2. The Earth is out of thermal equilibrium, with the excess energy taken up by the ocean. As equilibrium is reached, the surface temperature rises (red).
If emissions go to zero, the CO₂ concentration declines, leading to a smaller energy imbalance & less warming (blue).
3. Despite the non-linearities, these two factors almost perfectly cancel!
In the very long run – over many hundreds to thousands of years – carbon sinks become dominant & global temperatures would eventually fall – as long as anthropogenic CO₂ emissions remained at zero.
4. The behaviour is not perfectly symmetric in all models.
The amount of ocean heating & carbon uptake varies across model, but the key processes are the same across models.
Some models will warm slightly & others cool slightly at zero emissions.
5. The world emits more than just CO₂, and the response to the system differs if CO₂ is zeroed out, versus all GHGs, versus any other combination.
Different species have different lifetimes & different radiative efficiencies, so the temperature response at zero differs.
6. Putting air pollutants to zero could warm ~0.4°C, while putting all GHGs to zero could cool ~0.5°C!
On balance, zero air pollution & GHG emissions leads small changes in temperature.
All components are coupled via technology, not possible to zero one without the other.
7. This is an important issue, and one that is often misunderstood. It is also the background to why we talk about 'carbon budgets' & zero emissions, as CO₂ is a cumulative pollutant.
Cumulative CO₂ emissions explain most future global warming, assuming sufficient action on non-CO₂.
Distributing a remaining carbon budget of 500GtCO₂ with a linear decline (black) leads to net-zero ~2045.
Depending on short-term action, many net-zero years are possible.
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If emissions decline exponentially, also in the 'Raupach curve', net-zero emissions never occur & the remaining carbon budget is never exceeded. Net-zero is not a necessity, but a modelling outcome.
Most scenarios are based on cost-optimisation to a 2100 target, which means the temperature can peak & decline, overshooting the 2100 target before returning to it by 2100.
These scenarios are where the net-zero ~2050 come from (2059 in this figure).
"Human-caused climate warming stops when humans stop adding CO₂ to the atmosphere, & emissions of other greenhouse gases are declining sufficiently" (text from @KA_Nicholas)
"CO₂-induced global warming stops when anthropogenic CO₂ emissions balance with anthopogenic CO₂ removals"
is a long version of
"CO₂-induced global warming stops with (net-)zero CO₂ emissions"
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In short-form: "...emissions balance with removals"
In long-form: "anthropogenic emissions from sources balance with anthropogenic removals from sinks"
I have basically used UNFCCC language, not IPCC language. These policy makers had it right all along!
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Twitter was very divided on zero versus net-zero. Though, there are many reasons. Clearly, science & policy are getting blurred here. Many say "zero" because "net" means continued use of fossil fuels.
I suspect technically, "net" is more correct, but not sure 100% correct.
Great article by @JamesGDyke et al on the lack of climate action over the last 30 years.
I don’t see the article so much as a critique of "net-zero", more an elegant critique of lack of action. The title does not represent the article (IMHO).
“With hopes for a solution to the climate crisis fading again, another magic bullet was required”
The list of bullets:
* Afforestation
* CCS
* BECCS
* Other CDR (eg, DACCS, EW, ...)
* Overshoot scenarios
* Geoengineering
Always a technofix to keep it 5 minutes to midnight.
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"We struggle to name any climate scientist who at that time thought the Paris Agreement was feasible" [some exceptions]
"The price to pay for our cowardice: having to keep our mouths shut about the ever growing absurdity of the required planetary-scale carbon dioxide removal"
Global fossil energy CO₂ emissions:
2020: ⬇️ 5.8%, or ~2GtCO₂
2021: ⬆️ 4.8%, or ~1.5GtCO₂
Everyone wanted back to normal🤔
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My guestimate was 3.5% as of April 2021, so quite some lower.
My method is very aggregated, based on GDP & historical trends in CO₂/GDP. I would trust the IEA much more with disaggregated approaches... Or?
2/
Projections change over time. This is the projection I made in January, it was for 3% growth (not 3.5%). The difference? @IMFNews increased their projected GDP growth.
The study essentially argued that based on the AR5 carbon budgets, viewed from 2014, there was seven years (2021) until the 1.5°C carbon budget was used, & therefore 1.5°C was essentially a "geophysical impossibility".
They had a new method with a more realistic budget.
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The @CarbonBrief post was titled: "Why the 1.5°C warming limit is not yet a geophysical impossibility"
The authors wrote: "[A]lthough 1.5°C is not yet a geophysical impossibility, it remains a very difficult policy challenge."