Oceans cover 71% of the planet’s surface. They make up about 300 times more habitat (by volume) contribute 46% of global primary production and house far more biodiversity and biomass than terrestrial habitats. The oceans are also huge reservoirs for nutrients and gases, including CO2, and ocean currents redistribute heat around the planet, impacting atmospheric circulation, regional weather patterns and rainfall distribution.
Marine organisms are not merely passive occupants that are impacted by physical processes of the sea around them – they actively influence climate on a planetary level. They play important roles in the cycling of carbon, nitrogen and other key elements and influence the development clouds. A comprehensive review article published in Current Biology earlier this summer details the dramatic and perhaps irreversible effects that climate change is having on these life forms. These effects are being felt throughout ocean systems, from individual genes to whole ecosystems, from the smallest rock pools to entire ocean basins.
The direct consequences of global warming on the oceans are increasing sea surface and interior temperatures; disturbed weather patterns, including changes in the frequency location and intensity of storms; rising sea levels; increasing ocean acidity; expansion of low-oxygen (hypoxic) zones; changing nutrient availability; changes in salinity (fresher near poles, saltier near tropics); and altered ocean circulation. In many cases, the rate of change occurring faster than ever before. Some organisms may be able to respond quickly, many will not, depending on many different factors, such as their ability to move out of inhospitable areas into better ones and the other stresses they are experiencing, such as overfishing, habitat loss, or pollution.
Coral reefs are one of the marine ecosystems threatened by multiple effects of global climate. Corals and, in particular, their symbiotic zooxanthellae algae are highly sensitive to increases in temperature. Above 31 degrees Celsius, zooxanthellae are ejected and coral bleaching ensues. The intensity and scale of bleaching has increased since the 1960s, and major bleaching events in 1998 and 2002 affected entire reef systems. Waters of the Great Barrier Reef are expected to warm by between 1 and 3 degrees Celsius over the next 100 years, so the risk of high temperature press events that could be fatal to corals is increasing. Corals require calcium carbonate (in the form of aragonite) to build skeletons, but acidification is driving availability down. Calcification of Great Barrier Reef corals has declined by 14.2% since 1990. Weakened coral skeletons are prone to storm damage and more vulnerable to overgrowth by algae; algae-dominated reefs have much lower biodiversity than corals. As sedentary creatures, corals are unable to move to more hospitable waters. Coral reefs are habitats for many animal species so loss of coral will lead to reduced biodiversity and possible extinctions of organisms dependent upon reefs. Losses of fish and invertebrates will also have major impacts on fisheries, tourism and other human uses of reefs.
Compared to the land, seawater is normally a stable habitat; marine organisms may be less able to withstand change than those on land. Although mobile marine species may be able to shift distribution in response to climate change, for sedentary organisms and species with narrow ranges of temperature tolerance the rate of local change may be more rapid than their ability to adapt. If present-day change continues unchecked, the impact on marine systems could be as great as during the mass extinctions of the past.
For more on ocean acidification see “Ocean acidification: the “other carbon problem” posted August 12, 2009.
What ocean and climate change topics concern you the most? Have you seen issues written up in the papers or journals that you’d like to see discussed here? Let me know!