With most extra heat that is trapped in the atmosphere going into the oceans, how they react to it is critical. The heat leads to an expansion of the oceans, and consequently an increase in sea levels.
Previous research has said that this “thermometer effect” has solely been responsible for sea levels rising by between 0.7mm and 1mm a year. But a team from the University of Bonn, in Germany, said this week that in the past 12 years, sea levels have risen by double that – 1.4mm a year.
This conclusion was made after the team studied satellite data between 2002 and 2014.
“To date, we have underestimated how much the heat-related expansion of the water mass in the oceans contributes to a global rise in sea level,” said Jurgen Kusche, a professor at the University of Bonn.
The United Nations’ own climate agency says the effect has seen a sustained increase in recent years, thanks to greenhouse gas emissions trapping more heat in the atmosphere and oceans.
The overall impact of oceans is still not fully understood because they change temperature and levels slowly and over long time periods. But when ocean temperatures do change, they have a profound effect on weather systems.
The current El Niño – which has rapidly warmed the Pacific Ocean by 3°C – has seen drought in the southern hemisphere and flooding in the northern hemisphere. It has also been linked to more intense storms and an increase in the intensity of other weather phenomena.
Longer-term warming of the oceans is blamed for a multitude of problems, alongside the damage done by increased sea levels. Warmer oceans mean more energy for cyclones, which can also carry more water as a result of the heat.
In 2012, Hurricane Sandy passed over water that scientists said was 0.6°C hotter thanks to global warming, gaining energy and holding more water. It then hit the eastern coastline of the United States, where the sea levels were already higher thanks to overall ocean warming, and flooded cities such as New York.
When combined with melting ice and other warming factors, the Bonn team found that the overall global sea level rise is 2.74mm a year.