Climate Change: Ocean Processes

The physical state of the ocean provides the backdrop for all aspects of marine health, cleanliness, safety and productivity, from defining the nature of available habitats to determining how hazardous materials spread, and how we can best make use of our marine resources. Climate change and increasing levels of atmospheric CO2 will affect many different aspects of the physical environment, and this will in turn have important consequences for all the other areas considered in this assessment.

Weather and climate

Weather varies on timescales from hours to years, and is governed in turn by longer-term changes in climate. The clearest evidence of climate change is a mean global rise in air temperature of about 0.75 °C since the late 19th century. Storms have changed on interannual to decadal timescales, but longer-term or future climate-related trends are not clear.

Models predict that over the 21st century all areas of the UK will grow warmer, with more warming in summer than in winter. There is predicted to be little change in the amount of precipitation that falls annually, but it is likely that more will fall in winter, with drier summers for much of the UK. The regional pattern is more complex with generally greater changes in the Southern North Sea and Eastern Channel than in the Atlantic North-West Approaches and Scottish Continental Shelf areas.

Figure 6.2 Temperature changes relative to the corresponding average for 1901-1950 (°C) from decade to decade from 1906 to 2005 over the Earth’s continents, as well as the entire globe, global land area and the global ocean (lower graphs). The black line indicates observed temperature change, while the coloured bands show the combined range covered by 90% of recent model simulations. Red indicates simulations that include natural and human factors, while blue indicates simulations that include only natural factors. Dashed black lines indicate decades and continental regions for which there are substantially fewer observations.

Temperature, salinity, circulation

The three basic physical parameters determining the marine climate are water temperature, salinity and circulation patterns, and climate change affects them all. UK waters have warmed over the past 50 years, at least partly because of human-induced climate change. The large-scale circulation of the Atlantic, which helps to maintain the relatively temperate climate of Northern Europe, shows high variability but no clear trend. The upper ocean to the west and north of the UK has become saltier since a fresh period in the 1970s, but trends within the shelf seas are less clear.

Climate models predict that the large-scale Atlantic Meridional Overturning Circulation will decrease over the 21st century, but not shut down completely. Although this will reduce the flow of warm Atlantic water past the west of the UK, it will not prevent a net warming of the UK seas. Under a medium greenhouse gas emissions scenario, UKCP09 projects that UK shelf seas will be 1.5 to 4 °C warmer by the end of the 21st century (Figure 6.3) and, with greater uncertainty, 0.2 salinity units fresher.

Figure 6.3 Projected change in seasonal mean sea-surface temperature for the NE Atlantic. The plots show the UKCP09 Marine and Coastal Projections for 1961-1990 (RCM-P; upper panels) and under a medium greenhouse gas emissions scenario for 2070-2098 (RCM-F; middle panels) and the difference between them (lower panels).

The UK seas will seasonally stratify in the same locations as at present but this stratification may be stronger, start earlier and breakdown later each year. Circulation patterns are likely to be as variable in time and space in the future as they are today, being mainly controlled by the complex topography of the seabed around the UK, as well as by highly variable tides, winds and density differences.

Carbon dioxide and acidification

The oceans play an important role in mitigating climate change, taking up and storing about a quarter of anthropogenic CO2 emissions through a combination of biological processes, solubility, and circulation patterns. However, dissolving excess atmospheric CO2 in surface waters has already noticeably increased their acidity and this may in turn affect the ocean’s ability to take up further CO2. The North Atlantic reduced its uptake of CO2 by half from the mid-1990s to 2005, but it is not yet clear whether climate change is the cause.

Further acidification of the oceans will depend on the emissions pathway society takes. On a ‘business as usual’ pathway, models suggest that by the end of the 21st century the acid (i.e. the active hydrogen ion concentration) content of the ocean’s surface will have doubled (a decrease of 0.3 in the measured pH). Warming will tend to reduce the seas’ ability to dissolve CO2 directly from the atmosphere and this will reduce the capacity of the ocean to soak up CO2. The projection of pH did not include this effect on the basis that the feedback would be small.

Climate change may also alter the natural sequestration of carbon on a scale and in a direction that is presently the difficult to predict. Changes in patterns of circulation on and off the shelf of the Northern North Sea may affect the natural passage of CO2-rich waters from the shelf into the deep ocean, for example via the Norwegian Trench.

Rising sea levels will allow larger waves to approach the shore

© Crown copyright 2010

Sea level

In the 20th century, the average level of the UK seas rose by some 14 cm. The IPCC Fourth Assessment Report projected that global mean sea level will rise by 18 to 59 cm during the 21st century through a combination of thermal expansion of the seawater and melting of glaciers and ice sheets. Because the rate and magnitude of ice sheet melting is highly uncertain, the IPCC also included a higher upper limit of 79 cm, but they ascribed no likelihood to this projection and could not discount significantly higher changes. UKCP09 Projections of UK coastal sea-level rise (not including land movement) for 2095 range from 12 to 76 cm. Given that the IPCC could not rule out greater changes in sea level, UKCP09 adopted an extreme scenario for sea-level rise in the range of 93 cm to 1.9 m by 2100. However, the regional response of sea level depends upon where the water melts from geographically, and means the top end of this range is most unlikely for the UK.

Models suggest that there will only be very limited increases in the size of storm surges around the UK over the 21st century; in most cases this trend cannot be clearly distinguished from natural variability. However, models are not good at simulating future storms. Acknowledging this uncertainty, UKCP09 presents a scenario giving large rises in the surge component ranging up to a (most unlikely) 95 cm increase in the 50-year return level surge in the Thames region. When combined with the extreme scenario for sea-level rise, the upper extreme level projected by UKCP09 reaches up to about 3 m for a 50-year return period event by the end of the 21st century (an even more unlikely scenario).

Waves, suspended particles and turbidity

The GOCE satellite was launched to improve understanding of ocean circulation, sea-level change and ice dynamics

© ESA – AOES Medialab

Wave heights around the UK depend on winds and storms both locally and in the wider Atlantic. There is considerable variability in waves, storm surges, and suspended particulate matter from year to year, and no clear trend since Charting Progress.

Climate change could affect wave heights by changing the intensity of storms, or their tracks, but there is very low confidence in storm projections. Models suggest that seasonal mean and extreme wave heights will increase slightly to the south-west of the UK, reduce to the north, and experience little change in the North Sea. UKCP09 projects changes in the winter mean wave height of between -35 cm and +5 cm, with changes in the annual maxima of between -1.5 m and +1 m. Projections of longer return period wave heights would reflect the same pattern but with larger errors. Rising sea levels will allow larger waves to approach the shore and may change the type and size of particles suspended in the coastal region as more upper beach and terrestrial sediment is added to the marine environment. Any change in storminess could also change the suspension of bottom material in shallow areas. However, as yet there are no detailed projections of change for suspended particles and turbidity.

Sedimentary processes and morphology

Rising sea level has caused almost two-thirds of the intertidal profiles in England and Wales to steepen over the past 100 years, particularly on protected coasts. In low-lying coastal regions of England, 40 to 100 hectares of saltmarsh are being lost each year in a process known as ‘coastal squeeze’ as intertidal profiles steepen and human pressures restrict the landward migration of saltmarsh. Models suggest that coastal squeeze, habitat loss, coastal erosion and steepening of intertidal profiles will all increase in the future because of further sea-level rise and possible changes to wave conditions.