Thermal inertia: how oceans continue global warming after Net Zero

Would global average surface temperatures stabilise if every country on Earth achieved net-zero greenhouse gas emissions? On the contrary, scientists say they could keep rising – but why?

Let’s imagine that we could snap our fingers and tomorrow, our global economy stops net-emitting greenhouse gases (GHGs). 

In practice, this would mean holding atmospheric GHG concentrations (relatively) steady over time, *not* reducing these concentrations. What effect will this have on that all-important number, global average surface temperatures?

Well, researchers have been estimating what our ultimate temperature would be and how long it might take until it’s reached. One possibility: we could continue to warm – for years.  

Many readers may be unaware of this possibility and even fewer will be aware of exactly why this is the case. Short answer: the oceans. Long answer …

Oceans warm more slowly than other parts of the climate

Firstly, most of us still have a fairly reductive understanding of what climatologists call radiative forcing, or how much solar heat is trapped within our atmosphere.

In most of our heads, there’s an image of a big fat ray of sun shining onto the Earth, with a slightly slimmer ray being reflected back into space. Whilst this is correct, such a picture can give the impression that everything happens pretty much instantly – and maybe it would, if it weren’t for the oceans.

You’re probably aware that the oceans are made of water, but what you might not know is that water absorbs more thermal energy per degree of temperature increase (high specific heat capacity) than many other substances.

In practice, what this means is that water takes longer to heat up to a certain temperature when exposed to the same amount of heat. As long as you keep applying the heat, it will eventually reach the same temperature as anything else.

For example, think about heating a saucepan of water on your hob. Within seconds, you won’t be able to touch the metal bottom of the saucepan – but if you add water, you can probably keep your finger dipped into the water for over a minute. This is water’s **thermal inertia**.

As a result of the oceans’ slow warming, their temperatures tend to lag behind that of the atmosphere, which causes heat to transfer from the latter (cooling it down) into the former (warming them up).

Thus, because of their thermal inertia, the oceans function as a planetary heat sink. By some estimates, ‘the ocean absorbs more than 90% of the excess energy stored by the Earth system’.¹

 In this way, the average temperature of the oceans anchors global temperatures; it is indicative of – though not identical to – our climate’s thermal equilibrium temperature.

Here's the kicker. As greenhouse gas emissions have altered our climate’s radiative forcing, more heat is being prevented from radiating out into space and most of it is being absorbed by our oceans. If we steadied our atmospheric concentrations of greenhouse gases for a number of years (which would require an immediate global transition to near-net-zero emissions), our oceans would still be getting all of this new heat and would continue their slow climb through the degrees.

So, until our global economy becomes carbon-negative (or at least, robustly carbon-neutral to allow the carbon cycle to start whittling down concentrations), temperatures will continue to rise. As one study has it:

‘Remarkably, the simulation stabilized at a greenhouse gas (GHG) level close to the present day (2025) exceeds the Paris Agreement goals of 1.5 and 2° warming above pre-industrial in the long term, and only the 1990 simulation leads to a stabilized climate below 1.5° warming.’²

In short, we’ve turned up the heat on our saucepan filled with water – but we’re still waiting for it to boil.

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¹ Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry. Marti et al. Earth System Science Data. 2022.

² Multi-centennial evolution of the climate response and deep-ocean heat uptake in a set of abrupt stabilization scenarios with EC-Earth3. Fabiano et al. Earth Systems Dynamics. 2024.