With estimates of a global temperature increase of between 3 and 10 degrees over the next century, many people are simply planning to turn up the air conditioner. But what if you’re a fish?

As this ScienCentral News video reports, it might not be land-dwellers, but those who live underwater that will be the first to feel the heat.

The Ripple Effect

As temperatures in the atmosphere rise, scientists expect the Earth’s aquatic system to be affected by more than just a corresponding rise in water temperature. One of the reasons global warming can be so confusing and controversial is that its effects can be myriad, but subtle, and even contradictory.

For instance, one definite concern of scientists is that cool- and cold-water species of marine life (such as trout) could disappear from certain waters, especially those at the southern boundaries of their habitat ranges. In that sense, the blue crabs of Chesapeake Bay should be fine—increased temperatures isn’t a problem for them. But the rise in the water level that scientists say is a result of global warming is a problem because it’s destroying the tidal marshes and grasslands of the bay. These are important habitats for the crab, particularly during its juvenile stages.

The streamflow of our waters might also change as a result of global temperature rise. If rivers start flowing from mountaintops earlier than usual, fish species like salmon that rely on these rivers during their spawning season could be threatened.

"Aquatic ecosystems depend on receiving certain amounts of water at certain times of year," says Peter Gleick, director of the Pacific Institute for Studies in Development, Environment, and Security. "They depend on certain water temperatures at certain times of year for spawning or for reproducing. They depend on certain kinds of plant life for survival. And ecologists are increasingly concerned that global warming will cause very dramatic impacts on fish and on aquatic ecosystems."

Changes in the level and flow of waters can also affect the salinity of waters near the ocean. Higher estuarine salinity has been cited by the Environmental Protection Agency (EPA) as a cause of declining oyster harvests in Chesapeake and Delaware Bays, and as a cause for wetland loss in Louisiana, Florida, and Maryland.

But sometimes it is simply matter of the rise in the water’s temperature. The EPA has warned that cold water fish habitats could be lost entirely in states like Maine, Massachusetts, Connecticut, Ohio and Nebraska, which would be devastating to those states’ recreational fishing industries. An 8-degree increase in mean annual air temperature could eliminate more than 50 percent of the habitat of the brook trout in the southern Appalachian Mountains.

Jellyfish flourish in unseasonably warm water and tend to adapt well to changes. A recent New York Times article pointed out that in several places around the world jellyfish populations are on the rise, due at least in part to the rise in water temperature. This will further complicate ecosystems because jellyfish compete with other kinds of fish for food. (Also, more people are being stung than ever before, including an Ohio resident who was fatally stung in Australia in March.)

And rising water temperatures is also the main culprit when it comes to coral bleaching, which is becoming alarmingly widespread across the globe. Sometimes referred to as the "rainforests of the sea," coral reefs are important fish habitats. They also protect coastal communities from storms and erosion.

"As we see these changes it should be of great concern," says Kenneth Rose, professor of oceanography at LSU. "We’re changing things that perhaps we cannot reverse, and we’ve not had a good track record in engineering nature to go back to the conditions we thought we wanted.

"I think we’re at a crossroads where there’s two paths we can take. One path is we focus on the uncertainties—what we don’t know—and not do anything until we know everything about everything. The other path, and the one that I hope we take, is that we focus on what we do know and we take a precautionary approach, take action."