Climate Change Impact on Oceans & Shallow Seas
The first comprehensive synthesis on the effects of climate change on the world’s oceans has found they are now changing at a rate not seen for several million years. According to the report by Hoegh-Guldberg & Bruno (2010): (summarized by the authors here and here)
“Concentrations of man-made greenhouse gases are driving irreversible and dramatic changes to the way the ocean functions, with potentially dire impacts for hundreds of millions of people across the planet. The impacts of climate change on the world’s oceans include decreased ocean productivity, altered food web dynamics, reduced abundances of habitat-forming species, shifting species distributions, and a greater incidence of disease. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.”
Oceans are critical for life for the following reasons: (Ibid)
- The produce about 50% of the oxygen that humans and other species breathe.
- Oceans absorb 30% of the carbon dioxide produced by humans and more than 85% of the extra heat trapped as a result of global warming.
- It is estimated that 80% of people now live within 60 miles/100 kilometres of the coast, and more than 3.5 billion people depend on the ocean for their primary source of food. In 20 years, this number could double.
The report concludes that ocean ecosystems are changing much faster than at any time in the previous several millions of years. All marine ecosystems from intertidal to deep ocean and from tropics to polar oceans are all changing rapidly, directly impacting the ability of fisheries and other coastal resources to provide support for coastal populations worldwide. Ocean ecosystems appear to be approaching a number of critical thresholds at which the risk of catastrophic changes increases sharply (Ibid).
The scientists found that key habitat-forming organisms such as corals, sea grasses, mangroves, salt marsh, and oysters are starting to disappear as a result of global climate change. Hundreds of thousands of species depend on these habitats, further emphasising the major threat that man-made global warming poses to biological diversity in the ocean. For example, half of the species that live on coral reefs need to have the carbonate structures that corals build, but corals have been disappearing at a rate of roughly 1% per year around the world. If this rate of loss continues, most reefs will have negligible numbers of coral on them by 2050, putting at risk the habitat of many hundreds of thousands of species (Ibid).
The study found that changes in sea temperature are having direct effects on the ocean’s species, with fundamental implications for reef connectivity and conservation. For example, many species have planktonic larvae which disperse using ocean currents. As temperature increases, the length of time and hence the distance these larvae will travel will get smaller. Reef systems may experience a loss of some species due to the fact that they no longer can disburse far enough to reach these locations (Ibid).
Temperature has a fundamental effect on biological processes. Moderate increases in temperature increase metabolic rates, which ultimately determine life history traits, population growth, and ecosystem processes. Organisms tend to adapt to local environmental temperatures with optimal physiological responses matching temperatures that are close to the environmental average. Organisms are able to acclimatize to a range of temperatures around these optimal values. Beyond this range, however, acclimatization fails, mortality risk increases, fitness is reduced, and populations decline or are driven to local extinction (Ibid).
The study found that warmer sea surface temperatures are reducing upwelling and consequently the resupply of nutrients in the upper layers the ocean. The reduced amount of nutrients means less primary productivity by marine phytoplankton, which is critically important to the basic food chains of the ocean. For example, the least productive waters of the Pacific and Atlantic oceans have expanded by 6.6 million km2 or by about 15.0% from 1998 through 2006 (Ibid).
The study found that diseases and troublesome invasive species are becoming more common, with many examples of diseases spreading rapidly as a result of warming conditions. Organisms are moving towards higher latitudes and are causing massive changes in ecosystems as they do. There are also numerous examples of novel communities within ecosystems that are popping up which have no known precedent. For example, the black spined sea urchin has migrated south from New South Wales and is increasingly removing Tasmania’s kelp forests and replacing them with “barrens”. Another examples include the spread of the oyster parasite Perkinsus marinus across a 500-km range of the northeastern United States during pronounced warming in 1990 and 1991 and the temperature susceptibility of red abalone in California to a fatal rickettsial infection (Ibid).
Sea ice plays a critical role in structuring the biodiversity of polar oceans. The spring melt has a major role in determining the timing of phytoplankton blooms which influences polar marine food webs. In addition, the loss of sea ice will drive additional changes through reductions in food webs that are dependent on sea-ice algae, which may explain the recent 75 ± 21% per decade decrease in krill. Sea ice also plays a critical role for a wide range of birds and mammals, functioning as a temporary or permanent platform from which crucial predatory, reproductive, or migratory activities are carried out. Many arctic mammals face serious declines, with polar bears projected to lose 68%(~700,000 out of 1 million km2) of their summer habitat by 2100. Ice-dependent Antarctic organisms such as penguins and seals are declining and, in some cases, face an escalating risk of extinction under the current projections for Antarctic warming (Ibid).
The research also revealed that temperatures of +2°C and atmospheric carbon dioxide concentrations of 450 ppm cause major changes to ocean ecosystems from polar to tropical oceans, meaning that it is now even more important to avoid exceeding these levels (Ibid).
In the words of one of the authors, Dr. John F. Bruno:
“What strikes me the most about the recent science coming out on this topic, is the degree to which we are modifying fundamental physical and biological processes by warming the oceans. The warming doesn’t just kill sensitive species, it modifies everything from enzyme kinetics, to plant photosynthesis and animal metabolism, to the developmental rate and dispersal of larval (baby) fish to changing the ways food webs and ecosystems function. And the big surprise, at least to me, is how quickly this is all happening. We are actually witnessing these changes before we predict or model them. This isn’t theoretical; this is a huge, real-world problem. Moreover, we, not just our children, will be paying the price if we don’t get a handle on this problem very soon.” (Bruno, 2010)
Schofield, et al. (2010) studied the effect of rapid climate change on ecosystems in the West Antarctic Peninsula and found:
- The magnitude of large phytoplankton blooms over the past 30 years has decreased by 12%.
- There is evidence that the algal community composition has shifted from large to small cells.
- Krill are being replaced by salps – a phenomenon that can be magnified over time because salps consume krill eggs and larvae.
- The spawning the spawning behavior of Antarctic krill depends on sea ice. Because krill form a critical link between primary producers and upper-level consumers, the shift in zooplankton community structure suggests that there should be dramatic changes in the higher trophic levels (fish, seals, whales, and penguins and other seabirds).
- These changes have been documented most dramatically in Antarctic pygoscelid penguins. In the past 30 years in the northern WAP, populations of ice-dependent Adélie penguins have fallen by 90%, whereas those of ice-intolerant Chinstrap (P. Antarctica) and Gentoo (P. papua) penguins have risen.
- Shifts in climate have had a cascading effect, with altered sea ice distributions disrupting the evolved life strategies of resident species, leading to changes in community structure and in the abundance of populations, and ultimately altering the nature of local and regional food webs.
The IPCC (2007) WGII (Ch. 4.4.9) had previously identified warm and cold water coral reefs, the Southern Ocean, and sea-ice margin ecosystems as key areas of vulnerability. The IPCC also identified ocean acidification as a serious issue facing ocean ecosystems especially coral and shelled species.
According to the Millennium Ecosystem Assessment, (Reid et al., 2005) approximately 20% of the world’s coral reefs were lost and an additional 20% degraded in the last several decades of the twentieth century.
Reefs have deteriorated as a result of a combination of human impacts such as over fishing and pollution from adjacent land masses, together with an increased frequency and severity of bleaching and ocean acidification associated with climate change. (See my blog post about ocean acidification titled: The 800 lb. Gorilla in the Ocean)
Coral bleaching occurs with the loss of symbiotic algae and/or their pigments and has been observed on many reefs since the early 1980s. It may have previously occurred, but gone unrecorded. Slight paling occurs naturally in response to normal seasonal increases in sea surface temperature (SST) and solar radiation. Corals bleach white in response to unusually high SST (~1oC above average seasonal maxima, often combined with high solar radiation). Some corals recover their natural color when environmental conditions improve but their growth rate and reproductive ability may be significantly reduced for a substantial period. If bleaching is prolonged, or if SST exceeds 2oC above average seasonal maxima, corals die. Branching species appear more susceptible than massive corals (IPCC, 2007).
Major bleaching events were observed in 1982-83, 1987-88 and 1994-95 and some more recent bleaching outbreaks are shown in Fig. 9 (Ibid) below:
According to the IPCC (2007) global climate model results imply that thermal thresholds will be exceeded more frequently with the consequence that bleaching will recur more often than reefs can sustain, perhaps almost annually on some reefs in the next few decades. If the threshold remains unchanged, more frequent bleaching and mortality seems inevitable. Bleaching events reported in recent years have already impacted many reefs, and their more frequent recurrence is very likely to further reduce both coral cover and diversity on reefs over the next few decades.
In their comprehensive study, Silverman, et al. (2009) estimated the global decline of coral reefs as a result of increase in sea surface temperature and partial pressure of CO2. Calcification rates were calculated for more than 9,000 reef locations. As the figure below shows, by the time atmospheric CO2 concentrations reach 560 ppm all coral reefs will cease to grow and will start to dissolve (red regions).
Many reefs are affected by tropical systems such as hurricanes, typhoons, and cyclones. The impacts range from minor breakage of fragile corals to destruction of the majority of corals on a reef. These severe storms greatly affect species composition and abundance, from which reef ecosystems require time to recover. An intensification of tropical storms (which is expected) could have devastating consequences on the reefs themselves (IPCC, 2007).
The annual recreational value of the coral reefs of each of six Marine Management Areas in the Hawaiian Islands in 2003 ranged from $300,000 to $35 million. The total damages for the Indian Ocean region over 20 years (with a 10% discount rate) resulting from the long-term impacts of the massive 1998 coral bleaching episode are estimated to be between $608 million (if there is only a slight decrease in tourism generated income and employment results) and $8 billion (if tourism income and employment and fish productivity drop significantly and reefs cease to function as a protective barrier) (Reid et al., 2005).
For a more technical discussion of how climate change impacts various ecosystem processes, please read On the processes linking climate to ecosystem changes (Drinkwater, et al., 2010)