Fascinating new paper by @AndrewDessler and colleagues arguing committed warming might be higher than expected given historical pattern effects. Its combining a lot of different concepts together, so lets spend some time disentangling them nature.com/articles/s4155…

A thread: 1/19
The paper's headline number is that we previously thought the world was committed to 1.3C warming, but that number is actually over 2C (> 1.5C by 2100). This is quite a different message than we get from Earth System Models, which suggest committed warming is only ~1.2C. 2/19
This would imply that the 1.5C by 2100 target is effectively impossible, and that long-term warming of >2C would be very difficult to avoid. However, the devil is in the details, and the picture is not quite as dire as it would seem at first glance. 3/19
When we talk about "committed warming" folks are generally talking about one of two scenarios: either constant CO2 and other GHG concentrations (and forcings), or getting all emissions (or just CO2 emissions) down to zero immediately. 4/19
In the first method – constant concentrations – we find that the world warms up another 0.5C or so, as the oceans continue to take up heat as more energy is being trapped by greenhouse gases than is being emitted back to space. Much of this additional warming happens by 2100 5/19
In the second method – zero emissions – atmospheric concentrations of CO2 start to fall, as the ocean and land continue taking up some of the CO2 that humans have previously emitted. 6/19
(short-lived greenhouse gases like methane are also quickly removed from the atmosphere, but so are short-lived aerosols that tend to cool the planet. To a first order approximation these cancel eachother out, though there are some temporal differences). 7/19
Falling atmospheric CO2 causes enough cooling to balance out the warming "in the pipeline" due to slow ocean heat uptake, and global temperatures remain relatively flat after net-zero emissions are reached. 8/19
This flat-temperatures-at-zero-emissions finding is quite robust, first appearing in Matthews and Caldeira in 2008, highlighted in the IPCC SR15, and more recently being found in 18 different Earth System Models in the CMIP6 ZECMIP: bg.copernicus.org/articles/17/29… 9/19
What Dessler and colleagues look at in the new paper is the constant concentration scenario rather than the zero emission scenario, so its hard to directly compare the two. 10/19
Furthermore, their two numbers (1.3C vs 2C+ committed warming) conflate two different factors: higher climate sensitivity and changing warming patterns over time. 11/19
Modern climate models expect more then 1.3C warming at equilibrium in a constant concentration scenario. Rather, at 2.2 w/m^2 constant forcing they would expect around 1.7C warming (assuming a 3C ECS). 12/19
The 1.3C number in the paper is based on an observational-derived ECS of ~2C per doubling CO2, while the > 2C number is based on a revised ECS estimate of ~3.5C plus warming due to pattern effects. 13/19
Pattern effects themselves are a bit complicated. In short, climate models expect both the Western and Eastern Pacific Ocean to have similar rates of long-term warming. However, in the real world the Western Pacific is warming a lot faster than the Eastern. 14/19
Warming in the western Pacific tends to generate a lot more low-altitude clouds that reflect light back to space and cool the surface, while warming in the Eastern Pacific does not. This warming pattern is likely due to natural variability and may not persist in the future. 15/19
If the warming pattern in the Pacific changes to be more similar to that in climate models, Dessler and colleagues argue that it would result in between ~0.3 and ~0.6 w/m^2 additional radiative forcing (or 0.2C to 0.5C more warming, assuming 3C ECS). 16/19
So, in essence, Dessler and colleagues results would suggest that the world could warm 0.2C to 0.5C more under our best estimate of ECS even in a zero emissions scenario due to pattern effect changes. 17/19
However, not all of this warming would happen by 2100 (indeed, we really don't know when the Pacific warming pattern might shift!). The impact on meeting Paris Agreement goals would be smaller, though exactly how much depends on when the warming pattern changes. 18/19
I don't think this paper fundamentally changes our understanding of committed warming, and pattern effects are still an area of active research. But it should make us a bit cautious about being too confident in predictions of zero warming after emissions reach net-zero. 19/19

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More from @hausfath

5 Jan
Lets clarify something about "committed warming". A world where concentrations of CO2 and other GHGs remain constant in the atmosphere is not the same as a world where emissions go to zero. The former has ~0.5C or more warming "in the pipeline", while the latter is closer to 0C.
If concentrations stay constant (e.g. atmospheric CO2 remains at 412 ppm indefinitely), the oceans continue to heat up for the next few millennia. The vast heat capacity of the deep oceans currently buffers warming, as some of the heat diffuses down to the deep ocean.
If emissions actually fall all the way to zero (or net-zero), atmospheric CO2 concentrations start declining. This mostly counteracts the warming in the pipeline as the oceans continue to warm to reach equilibrium. agupubs.onlinelibrary.wiley.com/doi/full/10.10… Image
Read 7 tweets
13 Nov 20
Climate scientist here; stuff like this is dangerously inaccurate.

If we stop emissions tomorrow, the earth will remain around 1.2C above preindustrial temps. If we get to net-zero emissions by 2060 or so, we can still limit warming to well below 2C.

A wide range of studies – including the latest state-of-the-art Earth System Models – all show there isn't much additional warming "in the pipeline" if emissions go to zero. Saying its too late makes folks give up hope; the warming we get is still up to us biogeosciences.net/17/2987/2020/
To elaborate a bit, if we held atmospheric concentrations of CO2 constant, the world would indeed warm another 0.5C or so. But if emissions go to zero, falling levels of atmospheric CO2 counteract additional warming as the ocean equilibrates with the atmosphere.
Read 5 tweets
4 Nov 20
We've done a major update of our @CarbonBrief CMIP6 explainer to include the latest ScenarioMIP results; up to 36 different CMIP6 models included now, with between 28 and 35 models available in each of the "Tier 1" scenarios (2.6, 4.5, 7.0, and 8.5): carbonbrief.org/cmip6-the-next… 1/8
Here are the latest warming projections (late 21st century vs preindustrial) from CMIP6 models. Note that SSP1-1.9 is included as it has 12 models available, but others (3.4, 6.0) are not shown as they still have very few (< 10) models reporting. 2/8
Some notable changes: the SSP1-1.9 scenario which was designed to limit warming to 1.5C actually succeeds in doing that in the multimodel mean (which was not the case when we had fewer model runs in our initial analysis). Similarly, SSP1-2.6 MMM (barely) limits warming to 2C. 3/8
Read 8 tweets
4 Nov 20
Ambitious climate policy is going to be a major challenge in a divided government with a conservative court. But that doesn't mean we can't make real progress over the next four years.

A short thread: 1/7
We can continue to make clean energy cheap. There is a strong bipartisan consensus for things like ARPA-E and more spending on RD&D, which is particularly important outside the power sector where clean energy alternatives are less readily available. 2/7
We can still likely pass some big infrastructure bills, modernizing our power grid, enhancing our resiliency to climate change, and create jobs in the process. 3/7
Read 8 tweets
20 Oct 20
We (@Peters_Glen and I) have a new letter in the Proceedings of the National Academy of Sciences responding to a recent article by Schwalm et al on whether or not the RCP8.5 scenario is appropriate to use for near-term emissions (through 2050): pnas.org/content/early/… 1/15
Their original article suggested that RCP8.5 best matched historical emissions and what they identified as likely future emissions based on the IEA WEO fossil emission data and their own land use assumptions, at least through 2030 (and still reasonably close through 2050): 2/15
We point out that when you only look at fossil emissions (e.g. fossil fuels and industry) this is not the case; IEA projections are much more in line with RCP4.5 or RCP6.0: 3/15
Read 16 tweets
19 Oct 20
Today I published an op-ed in Politico with @atrembath on why @JoeBiden and @KamalaHarris are right to be skeptical of a fracking ban. It risks reviving coal when we need to phase it out ASAP and could perversely slow clean energy if not done carefully politico.com/news/agenda/20… 1/
As an aside, I really wish op-ed departments would stop rewriting headlines to make them more edgy without your permission. The title we submitted "Why Biden and Harris Are Right to Be Skeptical of a Fracking Ban"... 1.5/13
The op-ed is in part a distillation of this exceedingly long twitter thread the other week, so I'd suggest checking that out for details if you haven't seen it yet: 2/13
Read 14 tweets

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