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
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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
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)
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
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
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
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
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!
@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
@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
@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)
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
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
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) >