The water chemistry of the Southern Ocean appears to be changing at a faster rate than previously estimated, particularly in the deep ocean layers. In the cold Southern Ocean, carbon dioxide is being sequestered at a higher rate than in subtropical waters. Increases in carbon dioxide cause an acidification of ocean waters that make it difficult for shell-building organisms to extract the calcium they need from the ocean (see Section 2.3.1).
Changes in the physical ocean environment are likely to affect the ocean’s productivity, which influences the survival of higher order predators. However, the degree and nature of the effects of global warming on various levels of productivity, as well as on ocean circulation and chemistry, are still unclear. These uncertainties limit the degree to which we can predict the effects of changes in the physical environment and biological production, the rate and direction of change, or the relative importance of various pressures.
Wildlife populations have been exposed to change in their environment throughout the history of our planet. Some extreme events led to mass extinctions. However, other changes (for example, changes in atmospheric carbon dioxide) took place slowly over centuries or longer, and often enabled vertebrate species to evolve certain adaptive traits.195 By contrast, the current climate change is occurring at an unprecedented and increasing rate, leaving many species vulnerable because their capacity to adapt operates much more slowly. Also, the changes are not constant but often vary with region and may differ in their timing and scale. Species differ significantly in their ability to adapt, their generation time and longevity, reproductive output and success, and more. A particular problem occurs where the lifecycle of a prey species loses its synchronicity with dependent predators and food becomes less available at key times (e.g. onset of breeding, weaning or fledging of young). The inherent differences of species, plus a lack of understanding of how various environmental factors may interact, make it almost impossible to predict the fate of particular species and populations.195
Organisms can react to their changing environments in three main ways:
- Species shift to areas where the conditions are still similar to those they encountered previously and where adaptations are not required. Movement of species at the Antarctic Peninsula is possible: as the northern parts become warmer, affected species may move further south. However, the size of the Antarctic continent and access to food limit how far they can go. In the southern Indian Ocean, wildlife populations breeding on the subantarctic islands have far fewer options to move south, because there is no intermediate location between the islands and the Antarctic continent. Thus, if they were to shift their distribution, they may have to endure colder conditions than they have so far experienced.
- Species adapt to live under warmer and perhaps more marginal conditions at their current breeding locations. This might require a shift in their behaviour and physiology to allow them to adjust, for example, the timing of their breeding season, the growth rate of their offspring or even the age of first breeding. In all likelihood, these changes would require a change in their genetic make-up. Which strategy species ultimately choose depends on their degree of adaptability, as well as the rates of change of the various parameters.
- If species fail to move or to adapt to their altering environment, they will become extinct.163 Some species are clearly more threatened by the environmental changes than others.
The effects of ocean acidification are likely to have severe biological impacts within decades and could dramatically affect the structure and function of marine ecosystems.27,80,196-199 Such changes would have profound effects on ecosystem services, including the productivity of fisheries. These changes are most pronounced in the polar regions where the acidity of the water is changing twice as fast as in warmer, tropical and subtropical regions.
Antarctic invertebrate communities form a significant part of the marine food web. The responses by invertebrates to ocean acidification are expected to vary with species. Experimental work on temperate marine organisms has demonstrated a wide variety of responses ranging from potentially positive effects, such as increased metabolic rates in autotrophs (organisms that produce their own food from inorganic sources) to negative effects, such as decreased growth rates in sea urchins (see Hendriks et al.200 for review). Ocean acidification affects the life stages of organisms in different ways.201 For example, fertilisation of the Antarctic nemertean (ribbon) worm (Parborlasia corrugatus) may not be affected by a lowering of the pH, and experimental work showed that even egg development appeared resilient when seawater pH was reduced to neutral.202 However, abnormalities occurred at a later stage (blastula stage) of the embryos' development.202 While the pH changes that produced the abnormalities are not predicted to occur in the near future (by 2100) they are expected if the oceans continue to acidify in the long term (by 2300).202
The benthic invertebrate communities of Antarctica, especially those living outside the intertidal zone, exist in a very stable environment where temperatures fluctuate - for example, in the high Antarctic - as little as 1.5 °C throughout the year.203 These stenothermal (narrow temperature) environments came into existence about 4-5 million years ago as the waters surrounding Antarctica cooled.204 How a warming of the ocean may affect organisms adapted to live in a very narrow temperature range is difficult to predict. Many invertebrates die or cannot perform crucial biological activities when temperatures are raised 5-10 °C.204 However, these results are based on experiments during which temperatures are increased rather quickly. The more gradual the environmental change, the better are the chances of at least some species adapting to the modifying conditions.