The impact of global climate change in polar marine environments

This page is part of the topic Marine biology over the next 100 years

Given the differences in topography, substrates, freshwater input and glaciation history, the Antarctic and Arctic Oceans and their organisms are likely to respond differently to climate change. Laboratory experimental work has suggested that some of the more common species in the shallows are highly stenothermal (Peck, 2005). Small temperature differences (just a degree or two) may have great impacts on the physiology of stenothermal organisms as well as on the extent of sea ice, hence on the life history and biology of many species (but see Barnes and Peck, 2008). Although projected climate change should alter the situation, the polar regions currently offer an important opportunity to study species biodiversity and ecosystem functioning in environments largely undisturbed by humans. This is mainly the case in the Antarctic, where national territorial claims are still not applied, and international initiatives and organisations, e.g. the Antarctic Treaty System and the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR), prevent, or at least limit, commercial activities (exploitation of natural resources, industry, fishery, etc) with their consequent anthropogenic impacts. The main direct influences on the Antarctic marine ecosystem are likely to come from global climate change in the mid- to long term. It has to be recognised from the outset that the ecosystem has been radically disturbed by the effects of historic whaling and sealing, and that while much of the fur seal population may have recovered since then, whale populations are still severely depleted compared with former times. After a short phase of overfishing demersal fish stocks (rock cod and icefish) around the Antarctic Peninsula and Scotia Arc, the populations collapsed, but legal and illegal fishing continues, especially for Patagonian and Antarctic Toothfish as well as for krill.

Climate change is already having significant impacts on global marine and terrestrial systems (Hughes, 2000; Walther et al., 2002), and will continue to influence biological diversity. Many species are susceptible to climate change, and those of the marine environment are particularly vulnerable, even though warming is more evident in the air than in the sea as is evident from IPCC reports. The polar regions are undergoing more rapid environmental changes than elsewhere, in many instances due to the combined effects of natural climate change and human activity. However, these changes are much more evident in the Arctic than in the Antarctic except west of the Antarctic Peninsula.

Present patterns of biodiversity and distribution are a consequence of processes working on both evolutionary and ecological timescales. Among the physical and chemical factors controlling distribution and biodiversity of the modern polar marine fauna, the most important are ice scour, topography, substrate, temperature, currents, ice cover, oxygen, light, UVB, wind, and nutrients. Besides being largely interconnected, some of these factors are not constant, and vary over a range of temporal scales from less than daily through seasonal to inter-annual. Variability is of fundamental importance to ecosystem dynamics. The system may be disrupted if the pattern of environmental variability is upset.

The most important anthropogenic changes currently affecting the Antarctic are accelerated global warming and increased UV-B levels resulting from the ozone hole that develops in spring. Illegal and unregulated fishing and the introduction of alien species constitute further threats, although they are more limited in geographic scope. Pollution associated with scientific activities and ships, and visitor pressure from the growing tourism industry have very localised effects on community structure and diversity. Many of these changes have complex and interacting effects. For example, an impact on the lowest or highest level in a food web can propagate through to affect other taxa indirectly. Thus UV-B impact on primary producers may affect consumers and higher levels in the food web, while the extraction of the great whales undoubtedly had an effect that has cascaded down through lower levels.

Around the western side of the Antarctic Peninsula, which is currently subject to one of the fastest rates of climate change anywhere on the planet (Cook et al., 2005), there has been a considerable reduction of annual mean sea ice extent (reviewed in Clarke et al., 2007). There are indications that populations of Pleuragramma antarcticum, a key fish species of the trophic web, and whose reproduction is closely associated to sea ice, declined locally, to be replaced by myctophids, a new food item for predators (M. Vacchi, pers. com.; W.R. Fraser, Regional loss of Antarctic Silverfish from the western Antarctic Peninsula food web, in preparation). This change is thought to have been caused by seasonal changes in sea ice dynamics compromising reproduction processes.

Temperature trends elsewhere in Antarctica show little change or, in some places, a cooling that may be accompanied by local impacts, as in the lakes and soils of the Dry Valleys. There is no evidence of a continent-wide “polar amplification” similar to that predicted in the Arctic (Dyurgerov and Meier, 2000; Oechel et al., 2000; Romanovsky et al., 2002; Lemke et al., 2007). On balance this overall lack of change would not be expected to result in significant biological change, even in the open Southern Ocean, which has warmed by some 0.2ºC. Sea ice, which has a significant relation to the ecosystem, has increased in the Ross Sea, and decreased in the Bellingshausen and Amundsen Sea, with local effects on the ecosystem. Acidification is beginning to occur in the ocean, slightly changing the chemistry, which on the basis of laboratory experiments is expected to first affect organisms with aragonite skeletons, such as pteropods, and ultimately to reduce the uptake of carbon dioxide from the atmosphere. As yet there is no evidence for any large-scale change in the Antarctic ecosystem associated with this effect. For many species, uncertainty in climate predictions leads to uncertainty in projecting impacts; however, continued warming and winter sea ice decrease are likely to affect reproduction cycles and the growth of fish, krill and benthos, possibly leading to declines in some populations and changes in their distributions.

In areas experiencing warming, increases have been recorded in sponge species (with extreme natural variability in recruitment and exceptionally fast growth) and their predators, and decreases in krill, Adélie and Emperor penguins and Weddell seals (Ainley et al., 2005). The reduction in krill biomass and the increase in abundance of salps (gelatinous pelagic organisms) have been suggested to be linked to regional decreases in sea ice (Loeb et al., 1997) that may also underlie recent changes in the demography of krill predators, e.g. mammals and birds (Fraser and Hofmann, 2003). Examination of growth of some seabed suspension feeders over the last two decades has revealed recent increases in annual growth rates in one species but little change or decreases in other similar species (see Barnes et al., 2007). In all of these cases it is hard to state that any change is definitively due to climate change, but evidence is mounting. There are signs that warming air temperatures have had negative impacts on the local biota on some sub-Antarctic islands. Signy Island and some sites at the West Antarctic Peninsula have witnessed an explosion of the fur-seal numbers that may be related to decreased ice cover resulting in increasing areas available for resting and moulting, but which may also be related to population increases on South Georgia; the growing seal population has had deleterious impacts on the local terrestrial vegetation.

The large reductions in the extent of cover and thickness of sea ice on the western side of the Antarctic Peninsula are potentially devastating to some species. The warming of the sea surface has been accompanied by an increase in phytoplankton in cooler regions (which might actually be beneficial to commercial fish stocks) and a decrease in warmer regions (Richardson and Schoeman, 2004; Montes-Hugo et al., 2009). If the sea ice cover continues to decrease, as models suggest, such responses to changes will widen, impairing predation processes and affecting community composition and levels of primary and secondary producers. For instance, marine ice algae would disappear due to loss of habitat. That may cause a cascade through higher trophic levels in the food web, diminishing the zooplankton that feed on algae, the fish that feed on zooplankton, and the sea birds and mammals that feed on the fish.

The responsiveness of species elsewhere to recent and future climate change raises the possibility that human influence may cause a major extinction event for some vulnerable species; we must consider such a possibility, though with due care. Thomas C D et al. (2004) analysed the global extinction risk from climate warming and concluded that many of today’s species could be driven to extinction by climate change over the next 50 years. Such analyses provide compelling arguments for the development of policies aimed at reducing the impact of warming due to human activity. Greater and more rapid warmings have occurred around Antarctica before, during interglacial periods. Extinctions and radiations of species occur continuously, and most current species have probably survived through the climate changes of one or more glacial cycles (we can’t be certain how many because the fossil record of many areas around Antarctica is very poor). However, projected warming and rates exceed those of the last eight interglacial warm periods. Given complete disappearance of sea ice we would expect extinction of those species that currently depend on it for survival. Climate models suggest that in the Antarctic such a reduction is unlikely within the next 100 years, when instead a 33% reduction in sea ice cover is projected.