The global mean surface temperature of the earth is projected to increase by about 2oC (between 1 and 3.5oC) by the year 2100. (Note that these are global averages, and considerable regional differences would be expected.)
That average rate of warming would be greater than any seen in the last 10,000 years -- but the actual annual to decadal changes would include considerable natural variability. In most regions and most seasons, night-time temperatures will rise more than day-time ones.
Warming is projected to be greater over land than over the oceans, and the maximum warming is expected to occur at high northern latitudes, particularly in winter.
Minimum warming is estimated to occur over the central North Atlantic and over the Southern Ocean near Antarctica. Regional winds may increase in intensity.
Sea-level rise would occur primarily as a result of thermal expansion of the ocean, as well as from the melting of glaciers and ice caps. Increased melting of sea ice is also possible. IPCC [the Intergovernmental Panel on Climate Change predicted in 1996] a sea-level rise of between 13 and 94 cm by 2100.
Climate change is an atmospheric phenomenon which affects land-based communities through changes in temperature, rainfall patterns, etc., thus leading to alterations in ecosystem structures. This could lead to the loss of more sensitive species and gains by organisms better suited to the new conditions.
Though numerical models of climate and climate change clearly incorporate oceanic phenomena -- particularly heat fluxes -- and have been concerned with sea-level rise and winds over the ocean, relatively little attention has been paid to the impact of climate change on marine environmental quality. However, there are a number of potential environmental changes involving the health of the marine environment that will or may occur as a result of global warming. Several of these warrant future attention and are outlined below.
One potential consequence of the response of climate to anthropogenic forcing is projected to be a global increase in the number and magnitude of extreme events (droughts, floods, hurricanes, etc.).
For example, the number of extreme precipitation events (heavy rainstorms and blizzards) has increased by 20% since 1990. The effects of the very strong El Niño event in 1997-1998 had a significant impact on both sides of the Pacific and elsewhere, including many deaths and much homelessness.
These events also have many consequences for marine communities . In particular, damage to nearshore coral reef and intertidal ecosystems may be devastating, with potentially significant secondary effects. Severe storms can destroy structures and contaminate water systems, and simultaneously create breeding sites for organisms carrying infectious diseases.
Changes in sea-level have clearly had a major influence on terrestrial and coastal systems over geological time scales. Predicted future sea-level rises would be much more rapid than those observed previously. [...]
Low lying coastal habitats, particularly those in densely settled deltas and small islands, are particularly vulnerable. They are of major significance to coastal marine ecosystems since they are frequently key places for the reproduction of marine organisms.
Estuaries, mud flats, mangroves, coral reefs, and coastal wetlands in general -- and densely settled deltas and small islands -- besides being the most at risk from sea-level rise, are particularly important in this way and provide essential food supplies for terrestrial birds, reptiles, amphibians and mammals.
Increased coastal erosion and changes in currents and waves will also have adverse effects on coastal ecosystems. Since a large proportion of the world’s population lives close to the coast, there is certainly risk of direct contamination (e.g., by sewage, toxic metals and toxic organic compounds) resulting from the inundation of portions of coastal towns, cities and associated industrial and power-generating plants.
Many of the major cities of the world are coastal, and significant fractions of the area of some (e.g., cities in the Netherlands, Bangkok) are below current sea-level. Many major industries -- including oil refineries, power stations, chloralkali plants, sewage treatment plants, chemical manufacturing plants, and metal refineries -- are sited along the coast because of their requirements for both cooling waters and access to shipping.
In addition to the health effects related to marine environmental pollution [...], many organisms and processes linked to the spread of infectious diseases are influenced by temperature, precipitation and humidity and thus would be affected by climate change.
Over the past century, average sea surface temperature has increased approximately 0.7° C, and water temperature is an important factor in the growth of many marine algae. Red tides, which can cause paralytic shellfish and diarrhoeic shellfish poisoning, are blooms of toxic dinoflagellates, whose growth is favoured by warm water. Global warming is also expected to cause widespread shifts in the pattern of faecal-oral infections and foodborne diseases.
It is expected that the wider geographic distribution (both by altitude and by latitude) of organisms that transmit diseases (i.e., vector organisms) would increase not only the potential for disease transmission, but also change the lifecycle dynamics (e.g., reproduction, survival and infectiousness) of vector organisms and infectious parasites.
Disturbances of ecological relationships due to climate change may disrupt the natural control mechanisms of vector organisms and their host organisms, as well as parasite populations. This could lead to changes in population dynamics and may result in an acceleration of pesticide resistance in vector organisms and drug resistance in infectious bacteria.
Additionally, more frequent droughts and rising sea-level might force human populations into areas where infectious organisms are located but currently have little impact on people.
[It has been] argued that the relationship between global warming, the occurrence of marine algal blooms and outbreaks of cholera warrants attention. However, [others] have argued that at present the contention that global warming will increase the risks to human health as a result of increased incidence ofVibrio cholerae is speculative.
Whether the frequency of marine algal blooms on a global scale is increasing still remains a matter of scientific debate. Furthermore, the causal association between global climate change, bloom frequency and associated risks to human health has not yet been firmly established. With regard to the ability of Vibrio cholerae to survive in water, long-term survival has been shown in laboratory studies at salinities ranging from 1 to 30 parts per thousand, representing the spectrum from freshwater through estuaries to coastal seawater.
Further, survival in fresh waters occurs in association with a variety of fresh-water algae. Vibrio cholerae can attach to seaweed in laboratory studies. However, although it is accepted that Vibrio cholerae is a member of fresh water and estuarine microbiota, it remains uncertain whether coastal marine reservoirs of Vibrio cholerae play a major role in outbreaks of disease globally.
There is considerable uncertainty about the specific impacts of climate change on marine life. As an example, the potential impact on the dynamics of marine fish populations or projections of the effects of such change on fisheries are discussed below. Sufficient warming could lead to disruption in the population of many fish species because:
Fish tend to have complex life cycles in which the success of survival at certain stages in the development often appears to be dependent on specific environmental conditions. In some cases, fish may be able to develop effective adaptive responses to changed environmental conditions, but in others they may not.
As [has been] pointed out [...], global warming is likely to have a relatively greater effect along the eastern boundaries of oceans, which tend to be drier than the western zones. In the Pacific Ocean, for example, a warming of the eastern equatorial zone relative to the west would tend to shift the tropical system to the "elevated El Niño" state experienced in the mid-1970s to mid-1980s. Such conditions would be disadvantageous, for example, to the Pacific albacore.
On the other hand, they might be advantageous to northern ground fish stocks, as they were in the mid-1970s to mid-1980s. If warming has greater impacts on the less humid eastern sides of oceans, it is likely that the great upwelling systems in these regions will tend to intensify significantly.
These conditions could exist at the same time as the speed of the circulation in both the atmosphere and the ocean in these regions is reduced. This reduction results from increased warming in the polar regions, leading to a slowing down of the global atmosphere/ ocean "heat engine", which would change the flow of major oceanic current systems.
However, increases in regional wind speeds might be expected to increase the prevalence of nutrient-rich ecosystems by increasing, for example, the rates of coastal upwelling and open ocean mixing.
Thus, the dynamic effects of global climate change on various marine ecosystems will be quite complex and are likely to depend on the relative importance of these possible changes to ocean and atmospheric processes in each region.
[A 1996 study] indicates that another consequence of global warming that may affect fish productivity would be a seasonally earlier run-off of snow-melt in areas where much of the winter precipitation, in the form of snow in the mountains, currently contributes to river flow in the dry spring and summer months. Such changes in flow may make rivers unavailable to fish such as salmon, leading to a decline in population. Salmon may also be exposed to another threat through increased ultraviolet radiation: salmon fry nursery areas tend to be in very shallow, transparent waters, often at higher altitudes where less of the ultraviolet portion of the solar radiation is removed.
There are many other aspects of climate change with potential for major effects on marine ecosystems and fish resources. Interactions among species, notably within predator-prey systems, make it extremely difficult to model the likely consequences of any change in global climate.
For example, the northern California Current anchovy spawns in a finely balanced habitat that is apparently subject to disruptions by such changes as run-off patterns, water temperature, etc. Major fish predators on the anchovy are salmon and albacore tuna. The reproductive success of the albacore depends on conditions existing many thousands of kilometres away, while those of the salmon are dependent primarily on continental conditions in the Rocky Mountains.
The oceans play a major role in the atmospheric budgets of carbon dioxide and dimethyl sulfide, the latter in part influenced by eutrophication processes.
A key question is whether there is any feedback process in which oceanic gas exchange alters climate, and whether the altered climate then in turn alters oceanic gas exchange or other oceanic processes.
For example, it has been suggested that primary producers might bloom earlier in a warmer climate, because a warmer ocean would provide a shallower, more stable, stratified surface water layer. Organisms that graze on these phytoplankton, on the other hand, might develop at the ‘normal’ time of year because their natural cycles are determined by the length of the day.
This mis-match of the timing of predator and food development might significantly disrupt marine ecosystems and change the pattern, timing, and amount of the exchange of such climatically important gases as carbon dioxide and dime-thyl sulfide.
This could, furthermore, result in a greater proportion of organic carbon being recycled by bacteria and photo-oxidation, leading to a greater proportion of the photosynthetically fixed carbon being returned to the atmosphere as carbon dioxide.
The calcification of coral reefs is another potentially important marine issue related to increasing atmospheric carbon dioxide and its exchange with the ocean. Increased sequestering of atmospheric CO2 in the ocean would result in a lowering of the oceanic carbonate (CO3=) concentration. The calcification of coral reefs depends on the saturation of the carbonate mineral aragonite.
[It was suggested in 1999] that by the middle of the 21st century increasing atmospheric CO2 could result in a decrease in the aragonite saturation state in the surface ocean by 30% and biogenic aragonite precipitation by 14-30%. This could result in a significant decrease in the reef-building process. The authors point out that other calcifying marine ecosystems could also be affected by decreasing carbonate concentrations in the ocean.
Changes in stratospheric ozone can lead to significant alterations in the wavelength and intensity of light reaching the earth’s surface. High-latitude ecosystems will be the most exposed to increased ultraviolet irradiation because of the lower concentration and greater variability of stratospheric ozone in polar regions.
It is, however, possible that the effects of enhanced ultraviolet light on aquatic organisms has been overstated because of its very limited penetration into water. For example, in the clearest open ocean water, UV-B radiation is reduced to 86% of its surface level intensity at a depth of 1 metre and 22% at a depth of 10 metres. In moderately productive water, the respective percentages are 40% and 0.01%.
Animals which spend time out of the water and on ice (seals, penguins, polar bears, etc.) will be more vulnerable, both because of the increased UV-B radiation and because of the possibly of reduced ice cover as a result of climate change. Polar bears that hunt seals on the ice for their main source of food may be driven back onto land for longer periods, where their ability to find nourishment may be severely reduced. More important will be changes in the timing and possibly abundance of primary production through possible earlier removal of ice cover. It is also worth noting that the reduction in ice cover might have the effect of encouraging increased fishing activity in these waters.
While other threats to coral reefs have been known for some time[...], increased incidences of coral reef diseases(1) and coral bleaching have been a more recent concern.
Although diseases of reef-building corals have been known since the early 1970s, there are emerging concerns that their impacts on reef communities are increasing. New diseases, apparently unprecedented disease outbreaks which sometimes lead to mass mortalities, and the occurrence of coral diseases in locations where they were previously unknown all continue to be reported.
A key question is whether diseases are actually having increased impacts on reef systems, or whether the apparent increase is an artefact of more intensive observation and reporting. As scientific observation of coral reefs has unquestionably increased in the past decade, an increased number of observations of coral disease would be expected even if the actual frequency of occurrence of disease were constant.
There are, however, reasons to believe that the frequency and severity of coral diseases are increasing, and that they are having significant negative impacts on reefs. New observations of coral disease have been made even in areas with relatively good scientific baselines. A new disease variant dubbed "White Plague Type II", for example, was first observed in the Florida Keys in 1995 and has subsequently caused substantial coral mortality.
While it is unlikely that coral pathogens have arisen de novo (such as through mutations), it is quite possible that they have been transported beyond their natural ranges. For example, a fungus, Aspergillus sydowii, -- which is believed to originate on land -- has significantly infected sea fans throughout the Caribbean: it may have entered the marine environment through sediments from land runoff.
Many reefs are being placed under increasing anthropogenic stress, which may both render corals more susceptible to pathogens and itself be a cause of some diseases. It has been speculated that there has been a global increase in the occurrence of coral diseases in response to increasing anthropogenic stress from sedimentation, eutrophication, and other forms of pollution: evaluation of this impression requires better understanding of the causes of coral diseases. This would include determining whether all of the conditions described as "disease" actually represent abnormal physiological responses against the background of natural variability.
Coral bleaching is a generalized reaction to environmental perturbations of many kinds . Like coral disease, it is a natural disturbance to reef communities. If there is no extensive mortality, natural recovery can take place in a matter of months. However, if there is mass mortality, natural recovery may only occur on decadal time scales.
As with coral disease, concerns have emerged about increases in coral bleaching due to land-based activities.
The possible effect of global warming on coral bleaching is another scientific concern. Corals on most reefs live near their upper limits of thermal tolerance, making them potentially vulnerable to sea-surface warming. Significant increases in sea surface temperature over the last 50 years have been observed in some tropical areas. Corals have considerable ability to acclimatize to elevated water temperatures, but it is not known whether they will be able to adapt to the projected rate of temperature increase.
It is worth noting, therefore, that any anthropogenic component of global warming could negatively affect reefs by increasing the rate, as well as the magnitude, of ocean warming. Until recently, the scientific consensus was that, although mass bleaching occurs in response to local episodes of high water temperature, available evidence did not support the occurrence of widespread coral bleaching in response to global warming.
A new consensus is emerging, however, that global climate change may indeed threaten the long-term viability of coral reefs on a global basis. The most geographically widespread, and probably most severe, bleaching ever recorded occurred during the 1997-98 El Niño Southern Oscillation (ENSO) event, although not all of the bleaching can be attributed to ENSO-induced elevation of water temperatures.
[Marine scientists in 1999] reported the extensive coral bleaching and mortality that took place in 1998 in the Indian Ocean, where water temperatures were often 3 to 5 o C above normal in this ENSO year. Mortalities of up to 90% were observed in many shallow areas of Sri Lanka, Maldives, India, Kenya, Tanzania, and Seychelles, while mortalities of 50% were common in other parts of the Indian Ocean and in waters below 20 metres.
As these authors point out, the socio-economic impacts of such losses are very significant, with potential reductions in fish stocks, negative impacts on tourism and future problems with coastal erosion.
While the economic loss resulting from reef damage is quite difficult to determine worldwide, [a 1996 study] estimated that the societal costs of a number of activities which result in reef damage are up to 50 times the private benefits obtained from them (using a 10% discount rate over a 25-year term). Intervention in this case would be reef management -- including, inter alia, restriction on access to reefs -- and the costs would be those of implementing the required management measures and the lost individual benefits.
The 1997-98 ENSO event may fall within the bounds of natural variability rather than be an indication of anthropogenically induced climate change. The extremity of the associated bleaching event, however, is indicated by the bleaching-induced death of some coral colonies on the order of 1000 years old.
Since a possible consequence of global warming is an increased frequency of extreme climatic events such as the 1997-98 ENSO, this would presumably cause more frequent coral bleaching, altering the balance between disturbance and recovery. The problem will be exacerbated to the extent that anthropogenic stresses compromise the ability of reefs to recover from bleaching events. Contamination and other stresses interfere with natural recovery from bleaching and other natural disturbances, and could lead to reef degradation even in the absence of an increase in such disturbances.
1 "Disease" is defined as "Any impairment (interruption, cessation, proliferation, or other disorder) of vital body functions, systems, or organs." Thus, abnormal conditions caused by physiological stress, poor nutrition, genetic mutation, or other factors are considered diseases as well as those conditions caused by pathogens.