Earth’s energy imbalance, a key measure of global warming, has doubled in the past 20 years, raising concerns about how much heat the oceans are absorbing
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| More of the Sun's heat is getting trapped in the Earth's atmosphere (Credit: muratart/Shutterstock) |
Earth’s atmosphere is trapping more than twice as much excess heat now compared to 1993, making scientists increasingly concerned about the impact on the world’s oceans.
The planet’s energy imbalance is a measure of how much surplus heat there is in the climate system. It is the difference between how much energy enters Earth’s atmosphere from the sun, and how much is radiated back into space.
Earth has been absorbing more energy than it emits for decades, as increased concentrations of greenhouse gases in the atmosphere trap more heat from the sun.
This imbalance has increased sharply in recent years, according to estimates based on data from ocean buoys, published in the latest Indicators of Global Climate Change study. Between 1974 and 1993, Earth’s average energy imbalance was 0.42 watts per square metre (w/m2), but between 2004 and 2023 it more than doubled to 0.87 w/m2.
“The Earth is out of energy balance, which is due to the greenhouse gas effect. There is less energy leaving the system,” says Karina von Schuckmann at Mercator Ocean International, a non-profit organisation based in France.
The Indicators of Global Climate Change, co-authored by von Schuckmann and more than 50 other climate scientists, is published annually to provide an update on the state of the world’s climate.
It follows the same methodologies as the Intergovernmental Panel on Climate Change and aims to act as an interim update to the IPCC’s comprehensive, but infrequent, assessments.
The most recent IPCC assessment, which was published in 2021, estimated Earth’s energy imbalance at 0.79 w/m2 during the period 2006 to 2018.
The energy imbalance has been increasing since the 1970s, due to increased greenhouse gas emissions and lower air pollution, which helps bounce heat back into space. Previous studies, using satellite sensors, have concluded that Earth’s energy imbalance has doubled since 2005.
This is worrying news for the world’s oceans, which are absorbing around 90 per cent of this excess heat. The oceans have been warming at record-breaking rates every year since 2019, research shows. The oceans are “our sentinel for planetary warming”, says von Schuckmann.
The additional heat is warming ocean waters, making them less effective at capturing CO2 from the air, driving sea level rise and severely affecting marine life. Some of the changes could be permanent.
Recent studies also suggest the excess warmth is penetrating into the deep ocean, to depths of up to 2000 metres. This heat will take thousands of years to resurface, and in the meantime could affect ocean currents, oxygen levels and marine chemistry.
“Earth energy imbalance is not just some sort of esoteric quantity that scientists like to look at. It’s got real world implications,” says Norman Loeb at NASA Langley Research Center in Virginia.
Studies also suggest the rate of acceleration in global ocean heating has increased over the past 20 years, indicating the rate of change for Earth’s energy imbalance is also rising. This may be in part due to a diminishing albedo effect – as warmer temperatures melt snow and ice, there is less white surface on Earth to reflect the sun’s heat.
The rate of increase has jumped sharply even since the 2022 Indicators of Global Climate Change report, which estimated the energy imbalance at 0.89 w/m2 for the 2010-2022 period. This year’s paper puts it at a record 0.96 w/m2 over the 2011-2023 period.
2023 was a record-breaking year for air and ocean temperatures, driven primarily by climate change and a global swing to El Niño, a natural climate variation that brings warmer temperatures.
The jump in energy imbalance in 2023 is probably due to the sudden swing to El Niño, says Loeb. “Traditionally, when you have La Niña, the climate system takes up energy. And then when you have El Niño, it releases energy,” he says. “When you go from La Niña to El Niño, it packs a big punch.”
Alongside warnings about the increase in energy imbalance, the latest Indicators of Global Climate Change study also cautions that global average temperatures are rising at 0.26°C per decade, double the pre 1983 warming rate. With emissions at the current level, the world has just five years left before the 1.5°C temperature goal is broken.
Journal reference:
Earth System Science Data DOI: 10.5194/essd-16-2625-2024
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