Ocean heat uptake and the global surface temperature record - Grantham Briefing Paper 14

Authors: Dr Flora Whitmarsh, Dr Jan Zika, Dr Arnaud Czaja

The  last century has seen a long term increase in global surface temperatures. A slowdown, sometimes referred to as a hiatus, in this long term trend has been observed over approximately the last 16 years. Whilst there is ongoing debate as to the severity of the hiatus, a slowing or even a reversal of the temperature trend is not unprecedented. 

This paper considers the contribution of changes in ocean circulation and heat content to the recent slowdown, examining recent variation in both Pacific and Atlantic ocean currents and their heat uptake.

Key points

• Over the last 16 years there has been a slowdown in the long term increase in global surface temperatures. A number of factors have contributed to this hiatus including ocean circulation processes.
• Due to its large mass and high heat capacity, the ocean has taken up most of the excess heat stored in the Earth system as a result of greenhouse gas emissions.
• Observations are insufficient to pinpoint exactly where the relevant changes in ocean circulation or heat content have occurred.
• A change in Pacific surface temperatures, driven by stronger than usual Pacific trade winds, is the leading hypothesis to explain the slowdown.
• This change is driven by a natural process that changes phase every few decades, implying that if this hypothesis is correct the rise in global surface temperatures will resume next time it changes phase.
• Changes in Atlantic Ocean circulation may also have reduced heat uptake in the deep ocean.
• Because fluctuations in the rate of warming have been observed throughout the historical record, the slowdown does not alter our understanding of the sensitivity of temperatures to carbon dioxide, and future temperature projections remain unaltered.

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