Volcanic eruptions may have masked rate of rising sea levels

Sea levels could be rising faster than anticipated, as the first two decades of satellite measurements monitoring melting land ice may have been masked by volcanic eruptions like the one at Mount Pinatubo in the Philippines, according to the National Center for Atmospheric Research in Colorado. Large volcanic eruptions temporarily block the sun and cooled temperatures, factors that the centre says need to be taken into account when studying the effects of climate change. Melting land ice poses significant risks, initially to lower-lying areas such as the Maldives, and longer-term to large cities such as New York and London. 

It’s well known that global warming is pushing up sea levels – but alarming new research suggests that the ocean has begun rising at an accelerating rate.

Rising temperatures increase the sea level by melting land ice, which makes its way into the ocean and by expanding the water, which swells as it warms.

This poses a huge threat to large swathes of the world, initially to low-lying island such as the Maldives which are slowly sinking and, in the longer term, to numerous metropolitan cities such as London and New York.

3 mm’s a year

The sea has been rising at around 3 millimeters per year, or 1.2 inches a decade, since the official TOPEX/Poseidon measurement satellite launched in 1992.

The rate of increase has been gradually increasing, but the rate of that increase had been slowing down. Until now.

“When we used climate models which removed the effect of the Pinatubo eruption we saw the rate of sea level rise accelerate” John Fasullo

A new study by the National Center for Atmospheric Research in Colorado has found that the eruption of Mount Pinatubo in the Philippines in 1991 blocked out the sun to such an extent that the Earth temporarily cooled and sea levels actually dropped for a while.

This meant the eruption artificially masked the true effect of the melting ice on the sea level for the first two decades of satellite measurement – suggesting the underlying trend was a slowing down of the rate of increase, when in fact it should have been speeding up.

But Mount Pinatubo effect has essentially worn off and the sea level is now starting to accelerate in line with original forecasts, according to the study, published in the journal Scientific Reports.

“When we used climate models which removed the effect of the Pinatubo eruption we saw the rate of sea level rise accelerate,” said lead study author John Fasullo.

“Now that the impacts of the Pinatubo have faded, this acceleration should become evident in the coming decade, barring another major volcanic eruption,” he added.

The Pinatubo eruption was the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in the Alaska Penisula. It ejected roughly 10 billion tonnes of magma – molten rock – and 20 million tonnes of sulphur dioxide, the impact of which was magnified by the arrival of a giant typhoon. Tens of thousands of people were evacuated.

Steve Nerem, a co-author of the study, said: “This study shows that large volcanic eruptions can significantly impact the satellite record of global average sea level change. So we must be careful to consider these effects when we look for the effects of climate change in satellite-based sea level record.”

The main contributor to rising sea levels is the melting of the giant land-based ice sheets of Greenland and Antarctica.

The Antarctic ice sheet is the biggest, covering an area of about 5.4 million square miles, or about 98 per cent the continent. It contains about 61 percent of all fresh water on the Earth.