Where do we need abyssal ocean observations to estimate planetary energy imbalance from ocean heat content?

The oceans absorb the vast majority of heat accumulating in the climate system and so ocean heat content (OHC) estimates reflect global climate change. The sparseness and infrequency of ocean temperature measurements below 2000 m hinders precise evaluation of full depth OHC and change. We use global climate simulations from the Coupled Model Intercomparison Project 5 (CMIP5) to reveal the regions where below 2000 m temperature observations are necessary to accurately estimate OHC and adequately resolve the planetary energy imbalance over the twenty-first century (under the RCP 8.5 forcing scenario).

Consistent with available observations, we find that OHC variability and the emergent patterns of climate change in the deep oceans (2000 – 4000 m) are dominated by signals in the Southern Ocean and Atlantic Ocean. We estimate the bias in OHC estimates when sampling to specific depths compared to full depth OHC and quantify how these biases evolve during the twenty-first century. Our results highlight that current observing strategies must be rapidly extended to include sampling below 2000 m as heat penetrates ever deeper into the oceans. For the purposes of constraining the planetary energy imbalance over the coming decades, 2000 – 4000 m ocean observations should initially be focused in the Southern and Atlantic oceans.