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Global climate and atmospheric change

Part of the fishing industry's inadvertent bycatch - a seal ensnared in broken fishing net

Source: UNEP, L. K. Nakasawa, Topham Picturepoint
The rapid global warming caused by human-induced changes in the atmosphere that is projected by the IPCC would have dramatic effects on the ocean (IPCC 2001), threatening valuable coastal ecosystems and the economic sectors that depend upon them. Other potential impacts are complex and poorly understood. Polar warming, and melting of the ice caps, could slow down the global atmosphere/ocean 'heat engine', potentially altering the flow of major ocean currents (Broecker 1997). The warming of the ocean's surface layers, and an increased input of fresh water, could reduce the upwelling of nutrients that supports much of the ocean's productivity. On the other hand, the highly productive upwelling on the eastern side of some oceans could intensify if, as some projections predict, relatively greater warming occurs there (Bakun 1996). The IPCC predicts that storms and other extreme weather events will increase in frequency and intensity (IPCC 2001), increasing natural disturbances to coastal ecosystems and perhaps reducing their ability to recover.

There is particular concern about the possible effects of global warming on coral reefs. During the intense El Niņo of 1997-98, extensive coral bleaching occurred on coral reefs worldwide (Wilkinson 1998, Wilkinson and others 1999). While some reefs quickly recovered, others, particularly in the Indian Ocean, Southeast Asia and the far western Pacific, suffered significant mortality, in some cases more than 90 per cent (Wilkinson 1998, 2000).

Some models predict a long-term shift to an increased frequency and intensity of El Niņo events or similar conditions. If this occurs, bleaching could also become more frequent and intense, with irreversible damage to reefs. There is evidence that a long-term decline of reefs in the remote Chagos archipelago in the Indian Ocean is related both to El Niņo events and to a long-term rise in surface temperature (Sheppard 1999). Mass bleaching of reefs in various parts of the world was also observed in 2000, a possible sign that bleaching is becoming more frequent. Reefs may also be threatened by a higher concentration of CO2 in seawater which impairs the deposition of their limestone skeletons.

Proposed protection measures to address a sealevel rise caused by climate change have shifted away from solid constructions such as seawalls in favour of a mix of soft protection measures (such as beach nourishment and wetland creation), adaptive planning (such as new building codes), and managed retreat, including cessation of new coastal construction (IPCC 2001). Some proposals to address global climate change are themselves a cause for concern, particularly those to short-circuit the natural transfer of CO2 from the atmosphere to the ocean by fertilizing large areas of the ocean surface with nitrogen or iron to enhance phytoplankton growth, or to inject CO2 directly into deep waters. The effects of these large-scale measures cannot be predicted but are potentially enormous.

Small island developing states (SIDS) and lowlying coastal areas are particularly vulnerable to the effects of rising sea levels and more extreme weather. Furthermore, they are essentially entirely coastal and therefore more dependent upon coastal and marine resources. Recognition of this special vulnerability in Agenda 21 of the UN Conference on Environment and Development (UNCED) led to the adoption in 1994 of the Barbados Programme of Action on the Sustainable Development of Small Island States.

Jellyfish in the Black Sea

The effect of a jellyfish invasion on the Black Sea is one of the best documented examples of the far reaching economic and ecological consequences that can follow the introduction of an alien species into an environment favouring its almost unlimited expansion.

Mnemiopsis leidyi, a comb jellyfish, originates on the eastern seaboards of both North and South America. It abounds in ports and harbours, and is pumped in ballast water into cargo ships. These jellyfish can live for 3-4 weeks without food, by reducing the size of their bodies, so they can easily survive the 20-day voyage to the Black Sea. They were first found in the Black Sea, off the south-east Crimea, in 1982.

Damaging human activities - including overfishing, pollution, water extraction and barrages on rivers running into the sea - had set the stage for its entrance. Overfishing and eutrophication seem to have combined to remove top predators such as turbot, bluefish and monk seals and to cut the numbers of plankton-eating fish severely, opening up a niche for the jellyfish. Meanwhile plankton proliferated.

Hermaphroditic and self-fertilizing, the numbers of jellyfish exploded from 1988 onwards. The populations of plankton crashed as the invaders ate them. Fish stocks collapsed, partly because the jellyfish deprived them of their food and ate their eggs and larvae. The catch of the former states of the Soviet Union plummeted from 250 000 to 30 000 tonnes a year, and it was much the same story in Turkey. At least US$300 million was lost in falling fishery revenues between the mid-1980s and the early 1990s, with grave economic and social consequences. Fishing vessels were put up for sale, and fishermen abandoned the sea.

Source: GESAMP 2001b

 

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