The world's oceans perform a great task that's impossible to see. In addition to providing a home for countless organisms, these vast bodies of water regulate earth's temperatures and absorb greenhouse gases.

"The oceans always functioned as a refrigerator and as storage for carbon dioxide," says Hans-Otto Pörtner, a scientist at Bremerhaven's Helmholtz Centre Polar and Marine Research at the Alfred Wegener Institute and co-author of the study "Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios" published Thursday.

"The oceans have saved about 93 percent of the additional heat caused by the greenhouse effect since the 1970s. They slowed the warming of our planet," says Pörtner.

But the scientist and his colleagues warn that "the oceans can't go much longer." Their cooling effect is at risk. The seas are now getting warmer, not only on the surface but also to depths of 700 meters. And they could undergo an irreversible change even if greenhouse gas emissions are reduced effectively, according to the study, which looks at the effect of climate change on the seas.

Migrating species

Rising temperatures have already had dramatic consequences for marine life with limited tolerance for warmer climes. Many have been driven from their native habitats to cooler regions, such as the poles or further into the depths of the sea.

Cod, for instance, can no longer stand the heat in the North Sea. Since the 1960s, water temperatures there have risen by an average of 1.7 degrees Celsius. With hot summers becoming a more regular feature, the fish started to make their way northward, moving seven kilometers closer to the Arctic each year. Their young's main source of food - a kind of crustacean called copepod - is also disappearing.

The humble cod is not alone. Scientists estimate that almost two thirds of all marine fish species are on the move to cooler waters - even in the southern hemisphere. There, the striped marlin, a fish usually at home in tropical and subtropical waters, can now be sighted off the cooler coast of Tasmania.

On their way to the poles, the migrating species clash with native species, competing with them for habitat and food. The northbound cod, for instance, is displacing the Arctic cod - a favorite prey for seals - as it moves into new territory.

Warm and acidic oceans

Concentrations of carbon dioxide in Earth's atmosphere have increased by 40 percent on pre-industrial levels, write Pörtner and his colleagues. And rising temperatures are only one consequence of this development. The ocean's handy ability to absorb more than a quarter of the carbon dioxide currently produced by humans each year comes at a price. Absorbed carbon dioxide dissolves in water and forms carbonic acid, in turn changing the ocean's PH value from slightly alkaline to acidic. The effects of this change are already visible, the researchers add.

Corals, mussels and other marine life-forms, such as sea snails, with skeletons and shells made of calcium carbonate are in serious trouble in acidic waters, which eats away at their outer layers.

Damage to corals puts whole ecosystems at risk - they are home not only to hundreds of thousands of species but also provide food to a large number of people. But an optimal solution to stop both increasing water temperatures and acidification does not exist, say the researchers.

Bad or very bad?

Pörtner and his colleagues simulated two possible scenarios for the planet in their study. The consequences of the first may not be nice but are acceptable given the circumstances. In this scenario, air temperature will rise less than two degrees Celsius by the year 2100.

Although, the "risk especially for tropical corals and shells climbs to a critical level, other risks remain rather moderate," explains the study's lead author, Jean-Pierre Gattuso. For this scenario to be realistic, we would have to act quickly to reduce the carbon dioxide emissions.

The second scenario is much more pessimistic and assumes that it will not be possible to limit an increase in temperature.

"If we continue with business as usual we will lose large areas of coral reefs up to a few safe havens. Temperature and ocean acidification reinforce the negative effects. Add to that the lack of oxygen in some areas and we will see even more habitats shrinking," Pörtner says. That would have a dramatic impact on all ways in which humans use the ocean.

The researchers want the results of their study to be understood as a plea to the international climate conference, COP21, being held Paris in December 2015. While the problems oceans face do have a higher profile today thanks to the IPCC world climate report, more awareness is needed - negotiators should be aware of the importance of oceans for Earth's climate system, Pörtner says.

"This is especially true when it comes to risks such as rising sea levels. However, just a few are aware that rising temperatures above 1.5 Celsius compared to pre-industrial times, significantly increase the risk that the sea level rises over the turn of the century. We are talking about several meters in about 300 year's time," says Pörtner.