The sea has quietly absorbed most of the heat and much of the carbon we have released. That service is real, and it has limits.
When we talk about a warming world, we usually mean the air. But the atmosphere has taken only a small share of the extra heat trapped by our emissions. The overwhelming majority — more than ninety percent of it — has gone into the ocean, which has warmed slowly and hugely while shielding the surface where we live from the full force of the change.
The ocean does us a second favor, too. It has absorbed a large fraction of the carbon dioxide we have put into the air, dissolving it into seawater and dampening the pace of atmospheric warming. Between soaking up heat and soaking up carbon, the sea has been the planet great buffer, quietly softening a blow we would otherwise have felt far more sharply by now.
But a buffer is not a bottomless sink. As seawater warms it holds less dissolved gas, so its ability to keep absorbing carbon weakens even as we ask more of it. The carbon it does absorb makes the water more acidic, straining the corals, shellfish, and plankton that anchor entire food webs. The favor comes with a bill that is already being paid in the biology of the sea.
The hardest part to grasp is the lag. The ocean is so vast and so slow that it responds to today emissions over decades and centuries. Heat committed now will keep surfacing long after the air stabilizes, which means the sea is both our greatest ally against rapid change and a reservoir of consequences we have already set in motion.
Add your own version here: the ocean measurement that reframed the problem for you, the buffer effect most people overlook, and the number you wish were in every headline about a warming planet. — Written by the editors.