Rising sea temperatures, ice-free Arctic summers and a changing marine food chain

Global sea surface temperature is approximately 1 degree C higher now than 140 years ago. Projections show the temperature increases will persist throughout this century. Ice-free summers are expected in the Arctic by the end of this century, if not earlier.

Sea surface temperature in European seas is increasing more rapidly than in the global oceans. Over the past 25 years the rate of increase in sea surface temperature in all European seas has been about 10 times faster than the average rate of increase during the past century. In five European seas the warming occurs even more rapidly. In the North and Baltic Seas temperature rose five to six times faster than the global average over the past 25 years, and three times faster in the Black and Mediterranean Seas.

Sea surface temperature anomaly for period 1870–2006

Projections made in 2007 by the International Panel for Climate Change (IPCC) show that sea surface temperatures will continue to increase globally throughout the 21^st century. In the early part of this century, ocean warming will be the greatest in the upper 100 m of the ocean. Towards the end of the century temperatures will also increase further down into the deep ocean.

The IPCC also suggests ocean warming will be relatively large in the Arctic and along the equator in the eastern Pacific and that the North Atlantic and the Southern Ocean will warm to a lesser extent. Currently it is not possible to project changes in sea surface temperatures for the different geographic regions across Europe because the spatial resolution of the ocean-climate models is not high enough.

Towards ice-free Arctic summers

Sea surface temperature plays an important role in the oceans heat content, and regulates climate. This is illustrated in the Atlantic Ocean currents which play a key role in regulating the climate in Europe. Heat is transported northward with the Gulf Stream and influences precipitation and wind regimes. Heat also transfers from the atmosphere into the ocean, causing temperature to increase both at the sea surface and in deeper waters.

One of the most visible ramifications of increased temperature of the ocean is the reduced area of sea ice coverage in the Arctic polar region. If this continues, a feedback mechanism is expected, i.e. the reduced polar ice coverage will speed up global warming, which will further affect ocean circulation and weather patterns.

Although there is an element of uncertainty, projections show that summer ice is very likely to continue to shrink in extent and thickness, leaving larger areas of open water for an extended period. In winter, however, it is very likely that sea ice will continue to cover large areas.

Until recently several international assessments concluded that mostly ice-free late-summers could occur by the end of this century. However, the actual melting has been faster than the average trends simulated by the climate models used for these assessments. New model studies suggest that ice-free summers may occur much sooner. Exactly when is impossible to predict with confidence, due both to the limited understanding of the processes involved and the large variability of the system.

Observed and projected Arctic September sea-ice extent 1900–2100

Impacts on marine ecosystems

There is an increasing body of evidence suggesting that many marine ecosystems in European seas are affected by rising sea temperature and the warming of the air. Some organisms are now appearing earlier in their seasonal cycles than in the past.

In the North Sea, for example, the seasonal peak of abundance of decapod larvae, an organism at the bottom of the marine food web, is now four to six weeks earlier than in 1958.  This can seriously impact the ability of higher organisms, such as fish and sea birds to survive in certain areas.

Changes in the timing of seasonal cycles have important consequences for the way organisms within an ecosystem interact. Ultimately, this also has affects the structure of the marine food-webs at all levels in the food chain.

The consequences include:

• increased vulnerability of North Sea cod and stocks to over-fishing;
• decline in seabird populations

Fish and plankton have expanded their geographical distribution further north in response to increasing temperatures. Depending on the species this expansion occurs at an average rate of 30 -100 km per year.

Marine species are able to adapt genetically to changed conditions. However, with the current pace of climate warming this may be hampered because genetic changes require several reproductive cycles to occur.

Live maps

More information

The EU has many activities to monitor ocean temperature.  The Marine Core Service, which is being developed as part of the Global Monitoring for Environment and Security (GMES) initiative, delivers among others daily maps of sea surface temperature for all European Seas.