The massive amounts of carbon dioxide and other greenhouse gases that have entered our atmosphere since the Industrial Era began in the 18th century have had significant effects on the world’s oceans.
Solar energy striking Earth is either reflected back into space or absorbed and then radiated back as heat. Greenhouse gases trap some of that heat. Because they are accumulating in the atmosphere, excess heat is accumulating too, and the Earth is warming.
“Greenhouse gases like carbon amplify the amount of excess heat left over because they prevent heat energy from releasing from Earth’s system,” says oceanographer Tim Boyer of the National Oceanographic and Atmospheric Administration.
Excess heat trapped in the atmosphere by our greenhouse gases spreads into the oceans. They've absorbed about 90 percent of that heat. As a result, they've been warming steadily for a long time.
Sea surface temperatures over the last several decades reflect such warming, but are also sensitive to weather events like hurricanes and El Nino. That explains why temperatures fluctuated from one year to the next as far back as the mid-1800s.
Along with the warm air itself, the heat absorbed by the oceans melts ice in the polar regions, releasing fresh water that accounts for more than half of all sea level rise; the rest is attributed to the expansion of seawater as it warms. “This has obvious effects on coastal area flooding and real estate,” says NOAA oceanographer Andrew Allegra, as well as implications for marine life.
The oceans don't just soak up excess heat from the atmosphere; they also absorb excess carbon dioxide, which is changing the chemistry of seawater, making it more acidic. “Ocean acidification is one simple and inescapable consequence of rising atmospheric CO2 that is both predictable and impossible to attribute to any other cause,” says oceanographer John Dore of Montana State University.
“Almost every aspect of marine biology—from bacteria to blue whales—is in some way influenced by the acid-base balance of seawater itself,” he says. “The effects on other marine life are harder to predict, but it could take thousands of years or more to undo what we are presently doing to ocean pH.”