Earth’s climate and ocean systems are in a continual state of change. Such variability occurs on different scales of space and time, from daily weather to decades-long patterns in regional sea surface temperatures. Although much of this fluctuation is natural, it is now believed that change can be induced by human activity as well. Greenhouse gases from the burning of fossil fuels, for example, can cause warming of the planet’s surface, including its oceans.

Average ocean temperature, from sea surface down to 10,000 feet (3050 meters), has risen by 0.05 degrees Celsius since the 1950s, according to researchers. Whether or not this is a result of human greenhouse gas emissions, the warming presents potential long-term challenges for MPA management, including coral bleaching, shifting habitats, and even disappearing species. MPA News covered the science of ocean warming and the challenges it poses for MPAs in July 2001 (MPA News 3:1). In light of the Kyoto Protocol taking effect last month, we revisit the subject of ocean warming, examining how managers might prepare for it.

Coral bleaching and the future of the Great Barrier Reef

Of the impacts of ocean warming on marine ecosystems, the one that is perhaps most easily visible is coral bleaching. A condition in which reef corals turn white, bleaching occurs when corals experience stress. Any number of stressors – siltation, pollution, destructive fishing practices, exposure to freshwater, and increased temperatures – can result in the loss of corals’ symbiotic algae (zooxanthellae), whose photosynthetic pigments give coral reefs their color. Loss of the zooxanthellae removes a source of energy for the corals. Although bleached corals can survive for some time, they can die if conditions do not return to normal. In 1997-98, a massive worldwide bleaching event triggered by elevated sea surface temperatures resulted in widespread coral mortality.

The Great Barrier Reef Marine Park in Australia escaped the worst of that event, but has faced the challenge of bleaching in ensuing years, including a major bleaching event in 2002. “While the Great Barrier Reef was fortunate in that it did not experience catastrophic coral mortality in 1998, bleaching has caused widespread stress to many reefs in our park’s ecosystem, particularly in shallower areas,” says Paul Marshall, manager of the Climate Change Response Program for the Great Barrier Reef Marine Park Authority (GBRMPA). “Bleaching-related stress was evident on 60-95% of our reefs in 2002.”

Marshall says monitoring studies suggest recovery of these reefs may take years, if not decades. Further warming of the sea surface could delay or even halt that process. “Repeated bleaching events, even if they are no more severe, have the potential to cause significant declines in the condition of the Great Barrier Reef over coming decades,” says Marshall.

The marine park received media attention in February 2005 when a prominent Australian reef biologist – Ove Hoegh-Guldberg of Queensland University – told journalists that much of the park’s shallower reefs could die off within 20 years due to increasingly frequent bleaching events. He warned of “a complete devastation of coral communities on the reef”. (This echoed a 2004 report by Hoegh-Guldberg, released by WWF Australia and the Queensland Tourism Industry Council [MPA News 5:8].)

Marshall says GBRMPA is “certainly concerned” about the implications of climate change for the future of the Great Barrier Reef. The question becomes, how do you manage for it? “The ultimate cause of mass coral bleaching is anomalously warm sea temperatures in combination with UV light, and neither of these factors is amenable to management intervention over ecological spatial scales,” he says. “However, an emerging understanding of the interactions between stressors, and about the factors that confer resilience to reef ecosystems, is proving an important basis for management actions in response to coral bleaching.”

In particular, he says, it is becoming clear that good water quality, abundant and diverse herbivores, high biodiversity, and well-protected refugia are critical to optimizing the productivity and natural values of reef ecosystems. GBRMPA has launched several major initiatives aimed at restoring and maintaining these ecosystem qualities, thus moderating the effects of warmer waters on the Great Barrier Reef. These initiatives include a reef water quality action plan (MPA News 3:7), a substantial expansion of no-take zones (MPA News 5:10), and a partnership with the Florida Keys National Marine Sanctuary in the US to explore the science and management of reef resilience (MPA News 6:7).

Climate change may be impacting more than just the corals of the Great Barrier Reef, says Marshall. “There is research underway that suggests that high sea temperatures during warm years are impacting the ability of some seabirds to provide enough food for their chicks, leading to mass mortalities of fledglings on some islands,” he says. “Other work is examining the effect of increased sand temperatures on sex ratios in sea turtle hatchlings.” Over the next four years, he says, a major goal of the GBRMPA Climate Change Response Program is to complete an assessment of risk from climate change for key species and habitats.

Shifting of habitats

Fluctuations in ocean temperature – even as part of natural cycles like El Nino, the rapid warming trend in the Pacific Ocean – can cause shifts in the spatial distribution of species. When the waters of a particular site warm up, the site generally becomes more hospitable to warmer water species. When the waters cool down again, colder water species return.

The 4,292-km2 Channel Islands National Marine Sanctuary (CINMS) stretches along the Pacific coast of the US state of California. Comprising a “mixing zone” where cool and warm ocean currents intersect, CINMS features a diverse array of temperate and sub-tropical species and habitats. The temperate ones are concentrated more northward in the sanctuary, while sub-tropical ones are predominantly southward. During El Nino cycles, the boundary between the temperate and sub-tropical portions of the sanctuary may shift tens of kilometers northward, resulting in local increases or decreases of some species. In essence, during these periods, the sanctuary becomes more sub-tropical and less temperate. (During La Nina cycles, the opposite is observed.)

In 2001-2002, when the multiple-use CINMS underwent a process to create a network of no-take marine reserves within its boundaries (MPA News 4:6), connectivity of habitats was a consideration in the planning. That is, to help ensure that species of interest would be protected throughout their life history, reserves were placed at distances that did not exceed larval and adult dispersal distances for these species. However, should ocean warming cause sanctuary habitats to move consistently northward over, say, the next 50-100 years, species dispersal could change and the reserves could conceivably lose some of their effectiveness.

Satie Airame, who served as CINMS liaison to a science advisory panel for the reserve-planning process, says the reserve design accounts for some of the natural variability in ocean temperature, but may not be enough to handle persistent habitat shifts. “The distribution of the reserves spans the entire island chain and the full range of habitats and species in the region,” she says. “But if the region becomes completely sub-tropical as ocean warming proceeds, those species currently protected in cooler waters may persist only at northerly latitudes.” Such latitudes could, perhaps, be only outside the sanctuary.

To protect the species of the region over the long term, Airame says, it may be necessary to establish reserves along the coast northward of CINMS. Indeed, the state of California is working to build a network of marine reserves throughout its waters, a process that could yield a functioning network by 2011, according to officials. “Because the scale of global warming is much larger than the Channel Islands region, complementary regulations that cover a much larger area – the entire state of California – are likely necessary,” says Airame.

David Obura is East Africa regional coordinator of CORDIO, an international research program to respond to coral reef degradation in the Indian Ocean. He agrees with Airame that in the context of climate change, management of habitats and species must follow those habitats and species as they shift over the long term. But creating new MPAs to do it, or expanding the boundaries of existing ones, could face challenges from stakeholders, he says. Specifically, accusations of governmental “land grabs” could make such policies difficult to carry out. Instead, he would prefer to see comprehensive marine zoning, with up- or downgrading of protective measures in various zones as needed over time.

“If done cleverly, the boundaries of zones within such a system may not need to be changed – just the management regime,” says Obura. “A reserve area within a buffer zone might be better protected if the activities in the buffer zone are more stringently licensed during a recovery period, such as after a coral bleaching event. Conversely, in an instance of total reef mortality in an area with ‘reserve’ status, this site would be reclassified as a fishery area; in exchange, a healthy area would be reclassified as a reserve.”

Obura continues, “Viewed from a larger scale, the important thing is that proportional cover of different zones is maintained or altered according to specified guidelines. At a smaller scale, it amounts to trading areas. In fact, if the ability to do this is encoded in management and legislation, it could become a trading system along the lines of individual transferable quotas in fisheries.”

Disappearing habitat of freshwater species

With ocean warming, colder water fish species face the challenge of shrinking habitat. As their preferred waters shift farther from the equator, these species must move as well. Many marine and anadromous species have the mobility to respond to such change, at least until it hypothetically becomes so extreme that there are no longer any suitable habitats available to them. (Anadromous species are those that spend all or part of their adult life in saltwater and return to freshwater streams and rivers to spawn.)

Peter Maitland, a biologist at Scotland’s Fish Conservation Centre, says such migration has occurred in the past, as ice ages have come and gone. “Indeed, we know that it is happening at present as salmonid fish move into virgin waters in Alaska, opened up by glacier retreat,” he says.

But for populations of freshwater fish species, climate change offers real potential for extinction as species become trapped in lakes and river catchments, unable to migrate to colder climes. “We can expect that southern populations of northern freshwater species, especially those at low altitudes, will steadily be eliminated as global warming proceeds,” he says. “From a conservation perspective, protection for any one species should probably be given preferentially to sites at higher altitudes and those in the north, as being the most likely to survive.” He points out that conservation organizations in Scotland are protecting populations of certain rare species not only through appropriate site management but also by creating “safeguard” populations in new waters, preferably at high altitudes.

Maitland says a major challenge to colder water species from global warming is outcompetition by warmer water species that move into their habitat. “This is one of the greatest threats to more sensitive northern fish species in Scotland and the rest of northern Europe, and is something we are battling,” he says. “Not only do many southern species find waters in the north very suitable as they become warmer, but many of the species concerned are much more tolerant of the other anthropogenic changes that are affecting many waters, such as eutrophication and siltation. Thus, an important part of the management of waters with important populations of northern fish species is the prevention of alien species ever reaching them.”

Maitland acknowledges that the phenomenon of alien species – along with changes in climate over history – are partly responsible for the “native” species assemblages we have today. “The majority of our northern freshwater fish species originated from anadromous stocks, as these were the only fish able to access most fresh waters after the ice retreat from Europe,” he says. “These have eventually become isolated as the climate warmed up or barriers to migration appeared.”

Asked whether it is a fruitless effort by humans to protect northern fish species in the face of global warming, he says, “It may be. But that should not prevent us from trying, and believing that today’s conservation efforts will prevent tomorrow’s extinctions – or at least some of them.”

For more information:

Paul Marshall, Climate Change Response Program, GBRMPA, PO Box 1379, Townsville QLD 4810, Australia. Tel: +61 7 4750 0771; E-mail:

Satie Airame, Marine Science Institute, University of California, Santa Barbara, CA 93106-6150, USA. Tel: +1 805 893 3387; E-mail:

David Obura, CORDIO East Africa, 8 Kibaki Flats, Kenyatta Beach,Bamburi Beach, P.O. Box 10135, Mombasa, Kenya. Tel: +254 41 548 6473; E-mail:

Peter Maitland, Fish Conservation Centre, Haddington, Scotland. E-mail:

BOX: Advice for coral MPA managers on managing for climate change

David Obura is East Africa regional coordinator of CORDIO, an international research program to respond to coral reef degradation in the Indian Ocean. He offers the following advice to coral MPA managers on protecting their resources from the effects of climate change:

  1. Educate local stakeholders about the threat of climate change so that when a bleaching event occurs, there is greater opportunity for a considered response.
  2. Prepare response options for bleaching events that include press releases, alerting stakeholders of what is happening, calling scientists to track the event, and planning potential closures or “additional care” measures that divers/fishers can take during and after a bleaching event. Discuss these openly (#1 above) so that there is prior acceptance.
  3. Minimize the ecosystem stressors that can be minimized, particularly those that affect individual coral health (pollution) and ecological resilience/recovery processes (water quality, herbivory).
  4. Establish basic monitoring programs involving rangers and frequent visitors so that the manager knows what is happening in real time; the network can serve to channel information to stakeholders as well. Bring in scientists to conduct more detailed monitoring programs to assess reef health, and link this with the less formal monitoring.
  5. Promote sound management and protection of reefs outside the MPA to maximize the overall health of the system; this way, healthy patches of reef may survive somewhere to facilitate recovery. Focus particularly on water quality issues and fishing/extraction.
  6. Join national and regional processes that publicize what is happening to reefs (i.e., MPA and conservation networks, tourism marketing initiatives, artisanal fisheries advocacy programs) in order to pressure governments to consider the problem and advocate internationally for greenhouse gas reductions.

BOX: Other effects of ocean warming

With persistent warming of the world’s oceans, sea level would continue to rise as it has over the past century: as a result, coastal regions could be subject to increased wind, flood, and erosion damage, and low-lying islands would be submerged. Many researchers predict that warmer sea surface temperatures could also spur an increase in the frequency and severity of storms, particularly in tropical regions. And there is speculation that, if allowed to progress unchecked, the rising of ocean temperatures could even cause changes in major ocean circulation patterns, leading to abrupt shifts in climate.

For more information on the potential impacts of climate change, including on the marine environment:

Intergovernmental Panel on Climate Change (IPCC)
Climate Change 2001: Impacts, Adaptation and Vulnerability

US National Research Council
Climate Change Science: An Analysis of Some Key Questions

Pew Center on Global Climate Change
Coral Reefs and Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems