It is probably impossible to reverse global warming, say scientists

How much more urgent would stopping global warming be if it was more widely know that temperatures can only stabilise, not decline?

All that Net Zero means is that carbon emitted matches carbon drawn down.¹

In practice, this might mean bringing annual carbon emissions exactly square with maximum annual sequestration rates, in which case we’ll not be adding any carbon to the atmosphere – but concentrations won’t decrease.

So, let’s say we manage to achieve some form of Net Zero where annual emissions are below the maximum annual sequestration rate, freeing up enough draw-down capacity to actually decrease atmospheric concentrations of carbon dioxide.

Would we see falling global average surface temperatures over time?

Warmer oceans take centuries to cool

Whilst declining atmospheric CO2 concentrations change radiative forcing to allow more radiation of heat back into space over time, the warming of the oceans will begin to slow – until, finally, it stops.

However, because of water’s high specific heat capacity, the oceans require far more thermal energy to warm up by a degree than other aspects of the climate. As a result, the oceans tend to lag behind atmospheric temperatures, remaining the coldest part of the climate over time – it’s why we always gravitate to beaches on hot days.

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This lag ensures that the oceans function as a planetary heat sink that cools the faster-warming atmosphere, setting the baseline surface temperature for the planet. Helpful, right?

Well, unfortunately for us, the flip side of this process is that the ocean doesn’t transfer its heat to the atmosphere on any significant scale, which means it doesn’t really cool down – at least, not on human timescales.

Sure, radiative cooling at night will be more significant if there’s a lower concentration of greenhouse gases in the atmosphere that prevents less of the ocean’s heat escaping into space.

But typically, the night’s heat loss simply balances the day’s heat gain – otherwise, if the ocean lost more heat overnight than it gained, we’d be surrounded by ice right now.

So, whilst the ocean will be exposed to far less heat in a future scenario where more heat escapes into space, the fact that the ocean can’t really transfer any of its past warming to the atmosphere means that it will be only this new heat that will be added every day and removed every night, in balance.

Thus, a consensus has formed amongst climatologists that if we ever manage to get our global economy into a position where atmospheric concentrations of carbon dioxide begin to fall, the rate of this fall will match the ocean’s natural delay in warming such that from the moment of this net-negative emissions scenario, global temperatures will flatline – for centuries, even millennia.

One of the first comprehensive studies to model this dynamic states unequivocally:

'Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years.'²

And then, in 2013, the clearest expression of the field’s consensus came when this perspective was put forward in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report:

‘For scenarios driven by CO2 alone, global average temperature is projected to remain approximately constant for many centuries following a complete cessation of emissions.’³

From the horse’s mouth, then.

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¹ The meaning of net zero and how to get it right. Fankhauser et al. Nature Climate Change. 2021.

² Long-Term Climate Change: Projections, Commitments and Irreversibility. Collins et al. Fifth Assessment Report of the Intergovernmental Panel on Climate Change. 2013. p. 1033. 

³ Irreversible climate change due to carbon dioxide emissions. Solomon et al. Environmental Sciences. 2009.