Looking for Global Warming? Check the Ocean

We tend to focus on land surface temperatures, because, well, that’s where we live. And human greenhouse gas emissions have ensured their steady rise since the start of the Industrial Revolution, punctuated by 2014 setting therecord for hottest year.

But surface heat is but a fraction of the climate change equation. Only 7 percent of the heat being trapped by greenhouse gases is sticking around in the surface and atmosphere of the planet. The other 93 percent? That's ending up in the ocean, though some scientists expect some of that heat will eventually find its way back to the surface andtrigger even more warming.

Research indicates that since 1970, the world's oceans have absorbed 251 zettajoules of energy. But zettajoules are a pretty esoteric way of talking about heat. In more relatable terms, just one zettajoule would be enough to power 25 billion average American homes for a year. Or put another way, the average annual energy absorbed by the upper ocean alone equals 43 times the amount of energy the U.S. consumed in 2012, the most recent year with data available. Either way, we're talking a whole lot of heat.

“We continue to be stunned at how rapidly the ocean is warming,” Sarah Gille, a Scripps Institution of Oceanography professor, told Climate Central last year. 

Not only has the ocean been absorbing more heat than the surface of the earth, it's been absorbing ever greater quantities of it. Earlier this year, the ocean's heat literally went off the charts. The y-axis on a graphic that the National Oceanic and Atmospheric Administration uses to measure ocean heat wasn't high enough to accomodate the latest measurements. Luckily, it was an easy fix to just add a few more zettajoules to the axis. 

Unfortunately it won't be quite as easy to fix the ocean, which has a long memory. It will take decades or even centuries after humans stop burning fossil fuels and emitting greenhouse gases for the ocean to reach equilibrium. In the meantime, all that extra stored heat will cause seawater to expand, raising sea levels, and altering the ecosystems that marine plants and animals have adapted to for eons.