Reconciling Earth’s growing energy imbalance with ocean warming

By: Prof. Richard Allan (Professor of Climate Science)

The exceptional global warmth of 2023 and 2024 generated much idle chit chat in Meteorological circles and following a summer Met department coffee room chinwag with Chris Merchant, an ill thought out plan was devised to solve all the world’s problems… after which we narrowed our focus slightly to investigate the role of Earth’s growing accumulation of heat on the record global temperatures and implications for ongoing climate change. The rebel professors decided to roll up their sleeves, recall basic python (and IDL, sorry) and make some top plots. Chris got all the press with part 1 so I’ll just have to settle for a blog on part 2.

There is more energy arriving than leaving the planet (an energy imbalance) which is driving a warming of climate. But this imbalance is growing – Earth’s rate of heating has doubled over the past 20 years as less sunlight is being reflected back to space by dimming clouds and melting ice over the oceans, adding to heat trapped by rising greenhouse gas levels (interestingly, although more outgoing infrared heat is trapped by rising greenhouse gas levels, this is offset by greater infrared heat loss to space as the surface and atmosphere warms). The source of the growing energy imbalance is illustrated in Figure 1 which shows the increase in net energy imbalance anomalies is mostly explained by increases in absorbed shortwave radiation over the ocean and the signal is mostly absent for clear-sky conditions.

We analysed the satellite record going back to 1985 and compared with the ERA5 reanalysis, a model-observations hybrid. Heating of the planet reached record values in early 2023, equivalent to every person currently alive using SIXTY 2-kilowatt kettles to boil the ocean (including babies that may need supervision)! Although ERA5 can capture the month-to-month fluctuations in Earth’s energy imbalance, it does not pick out the increase, particularly since the mid-2010s (Fig. 2).

Figure 2 – Time series of seasonally corrected Earth energy imbalance where positive values show a heating of the planet and data is from CERES satellite measurements, the DEEP-C reconstruction and ERA5 reanalysis. See Allan and Merchant (2025) for details.

By analysing the growing difference between the satellite data and ERA5, could we find what was going on? Of course not, but there were some interesting signals across the globe, with hotspots over the vast expanses of low altitude stratocumulus clouds off the coast of California and Namibia where more sunlight was being absorbed. But the discrepancy seems to show up across much wider areas of the oceans in the satellite data (Fig. 3, left). And when only clear-sky scenes were analysed much of the differences were wiped clean suggesting that the root cause of Earth’s growing energy imbalance is explained by cloudy ocean regions soaking up more sunshine (see Fig. 1). In fact, the only remaining discrepancy for clear-sky conditions aside from some funny business in the Arctic is a suspicious blob over eastern China (Fig. 3, right). This is somewhat interesting as ERA5 used older estimates of aerosol particle pollution changes which assumed more dirty skies in this region than in reality as China acted to clean up its cities and industry. These land-based emissions of aerosols could be part of the story as they can blow over vast areas of the north Pacific ocean and affect the clouds and wind patterns across a wide area of the globe.

Figure 3 – Maps showing differences between CERES satellite data and the ERA5 reanalysis in estimates of changes in absorbed sunlight (left) and the same for clear-sky conditions (right). See Allan & Merchant (2025).

Another possible cause of the dimmer Earth is declining aerosol emissions from shipping, which has been surreptitiously cleaning up its act over many years, but particularly since 2020 when International Maritime Organisation limits on ship fuel emissions over the open ocean were added to earlier regulations on pollution within ports.  Aerosol haze can directly reflect sunlight back to space but its real superpower is acting as condensation seeds for cloud that cause water to be distributed across more numerous, smaller droplets, making clouds more reflective like a polished mirror. Taking away this artificial cleaning spray effect leaves the usual dusty mirror we tend to have in our house. So declining aerosol pollution, either from land based or ocean-based aerosol particle emissions, could be making our clouds dimmer again.

On the other hand, a warming ocean and the pattern of this warming can make the most reflective low altitude clouds break up, causing more sunlight to reach the sea surface, adding to the warming. Previous research has shown this to be a thing and clouds, along with increasing gaseous water vapour and diminishing ice coverage, are known to amplify the amount of climate change through feedback loops. Because there is only so much cleaner we can make our air, while clouds can continue amplifying climate change as long as greenhouse gas emissions continue to add heat, working out how much of the growing imbalance in Earth’s energy budget is due to cleaner air, cloud feedback loops or natural ocean fluctuations is critical to working out what sort of a hell hole future societies will be living in.

And that brings us to this somewhat surprising surge in global warmth up to the record global temperatures in 2023 and 2024. The general consensus is that the record global warmth was driven primarily by greenhouse gas emissions with an extra boost from the El Niño conditions that developed in 2023. A bright sun at the peak of its 11-year cycle only contributed marginally while the consensus is that the Hunga Tonga undersea volcanic eruption early in 2022 barely influenced global temperatures since heating from water vapour injected into the bone dry stratosphere was offset by cooling from sulfate aerosol particles also emitted. Effects from reduced Sahara dust, wildfires or other volcanoes may have influenced regional climate but were probably not important globally. Nevertheless, we sidestep these niggly issues by focusing on the heat budget alone.

It takes just over 4000 Joules of energy to heat up a kg of salty ocean water by a degree Celsius. So, can the extra energy flooding into the climate system explain the large 0.27^oC rise in ocean temperatures from 2022 to 2023, the amount of warming we’re more used to observing in a decade? We did a dull-as-dishwater accounting exercise, to estimate how much heat was used up warming the atmosphere and land or melting ice (Figure 4). We estimated this to account for a larger than normal proportion of the total heating (20%) but this still leaves a whopping 24,000,000,000,000,000,000,000 Joules of energy to simmer the oceans over a year. Further calculations suggest that even this is not enough to explain the annual warming so we propose two possibilities: either this heat is being focused on a narrower ocean layer (Fig. 4c) or the usual uptake of heat to deeper layers reverses (Fig. 4b) and instead bubbles up to return energy to the upper layers. They are probably both part of the story. It is known that the deeper ocean loses its breath and is less efficient at sequestering heat as the surface warms. Also, a return of heat from deeper ocean layers is consistent with the transition from an extended La Nina 2020-2022 (where heat builds below the ocean surface) to a moderate El Nino that developed in 2023 (when some of the stolen energy returns to the surface layers).

Figure 4 – Schematic of energy entering the ocean upper layers for (a) climatological conditions and plausible scenarios for the large warming period 2022-23 (b-c). See Allan & Merchant (2025) for details.

The planet is becoming dimmer in so many ways, and one of these seems to be caused by less shiny clouds over the ocean which is adding to the growing greenhouse effect to heat up our planet at ever increasing rates, accelerating climate change. The temporary help from ocean fluctuations will fade but inevitably pop up uninvited again sometime in the future to wreak havoc. But how much Earth’s heating rate increases will depends on whether less reflective clouds are a temporary response to declining particle pollution or a response to, and amplifier of, the ocean warming – this is a crucial outstanding question and one that affects the trajectory of climate change.  Current levels of global temperature suggest that surpassing the 1.5^oC threshold above pre-industrial conditions is now inevitable yet along with uncertain cloud effects underscores (in marker pen) how essential rapid and massive cuts in greenhouse gas emissions are for limiting further warming and associated impacts on societies and ecosystems.

Journal Article:

Allan RP & Merchant CJ (2025) Reconciling Earth’s growing energy imbalance with ocean warming, Environ. Res. Lett.20, 044002, https://doi.org/10.1088/1748-9326/adb448