Download3 Climate Change

Climate change is widely recognised but its rates and impacts are uncertain

The UN Intergovernmental Panel on Climate Change (IPCC) has warned that continued emissions of green­house gases at or above current rates will cause further warming and will lead to many changes in the global climate system during the 21st century which can be expected to be greater than those observed during the 20th century Figure 3.1. The changes may exceed natural multi-decadal variability and lead to permanent changes in ecosystems.

The changing climate has been linked to a wide range of impacts on marine ecosystems
Figure 3.2, either directly (through changes in sea temperature) or indirectly through impacts on the seasonality, distribution and abundance of species. The impacts of future climate trends on marine ecosystems are difficult to predict due to a number of uncertainties, including those in the scenarios for future greenhouse gas emissions. There is also a need for a better understanding of how marine ecosystems will respond to change.

The range of climate change impacts projected for various components of the marine ecosystem are listed in Table 3.1 (physical and chemical environment) and Table 3.2 (biological environment), together with a summary of what has been observed to date.

Many of the observed physical and chemical changes are consistent with increasing atmospheric CO₂ and a warming climate (rising sea temperature, reduced sea ice, acidification), but many of the causative links to climate change are still not well understood. It is difficult to predict the precise rate, magnitude and direction of change, for example for ocean uptake of CO₂, salinity, storminess and nutrient enrichment, and to map impacts at the local level. Physical and chemical changes have been directly linked to impacts on marine organisms (range shifts in plankton, fish and intertidal species communities) and are suggested to have important secondary effects such as on prey availability for seabirds. Uncertainties about physical changes make it difficult, for example, to predict the effects of changes in stratification on primary production, storminess on seabird nesting sites and nutrient enrichment on harmful algal blooms. Understanding of the links between climate change and impacts on marine ecosystems is also limited due to insufficient data (e.g. relating to marine mammals, benthic ecology, and intertidal communities) and to difficulties in establishing local effects. Synergies and trade-offs between impacts and feedback mechanisms add further to uncertainties in projections.

Clear evidence of physical changes

Annual sea surface temperatures for the period 1999–2008 were warmer than in the period 1971–2000 across the whole OSPAR area Figure 3.3. Region II has warmed the most, with temperatures increasing by 1 to 2 °C over the past 25 years. Temperatures in 2002 were the warmest since sea surface temperature records for the North Sea began in 1968. Summers in Region II have generally become longer and warmer while winters have become shorter and less cold. Regional patterns in weather and water circulation reduce the global warming signal in some areas. For example, in Region IV the temperature increase in the south is lower than expected, due to the upwelling of colder water. In the Arctic, both the maximum (March) and minimum (September) sea-ice extent decreased by around 2.5% and 8.9% per decade respectively in the period 1979–2009.

The observed decrease in salinity in the deep North Atlantic and the Nordic Seas is likely to reflect higher levels of precipitation in the northern regions as well as higher river run-off, ice melt, advection and an overall speeding up of the global water cycle.

These changes have been linked to a possible slowing down of the large-scale circulation in the North-East Atlantic. It is unclear whether the observed increase in storm frequencies and higher sea levels are due to natural variability or whether there is some link with climate change. Rates of relative sea-level rise may be partly compensated in areas where the land rises in response to the loss of ice cover.

Evidence of biological impacts is growing

Climate is an important factor driving changes in the distribution, abundance and seasonality of marine biota. Evidence suggests that species are expanding their ranges under a warming climate in marine systems. The changes in distribution and abundance, which are expected to continue in the near future, have been sufficiently abrupt and permanent to be termed ‘regime shifts’ with ecosystems reorganising rapidly in terms of changes in predator-prey relationships and the spread of non-indigenous species.

The seasonal timing of phytoplankton and zooplankton production has altered in response to recent climate change with some species present up to four to six weeks earlier than twenty years ago, which affects predators such as fish. Changes in the timing of planktonic production and the distribution and composition of planktonic communities Figure 3.4 have been linked to changes in the distribution of many fish species. For example, the earlier occurrence, reduced abundance and increasing dominance of smaller species in zooplankton communities have been linked to the decline in cod in the North Sea. Loss of summer sea ice will have profound implications for ice-associated plankton and the organisms that rely on them.

All OSPAR Regions have experienced range shifts and changes in fish distribution and abundance consistent with what is expected as a result of climate change, with northward shifts in distribution and lower levels of abundance in the southern part of the range. The rate at which cod stocks in the North Sea have decreased cannot be explained by overfishing alone. Southern species such as the silvery John dory, sea bass, red mullet and European anchovy have all become more common further north. In the UK, expansions in the range of intertidal species have been observed towards previously cooler areas (i.e. eastward and northward).

Climate change is likely to encourage species to spread into and establish in new areas. Several non-indigenous species are now established in the OSPAR area; two of these (the Pacific oyster and the barnacle Elminius modestus) as a direct result of regional warming. As Arctic sea ice decreases, organisms may spread into the North Atlantic from the Pacific. The Pacific diatom Neodenticula seminae was discovered in the North Atlantic in 1999 and may provide the first evidence of trans-Arctic migration. There is also a risk that loss of sea ice will lead to loss of ice-dependent Arctic species.

Ocean acidification is a key threat

With increasing amounts of anthropogenic atmospheric CO₂ dissolving into the sea, the pH of seawater is decreasing and the ocean is becoming more acidic. Decreasing pH reduces the ability of the ocean to take up CO₂ and provides a potential feedback effect on climate change.

There has been an average global fall in ocean surface water pH of 0.1 units since the start of the industrial revolution which reflects a 30% increase in acidity. The trend is also reflected in the OSPAR area, for example in the Kattegat and Norwegian Sea.

Current changes in ocean carbon chemistry are at least 100 times more rapid than any over the last 100 000 years. Little is known about the ecological and economic impacts of marine acidification but they could be severe, affecting the many biologically mediated processes that transport carbon from the ocean surface to the depths. Experimental data indicate that lower pH (at the levels predicted) is expected to have a range of effects on marine organisms, including dissolution of calcium carbonate (aragonite or calcite) shells and skeletons (decalcification) in calcareous plankton and corals, and acidification of body fluids in fish and invertebrates. Many species with critical ecological roles in pelagic and benthic systems will be affected. Ecosystem-wide effects are expected within 50 to 100 years, including the undersaturation of calcium carbonate in sea­water – a condition where there is a risk of decalcification occurring. There have been some recent projections that undersaturation of surface water with aragonite may happen in parts of the Arctic by as early as 2016 in winter and 2026 throughout the year. More than 150 scientists under the umbrella of UNESCO’s Intergovernmental Oceanographic Commission (IOC) support projections that most regions of the ocean will be inhospitable for coral reefs by 2050 if atmospheric CO₂ concentrations continue to increase. They urged policymakers through the 2009 Monaco Declaration to develop plans to drastically cut CO₂ emissions.

Continue to "What has been done?"