Threats: Climate change

Publication - April 8, 2008
The world’s climate is changing, and our oceans are feeling the heat. Oceans cover 70 per cent of the Earth’s surface, and small changes in the oceans’ temperature mean severe and wide-ranging impacts on the climate and marine ecosystem stability.

Missing links in the conveyor belt

The water in our oceans is constantly moving, not just on the surface, but deep below in a global ocean ‘conveyor belt’ that slowly circulates warm and cold waters around the globe. This process, known as thermohaline circulation (THC) is powered by differences in water temperatures and salinity and redistributes heat and nutrients so that life in the marine environment can thrive. It also helps regulate the earth’s climate, transporting heat absorbed at the equator toward the poles.

In the Arctic, ice shelves and glaciers are melting at an unprecedented rate and dumping huge amounts of fresh water into the North Atlantic Ocean. The North Atlantic is a vital point in ocean circulation, where water that has been pushed north by the North Atlantic Current is cooled and sinks up to four kilometres in a giant underwater waterfall. The sinking water then begins a long journey south, deep beneath the ocean’s surface.

Disruption of this process could have very serious implications for marine ecosystem health. Deep cold water is richer in nutrients than warm surface water, which is in turn richer in oxygen. Ocean circulation causes upwellings of nutrient-rich cold water to surface animals such as plankton, which forms the basis of the marine food chain. Without these upwellings, vertical stratification can occur, which essentially suffocates the bottom layers of the ocean, while depriving the top layers of nutrients.

Through the process of THC, the ocean has also been able to absorb and sequester a large amount of the CO2 that humans have emitted over the last two centuries. But our oceans have reached a point where they can no longer keep up with the rate that CO2 is being emitted into the atmosphere, leading to an upset in its pH balance and acidification of our oceans. Through chemical processes, CO2 is converted to carbonic acid when it dissolves. This can lead to corrosion of the limestone structure of existing coral reefs, and prevent the formation of new corals and shelled organisms.

Impacts of warmer water