By Matthew D. Palmer, Susan Wijffels, and John A. Church
21 March 2016
(WMO) – In a stable climate, the amount of energy that the Earth system absorbs from the Sun is balanced by the amount of energy emitted back to space by the Earth as thermal infrared radiation. However, increases in greenhouse gas concentrations have created an imbalance by reducing the emitted radiation and causing energy to accumulate in the Earth system over time. The rate of energy increase in the climate system – Earth’s energy imbalance – is the most fundamental metric that defines the rate of global climate change.
On timescales longer than about a year, the vast majority (more than 90%) of Earth’s energy imbalance goes into heating the oceans. Thus tracking ocean temperatures and associated changes in ocean heat content (OHC) allow us to monitor variations in Earth’s energy imbalance over time. Observations of OHC have been pivotal for the assessment of climate models and the detection and attribution of human climate change. They are also essential to “anchor” satellite-based estimates of changes in Earth’s energy imbalance.
As the oceans warm, they expand, resulting in both global and regional sea-level rise. Increased OHC accounts for about 40% of the observed global sea-level increase over the past 60 years and is expected to make a similar contribution to future sea-level rise. The warming of ocean waters adjacent to the ice sheets can also affect the flow of ice into the ocean, which is another key component of sea-level rise. Therefore, monitoring global and regional OHC, along with tide gauges and satellite measurements of sea level and ocean mass, is essential for understanding historical and future sea-level change.
In the past, one of the challenges for estimating the rate of OHC change has been the historical sparseness of the ocean observing system. Ocean subsurface temperature measurements have mostly relied on ship-based instruments, which often sample only the upper few hundred metres. As a result, many historical estimates of global OHC change are limited to about the upper 700 m, with large uncertainties prior to the 1970s, when widespread ocean sampling became possible through more affordable observing technologies.
The early 2000s saw a revolution in our ability to monitor global OHC and freshwater content through the inception of the Argo array of autonomous profiling floats. This array reached maturity in 2006 with approximately 3,000 floats distributed around most of the globe; they measure temperature and salt content over the upper 2 km of the oceans every 10 days. The Argo observations usher in a new era of monitoring Earth’s energy imbalance and the various factors that shape its evolution over time.
While ocean temperatures below 300 m (Figure 18 b, c, and d) show a relatively steady increase over the period 2006–2015, ocean surface temperatures (Figure 18 a) show additional multi-year variability primarily due to variation in the tropical Indian Ocean and Pacific Ocean associated with the El Niño Southern Oscillation. Most of these near-surface oscillations are balanced by deeper, opposing changes between 100 m and 300 m, and the 0–2,000-metre OHC rises steadily across this period. Land-surface temperatures show more variability again, with particularly large variations from year to year. According to estimates shown in Figure 18 e, between 75% and 99% of the warming occurs in the southern hemisphere, predominantly between 30°S and 50°S.
The observed OHC increase implies that Earth’s energy imbalance is nearly constant at 0.65–0.80 W/m2, expressed as an average value over the surface area of the planet. About 75%–80% of this value comes from the upper 2,000 m, as illustrated in Figure 18, with the remaining 20%–25% coming from the deeper ocean. The observed energy imbalance inferred by OHC change is within the range of 0.6–1.0 W/m2 projected for the 2006–2015 period using climate models.
Monitoring OHC enables us to better track the underlying rate of climate change on decadal and shorter timescales and to better quantify the effect of other climatic factors on Earth’s energy imbalance, such as changes in man-made and volcanic aerosols. While Argo observations are currently limited to the upper 2,000 m of ocean depths, the technology now exists to profile over nearly the full ocean depth (up to 6 km). These new observations will become essential for monitoring climate and sea-level change as the impact of Earth’s energy imbalance becomes apparent at ever greater depth in the global oceans.
Wijffels, S. et al., 2016: Ocean temperatures chronicle the ongoing warming of Earth. Nature Climate Change, 6:116–118, doi:10.1038/nclimate2924.
von Schuckmann, K. et al., 2016: An imperative to monitor Earth’s energy imbalance. Nature Climate Change, 6:138–144, doi:10.1038/nclimate2876.