Global Sea Levels Rising

(CORDIS) — A team of researchers from France and the United States has discovered that the planet’s oceans are rising as ice melts. Presented in the journal Nature, the study is the first of its kind to determine just how much of Earth’s melting land ice is fuelling the global sea-level rise.

Led by the University of Colorado Boulder in the United States, researchers used satellite measurements from the Gravity Recovery and Climate Experiment (GRACE), a joint mission between the National Aeronautics and Space Administration (NASA) and the German Aerospace Center, to measure ice loss across the planet between 2003 and 2010. The team focused especially on glaciers and ice caps outside Greenland and Antarctica.

The data show that overall global ice mass lost from Greenland and Antarctica as well as the world’s other glaciers and ice caps over the 7-year period was around 4.3 trillion tonnes. The result is that the global sea level has increased by 12 millimetres. The experts say that is enough ice to cover the United States 0.5 metres deep.

‘Earth is losing a huge amount of ice to the ocean annually, and these new results will help us answer important questions in terms of both sea rise and how the planet’s cold regions are responding to global change,’ explains Professor John Wahr of the University of Colorado Boulder, one of the authors of the study. ‘The strength of GRACE is it sees all the mass in the system, even though its resolution is not high enough to allow us to determine separate contributions from each individual glacier.’

According to the researchers, around 25% of the average annual ice loss (about 148 billion tonnes) resulted from glaciers and ice caps outside Greenland and Antarctica. They add that ice loss from Greenland and Antarctica and their peripheral ice caps and glaciers averaged 385 billion tonnes each year.

In previous studies, researchers had used ground measurements from a small number of glaciers to deduce the status quo for the globe’s unmonitored glaciers. Just a few hundred of the some 200 000 of the planet’s glaciers have been monitored for more than 10 years.

In this latest study, the GRACE team found that the estimated ice loss from high Asian mountain ranges, such as the Pamirs and the Himalayas, was just some 4 billion tonnes of ice each year.

‘The GRACE results in this region really were a surprise,’ Professor Wahr says. ‘One possible explanation is that previous estimates were based on measurements taken primarily from some of the lower, more accessible glaciers in Asia and extrapolated to infer the behaviour of higher glaciers. But unlike the lower glaciers, most of the high glaciers are located in very cold environments and require greater amounts of atmospheric warming before local temperatures rise enough to cause significant melting. This makes it difficult to use low-elevation, ground-based measurements to estimate results from the entire system.’

Commenting on the findings, NASA scientist Tom Wagner says: ‘This study finds that the world’s small glaciers and ice caps in places like Alaska, South America and the Himalayas contribute about 0.02 inches per year to sea level rise. ‘While this is lower than previous estimates, it confirms that ice is being lost from around the globe, with just a few areas in precarious balance. The results sharpen our view of land-ice melting, which poses the biggest, most threatening factor in future sea level rise.’