Climate Change Devastates Oceans
|Our Correspondent||Nov 4, 2010|
Legislators, scientists and conservationists meet again in Nagoya, Japan, under the gathering clouds of species extinction to discuss the state of Earth's biodiversity at the Tenth Conference of Parties for the Convention on Biological Diversity.
It takes place amidst discouraging news.
The convention's Global Biodiversity Outlook 3 reports that the target, set in 2002, to achieve significant reduction in the rate of biodiversity loss by 2010 has failed. Underlying causes of continued loss of biodiversity include habitat loss, unsustainable use and overexploitation of biological resources, climate change, invasive species and pollution.
There is some good news in this report on localized or partial success in stemming some of these pressures. However, while climate change is viewed as a threat of "increasing significance," it's remarkable that this document does not call for international agreement on an effective course of action to tackle climate change with utmost urgency.
Without such an agreement, humankind will be the causative agent of a planetary extinction expected to rival the five great extinctions recorded in geological history.
Coral reefs are the most species-rich marine ecosystems on Earth. Despite only comprising about 0.2 percent of the area of the oceans, coral reefs host a quarter of all marine fish species and perhaps 1 to 3 million marine species in total. In economic terms, they provide goods and services estimated up to $375 billion per annum. Around 500 million to 1 billion people rely on coral reefs for food, and 30 million of the world's poorest people in coastal communities depend entirely on reefs as their primary means of food production and livelihood.
The impacts of climate change are already apparent on coral reefs. There's no need to resort to models that forecast the effects of increased global temperatures on coral-reef ecosystems and the species associated with them, the evidence is there. In the late 1970s the first mass coral-bleaching events were recorded. This phenomenon is associated with abnormally high sea-surface temperatures and results from the corals ejecting algal symbionts from their tissues.
The symbionts, microscopic plants, produce energy from photosynthesis and provide corals with most of their nutrients. Once these are ejected from coral tissues, the corals change from various shades of brown to a bleached white color; if warm conditions are prolonged, the corals die.
Mass coral bleaching has increased in frequency as sea-surface temperatures have steadily increased in response to global warming. In 1998, a single mass bleaching event reached across the tropics and killed 16 percent of all the world's shallow-water coral reefs. At the Nagoya conference, news that another large-scale mass coral bleaching event affecting the Indian Ocean, Pacific Ocean and Caribbean has occurred in 2010, potentially rivaling that of 1998 in severity, is particularly unwelcome. By the middle of the century the steady increase in SSTs will make bleaching annual events, laying waste to corals reefs globally.
However, global warming is not the whole story of CO2-induced climate change. The oceans have been absorbing a large proportion of the CO2 produced by humankind. When absorbed by seawater, CO2 forms carbonic acid and reduces pH, a measure of the acidity. So far a pH reduction of 0.1 units has been recorded by long-term ocean monitoring stations around the globe suggesting the oceans are becoming more acidic.
This change may seem tiny, but in fact represents a major shift in the oceans' chemistry. A pH change of 0.1 represents 30 percent more hydrogen ions in surface waters. These hydrogen ions react with calcium carbonate, the raw material that corals use to build their skeletons, converting it to bicarbonate, a process known as ocean acidification. Reduction in the availability of calcium carbonate has profound implications for coral reefs. Both corals, and other organisms that build and connect intricate structures of a reef, grow shells or skeletons out of calcium carbonate. As concentrations of this mineral decline in seawater, the growth rates of corals and other organisms also decrease. Already growth of corals is slowing down in several major reef areas of the world, and acidification is thought to be at least partially responsible.
Ocean acidification has been found to have other unexpected effects on the marine animals. For example, clown fish lose their ability to discriminate between the reef on which they were spawned and other reef habitats, possibly less favorable for their growth and survival.
Ocean acidification not only affects the tropics. If CO2 emissions continue to increase at the present rate, parts of the polar-ocean surface become under-saturated with calcium carbonate by the middle of the century. This means they will actually become corrosive to calcium carbonate. The consequences on marine food webs in high latitudes are not understood.
We know that careful management of coral-reef ecosystems helps them to recover from the impacts of a mass-bleaching event. Fishing has a profound influence on reef recovery because the removal of grazing fish species allows algae to smother a damaged reef and prevent re-colonization of the reef substrata by coral larvae. Sustainable management of fishing and prevention of destructive fishing practices maintain reef health and improve resilience to climate-change impacts. Likewise, the presence of sediments and pollutants can also damage reef health and inhibit reef recovery.
The Global Biodiversity Outlook points out that careful management of the largest reef, the Great Barrier Reef, will give it "the best chance of adapting to and recovering from" serious threats, "especially those related to climate change." This statement is wholly misleading and underlies a fallacy that permeates current discussions on action to reduce CO2 emissions.
Observations indicate that already, at an atmospheric CO2 concentration of ~380ppm, coral reefs are in decline as a result of climate-change effects combined with direct human impacts. At a CO2 level of 450ppm and beyond, which will be reached by the 2030s, mass coral bleaching will progressively destroy most coral reefs in shallow waters. Sometime in the third quarter of this century, CO2 levels will move beyond 560ppm, a point at which ocean acidification will adversely affect carbonate levels at the surface of most oceans.
Coral reefs at this point will be in an uncontrolled decline from which recovery will be unlikely. Climate negotiations are currently discussing 450ppm atmospheric CO2 level as a target for stabilization, not peak emissions. This means that, at best, careful management of coral reef ecosystems will postpone demise by a few decades. This may give human populations some chance of adaptation to the destruction of these ecosystems, but it will not save the biodiversity associated with coral reefs. By the end of the century we will have lost the most beautiful, most diverse and in socioeconomic terms, one of the most valuable marine ecosystems on the planet.
At the Tenth Conference of Parties for the Convention on Biological Diversity, there will be calls for more protected areas, greater action to restore populations of threatened species, more efforts to reduce consumption of biological resources and destruction of habitats.
But the elephant in the room that is climate change cannot be ignored. Beyond direct actions to protect biodiversity the message must be delivered clearly, without compromise, that failure to take action on climate change and curb CO2 emissions immediately and drastically will result in ecological catastrophe. The consequent loss of biodiversity and ecosystem goods and services will significantly impact the Earth and along with it humankind.
Alex David Rogers is a professor with the Department of Zoology, University of Oxford. This is reprinted with permission from YaleGlobal, the magazine of the Yale Center for the Study of Globalization.