By Nanci Bompey

WASHINGTON, DC – Human-generated emissions are largely responsible for warming the top layer of the ocean over the past four decades, causing water to expand and sea level to rise, according to a new study.

Climate models show that human activities, like burning fossil fuels, are responsible for 87 percent of the sea level rise since 1970 that’s been caused by swelling volume of the upper ocean. Natural forces, like solar radiation and volcanic activity, are responsible for the remainder of the increase in the upper ocean’s share of warming-induced, or thermosteric, sea level rise, according to a new study recently published in Geophysical Research Letters, a journal of the American Geophysical Union.

“The fact that greenhouse gases from human activity have an impact (on thermosteric sea level rise) is well known but nobody up to now has been worried about putting a number on that,” said Marta Marcos, a researcher at the Mediterranean Institute for Advanced Studies in Mallorca, Spain and the lead author of the new research. The analysis is the first to quantify how much of an influence human activity has had on thermosteric sea level rise in the top layer of the ocean, she added.

Figuring out how much of an influence humans are having on specific climate variables, like thermosteric sea level rise, is important for assessing what will happen as the planet warms, Marcos said. Sea level rise can cause flooding and erosion in low-lying coastal areas.

Thermosteric sea level rise is the increase in ocean height caused by the increasing volume of the ocean that occurs when seawater warms up and expands. Since 1970, global sea level has been rising by 2 millimeters (.08 inches) per year, of which .8 millimeters (.03 inches) per year* is due to increased warming in all layers of the ocean. More than half of that rise, or .46 millimeters**, results from thermal expansion specifically in the upper-ocean layer, or the top 700 meters of seawater. The rest of the increase in global sea level is caused by melting ice, and changes in reservoir levels and the shape of the Earth.

The new study, showing that the top layer of the ocean is warming mainly because of human activity, “means that all the consequences of this warming – sea level rise, higher storm surges – are because of our activity,” Marcos said. “There’s almost nothing natural going on.”

To calculate the human influence on thermosteric sea level rise, the researchers used the latest climate models that simulate the rise in thermosteric sea level in the top 700 meters (2,297 feet) of the ocean from 1950 to 2005. This historical simulation included both the influence of natural variability, and greenhouse gases and aerosols generated by human activity. They then looked at another simulation that examined only the contribution of natural variability to thermosteric sea level rise over this same time period. The difference between these two simulations allowed the researchers to calculate how much of the thermosteric sea level rise can be attributed to emissions of gases associated with human activity.

The researchers found that human activity was responsible for an increase in thermosteric sea level rise of .40 millimeters (.016 inches) per year from 1970 to 2005, compared with an increase of .46 millimeters (.019 inches) per year during the same period as a result of both natural variability and human activity. This means that 87 percent of the observed global thermosteric sea level rise of the upper water column of the ocean since 1970 is associated with human activity, according to the paper.

“The difference is so big, the number is so high, that quite clearly the impact of human activity is evident,” Marcos said.

The study also found that the human contribution to thermosteric sea level rise varies by region. Human activity is responsible for all of the increase in thermosteric sea level rise in the Tropical Pacific and the Southern Ocean, while natural variability accounts for 35 percent of the warming in the North Atlantic, according to the study.

Marcos said although the concentration of greenhouse gas and aerosol emissions has increased ocean temperature globally, the entire ocean does not warm at the same rate. Changes in wind conditions and ocean circulation can affect how much heat each region of the ocean takes up and how it is distributed around the world. Each ocean region has its own characteristics and will respond differently to global warming, she said. Marcos said she plans to more closely examine these regional differences.

* This number comes from Intergovernmental Panel on Climate Change, Fifth Assessment (2013) and covers the period from 1970 to 2010.

**This number is based on numerical modeling of the period from 1970 to 2005 – the time period studied by Marcos and her colleagues. Marcos says that the numerical models on which her study is based stop in 2005.

Press release from the Mediterranean Institute for Advanced Studies (in Spanish): http://www.imedea.uib.es/noticias.php?nid=MTMwMA==

– Nanci Bompey, AGU Public Information Specialist – Writer