Taking the temperature – vital signs from the world’s oceans

Answers may be found in the world’s oceans, writes Jon Turney.Is it getting warmer yet? That’s often a simple enough question. But when you are asking about the entire Earth system there tend not to be any simple questions.

The early rounds of reporting from the Intergovernmental Panel on Climate Change did offer a simple answer – the global average surface temperature was creeping up and they predicted that it would go on the same way.

Collating and analyzing worldwide observations to produce a figure for the average, and plotting how it is changing, is itself a major scientific and organizational feat. But the result is only one indicator of global change, and not necessarily a good one over the medium term. The search for better indicators has gained urgency because the pace of warming has slowed when compared to the previous couple of decades. That is not unexpected, scientifically speaking, as both climate models and records of past climate show other instances of medium-term departures from longer-term trends as the complexities of the climate system respond to change. However it has been seized upon by those who argue that man-made greenhouse gas emissions aren’t that much of a problem as support for their case.

So what research will provide more useful indicators of the climate system? Three recent publications offer answers. Sonia Seneviratne, co-chair of the GEWEX (Global Water and Energy Exchanges) project, and colleagues reported in Nature Climate Change in March that the “pause” is not reflected in changes in climate extremes: quite the reverse, in fact. The team studied local peaks in temperatures over land; highly relevant to studying the impacts of climate change because land is where people live and grow their food. These temperature extremes have continued to climb. The analysis additionally suggests that “the most ‘extreme’ extremes show the greatest change”.

This paper calls for “a better understanding of the full complexity of changes resulting from greenhouse gas emissions, focusing on the identification of individual processes contributing to the global-scale response.” Elements of that better understanding also come in when researchers are trying to account for the effect of the solar radiation trapped by increased levels of greenhouse gases in the atmosphere. If it is not generating increased warming, on average, on the surface, where is the heat going?

Into the ocean, argues Lisa Goddard, co-chair of the Climate Variability and Predictability project (CLIVAR). In a commentary in the same issue of Nature Climate Change she emphasises that the radiative imbalance at the top of the atmosphere – which drives global temperature increases – is still increasing. But the energy that accumulates is distributed within the climate system. That distribution is complex, but is heavily influenced by movements and mixing in the great mass of water in the oceans. In particular, the Pacific Decadal Oscillation (PDO) has phases when more heat reaches the deep ocean, and stays there.

This leaves plenty more questions to answer, including how the PDO itself may be influenced by climate change, and how broader linkages such as those between the Pacific and Atlantic oceans might work. They will only be answered, Goddard argues, if observing systems are sustained and enhanced, especially in the ocean depths. “We still lack the long-term and continuous observations, and their synthesis, that are critical to understanding the climate system as a whole”, she writes.

Martin Visbeck of GEOMAR and Kiel University, Chair of the German Future Earth National Committee and former co-chair of CLIVAR, argues in a third commentary – appearing last month in Nature Geoscience – that decadal climate prediction, on just the kind of timescale affected by ocean-atmosphere exchange phenomena like El Niño or tropical Atlantic climate variability – is crucial for improving climate adaptation strategies. He sees an urgent need for “a fully-implemented ocean and climate-observing system”, which would include full global data sharing. Calculating the average surface temperature is a start, but does not give insight into a variable climate record that “we need to understand and document at a more complete and fundamental level”, he says.