Thread by @ClimateBen

CO₂

1. We're at 417ppm
J. Hansen: that's 3.5°C eventually

2. We face 427ppm by 2025
That's like climates +3M yrs ago

3. Corporate plans: +450ppm by 2036
Many scientists: worst impacts likely

4. Trajectory: 550ppm by 2047
Unsurvivable 5°C sooner or later?

5. We can still act

'Exactly when..feedbacks seriously kick in is the rub..somewhere between 400 and 500 ppm seems most likely

world’s top climate scientists, now believe 450 ppm is the upper bound

450 needs a World War II-scale effort starting in the next decade

From 2008:grist.org/article/partin…

'the eventual warming for 407ppm CO2 will be about 3.5°C'

~James Hansen (who hasn't given up)

3.5°C is horrific, hard to survive. We must suck carbon from the atmosphere, but forests are losing their ability to do this. Industrial methods seem unfeasible.monthlyreview.org/2019/02/01/mr-…

CO2 in Earth’s atmosphere 'approaching a level not seen in 15m years and perhaps never previously experienced by a hominoid

within five years atmospheric CO2 will pass 427 parts per million.. probable peak of the mid-Pliocene warming period 3.3m years ago'dumptheguardian.com/environment/20…

'Many scientists argue.. the CO2 concentration must be stabilized at 450 ppm to avoid the worst impacts of climate change. Some activists argue for a more ambitious goal of 350 ppm'

Yes, because respected scientists say that.

'worst impacts' : euphemism? nationalgeographic.com/pages/article/…

From 2018:

'Levels of CO2.. are expected to reach 550ppm in the next 30 to 80 years'

Emergency system change action needed now to have any chance of decent human survival.

(Ecomodernist-friendly Carbon Brief assumes 550 ppm isn't the end of the world.) carbonbrief.org/rising-co2-lev…

'Not only are the emissions consistent with RCP8.5 in close agreement with historical total cumulative CO2 emissions (within 1%), but RCP8.5 is also the best match out to midcentury under current and stated policies..'

Emissions are rebounding.

pnas.org/content/117/33…

At ~550ppm:

'up to an 18% chance that temperatures will rise to 4.5°C above pre-industrial levels, and a no more than 5% chance that they will go above 5.7°C' theconversation.com/just-how-sensi…

6C: 'most of the planetary surface would be functionally uninhabitable'
edition.cnn.com/2015/05/21/opi…

“Of course, C02 concentrations aren’t stopping today.. We’re probably going to blow through 550 to 600 ppm"

Some argue today’s climate has the highest concentration of ghgs in 20M yrs:

“carbon dioxide equivalent” has some support in the climate community'mashable.com/article/climat…

'if atmospheric concentrations of CO2 are capped at 450 ppm.. but concentrations of CO2-eq turn out to be closer to 550 ppm when non-CO2 forcings are taken into account, the world might end up with a 3°C warming' ⚠️

Worse still, 450ppm may lead to +3.5°C.yaleclimateconnections.org/2009/01/common…

Somewhere around 5-6C looks unsurvivable.

“In the case of global warming in particular, the combination of intolerance to heat combined with co-extinctions mean that 5 to 6 degrees of average warming globally is enough to wipe out most life on the planet.”truthout.org/articles/killi…