Editor’s note: This post was updated on Aug. 23, 2018
Climate change is taking a toll on forests, farms, freshwater sources and the economy – but ocean ecosystems remain the epicenter of global warming.
Even with their vast capacity to absorb heat and carbon dioxide, oceans were 0.17 degrees Celsius (0.3 degrees Fahrenheit) warmer in 2017 than in 2000, and the warming trend appears to be accelerating. More than 90 percent of Earth’s warming since 1950 occurred in oceans, so it’s easy to see why scientists are concerned.
Here are five ways these ever-warmer temperatures are affecting our oceans:
1. Coral bleaching
As early as 1990, coral reef expert Tom Goreau and I pointed out that mass coral bleaching events observed during the 1980’s were probably due to anomalously warm temperatures related to climate change.
It’s now evident that many coral reefs, including Australia’s Great Barrier Reef, are dying. Mass coral bleaching results in the starvation, shrinkage and death of the corals that support the thousands of species that live on coral reefs.
Coral reefs are particularly sensitive to increases in temperature. And new research shows that oceans are now experiencing longer and more severe “marine heat waves” that could push even more ocean animals and ecosystems to their limits.
2. Fish migration
In addition, many fish species are moving toward the poles in response to ocean warming, disrupting fisheries around the world.
A recent study predicts that climate change will force hundreds of ocean fish northward, hitting North American fisheries that depend on Pacific rockfish, Atlantic cod and black sea bass especially hard.
3. Drowning wetlands
Rising sea levels, partly the result of heat absorbed by the ocean, is also “drowning” wetlands. Such areas normally grow vertically fast enough to keep up with higher water levels. But recently, this rise has accelerated to the point where wetlands can no longer keep their blades above water.
Coral reefs and sea grass meadows are also in danger of “drowning” since they can only photosynthesize in relatively shallow water.
Whether wetlands, coral reefs and seagrass meadows can keep up with rising seas will depend on many factors, however, including how healthy they are and how clean the water is.
4. Ocean acidification
Oceans today absorb about one-third of the carbon dioxide humans send into the atmosphere, about 22 million tons a day.
This great service, which has substantially slowed global warming, has been accomplished at great cost: The trend in ocean acidification is about 30 times greater than natural variation, and average surface ocean pH, the standard measure of acidity, has dropped by 0.1 unit.
That’s a 25-percent increase in acidity, which is significant.
Higher acidity is damaging many ocean species that use calcium carbonate to form their skeletons and shells. Studies have shown that calcium carbonate formation is disrupted if water becomes too acidic.
Ocean acidification also appears to be affecting whole ecosystems, such as coral reefs, which depend on the formation of calcium carbonate to build reef structure, which in turn provides homes for reef organisms.
5. A disastrous positive feedback loop
Finally, acidification also appears to be reducing the amount of sulfur flowing out of the ocean into the atmosphere. This reduces reflection of solar radiation back into space, resulting in even more warming.
This is the kind of positive feedback loop that could result in run-away climate change – and of course, even more disastrous effects on the ocean.
Oceans are at the brink
For decades, the ocean has been absorbing carbon dioxide dumped into the atmosphere by burning fossil fuels, faithfully capturing the extra heat that elevated atmospheric carbon dioxide levels produce.
But even the ocean has limits. We are bumping up against them now, with damaging consequences for the whole world.