Most scientists believe that climate change is a reality and will have profound effects on the environment. Most governments accept this, but struggle to agree a mechanism for addressing the causes. Meanwhile the planet is warming, and the marine environment is as vulnerable as any other ecosystem.
However, it is not just that warming surface waters are forcing changes in distribution of cetaceans and seabirds in response to change in food supply, although this is a real threat. A number of other impacts are being researched:
The ocean absorb a huge proportion of global CO2 and emit oxygen. However, with increased CO2 in the atmosphere more is dissolved by the sea, which increases its acidity. This has at least two important effects.
1) acidification reduces the availability of iron - an essential nutrient for phytoplankton growth. This is likely to lead to reduced phytoplankton in the upper surface of the ocean. Phytoplankton produce vast amounts of oxygen which is released to the atmosphere, if this declines the effect of climate change is exacerbated. Phytoplankton are also the base layer of the marine food chain on which all other life depends.
2) acidification impedes uptake of calcium by shellfish and coral, which is essential for building shells, so could slow their productivity and lead to a collapse in their ecosystems
It is not yet clear how this situation will develop; what scale of impact it might have and over what timescale. The picture is highly complex and a long way from being properly understood, however it is evident that without real action to stop climate change, the risks to the oceans ecosystems are immense.
As seawater temperature increases it expands in volume. It is this that will drive sea-level rise at difficult to predict rates. The potential impact on the marine ecosystems are difficult to anticipate and will be influenced by the scale of sea-level rise. There may even be some benefits; many fish and shellfish spawn in shallow coastal waters, and new habitat might be created by sea-level rise, however this might be more than offset by the loss of current spawning habitat lost as water depth increases.
Change in oceanic currents
Increasing rates of ice melt and glacier calving increases the amounts of freshwater entering the oceans at high latitudes. Although controversial (markedly differing scientific views), it is possible that this could have in important effect on oceanic currents, which move massive amounts of water. Freshwater is lighter than salt water, and one possibility is that when cold freshwater meets warm saline water the warm water current could sink. In the north Atlantic, the main surface current is the Gulf Stream which brings warm water to western Europe and gives us a moderate if damp climate. If this was to be interrupted, the entire marine ecosystem of the north east Atlantic would change dramatically, even possibly leading to rapid falls in winter temperatures and sea freezes.