At the ocean surface, the ocean and atmosphere place a natural cap on water temperature thanks to convection, the force that causes strong vertical movement in the atmosphere and makes towering thunderstorms.
As ocean temperature rises in a given region, convection pulls moisture upward into the atmosphere and builds clouds. Those clouds limit the sunlight that reaches the ocean surface and thus cool it. This process begins to take place when ocean surface temperatures reach 26° C (79° F) or so.
Very small areas of water can get very hot for short periods of time but that doesn’t say much about the overall warmth of the ocean so let’s consider regional or larger scales and monthly averages. At the present time, the limit to which ocean surface temperatures can rise at those scales is estimated to be about 30° C (86° F), though monthly maximums in a few areas in the tropics have reached 31° C (88° F). Around the world, the temperature of the sea surface and overall average temperature of the ocean are rising because there is slightly more incoming solar radiation globally than there is outgoing heat radiation.
That imbalance of heat energy results from greenhouse gas emissions caused primarily by human activities. More than 90 percent of that energy is stored in the oceans.
Scientists consider the limit to 30° C under current climate conditions but as global temperatures continue to rise, the maximum possible temperature of the surface ocean might increase over time. Some scientists have projected that if current global warming trends continue, ocean temperatures could rise between 2° C (3.6° F) and 5° C (9° F) by 2100.
- Shang-Ping Xie is a climate modeler and the Roger Revelle Chair in Environmental Science and Dean Roemmich is a physical oceanographer and co-chair of the international Argo network steering committee at Scripps Institution of Oceanography at the University of California San Diego