Prof Richard Betts Profile picture
Jan 12, 2022 9 tweets 5 min read Read on X
What will the iconic @Keeling_curve graph of atmospheric CO2 concentrations need to look like in future if we're to limit global warming to 1.5°C?

The build-up of CO2 in the air has been accelerating

This build-up needs to slow within a few years, then stop & reverse

Thread⤵️
Our extended version of the Keeling Curve is based on the SSP1-1.9 scenario

With this scenario (light blue) the IPCC central estimate has warming reaching 1.5°C on average over 2021-2040, overshooting but returning below 1.5°C by the end of the century

ipcc.ch/report/ar6/wg1… Image
Our @metoffice forecast of CO2 levels at Mauna Loa suggest that the rise from 2021 to 2022 will be 2.14 ± 0.52 ppm

This is slower than previous years, but not because human emissions are falling - it's due to stronger uptake of carbon by vegetation

metoffice.gov.uk/research/clima… Image
The size of year-by-year increases in CO2 concentration (annual increments) varies due to changes in natural carbon sinks impacted by climate variability

CO2 rise:

faster in El Niño years (hotter, drier - less carbon uptake)

slower with La Niña (cooler, wetter -> more uptake) Image
We're currently in a La Niña, so expect a smaller CO2 rise compared to most recent years (2.1 ppm vs average of 2.45 ppm / year over the last ten years)

Nevertheless, we expect the rise from 2021 to be faster than previous La Niña years because human emissions have increased Image
The CO2 rise of 2.1 ppm expected this year is close to the rate which the 1.5°C scenario (SSP1-1.9) shows to for the next 5 years

SSP1-1.9 then has the CO2 rise slowing to 1 ppm / year around 2030, reaching zero in the early 2040s

After that, the scenario has a fall in CO2 Image
But crucially, the expected slower rise this year is only happening because of the free bonus of stronger carbon sinks

Without La Niña the CO2 rise would be at higher levels

For CO2 concentrations to track the 1.5°C scenario, global emissions would need to start to falling now Image
Another important point about this year's CO2 forecast:

In April / May, monthly average CO2 concentrations at Mauna Loa will exceed 420 ppm for the first time

This will be the highest level of CO2 in the atmosphere for over 2 million years

ipcc.ch/report/ar6/wg1… Image
Incidentally, that splendid brainbox @chrisd_jones wrote this excellent paper on our CO2 forecasting for the scientific journal for young people Frontiers for Young Minds @FrontYoungMinds - worth a read!

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More from @richardabetts

Apr 17, 2023
@PatrickTBrown31 Hi Patrick, those IPCC WG2 figures of mine show the range of outcomes, but do not show the extent of model agreement on the sign of change. Working Group 1 Ch 11 Fig 11.1 shows that even at +2°C, there's ≥80% agreement on increased extreme precip over most land areas in CMIP6 Image
@PatrickTBrown31 Link to WG1 chapter 11 "Weather and Climate Extreme Events in a Changing Climate" ipcc.ch/report/ar6/wg1…
@PatrickTBrown31 Importantly, if your aim is to assess the level of consensus on the anthropogenic signal, it would be more relevant to look at higher levels of global warming not just 2°C where internal variability can offset the anthropogenic signal. Here's the full version of my IPCC figure Image
Read 12 tweets
May 4, 2022
Excellent article by @hausfath et al

When are fastest-warming climate models useful?

1. risk assessment
2. for Global Warming Levels if timing not important

When not? If you're interested in the more likely rates of warming

Thread on this and GWLs ⤵️

nature.com/articles/d4158…
IPCC no longer considers all models as equal

In AR6, other lines of evidence were used to assess likelihoods of different rates of future warming for each scenario of greenhouse gas increase

While many models project likely warming rates, several warm at unlikely fast rates

2/
So when using the model outputs, it's crucial to consider the question you're addressing

If you're interested in most likely outcomes, don't use the full set of CMIP6 models, or the average, as this is biased hot

But there are cases when the fast-warming models are useful

3/
Read 10 tweets
Dec 13, 2020
@flimsin @Peters_Glen @ClimateAdam @MichaelEMann @FrediOtto Yes, I was going to mention that more generally there is a misunderstanding that the political goals for limiting warming to round(fish) numbers (1.5C, 2C) represent some sort of physical threshold, beyond which feedbacks suddenly kick in or all is lost in some other way >
@flimsin @Peters_Glen @ClimateAdam @MichaelEMann @FrediOtto And as a variant on that, some folk seem to think that there is some sort of physical significance to warming of 1.5C, 2C at smaller scales, eg. individual countries. These numbers were established as targets to keep *global* warming below (+2C) or to try to limit to (+1.5C)
@flimsin @Peters_Glen @ClimateAdam @MichaelEMann @FrediOtto haha just noticed my typo after "round"! 🐟🤣
Read 4 tweets
Sep 6, 2020
@clivehbest @HG54 @Leigh_Phillips @CC_NoThanks @XRebellionUK @ziontree @afneil @ScientistsX 4C global warming by 2100 seems to be much more likely than a lot of people realise

It doesn't even need one of the highest emissions scenarios. RCP6.0 (considered likely with current policies) gives that much warming fairly near the middle of the range in our latest projections
@clivehbest @HG54 @Leigh_Phillips @CC_NoThanks @XRebellionUK @ziontree @afneil @ScientistsX For RCP6.0 and current policies, see this from the Hausfather & Peters Nature comment

They label it 3C but that's the central estimate, & I don't think it accounts for uncertainties in carbon cycle feedbacks like the UKCP18 probabilistic projections do

nature.com/articles/d4158…
@clivehbest @HG54 @Leigh_Phillips @CC_NoThanks @XRebellionUK @ziontree @afneil @ScientistsX Hausfather and Ritchie made their own "current policies" estimate of global warming by 2100 based on extending the International Energy Agency scenarios with different assumptions

Again the central estimate was +3C, but +4C by 2100 was within the range

thebreakthrough.org/issues/energy/…
Read 5 tweets
Sep 5, 2020
Are we on track to cause mass extinction of other species?

Yes

Huge numbers of species to be lost if we don't act now to change course

This includes improving conservation efforts and stopping global heating

Short thread⤵️ prompted by this new study

bbc.co.uk/news/science-e…
Here's this week's paper by Andermann et al on human impacts on mammal extinctions

"Based on current trends, we predict for the near future a rate escalation of unprecedented magnitude"

"still a window of opportunity to prevent many species extinctions"

advances.sciencemag.org/content/6/36/e…
They focus on the need for improved conservation efforts, which is right, especially for mammals

Habitat loss and degradation due to direct human impact are a huge driver of extinction, currently more than climate change

But climate change adds a further massive risk in future
Read 7 tweets
Aug 29, 2020
@PlanB_earth @IlanKelman @thetimes @bwebster135 I read the emails

Yes global warming of 4˚C this century is quite possible, & would bring massive risks to life & society (heatwaves, coastal & river flooding, drought etc)

Ilan does not seem to dispute that - he just says (correctly) that societal outcomes can't be predicted >
@PlanB_earth @IlanKelman @thetimes @bwebster135 Ilan's right that the DARA claim of 400,000 deaths per year due to climate change can't be verified - they don't give the source. It's possible that they have mis-typed a reference as there's a similar one in their bibliography, but even that doesn't seem to support the number >
@PlanB_earth @IlanKelman @thetimes @bwebster135 This is not to say that the number is less. It might be more. We just don't know, and IMO quoting numbers that can't be substantiated is just not useful because it undermines the concept of rigorous analysis (and this number is being quoted as if it's somehow authoritative) >
Read 7 tweets

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