRE value (1.65C per 1000 GtC) from the IPCC AR6. They assume that other forcings (e.g. non-CO2 GHGs and aerosols) remain constant. In more detailed scenarios we generally end up with ~0.05C-0.1C warming in all cases due to non-CO2 forcings.
Read 5 tweets
Dr. Zeke Hausfather Profile picture
15 Nov
A good summary of views of scientists coming out of COP26. Some like @ClimateOpp are ready to declare the 1.5C dead, while others hold out a bit more hope based on enhanced long-term net-zero promises that countries have made. apnews.com/article/climat…
As I told @borenbears, where we are headed coming out of COP26 based on more solid near-term commitments is probably ~2.3C (1.8C - 2.9C) – and that is assuming all 2030 NDCs are reached. We can – and hopefully will – do better as countries ratchet up commitments in coming years. Image
There is a lot of nuance in which outcomes (current policy, near-term commitments, or long term promises) folks decide to emphasize. Here is the full remarks I sent the AP that tries to sort it all out: Image
Read 9 tweets
Dr. Zeke Hausfather Profile picture
13 Nov
As COP26 comes to a close, its clear it will not put us on a path to 1.5C by itself. It does not ensure we remain below 2C, given gap between long-term ambition and near-term 2030 commitments.

But it does move the needle forward, and tee up another round of stronger commitments.
If folks were hoping for a dramatic breakthrough, this is not it. But nevertheless it is slow and steady progress towards a lower warming future, even if pace of action means that we may not avoid as much warming as we'd like.

At the end of the day every 0.1C still matters.
It also makes real progress on a lot of thorny issues that have bedeviled past negotiations, even though many outstanding issues still remain:
Read 6 tweets
Dr. Zeke Hausfather Profile picture
12 Nov
COP26 will not – by itself – put the world on track to meet Paris Agreement goals. But it does meaningfully move the needle. Commitments in the lead-up to COP26 likely reduced global temperatures by 0.2C and commitments during the conference by another ~0.1C.
Every 0.1C matters!
While long-term net-zero promises are encouraging, talk is cheap, and pledges around outcomes 30 to 50 years in the future are only meaningful if reflected in near-term commitments. The task as COP26 draws to a close is to ratchet these up to put us on a path to net-zero.
For more details, see the piece by @piersforster and I earlier this week over at @CarbonBrief on climate outcomes of COP26: carbonbrief.org/analysis-do-co…
Read 4 tweets

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