Topic: The ocean’s climate
Monitoring the oceans is absolutely essential to assessing the condition of the oceans themselves and everything that lives in them. Marine scientists started surveying Norwegian coastal waters and the Barents Sea sporadically as far back as in the 1860s, with regular surveys beginning in the 1930s.
Today we survey the whole food chain from phytoplankton to cod, count whales, observe seabirds, record litter, test for pollution and measure temperature and salinity. As a result, we have unique historical data series that enable us to assess the condition of the oceans and document the changes taking place. These series also help us to predict what the ocean’s climate will be like in the future, and ultimately they form the basis for managing our marine resources.
We normally define the climate as the average weather over a given period, and on land we describe the climate in terms of factors such as air temperature, precipitation and wind. With the ocean’s climate, the variables that interest us include the water temperature, salinity, ocean currents, acidification and sea level.
The ocean’s climate varies naturally on a number of overlapping time scales. Temperatures vary between summer and winter, from year to year and over periods of up to several decades. Longer term fluctuations in the climate are also well documented, and these fluctuations greatly affect the condition of ecosystems. Since 1900 there have been two long cold periods – 1900-1920 and 1960-1970 – while the warm periods have been 1930-1950 and 1980 until the present.
The weather in Norway in any given year is affected by changes taking place in the atmosphere over the North Atlantic, which in turn control how many low pressure systems pass through our waters. When many low pressure systems move into the Barents Sea, they result in warmer conditions there. Warm Atlantic waters and the species that live in them spread out over a bigger area, while cold, Arctic waters and species are forced to retreat northwards.
Human-caused climate change
In recent years, air temperatures have risen faster than expected, and the researchers in the UN panel on climate change (the IPCC) agree that this rise in temperatures has been partly caused by humans. The atmosphere, which wraps around the Earth like a blanket, is composed of gases that retain some of the heat from the Sun. Human emissions of these gases, and particularly CO2 and methane, mean less reflected heat radiation penetrates the blanket back into space, causing the heat to accumulate in the atmosphere. The oceans can absorb a significant amount of this heat, but not as quickly as the atmosphere is heating up.
Since the industrial revolution, the average global temperature has risen by almost one degree Celsius. That is more than you would expect from natural fluctuations. The increase is a result of the greenhouse gases such as CO2 and methane emitted by our industrial activities.
Human greenhouse gas emissions seem small compared with the amounts naturally present in the atmosphere, but as the human emissions always cause warming and never cooling, they eventually produce a lot of warming. This big impact is felt on top of the natural fluctuations.
When the air gets warmer, the oceans also warm up, but not at the same rate all over the world. Research shows that temperatures are rising fastest in the Arctic, with the Barents Sea heating up fastest of all and losing most sea ice in winter.
The Gulf Stream
The Gulf Stream plays a vital part in the climate in our part of the world. It brings us warm, salty Atlantic waters and means that northern Europe has a milder climate than you would expect at such a high latitude. The Gulf Stream won’t change direction due to climate change, because the landscape on the sea bottom and the rotation of the Earth determine the direction of the major ocean currents, but climate change may affect its strength.
One of the factors that produces the Gulf Stream is that salty Atlantic waters are cooled by the cold air in the Norwegian Sea and mixed with cold Arctic waters. When the water cools, it becomes heavier and sinks into the deep ocean, before returning to the North Atlantic as a heavy deep-ocean current. If the ice in the Arctic melts, there will be a layer of fresher, lighter water at the surface that will act as an insulating lid over the Atlantic waters. The water will not be cooled enough to sink down and form deep water masses. The process that draws the warm Atlantic waters up to our latitudes may then slow down, resulting in less heat reaching us. This is likely to be balanced by the effect of global warming, so there won’t be any big temperature changes.
The Gulf Stream is also affected by the frequent low pressure systems that sweep across the Norwegian coast from the south west, because the wind that accompanies the low pressure systems “drags” the warm, salty water towards Norway. So the wind also plays an important role in pushing warm Atlantic waters northwards, independently of whether or not the formation of deep water masses declines. Climate models do not agree on how these winds may change in the future.
The sea level is rising
Several factors can influence the sea level. The two biggest ones are ice-melt on Greenland and Antarctica, and the expansion of water as it warms up.
As the planet warms, the sea level therefore rises. Measurements taken along the world’s coasts and satellite data show that the average sea level has risen by over 20 centimetres since the start of the last century. Since 1993, oceans have been rising at over 3.2 millimetres per year, and since the middle of the 20th century the sea level has been rising faster than at any time over the past two thousand years. This process is expected to speed up even more as global temperatures rise.
While the two factors are equally important at the moment, the melting of land ice, like the ice on Greenland and Antarctica, will become the biggest factor in the future. Their ice caps store enough frozen water to raise the sea level by around 60 metres if all of it melts. Fortunately that won’t happen in the foreseeable future, but if greenhouse gas emissions continue at the current rate, the sea level is expected to rise by up to 80 cm this century. If warming is kept below two degrees Celsius, the sea level may rise around half a metre by the end of the century.
The oceans help to regulate the Earth’s climate. That is because the oceans are rather like a pump that drains the atmosphere of CO2, absorbing around a third of all emissions. This process slows global warming, but as the quantity of CO2 increases, the oceans lose their ability to absorb any more of it. Consequently, higher CO2 emissions will have an even bigger impact on the climate system.
The sea is currently slightly alkaline, but raising the CO2 content makes it less so. We call this “ocean acidification”. Initially, CO2 is dissolved at the surface, causing the pH value to decline there. This process spreads downwards very slowly, so the pH value will continue to fall for many decades to come. Acidification causes calcium to dissolve more easily, which creates problems for many marine animals, as they need calcium to produce their skeletons and shells.
Marine life is on the move
There are big changes taking place in the productivity of Norwegian waters, mostly due to the changing climate. Everything from plankton through benthic fauna to fish, seals, whales and sea birds is moving north with the warming oceans.
The sea temperature greatly affects the production of phytoplankton, which provide the basis for the productivity of the whole ecosystem. Temperature also affects the growth rate of zooplankton and fish larvae, and thus how long the fish spend in their most vulnerable life stages. The zooplankton species Calanus finmarchicus plays a particularly important role in the north as a food source for many of our biggest fish populations. With higher temperatures and a larger area of Atlantic waters, zooplankton that thrive in warmer waters will become more abundant, forcing Calanus finmarchicus further north.
The recruitment, growth, distribution and migration patterns of all of the major fish stocks are changing, including those of mackerel, blue whiting, herring, cod and capelin. For example, mackerel now migrate much further west and north in the summer, while cod go further north and east.
The ocean’s climate in the future
Climate models can help us to predict what the climate will be like in the future. In the same way as weather forecasts, climate models simulate average weather conditions over a given period, and the models can make predictions about the atmosphere, the oceans, ice, evaporation and the carbon cycle. The models are based on the laws of physics and an understanding of solar radiation, the motions of the Earth, and the properties of the atmosphere, oceans and land.
Our waters experience big natural fluctuations, which over the short term overshadow the slower warming caused by humans. Over the longer term (more than thirty years), global warming will dominate, and we can expect more extreme weather and bigger changes in marine ecosystems than the ones we have seen to date.
Areas at high latitudes will probably experience the biggest changes in their climates. Climate models show that temperatures will continue to rise, and that there will be less sea ice. This will affect the distribution of Arctic species, while enabling species that prefer warmer waters to take their place.