News in Science

Wet La Nina years mask sea level rise

Monday, 24 March 2014 Alister Doyle
Reuters

Heavy rains from the Amazon to Australia have curbed sea level rise so far this century by shifting water from the oceans to land, according to a study that rejects theories that the slowdown is tied to a pause in global warming.

Sea level rise has been one of the clearest signs of climate change — water expands as it warms and parts of Greenland and Antarctica are thawing, along with glaciers from the Himalayas to the Alps.

During the 1990s, global sea levels rose at a mean rate of around 3.5 millimetres a year. But from 2003 to 2011 this slowed down to 2.4 millimetres a year.

However, the rate would have been around 3.3 millimetres a year once natural shifts led by an unusually high number of La Niña weather events that cool the surface of the Pacific Ocean and cause more rain over land were excluded, report French scientists in the journal Nature Climate Change .

They analysed time-series data of global mean sea level from five prominent research groups, including the CSIRO, for the periods 1994 -2002 and 2003-2011.

“There is no slowing in the rate of sea level rise” after accounting for the natural variations, says lead author Anny Cazenave of the Laboratory for Studies in Geophysics and Spatial Oceanography in Toulouse, France.

The scientists found the largest cause of interannual sea-level variability is the El Niño-Southern Oscillation, which impacts upon the global water cycle through changes in land water and atmospheric vapour content.

Following the major El Niño event of 1997/1998, the past decade has favoured La Niña years.

In La Niña years, more rain fell away from oceans, including over the Amazon, the Congo basin and Australia, says Cazenave. It is unclear if climate change itself affects the frequency of La Niñas.

Rainfall over land only temporarily brakes sea level rise.

“Eventually water that falls as rain on land comes back into the sea,” says Anders Levermann, a professor at the Potsdam Institute for Climate Impact Research, who was not involved in the study.

“Some of it goes into ground water but most of it will drain into rivers, or evaporate.”

Hiatus in warming

The apparent slowing of sea level rise coincided with what the UN panel of climate experts calls a hiatus in global warming at the Earth’s surface, when temperatures have risen less sharply despite record emissions of greenhouse gases.

But the study finding that there has been no slowing of sea level rise between the 1990s and the 2000s “clearly advocates for no recent slowdown in global warming,” write the authors.

Many scientists suspect that the “missing heat” from a build-up greenhouse gases in the atmosphere is going into the deep oceans as part of natural variations in the climate.

But, because water expands as it warms, that theory had been hard to reconcile with the apparent slowdown in sea level rise.

Sea levels have risen almost 20 centimetres since 1900. The UN panel of climate experts expects an acceleration, with gains of between 26 and 82 centimetres over 100 years to the late 21st century.

Last year, another study said that unusually heavy downpours over Australia in 2010 and 2011 had curbed sea level rise, before a rebound reaching a rate of about one centimetre a year globally, partly as water flowed back into the sea.

“It has tailed off in the past 12 months or so” to above three millimetres a year, says John Fasullo of the U.S. National Center for Atmospheric Research who led the study.