If we can get emissions down to zero (or net-zero), the planet will likely stop warming. Good @guardian piece covering this issue – which is well understood by the scientific community but often missed in public discussions. theguardian.com/environment/20… 1/6
This is good news, because it means that warming that occurs this century is almost entirely under our control. We can decided how much CO2 and other greenhouse gases we emit, and the climate will respond accordingly. 2/6
However, the downside of this finding is that even if we get emissions all the way down to zero, temperatures will not fall, at least for the next few centuries. Without net-negative emissions climate change is largely irreversible. 3/6
Part of the public confusion around committed warming (or warming "in the pipeline") comes from people conflating scenarios with constant concentrations and those with zero emissions: 4/6
Theres some nuance and uncertainty here, as with all earth system science; for example, if current temperatures are lower due to natural variability (e.g. pattern effects in the Pacific), there could be some additional warming if or when that reverses: 5/6
But our models are pretty consistent in showing that, once emissions get to zero or net-zero, temperatures remain flat for the next few centuries (+/- a few tenths of a degree): biogeosciences.net/17/2987/2020/ 6/6

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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
4 Jan
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
Read 19 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

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