Researchers say that the world has seriously underestimated the amount of heat soaked up by our oceans over the past 25 years.
Their study suggests that the seas have absorbed 60% more than previously thought.
They say it means the Earth is more sensitive to fossil fuel emissions than estimated.
This could make it much more difficult to to keep global warming within safe levels this century.
What have the researchers found?
According to the last major assessment by the Intergovernmental Panel on Climate Change (IPCC), the world’s oceans have taken up over 90% of the excess heat trapped by greenhouse gases.
But this new study says that every year, for the past 25 years, we have put about 150 times the amount of energy used to generate electricity globally into the seas – 60% more than previous estimates.
This new calculation shows that far more heat than we thought has been going into oceans. But it also means that far more heat than we thought has been generated by the warming gases we have emitted.
Therefore more heat from the same amount of gas means the Earth is more sensitive to CO2.
What are the implications of the finding?
The researchers involved in the study believe the new finding will make it much harder to keep within the temperature rise targets set by governments in the Paris agreement. Recently the IPCC spelled out clearly the benefits to the world of keeping below the lower goal of 1.5C relative to pre-industrial levels.
The report suggests that to prevent temperatures rising above 2C, carbon emissions from human activities must be reduced by 25% more than previously estimated.
What have these scientists done differently?
Since 2007, scientists have been able to rely on a system of almost 4,000 Argo floats that record temperature and salinity in the oceans around the world.
But prior to this, the methods used to measure the heat in the ocean had many flaws and uncertainties.
“When the ocean warms, the amount of these gases that the ocean is able to hold goes down,” said Dr Resplandy.
“So what we measured was the amount lost by the oceans, and then we can calculate how much warming we need to explain that change in gases.”
How have other scientists responded to the findings?
With some concern.
“The authors have a very strong track record and very solid reputation… which lends the story credibility,” said Prof Sybren Drijfhout at the UK’s National Oceanography Centre in Southampton.
“The uncertainty in the ocean heat content change estimate is still large, even when using this new independent method, which also has uncertainties,” said Thomas Froelicher from the University of Bern, Switzerland.
“The conclusion about a potential higher climate sensitivity and potentially less allowable carbon emission to stay below 2C should stimulate further investigation.”
The study has been published in the journal Nature.
