Since the beginning of the Industrial Revolution a few hundred years ago, humans have burned massive amounts of fossil fuels; cut down huge swaths of forest; and undertaken many other activities that pump heat-trapping carbon dioxide into Earth’s atmosphere. In response, the planet has warmed up.
Only about one percent of all that trapped heat has stayed in the atmosphere, but it’s had a huge effect, warming up the air by Earth’s surface by about 1°F (0.6°C) on average over the past two centuries.
Most of the rest of the trapped heat has been absorbed into the planets’ vast oceans. Since the 1970s, the oceans have sopped up more than 90 percent of all the excess heat energy trapped by CO2. Because the oceans are enormous, and because water takes much more energy to heat up than air, that translates to a temperature increase of a little more than one degree Fahrenheit, on average, over the past century.
But the warming is speeding up. The top part of the ocean is warming up about 24 percent faster than it did a few decades ago, and that rate is likely to increase in the future.
Every little bit of warming, however small, has enormous impacts on marine life, storm intensity, and more.
Warming seas hurt marine life
The uppermost part of the ocean, down to about 2,300 feet (700 meters), has absorbed the bulk of the extra heat. The bottom few thousand feet of the ocean are not immune; they’ve sucked up another third of that excess warmth. But the uppermost skin of the sea, down to about 250 feet, is warming up the fastest, heating up by an average of about 0.11 degrees Celsius each decade since the 1970s.
Marine heat waves—the oceanic version of the sweltering heat events that ripple across Earth’s surface—are also increasing in frequency and strength, with the number of days that qualify as a heatwave increasing by more than 50 percent over the past century. During these hot events, temperatures near the surface of the ocean can spike up to several degrees above the average.
Most ocean dwellers, from plankton to fish to whales, live in the upper section of the ocean, squarely in the zone where temperatures are increasing quickest. Many of these marine organisms are sensitive to even slight or short-lived changes in temperature.
Corals, for example, are highly attuned to the temperature of the water in which they live. Warming of only about one degree Celsius can stress them out, causing them to “bleach.” That means they spit out the symbiotic algae that live inside them and usually provide them with much of their energy. Sometimes, corals can recover from these bleaching events. Other times, they can’t.
Warming seas make stronger storms
Scientists predict that warmer oceans will make storms like hurricanes and tropical cyclones more intense in the future, increasing the likelihood that they’ll reach category 4 or 5 on the Saffir-Simpson storm-strength scale; speeding up the rate at which they intensify; and increasing the likelihood that they’ll release enormous volumes of rain.
Warming seas drive sea levels higher
Warm water takes up more space than cool. As the oceans have heated up, they’ve expanded; as they get bigger, sea levels creep up.
Between 1971 and 2010, this heat-driven sea-level rise added about eight tenths of a millimeter to the height of the ocean each year. Thermal expansion has contributed to about half of all the sea-level rise observed across the planet so far—more, up until now, than contributed by melting ice from either Greenland or Antarctica or the other glaciers of the world. But those masses of ice are melting fast and are likely to overtake heat-driven water expansion as the primary contribution to global sea-level rise.