Surface temperature rise is often thought of as synonymous with climate change. However a recently published paper in Nature Climate Change argues that Earth’s energy imbalance (EEI) is what ultimately sets the pace of climate change and that substantive progress can be made by monitoring this key climate variable.
All the energy that enters or leaves the Earth system does so via radiation at the top of the atmosphere. For a stable climate, the sunlight absorbed by the planet must be balanced by thermal infra-red radiation emitted to space. Increased atmospheric greenhouse gas concentrations give rise to an imbalance in Earth’s energy budget by initially reducing the amount of emitted thermal infra-red radiation. The result of this imbalance is an accumulation of excess energy in the Earth system over time. The size of the imbalance, or equivalently, the rate of energy accumulation in the Earth system, is the most fundamental metric determining the rate of climate change.
The vast majority (>90%) of the excess energy is absorbed by the ocean, with much smaller amounts going into heating of the land, atmosphere and ice cover (Figure 1). Therefore, if we want to track the increase in Earth system energy content over time it is essential to have comprehensive measurements of temperature, and the associated heat content, throughout our vast oceans.
The ‘symptoms’ of EEI
As a result of the energy imbalance, the Earth system adjusts in a number of ways that have a direct impact on both the marine and terrestrial environment. The various elements of global warming that we are familiar with – including global surface temperature rise, reductions in snow and ice cover, and sea level rise – can be thought of as symptoms of EEI (Figure 2). In our thinking and communication around climate change, it is important not to confuse any of these symptoms with the underlying cause.
The global warming ‘hiatus’
A large part of the controversy around the recent slowdown in surface temperature rise, or ‘hiatus’, stems from the fact that many commentators view global surface temperature rise as the primary indicator of global climate change. If surface warming has paused, climate change has paused, right? Wrong. Both observational studies and computer simulations show that there is only a weak relationship between Earth’s energy imbalance and surface temperature change over a decade or so (Figure 3a). This is because natural climate fluctuations can re-arrange ocean heat content, to either offset or add to the long-term rate of global surface temperature rise over a decade or so.
Since the ocean becomes the dominant term in Earth’s energy budget on timescales longer than about 1 year changes in ocean heat content provide a much more reliable indicator of EEI (and therefore climate change) than surface temperature on decadal timescales (Figure 3). Indeed, time series of upper ocean heat content and satellite measurements agree on a fairly steady rate of heat uptake over the past 20 years or so, suggesting that EEI has also been relatively constant during this time (Figure 4). When viewed in terms of EEI, there is little or no evidence for a recent ‘hiatus’ in the rate of global climate change.
How do we measure EEI?
There are three ways in which we can use observations to track EEI and its variations over time. The first is to use satellite measurements of the variations in the radiative energy entering and leaving the Earth system. Second, we can use estimates of heat flows into and out of the ocean surface to infer the EEI because the atmosphere and land have comparably very little heat capacity. The third, and most robust, method is to track the rate of change of Earth system energy storage, which is dominated by ocean heat content for the timescales of interest here.
Each of the methods for estimating EEI has strengths and weaknesses, as discussed by von Schuckmann et al. The most promising strategy for advancing our monitoring capability lies in efforts to combine satellite-based monitoring of variations in EEI with estimates of ocean heat content change. Progress can be achieved through a multidisciplinary community effort to improve both Earth observations and devise optimal methods for combining the information on EEI that they provide.
An exciting advance under development is the extension of the Argo array of robotic profiling floats to sample the full ocean depth. Argo has revolutionised our ability to monitor changes in ocean heat and freshwater content, since its inception in the early 2000s. However, current generation floats can only sample the upper 2km – roughly 50% of the open ocean depth. New, deeper float technologies are currently in the testing phase and research is being carried out into what this deep array might look like.
Continued monitoring of Earth’s energy balance is paramount for understanding the evolution of climate change and has multi-century implications. Earth will maintain an energy imbalance long after greenhouse gas emissions have been reduced and surface temperature rise has stabilised. One of the important climatic consequences is that global sea level will continue to rise for many centuries after surface temperature rise has ceased, due to the continuing increase in ocean heat content and long term melting of giant ice sheets (Figure 5).
Global surface temperature rise and other key symptoms of EEI are essential foci of climate change science and assessing potential impacts of global warming. However, Earth’s energy imbalance is what ultimately sets the pace of global warming and it should be central to our thinking and communication on climate change.
The authors would like to thank Richard Allan and Jonathan Gregory for helpful suggestions to improve a previous version of this post.