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Fact-Check: is more than 1.5°C of global warming already locked in?

Fact-Check: is more than 1.5°C of global warming already locked in?

Claim: The lag between CO₂ emissions and warming means ~0.7°C of warming is yet to come, and aerosols are masking another ~0.7°C, meaning warming of much more than 1.5°C or even 2°C is already locked in even if we stopped all emissions right now.

Reality: If emissions stopped, falling carbon dioxide levels due to natural carbon sinks would counteract the climate lag. Immediately stopping aerosol emissions would cause a warming boost of ~0.2-0.4°C, but a slower partial phase-out can reduce this and can be more than countered by also reducing methane emissions. This means that the responsibility for future warming lies with current rather than past actions.

This is the sixth post in a new climatetippingoints.info series fact-checking claims that various climate tipping points have been crossed, and that sudden catastrophic warming is now inevitable. See the Introduction post for an overview.

It is sometimes claimed (e.g. 1,2,3,4,5,6) that more than 1.5°C or even 2°C of global warming is already geophysically locked-in even if we immediately stopped all greenhouse gas emissions, and that this has been ignored or covered up by scientists.

This is often based on two assumptions: firstly, that the lag between when emissions happen and when warming catches up means ~0.7°C of warming is in the pipeline already; secondly, that another ~0.6-0.7°C masked by cooling aerosols will emerge as emissions fall.

On top of a high estimate of ~1.3-1.4°C of current warming it’s therefore been stated that ~2.7°C of warming is already locked in (and some claim even more), breaching the Paris Targets already and risking triggering extra catastrophic feedbacks.

It’s also sometimes claimed that the planet is already beyond a specific threshold in CO₂ concentrations (e.g. 450500 ppm CO₂e) beyond which more than 2°C of warming becomes inevitable.

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