Earth’s energy budget, the delicate balance of incoming and outgoing energy that keeps the planet’s surface temperature relatively stable, is disrupted by human emissions of greenhouse gas. A sizeable amount, about 93%, of the excess energy trapped around the planet is absorbed by the oceans, which cover more than 70% of the globe. Despite the ocean’s importance in accounting for Earth’s energy budget (not to mention its weather and climate), much of its volume remains unexplored and unmonitored. Measurements of the temperature of the deep ocean, an expanse between 2000 and 4000 meters below the surface, and the abyssal ocean layer below, which extends down to 6000 meters, are relatively sparse.
Using hydrographic data collected from ship-bound instruments, previous research measured heat content trends between 1981 and 2010 in 32 deep-ocean basins around the world. Here Desbruyères et al. extended those estimates of deep-ocean temperature changes through 2015. The data, collected first through the World Ocean Circulation Experiment Hydrographic Program and now through the international Global Ocean Ship-Based Hydrographic Investigations Program, consist of temperature, salinity, and pressure profiles from the ocean surface down to between 10 and 20 meters above the ocean floor made by a conductivity-temperature-depth instrument. The team looked at temperature trends over the entire study period and on shorter timescales, dividing the data into pre-2000 and post-2000 periods, and determined the relative contributions of the deep and abyssal ocean layers to heat budgets.
The authors found that the additional data did not significantly change the previous estimates of ocean temperature trends. On a global scale, the deep ocean below 2000 meters warmed at similar rates before and after 2010, and the abyssal layer warmed at a faster rate than the deep-ocean layer, climbing at a rate of 0.53 ± 0.11 millidegrees Celsius (m°C) per year compared to 0.34 ± 0.19 m°C per year in the deep ocean.
Although research has found that warming in the uppermost surface layer of the ocean slowed after the year 2000, this study found little difference between the short-term pre-2000 and post-2000 periods in the deep and abyssal layers. Although there was some variation in warming trends within individual ocean basins, more generally, the higher warming rates in the North Atlantic and Southern oceans offset the lower warming rates in the South Atlantic, Pacific, and Indian oceans.
The new study reduces some of the uncertainty inherent in analyses of deep-ocean behavior. Despite the fact that humans rarely interact directly with the deep ocean, a clearer picture of the energy budget of the deep ocean is critical for accurately forecasting our planet’s changing climate. (Geophysical Research Letters, https://doi.org/10.1002/2016GL070413, 2016)
—Kate Wheeling, Freelance Writer