Changing oceans reveal clear human thumbprint

Climate heating must have already begun to result in changing oceans. The next step is to confirm and monitor this change.

REV Ocean’s research vessel
REV Ocean’s research vessel arrives in the Norwegian port of Ålesund. Image: Bogdan Vasilescu , CC BY-SA 2.0

Humankind has already begun to reshape the biggest available living space on the planet and to leave its mark in the changing oceans.

New research suggests that somewhere between 20 per cent and 55 per cent of the Atlantic, Pacific and Indian Oceans now have temperatures and salt levels that should be measurably different because of climate change driven by profligate human combustion of fossil fuels.

And forecasts suggest that by mid-century the scale of human impact will only have increased – to between 40 per cent and 60 per cent. By 2080, human impact on the oceans will have begun to change between 55 per cent and 80 per cent of the blue planet.

Although the researchers – they report in the journal Nature Climate Change – have based their predictions on computer models, they are confident that the thumbprint of human-induced climate change began to leave its mark on the seas of the Southern Hemisphere as long ago as the 1980s.

“We have been detecting ocean temperature change at the surface due to climate change for several decades now,” said Eric Guilyardi, of the University of Reading in the UK and the Laboratory of Oceanography and Climate in Paris, France.

Our results highlight the importance of maintaining and augmenting an ocean observing system capable of detecting and monitoring persistent anthropogenic changes.

“But changes in vast areas of the ocean, particularly deeper parts, are much more challenging to detect.”

The problem of measurement is simple: the ocean is enormous. It covers 70 per cent of the planet to an average depth of 3.7 kms. It defines the planet.

It is almost certainly where life on Earth first emerged; it was life’s only home for the first three billion years.

And it is in a state of constant change, constantly evaporating, and continually replenished with freshwater from rainfall, river flow and melting polar ice. So temperature and salinity change naturally, and with the seasons, and with much longer cyclic swings driven by the atmosphere.

Scientists have been measuring surface conditions for many decades. The ocean at depth is a bit more of a challenge. The question the researchers put was a simple one: could temperature and salinity levels in parts of the ocean have risen or fallen higher or lower than they would in normal peaks and troughs?

Beyond natural variability

It’s not an easy question: oceanography is expensive, the ocean is huge, much of it has never been studied and the ways in which the ocean layers mix is still a bit of a puzzle.

So the scientists started with two models, with and without the impact of human action. They then worked on an analysis of salt levels and temperatures to detect significant change, and then tried to predict the dates at which this change ought to declare itself.

Their readings tell them that changes beyond natural variability in the northern hemisphere – all the seas from the Arctic Ocean to the equatorial waters – could have emerged between 2010 and 2030. That is, change is already happening.

Their simulations also predicted that whatever shifts occurred at depth in the temperature and chemistry of the southern oceans, these could have been identified up to 40 years ago, had researchers had the technology, the funding, the people and the ships and submersibles to do so.

“Our results highlight the importance of maintaining and augmenting an ocean observing system capable of detecting and monitoring persistent anthropogenic changes,” they report. 

This story was published with permission from Climate News Network.

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