Abyssal Ocean Warming: How the climate system is transferring excess anthropogenic energy into the isolated deep ocean

The ocean is by far the largest sink for anthropogenic heat introduced into Earth’s climate system, currently absorbing over 90% of the global energy imbalance. How efficiently the oceans can continue to drawdown heat in the future will be determined by its ability to export heat from the surface into the interior ocean. Here, we present observational evidence of current Deep Ocean warming rates and the likely physical mechanisms driving this warming. Deep ocean warming trends are determined using all available ship based full depth high-quality deep ocean temperature measurements taken along ocean transects repeated multiple times between 1980 and present. These measurements reveal a global scale multi-decadal abyssal warming signal, with the strongest warming in the Southern Ocean near deep-water formation sites, and extending northward following the deep flow pathways. The integrated global deep warming below 3000 m over the past three decades is equivalent to a heat flux of 0.05 (±0.04) Wm^-2 over the entire surface of the earth, or roughly 5% of the global 1 Wm^-2 energy imbalance. In addition, this warming produces a 0.1 (±0.08) mm year^-1 increase in global average sea level from thermostatic expansion. The vertical distribution of the observed hydrography changes provides strong evidence that the warming is primarily driven by isopycnal heave, rather than an advected change. This could be driven by a decrease in the rate of abyssal ventilation. Transient tracer analysis provides additional evidence that this is indeed a contributing mechanism, suggesting a global scale slowdown of the bottom limb of the meridional overturning circulation.