The ‘Earth’s Energy Imbalance’ (EEI) is a disturbing sounding term. What does it mean to be out of balance? The EEI is really the net energy flux at the top of the atmosphere, and the ‘imbalance’ means it is not zero. If it is positive, the Earth is gaining energy, and the earth is warming.
An equation is worth a thousand words, and here is an energy balance of the Earth,
EEI = dQ/dt = F – T / λ
where all terms are assumed functions of time. This is a box model of the conservation of energy, where we consider the flow into the box (F), the flow out of the box (T/λ), and the change in energy in the box. The Earth is the box.
The Forcing, or its change since 1850 or so, is really inside the box. The atmospheric composition of the Earth’s atmosphere has changed due to greenhouse gas (GHG) and other emissions, and this means that a greater share of the incoming solar radiation stays in the box. The radiation back to space (T/λ) is a good term, as it gets energy out of the box, at least, it is good if λ is small.
We have estimates of all the terms in this equation, and we even have three estimates of the EEI. The EEI can be measured by satellite (CERES). It is also possible to estimate the energy in the system (Q), and how much it is changing (dQ/dt), see Section 6 of Forster et al (2023). We also have estimates of F and T, also see Forster et al (2023). Let’s plot all these terms on one figure!
The reason I wanted to make this figure is to see that everything agrees, given uncertainties. The thing with the term ‘imbalance’ is that it sounds like things are not behaving as they should. But they are. The ‘imbalance’ really means the system is out of equilibrium. We are adding more energy to the system by changing the atmospheric composition (F) then we can get out of the system via climate feedbacks (T/λ).
What does the imbalance mean about future warming?
We really care about the future? If we have an energy imbalance, we are not in equilibrium, what does it take to get to equilibrium. We often here about “unrealised warming”, “committed warming”, “warming in the pipeline”, or variations thereof. What do these terms have to do with the EEI?
Recall our equation:
EEI = dQ/dt = F – T / λ
This equation has one variable that we can influence, the forcing, F. Future warming is as much about assumptions on F as it is about the size of the EEI. Let us look at a few possible assumptions on F, all plotted below.
F=0. We could assume that F=0 from tomorrow onwards. This is a useful diagnostic, but it is rather meaningless as it is impossible to instantly take the atmosphere back to its preindustrial state. Hypothetically, if we did, the temperature would decline. It would decline rather quickly even, and we would get close to 0°C in the matter of decades, with the remaining temperature decaying over 100s of years. Perhaps zero forcing is a better definition of ‘unrealised warming’?
F constant. An alternative is to assume that the forcing is constant, by assuming the atmospheric composition does not change. This is sometimes called “committed warming”. It used to be a common diagnostic tool but has become rather uncool of late. Holding the forcing constant, is mathematically intuitive, but also rather obscure in reality. To hold the forcing constant requires emissions to continue in a rather artificial way, CO2 would decline but remain non-zero to keep forcing constant, methane would remain around constant, to maintain constant forcing. Explaining these dynamics is a different blog post. In any case, if the forcing is held constant, the temperature will continue to rise, as we keep ‘forcing’ the system.
T constant. We often hear about the Zero Emission Commitment, if you take CO2 emissions to zero, the temperature will be constant (pdf). For our question, we could think of the forcing that would keep temperature constant at today’s levels. This is also an abstract concept, when done overnight, as it has an unusual forcing pathway, and therefore, and even more unusual emissions pathway. But, it is useful for illustrative purposes.
1.5°C Pathway. There is an additional, perhaps more realistic option, and that is to follow a 1.5°C emission pathway. Even if we aggressively mitigate like nothing the world has ever seen before, the forcing increases for the next few decades in a 1.5°C scenario. There is basically nothing the world can do in the short term that would stop the forcing from increasing. That is because we have limited ways to quickly change the composition of the atmosphere, hence the forcing. Even if we could change our emissions ‘instantly’, we still have to wait for the atmospheric composition to change (forcing) and the climate to respond (temperature).
The following figures show these forcing pathways, and the corresponding temperature pathways.
Is it inevitable that we cross 1.5°C degrees?
In terms of exceeding 1.5°C, in the short term, options are limited. We can’t instantly change the atmospheric composition, and given 1) we have not reach equilibrium, 2) short term changes in forcing, even under deep mitigation, will put us further out of equilibrium, it is hard to see any way that we could geophysically avoid crossing 1.5°C of warming. And I mean in an average sense, after smoothing out interannual variability, and not crossing 1.5°C in a single year but not the next.
Given how close we are to 1.5°C, there seems to be very limited ways to change the forcing (atmospheric composition) sufficiently fast.
You could play some games with how 1.5°C is defined, changing the ‘probability’ of what crossing 1.5°C means, assume emissions went to zero overnight, etc. When you resort to those games, I think we can safely assume you clearly know we are crossing 1.5°C?
We must be careful not to conflate exceeding 1.5°C and the ‘1.5°C goal’. We might exceed 1.5°C, but that does not mean the ‘1.5°C goal’ is dead, as there are numerous ways to return below 1.5°C by sucking billions of tonnes of CO2 out of the atmosphere, more radical non-CO2 emission reductions could help, or we could get lucky with the climate system.
What does crossing 1.5°C mean?
We are crossing 1.5°C because the world has not acted on climate, not because the climate system has gone crazy. Greenhouse gas emissions continue to rise.
For me, failure does not mean giving up, it means try harder. The world needs to start what it was meant to have started 30 years ago. Rapidly reducing emissions. Otherwise, we will be back in five years having this same conversation about 1.6°C.
Methodological notes
I used HadCRUT5 for temperature, forcing and heat content from Forster et al (2023), and CERES for the Earth’s Energy Imbalance. The forcing to temperature calculations are based on a 2-layer box model, based on the IPCC (𝜏 = 4.6, 333 years, 𝑎 = 0.541, 0.459, λ=1/1.16). There are always a variety of choices. I used a 1850-1900 baseline for forcing and temperature. Because there are choices, and I used a rather simple approach, don’t overinterpret the figures. They are meant to be conceptual!
There are likely better ways to do what I did, more accurate methods, inclusion of uncertainty, etc, but this is a blog post, not a Nature / Science article. Happy to make changes if mistakes spotted!
Could you explain why outgoing radiation has increased since the last 40 years ?