Climate change: oceans "absorb more heat than expected"

Researchers say the world has seriously underestimated the amount of heat absorbed by our oceans over the past 25 years.

Their study suggests that the seas absorbed 60% more than previously thought.

They say that means the Earth is more sensitive to fossil fuel emissions than expected.

This could make it much more difficult to keep global warming at a safe level this century.

What did the researchers find?

According to the latest major badessment of the Intergovernmental Panel on Climate Change (IPCC), the world's oceans have absorbed more than 90% of the excess heat trapped by greenhouse gases.

But this new study indicates that every year, for 25 years, we have put about 150 times more energy used to produce electricity in the world, 60% more than previous estimates.

It's a big problem.

Scientists base their predictions on global warming by adding up all the excess heat produced by the known amount of greenhouse gases emitted by human activities.

This new calculation shows that much more heat than we thought was for the oceans. But it also means that the warming gases we emitted generate much more heat than we thought.

Therefore, more heat produced by the same amount of gas means that the Earth is more sensitive to CO2.

What are the implications of the discovery?

The researchers involved in the study believe that the new findings will make it much harder to maintain the temperature rise targets set by governments in the Paris agreement. Recently, the IPCC has clearly explained the benefits to the world of staying below the target of 1.5 ° C below pre-industrial levels.

This new study says it will be very difficult.

"This is a major concern," said project leader Laure Resplandy of Princeton University, New Jersey.

"If you look at the IPCC 1.5C, there are big challenges in keeping those goals, and our study suggests that it's even more difficult because we're closing the window for those lower lanes."

The report suggests that to avoid temperatures above 2 ° C, carbon emissions from human activities must be reduced by 25 per cent above previous estimates.

What does this mean for the oceans?

In addition to making it more difficult to maintain warming below 1.5 or even 2 ° C during this century, all this additional heat entering the oceans will cause significant changes in the waters.

"A warmer ocean will retain less oxygen, which will affect marine ecosystems," said Dr. Resplandy.

"There is also the sea level. If you warm up the ocean the more you will have more thermal expansion and therefore more sea level rise."

What did these scientists do differently?

Since 2007, scientists have relied on a system of nearly 4,000 Argo floats that record the temperature and salinity of the oceans around the world.

But before that, the methods used to measure heat in the ocean had many defects and uncertainties.

The researchers have come up with what they say is a very accurate method of detecting ocean temperature by measuring the amount of oxygen and carbon dioxide in the air. This allows them to accurately measure ocean temperatures globally since 1991, when accurate data from a global network of stations became available.

The key element is the fact that when the waters warm up, they release more carbon dioxide and oxygen into the air.

"When the ocean heats up, the amount of gas that it is able to contain decreases," said Dr. Resplandy.

"So, what we measured, is the amount lost by the oceans, and then we can calculate the magnitude of the warming needed to explain this gas shift."

Will the heat come back someday?

Yes, say the authors, but over a very long time.

"The heat stored in the ocean will eventually come back if we start cooling the atmosphere by reducing the greenhouse effect," said Dr. Resplandy.

"The fact that the ocean retains such a quantity of heat that it is possible to restore to the atmosphere makes it more difficult to maintain the temperature of the Earth's surface under a given target.

"The ocean circulation that controls the absorption / release of heat from the oceans operates at time scales of several centuries, which means that the heat of the oceans would be released for centuries to come."

How have other scientists responded to the results?

With some concern.

"The authors have a very strong track record and a very good reputation … which gives credibility to history," said Professor Sybren Drijfhout at the United Kingdom's National Oceanographic Center in Southampton.

"The updated estimate is indeed worrisome with respect to the likelihood that the company could reach the 1.5 and 2 degree targets as it moves the lower limit of climate sensitivity towards the top."

Others say that additional work is needed.

"Uncertainty in estimating the change in the heat content of the ocean is still great, even with this new independent method, which also involves uncertainties," said Thomas Froelicher of L & # 39; University of Bern, Switzerland.

"The conclusion that higher and potential climate sensitivity and potentially less carbon emissions to stay below 2 ° C should stimulate further investigation."

The study was published in the journal Nature.