Several complex physical and chemical processes in the sea are capable of resulting in the catastrophic destruction of sea life. Such processes could also be triggered off by man-made events such as nuclear explosions, generation of electricity by transference of deep ocean water to the surface, or by the large scale diversion of rivers. The effects can take the form of: lowering of temperature due to unusually cold weather, lowering of salinity from exceptional river discharge, displacement of ocean current (such as the El Nino off the coast of Peru), vertical mixing (bringing low oxygen content water to the surface as in Walvis Bay, Namibia, or high hydrogen sulphide content water as in the Black Sea). Related phenomena may result in explosive growth of micro-organisms which concentrate substances toxic to some sea life. Other threats are in the form of urban and industrial development and destruction of highly productive coastal wetlands and reef areas; chemical and radioactive pollutants washed from the land, discharged and dumped into the ocean, or deposited from the atmosphere; uncontrolled exploitation of ocean resources; and mounting pressure on the world's fisheries.
Such changes may be of disastrous significance to economies highly dependent on the fishing industry, or to bird populations dependent on fish (such as guano birds in Peru). The impact of such activity on climate depends on its extent. Ocean temperature anomalies of one degree or larger produce large local changes in the fluxes of sensible and latent heat to the atmosphere, modifying climates even at great differences. Proposed large-scale diversion of rivers flowing into the Arctic could well destabilize the Arctic ice pack.
As a consequence of the global warming, changes in currents in the North Atlantic pose the frightening prospect that the Gulf Stream might weaken or veer away, causing some European countries to cool, rather than heat up.