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.
I'd be remiss if I didn't note that I'm somewhat stealing this example from @KetanJ0, who calculated cumulate emissions (but not temperature response) in similar scenarios:
(and yes, the language of journey and destination in the first tweet is riffing off @BrandSanderson's Stormlight Archive books)

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More from @hausfath

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! Image
Read 5 tweets
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
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
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
10 Nov
COP26 has been full of future warming numbers – 2.7C, 2.4C, 1.8C, etc – with new estimates released daily.

In a new @CarbonBrief analysis, @piersforster and I take a deep dive into climate outcomes – what they mean and how seriously we should take them carbonbrief.org/analysis-do-co… 1/
Future climate projections coming out of COP26 broadly consider three different scenarios: current policies, 2030 commitments (e.g. both unconditional and conditional NDCs), and longer-term net-zero promises. 2/
Four different groups have produced updated estimates of climate outcomes across these different scenarios: the @UNEP, @climateactiontr (CAT), @IEA, and the relatively new @ClimateRsrc (CR) team. 3/
Read 21 tweets
5 Nov
There is a pretty strong claim here that anything without a 50/50 chance of avoiding 1.5C is "not compatible with the Paris Agreement". Well-below 2C was the main Paris goal with an aspirational goal of limiting warming to 1.5C, but this seems to conflate the two a bit.
Here is the actual language of the Paris Agreement: "Holding the increase in the global average temperature to well below 2C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5C above pre-industrial levels"
Of course, "well-below 2C" is not particularly precise. While some have defined it in the context of a 2/3 chance of avoiding 2C warming (e.g. in RCP2.6 or SSP1-2.6 scenarios), others have defined it more ambitiously (e.g. 90% chance of < 2C, which is effectively a 1.5C scenario)
Read 6 tweets

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