One Measure of Global Warming

Melting Glacial Ice

One story that has gained a lot of traction over the last 20 years is the one about the power of man-made global warming to change the planet. Is the story true or not? Scientists have wrestled for years with the problem of measuring climate change in the oceans since they cover 70% of the planet’s surface. The logic says that with that kind of coverage, it should be easier to spot the evidence there. Skeptics, therefore, crow that scientists’ limited success proves there is no such thing as global warming. Now a Canadian team has found a way to measure climate change by documenting what temperatures suit different fish species and following their migration to more comfortable venues.

As the world warms, fish that find the sea temperatures too hot for comfort tend to move north or south, away from the tropics-- or to deeper and therefore cooler waters. Dan Pauly of the University of British Columbia in Vancouver, Washington and his colleagues have found that the mix of fish in the world’s major fisheries has changed since 1970. For example, sea bass and the historic Mediterranean delicacy, red mullet, have moved north into British waters, Pacific salmon are now found in the frigid Beaufort Sea north of Prudhoe Bay and Atlantic cod are leaving the Grand Banks off Newfoundland devastating a historic North American fishing economy.

Red Mullet

Scientists say the average surface water temperature of major fisheries has increased 1° Celsius; and that confirmed fish migrations in or out of fisheries corresponds with water temperature change, not with fishing pressures or with oceanic features like ocean currents. Pauly’s colleague, William Cheung of University of British Columbia notes that it has been difficult to detect evidence of change because of the “over-exploitation of traditional fishing grounds” and greater pressure on more distant and deeper water fisheries. (More on this later as we discuss the history of the Grand Banks.)

Fish Thermometer 

Evidence of climate change has been well hidden by the more evident flux in species’ temperature preferences and over-fishing, so the researchers invented a “fish thermometer”. This is nothing more than meticulously tracking changes in the patterns and makeup of fish catches over time. As the mix of species changes, they were able to examine the temperature preferences of the species caught in the fishery and compare the mix of species to historical catches. The study spanned the years 1970 to 2006. This new scale of measurement recorded an overall warming rate of 0.17°C per decade. Non-tropical ocean waters, specifically, are warming at an even faster rate— 0.22°C per decade.

To further annoy skeptics, the measured rate of change was much faster in some regions. The north-east Atlantic, for instance, has been warming at 0.49°C a decade as measured by the fish thermometer, even though sea surface temperatures showed only a 0.26°C rise as measured by other instruments. Warm water species are on the move, to what were once considered cooler seas.

If such a change is inconvenient for global warming skeptics, it’s very bad news for fishermen in the tropics where many of the world’s poorer people subsist on local seafood. The more temperate zones will see a migration of species from the equatorial zones, but fish are not likely to migrate towards the tropics. So as global warming makes the equatorial seas too hot for comfort, fish catches are likely to fall, and yet another source of nutrition will dwindle.

Pretty Fish, Voracious Eaters

The migration of tropical fish, in particular, can be devastating to more temperate marine ecosystems. Invading tropical species are stripping kelp forests in Japan, Australia, and the eastern Mediterranean and mowing down sea grass in the northern Gulf of Mexico and Atlantic seaboard. Sea grass beds and kelp forests are known as the sea's nurseries because they have nooks and crannies filled with nutrients that feed and protect fish larvae and juveniles from marauding predators. The kelp and sea grass, however, are being replaced with other warm-water species such as coral that follow the arrival of tropical fish.

Parrot Fish

Many tropical fish species are "browsers" that clean coral of algae and plants that could otherwise choke the reefs. But when these herbivorous fish move toward more temperate waters, they eagerly unleash their voracious appetites on a bountiful harvest of kelp or sea grass. Once a tropical fish species arrives in a new area, it can quickly mow down vegetation and the lush habitat that protects other species. For instance, shrimp, crab, and other species that typically spend the first year of their lives hidden from predators in the protection of the grasses disappear without cover, leaving a void for the creatures that depend on them for food.

Kelp Forest

Loss of a kelp forest or sea grass bed can have devastating effects not only on native plants and animals, but also on commercial fisheries. In southern Japan, for example, the arrival of parrot fish destroyed as much as 40 percent of the kelp forests there. These lush areas were once thick with abalone and spiny lobster, which supported a famed fishery. When the kelp vanished, so did abalone and spiny lobster.

Over-fishing or Global Warming— Expediency at any cost?

U.S. and Canadian fishermen have harvested Atlantic cod since the 17th century. Cod was said to be so abundant then that you could almost walk across the ocean on their backs, and it was one of the most lucrative products traded during colonial times. During the 1950s and 60s, technological advances in trawler design and power modeled on the factory whaling ships that had devastated the last remaining whale populations began to lay waste to cod populations.

Map Showing the Grand Banks

These huge factory trawlers came from distant countries, attracted by a seemingly endless supply of cod. Their huge nets hoisted up massive quantities of fish; they rapidly processed and deep-froze the catch, working around the clock in all but the worst weather conditions. They could haul up as much as 200 tons of fish an hour, twice as much as a typical 16th century ship could catch in an entire season. This unbearable fishing pressure throughout the latter part of the 20th century led to commercial collapse of the cod fishery in the mid-1990s.  A concerted effort to rebuild these stocks began soon after. By 1992 the levels of northern cod were the lowest ever measured, forcing the Canadian government to close the fishery, throwing 30,000 people out of work and devastating many fishing communities.

So, says the global warming skeptic, the cod vanished because of over-fishing-- not because of climate change. While true, the over-fishing and ecosystem plundering is hiding the measurable fact that what few cod survive are migrating to colder water— to avoid warming seas. Despite a lengthy ban on cod fishing near the Grand Banks, the cod does not appear to be in recovery.

There is an interesting parallel to be drawn between global warming skeptics (oil companies and allied interests) and vanishing cod skeptics (commercial fishing industry and allied interests). Both subscribe to short-term expediency rather than the long-term health of the planet. Indeed, prior to the actual collapse of the fishery in the early 1990s, the Canadian government preferred to listen to the industrial fishing companies which claimed there was no problem with the declining catches— just like burning fossil fuels has little to do with global warming.

Scientific warnings in the late 1980s  went unheeded because any cut in catches or oil consumption would cause politically unacceptable job losses and cut oil company profits. It seems science is only as good as the money behind it.