Climate Change

Since 1850, average sea surface temperature (SST) has increased by 0.6 °C.. This ocean warming has been evident since 1980s and has been particularly rapid since 1998, with the ocean absorbing 93 % of Earth’s additional heat.. These changes are also observed in Europe’s seas.. Increases in SST lead to changes in species’ distribution ranges, abundance and seasonality, and it affects marine food webs.

Figure 1: Time series of annual average sea surface temperature (°C), referenced to the average temperature between 1993 and 2012, in the global ocean and in each of the European seas.(EEA, 2019d)

Extreme events such as marine heat waves are an additional concern, since they can cause immediate shifts in the distribution of (mobile) species, drive regime shifts and cause local extinctions, indicating that many, including temperate, marine ecosystems may not be resilient to extreme events.. One of the first documented mass mortalities in rocky benthic communities originated from the north-western Mediterranean Sea during the summer of 2003. Here, several thousand kilometres of coastline were affected by a marine heat wave with temperatures of 1-3 °C above the climatic values (mean and maximum), and the mass mortality (up to 80 % of the population) of at least 25 species of soft corals (e.g. sea fans) and sponges was observed. Marine heat waves are predicted to increase as a result of further anthropogenic climate warming.

There is increasing evidence of a northward shift in the distribution of marine plant- and animal species over recent decades (Poloczanska et al., 2016). This northward movement has been attributed mainly to global warming in the terrestrial, limnic and marine environments (e.g. Brander et al. 2016). However, the change in distribution might not be attributable to the changing climate alone, as other environmental and biotic factors influence species distributions. A combination of climate change and the increasing impacts of multiple anthropogenic activities are still poorly understood and are expected to escalate in the future..

Over the last 45 years, an increase in the number of fish species was observed in the Celtic Sea, the Greater North Sea and the Baltic Sea (Figure 2). This change is mainly related to an increase in the number of warm-favouring (Lusitanian, L) species and, to a much lesser extent, an increase in the number of cool-favouring (Boreal, B) species. Observed changes are significant in the North Sea and in the Skagerrak-Kattegat, where significant correlations were also found between the L/B ratio and increased temperature, indicating that changes in fish distribution are related to climate change. In the same period, there were no observed changes in the distribution of widely distributed fish species, which are less sensitive to temperature changes but are exposed to the same combination of increased sea temperature pressures related to human activities in the assessment areas.

It has also been documented that the seasonality of some species has changed, for example spawning occurs earlier for species such as mackerel and sole.. Temperature changes also occur in deeper or stratified areas, impacting benthic communities, but this is not covered here

Figure 2: Temporal development of the ratio between the number of Lusitanian and Boreal species. (EAA, 2019a)

In the period 1979-2019, the sea ice extent in the Arctic decreased by 42 000 km2 per year in winter (measured in March) and by 82 000 km2 per year in summer (measured in September).. The decrease in sea ice during the summer corresponds to a more than 10 % decrease per decade, which is unprecedented in the past 1,000 years based on historical reconstructions and paleoclimate evidence.Arctic sea ice is also getting thinner and younger, as less of the sea ice survives the summer to grow into thicker multi-year layers. The loss of Arctic sea ice is driven by a combination of warmer ocean waters and a warmer atmosphere.

Information available on sea ice extent in the Baltic Sea goes back to 1720. There has been a decreasing trend in maximum sea ice extent most of the time since about 1800. The decrease in sea ice extent appears to have accelerated since the 1980s, but large interannual variability makes it difficult to to demonstrate that this is statistically significant. The number of 'mild ice winters' increased from 7 in the period 1950-1979 to 16 in the 30-year period 1990-2019. In contrast, the number of 'severe ice winters 'decreased from six to one during the same period.

Melting of ice will enhance melting of permafrost and by that release of additional large amounts of greenhouse gasses stored in the soil in these areas.

Global sea level rise has accelerated since the 1960s. The average rate of sea level rise over the period 1993-2018, when satellite measurements have been available, has been around 3.3 mm/year. Global mean sea level in 2300 will likely be 0.6-1.1 m above current levels for a low-emissions scenario and 2.3-5.4 m for a high-emissions scenario. These values will rise substantially if the largest estimates of sea level contributions from Antarctica over the coming centuries are included. All coastal regions in Europe have experienced an increase in absolute sea level, but with significant regional variation. Most coastal regions have also experienced an increase in sea level relative to land (Figure 3). Extreme high coastal water levels have increased at most locations along the European coastline. This increase appears to have been predominantly due to increases in mean local sea level rather than changes in storm activity. All available studies project that damages from coastal floods in Europe would increase many fold in the absence of adaptation.
Public coastal protection consists of measures to protect the EU’s coasts from floods and erosion, such as dune and cliff stabilisation through construction of seawalls, dikes, revetments and bulkheads, as well as the use of nature-based solutions, such as restoring coastal wetlands due to their role in reducing coastal flooding. Rising extreme sea levels linked to anthropogenic climate warming and continued socio-economic development in coastal zones will lead to an increasing future flood risk along the EU coastline, requiring increased public expenditure to minimise it.

Figure 3: Spatial distribution of trends in mean sea level in European seas from January 1993 until February 2019 (EEA, 2019c).

The uptake of CO2 in the sea causes ocean acidification, as the pH of sea water declines. Ocean surface pH declined from 8.2 to below 8.1 over the industrial era as a result of an increase in atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %. In recent decades, ocean acidification has been occurring 100 times faster than during natural events over the past 55 million years. These rapid chemical changes are an added pressure on marine ecosystems.

Ocean acidification has wide-ranging impacts on marine ecosystems. Reduction of the  carbonate availability reduces the rate of calcification of marine calcifying organisms, such as reef-building corals, shellfish and plankton.  Ocean acidification has already affected the deep ocean, particularly at high latitudes. Change in pH affects biological processes, e.g. enzyme activities and photosynthesis, which effects primary production. These changes may be exacerbated by rising seawater temperatures. Changes in marine primary production will have an impact on the global carbon cycle and the absorption of atmospheric CO2 in the ocean and lower oceans capacity to mitigate climate change. The combined effects of elevated seawater temperatures, deoxygenation and acidification are expected to have negative effects on entire marine ecosystems, cause changes in food webs and marine production, and will also cause economic loss to human society.