climate change climate change: changing oceans

Transcription

1 climate change climate change: changing oceans

2 H.C Andersens Boulevard DK-1553 Copenhagen V Denmark telephone telefax [email protected] website Evidence of a changing climate from the IPCC ICES (the International Council for the Exploration of the Sea), coordinates, and promotes marine research in the North Atlantic Ocean and adjacent seas, e.g. the Baltic Sea. ICES acts as a focal point for a community of more than 1600 marine scientists from 20 Member Countries. Besides filling gaps in the knowledge of marine ecosystems, marine research is also developed into unbiased, non-political advice. This advice is then used by the Member Countries to help them manage human activities in the North Atlantic Ocean and adjacent seas. ICES plans and coordinates marine research through a system of committees, more than 100 working groups, science symposia, and an Annual Science Conference. Research on climate change and variability has been part of ICES work since it was established more than one hundred years ago, but during the past two decades, there has been a growing awareness of the importance of climate change among the marine science community. This brochure provides a state-of-the-art overview of ICES studies on climate change and variability. There is great confidence within the scientific community that climate change is a reality. Global atmospheric concentrations of the greenhouse gases carbon dioxide (CO 2 ), methane, and nitrous oxide have increased as a result of fossil fuel use and changing systems of agriculture. The increase in these gases has caused warming of the atmosphere and ocean, rising sea levels, and changing wind patterns. As greenhouse-gas emissions continue to rise, so will the global temperature, leading to further melting of ice and rises in sea level. In the last 100 years, the average global temperature has increased by 0.74 C. This temperature increase is widespread over the globe and is greater at higher northern latitudes. However, temperature anomalies may be patchy and vary regionally. Eleven of the 12 years from 1995 to 2006 rank among the 12 warmest years in the instrumental records of global surface temperature, which began in Global average sea level has risen since 1961 at an average rate of 1.8 mm year 1 and, since 1993, the rate has nearly doubled to 3.1 mm year 1, as a result of thermal expansion and melting glaciers, ice caps, and polar ice sheets. Observed decreases in snow and ice extent are also consistent with warming: satellite data since 1978 reveal that annual average Arctic sea-ice extent has shrunk by 2.7% per decade. According to the International Panel on Climate Change (IPCC), atmospheric concentrations of greenhouse gases have increased markedly since 1750, as a result of human activities, and now exceed pre-industrial values, as measured from ice cores spanning thousands of years. As a result of the greenhouse effect, average global temperatures have increased by ~0.6 C, and 11 of the 12 years from 1995 to 2006 rank among the 12 warmest years in the instrumental records, which date from Over the last century, as temperatures have increased, sea level has risen by 0.17 m, and the IPCC has predicted that sea level will have risen by a further m by 2100 (Figure 1). The sea is also predicted to become more acidic over the next few centuries as a consequence of the increasing level of CO 2 in the ocean. One of the most important implications of the changing acidity of the oceans is the problems that it may cause for the many marine photosynthetic organisms and animals, e.g. corals, bivalves, and some plankton species that make their shells and plates out of calcium carbonate (CaCO 3 ). The process of calcification, which for some marine organisms is important to their biology and survival, will be reduced as the water becomes acidic (less alkaline). From the evidence available, there is much uncertainty as to whether marine species, communities, and ecosystems will be able to acclimate or evolve in response to changes in ocean chemistry. At this stage, research into the impacts of high concentrations of CO 2 in the oceans is still in its infancy. Another major concern is that the circulation system of the Atlantic Ocean may change. The Atlantic thermohaline circulation is the process by which warm water moves northwards at the surface of the Atlantic Ocean and cold, deep water moves southwards. This process is driven by differences in water temperature and salinity, and any variations introduced by climate change (e.g. greater freshwater input from higher rainfall and melting ice) are likely to have an impact. The thermohaline circulation is unlikely to shut down, but some models predict a reduction of the overturning circulation as a result of climate change by 30%. On a more regional scale, altered atmospheric conditions may bring changes in storm tracks, winds, rainfall, evaporation, sea ice, and river run-off. Changing weather systems will affect ocean currents, ocean fronts, and upwelling and downwelling, which, in turn, will profoundly affect the distribution and production of marine ecosystems at all levels, from plankton to fish. In addition to potentially altering the distribution of productive areas, changing weather patterns are also predicted to increase the formation of vertically stratified water. Stratification is a natural process that occurs, for example, when the sun heats up the sea surface, creating a layer of warm water above the layers of much colder water below; this may also occur when river water enters the sea and forms layers of low density over the denser seawater. The major problem with stratification is that it restricts access to nutrients, which become locked into the stratified layers. This means, for example, that the nutrients in the surface waters are gradually used up by phytoplankton, but are not replenished from the depths. More nutrients are only released when the stratified water is mixed by wind action, and the more stratified the water, the more wind energy is needed to break it up. In the North Sea, for example, the duration of a stable seasonal stratification is predicted to increase as a result of climate change, because higher rainfall will increase fresh-water inputs, via rivers and the Baltic Sea, into the eastern North Sea. However, stronger winds will contribute to increased vertical mixing in the upper water layer.

3 Changes in temperature, sea level and Northern Hemisphere snow cover Figure 1: Observed changes in (a) global average surface temperature; (b) global average sea level from tidal gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). Source: IPCC 4th Assessment Report (AR 4, 2007) Coping with climate change A changing ocean climate has led to a biogeographical shift in certain crustacean plankton assemblages in European shelf seas over the past 40 years. This socalled regime shift, first detected by British and French scientists in the late 1990s, was reflected by a northward extension of warm-water species and a decrease in the numbers of colder water species. In 2005, this regime shift was confirmed for the North Sea by the ICES Regional Ecosystem Study Group for the North Sea (REGNS), which used a large variety of biological and other parameters. the trend in the north atlantic in the past decade ( ) has been of warming and increasing salinity in the upper ocean. The marine climate of Europe is predicted to continue to warm throughout the 21st century, with a forecasted rise in sea surface temperature of 0.2 C per decade. It is thought that warmer sea temperatures have already caused significant changes in zooplankton and phytoplankton populations, including changes in their abundance and distribution. In 2006, the ICES Working Group on Zooplankton Ecology (WGZE) reported that, in the North Sea in particular, the population of the previously dominant zooplankton species (the coldwater crustacean Calanus finmarchicus) had decreased in biomass by 70% between the 1960s and the post- 1990s. Warm-water species had moved northwards to replace C. finmarchicus, but their biomass was not as high. Farther south, other warm-water zooplankton species, e.g. the crustacean Temora stylifera (Figure 4), have been recorded moving northwards through the Bay of Biscay. In some regions, changes in plankton biomass and seasonal timing of blooms have already been linked to the poor recruitment of young fish to several stocks of commercial interest and to the low breeding success of seabirds in recent years. This is because the life cycles of many fish and seabirds are timed to make optimum use of peaks of particular prey species; if the timings are out of sync, because of changes in the plankton blooms, less food will be available for both fish and seabirds. Climate change is affecting not only the food that fish eat but also the distribution of fish. Fish are known to seek out their preferred temperature range. Therefore, if the sea temperature increases, fish at the northern limit of their temperature range will have new, more northerly areas to move into as formerly cold water becomes more habitable. In contrast, fish at the southern limit of their range will be forced to move northwards to escape the rising temperatures. Evidence shows that this is already happening. Alternatively, apparent shifts in distribution may be the result of more successful recruitment in the few individuals living at the edge of their temperature preferences. For example, the increased abundance of anchovy (Engraulis encrasicolus) in the southern North Sea is caused by recruitment in this area rather than movement from more southerly waters. climate change is affecting not only the food that fish eat but also the distribution of fish. The study of a 70-year time-series of fish, plankton, and intertidal animals in the English Channel revealed that warm-water species increased in large numbers during warm periods. The warm-water species moved up to 67 km northwards in order to take advantage of new habitat, then moved southwards during cooler periods. Another study, on the distribution of 20 fish species in the North Sea in the period , found that half of them demonstrated northward shifts coinciding with recent warmer conditions. One of the species most affected was the bib or pouting (Trisopterus luscus), which increased its northern range by more than 300 km. Analysis of Scottish trawl data, taken from research vessels and dating back to 1926, also found that catches of warm-water species, i.e. anchovy (E. encrasicolus), sardine (Sardina pilchardus), and striped red mullet (Mullus surmuletus), all increased in the North Sea, coinciding with increased temperatures after It is noteworthy that a similar extension of anchovy and sardine was observed in the 1950s o W 90 o W o W Figure 2: Overview of upper ocean temperature anomalies in 2007 from the long-term average across the North Atlantic. Dark red areas indicate warmer than average water. From ICES Report on Ocean Climate Figure 3. The trend in the North Atlantic in the past decade ( ) has been one of warming and of increasing salinity in the upper ocean. Warming is shown in a time-series of temperature from 2150 stations in the central Greenland Sea for the period Blue indicates cold water, and green, yellow, and red indicate warm water. Source: ICES Data Centre. 30 o W 0 o 80 o N 60 o E 30 o E 70 o N 60 o N 50 o N 40 o N

4 Other research has highlighted the northward movement of warm-water fish, e.g. the silvery john dory (Zenopsis conchifera) and rosy dory (Cyttopsis rosea), through Portuguese waters up to the southwest coast of Ireland, since the 1960s. Figure 4. Abundance of the warm-water copepod Temora stylifera in transects off Vigo, Coruña, and Santander, and other plankton surveys along the North Spanish coast. This crustacean was absent before 1978 but has been recorded in increasing quantities since the mid-1990s. cod stocks at the colder extremes such as those off eastern canada, greenland, and in the barents sea would benefit from increased recruitment and faster growth as the sea warmed. The changing distribution of the striped red mullet in the North Sea is shown in Figure 5. The small panels show the distributions in the periods (top left) and (top right). The main panel shows the changes in distribution between the two periods. A change from blue to green indicates an increase in density between the two periods, with dark green to blue indicating the largest changes. Yellow to red indicates a decrease in density, with red indicating the largest changes. The graph (top centre) shows the proportion of the total survey area where an increase and decrease occurred, broken down by the degree of increase or decrease (1 6). In the period , striped red mullet were rarely found by research vessels. By early , they were found as far north as Scotland. During the 1920s and 1930s, the warming of the air and ocean temperatures in the northern North Atlantic and the high Arctic led to reduced ice cover in the Arctic and Subarctic regions and to higher sea temperatures. Atlantic cod (Gadus morhua) and halibut (Hippoglossus hippoglossus) expanded northwards along the west coast of Greenland in response to the changes in the ocean climate. Other species were also observed to have undergone significant abundance and distributional changes. For example, cold-water species such as the beluga whale (Delphinapterus leucas) no longer migrated as far south in winter as in previous cold years. This was a clear case of an environmentally driven ecosystem response that became of paramount interest for fishery researchers at the time. This interest lead to the first scientific meeting by ICES on climate change held in 1948 at Copenhagen entitled Climate Changes in the Arctic in Relation to Plants and Animals. This was followed up by a new meeting in 1952 on Fisheries Hydrography. In the invitation to the first meeting, the conveners wrote, Thus it is clear that some of the largest fisheries in the northern hemisphere, e.g. some cod fisheries, during the coming years will be completely dependent on the course of the present mild period in the northern waters. This document shows that the ICES science community was concerned about the climate change impacts on the marine ecosystem for a very long time. Since 1992, the ICES/GLOBEC (Global Ocean Ecosystems Dynamics) Cod and Climate Change programme has focused on cod. This has yielded some key results on cod biology and ecology that have helped scientists to estimate the future impact of warmer temperatures. The results are a mixture of good and bad news. Climate models estimate that, by 2100, the temperature will increase by 2 C in most areas occupied by cod. Cod stocks are not found in waters with annual mean bottom temperatures greater than 12 C. It has been estimated that, if there is a sustained rise in sea temperature of just 1 C, several of the southern cod stocks will become stressed; for example, stocks in the Celtic Sea and English Channel will eventually disappear, while stocks in the Irish Sea, the southern North Sea, and Georges Bank will decline owing to decreasing recruitment. On the other hand, cod stocks at the colder extremes, e.g. those off eastern Canada, Greenland, and in the Barents Sea, will benefit from increased recruitment and faster growth as temperature increases. Temora stylifera, a planktonic crustacean species floating freely in the water column. Drawing by Giesbrecht, What Is Climate Change and Climate Variability? Any change in climate over time either is caused by natural variability or is a result of human activity. Following the IPCC, global climate change refers to changes caused by the accumulation of greenhouse gases in the lower atmosphere. The global concentration of these gases is increasing, caused mainly by human activities. In general, global climate change includes those changes that are attributed directly or indirectly to human activity through altering the composition of the global atmosphere and ocean, and that are in addition to natural climate variability over comparable time periods. The term climate variability is used when referring to variations not related to anthropogenic influences. Climate variability occurs over different time-scales such as seasonal, annual, decadal, and multidecadal and is caused by differences in solar input, volcanic eruptions, internal dynamics of the atmosphere, etc. These natural variations may act to either oppose or enhance those changes due to global climate change, e.g. they may impede or accelerate the warming due to anthropogenic effects. Climate change a greater impact in semi-enclosed seas? Baltic Sea predictions suggest decreases in salinity ranging from 8% to 50% and an increase in sea surface temperature of 2 4 C. North Sea predictions for salinity vary, from an expected increase in some areas to an expected decrease in others. Sea surface temperature is predicted to rise by about C in the northern North Sea and by C in the shallower southern North Sea.

5 Striped red mullet (Mullus surmuletus). Courtesy of Robert A. Patzner, Salzburg University. Figure 5. The change in distribution of striped red mullet (Mullus surmuletus) between the periods and in the first quarter (Q1) of the North Sea. (See text for explanation.) On the seabed, life generally moves at a slower pace, but the effects of climate change and warmer temperatures may also be felt there. The ICES Study Group on the North Sea Benthos Project (NSPB 2000) compared records of marine animals living in the sediment at the turn of the 20th century with records from a similar survey in As with work on fish species, it is difficult to separate the effects of climate change on distribution and abundance from the effects of fishing e.g. bottom trawling but the NSPB did note that some species appear to be responding to increased sea surface temperatures, e.g. the brittlestar Acrocnida brachiata, which occurred more frequently in the eastern North Sea, especially in German Bight and on the Dogger Bank, in the year 2000 than it did in The ICES Benthos Ecology Working Group (BEWG) has also reported on a general increase of warm-water species in the southern North Sea, e.g. the amphipod crustacean Megaluropus agilis on the Dogger Bank, whereas coldtemperate species, e.g. the polychaete worm Ophelia borealis, have decreased in abundance. Keeping an eye on climate change Although the implications of climate change may appear to be a fairly recent discovery, the potential for humans to alter the world s climate was foreseen over a century ago (in 1896) by the Swedish scientist Svante Arrhenius, who worked out that the burning of fossil fuels could cause a greenhouse effect that would warm up the climate. He made a series of simple calculations which showed that the temperature in the Arctic regions would rise by about 8 9 C if the CO 2 level were to increase to two or three times its value at the start of the 20th century. His predictions turned out to be correct, and now scientists all over the world are trying to follow changes in the climate and predict their future impacts. In the North Atlantic Ocean, scientists working through ICES are trying to understand the probable future for marine ecosystems. ICES is the oldest marine science organization in the world and, for more than a century, has been coordinating and promoting marine research with the help of scientists from its 20 Member Countries. ICES acts as a catalyst, bringing scientists together from all avenues of marine research and helping them to pool their knowledge and data, and then disseminate their findings. Rather than working on a national scale, ICES gives scientists a forum through which they can work on the larger, international picture. The following is a small sample of the key groups in the ICES community whose work has been focused on understanding the impacts of climate change on marine ecosystems. ICES/GLOBEC Working Group on Cod and Climate Change Role: The WGCCC has worked for over a decade running cooperative workshops and theme sessions, and undertaking comparative studies to bring researchers together and to develop a better understanding of the cod and how it is likely to react to climate change. Output: ICES Symposium Volume 198, entitled Cod and Climate Change (ICES Symposium, August 1993, Reykjavik), later in 2008 the publication of a book which draws together the current state of knowledge of cod and its response to climate change, newsletters, many papers, as well as ICES Cooperative Research Report No. 274 entitled Spawning and Life History Information for North Atlantic Cod Stocks ( Weblink: ICES Working Group on Zooplankton Ecology Role: The WGZE provides expertise on zooplankton in the ICES area and focuses on monitoring the impacts of climate change on plankton communities. Output: ICES Zooplankton Status Report, issued since These reports give a North Atlantic scale and visual overview of zooplankton distributions for the preceding years (in the form of time-series), with a brief interpretation of the ecological significance of the results. Weblink: ICES Working Group on Oceanic Hydrography Role: The WGOH provides expertise on physical oceanography, particularly on climate and variability in the ICES region. It also stimulates ICES involvement in international climate-monitoring programmes. Output: ICES Report on Ocean Climate (formerly ICES Annual Ocean Climate Status Summary), issued annually since These reports draw together oceanographic data from Member Countries into summary ocean weather reports ( marineworld/oceanclimate.asp). Weblink: ICES/IOC Steering Group on the Global Oceanic Observing System Role: The SGGOOS helps to organize the ICES contribution to the further development and implementation of GOOS in the North Atlantic Ocean. The aim is to merge ICES existing monitoring studies into a global network of standardized observations on climate and its effect on marine ecosystems. One aspect of ICES role in GOOS is the North Sea Pilot Project (NORSEPP), whose aim is to promote the use of operational oceanography for biological applications, e.g. fish-stock assessments. Output: NORSEPP currently produces quarterly reports on oceanography and fish stocks in the North Sea ( marineworld/norsepp.asp). Weblink: and

6 ICES Working Group on Operational Oceanographic Products Role: The aims of the WGOOP, which is currently being set up, is to translate oceanographic data into the most useful format for scientists working on other aspects of the marine ecosystem, e.g. fishery scientists. Output: forecast near-real-time status reports. ICES/GLOBEC Working Group on Life Cycle and Ecology of Small Pelagic Fish Role: The WGLESP will have an increased focus on the responses of fish to climate change, with the aim of helping fishery scientists and other scientists working on aspects of the marine ecosystem. Other recent work includes a workshop looking into the significance of changes of surface CO 2 and ocean ph in ICES shelf sea ecosystems as an upcoming issue. The work of ICES working groups is used to deliver advice now on climate change issues. For example, ICES examined the evidence for, and advised on the implications of, the effect of climate change on the distribution and abundance of marine species in the OSPAR Maritime Area linked to changes in sea surface temperature. Priceless data ICES maintains a huge bank of oceanographic data for its Member Countries, with records dating from the early 1900s. These data are now crucial because they allow scientists to monitor changes in the sea as a result of climate change; they also provide a guide as to what is normal variability and what is out of the ordinary range. Oceanographic data are now collected from a huge network of sampling stations and transects in all four corners of the North Atlantic Ocean This is organized by ICES Member Countries and makes the region one of the best monitored in the world. One of the key uses of these data is to create the flagship annual ICES Report on Ocean Climate. Another key data store is the records of fish and shellfish catches since 1903 and a database of trawling survey records (DATRAS) for the last 35 years, both of which help scientists to follow changes in fish communities as a result of climate change. Closing the gaps Although a lot of research has already been completed on climate change by scientists within ICES, and around the world, there is still a huge amount of work to be done. It is important not only to continue with the existing time-series but also to initiate new research in key areas where more efforts are needed into the impacts of climate change on marine ecosystems. These key areas are highlighted below. Improvement of circulation models Current global circulation models, driven by climate scenarios, need to become more focused so that they are more relevant at the regional scale. In addition, more research is needed into the impact of climate change on the behaviour of the thermohaline circulation. Special emphasis also needs to be placed on research into semi-enclosed areas, because these are the areas that will be greatly affected by climatedriven changes. Effects of climate change on marine life Further research is needed into the impact of climate change on marine species (e.g. fish, benthic animals) in the presence of non-climatic stressors, e.g. fishing. There is also a need for more studies on the impact of climate change at population and community levels, because direct climatic effects on individuals do not translate directly into changes in distribution and abundance of the population as a whole. More information is needed about the evolution of fish in the marine environment and their genetic adaptability to climate change. Furthermore, a better understanding of the links between plankton and larval fish in relation to future warming scenarios is crucial. Likewise, more research into the movement of species into European waters will lead to a better understanding of their impact on marine ecosystems. Ocean acidification: a subtle but worrying change Research is needed into the impacts of ocean acidification on marine species and their physiology, particularly cold-water corals, cephalopods, bivalves, and echinoderms, and especially fish larvae. Process studies, experimental work, and field studies should be integrated into biogeochemical, circulation, and climate models for the evaluation of the future impacts of ocean acidification. Managing marine ecosystems in a changing climate Research is needed into the sustainable exploitation and appropriate management of fish stocks including recovery strategies in order to give fish the best chance of adapting to environmental changes. The ecosystem approach to fishery management requires an insight into the infrastructure of fish stocks and how they function. This requires detailed information about the ecology of important marine species. Monitoring and evaluation of areas suitable for spatial closure as Marine Protected Areas (MPAs) is needed. In particular, MPA proposals will need to take into account the potential impacts of climate change on the distribution of target species. Further development of advanced ecosystem models and multidisciplinary research is needed to improve the detection, prediction, and forecasting of the response of the marine ecosystem to climate change.