As significant as these threats are, however, they represent only part of the problem created by the emission of 250 billion metric tonnes of greenhouse gases since the beginning of the Industrial Revolution. While half the world’s CO2 has remained in our atmosphere, the other half has dissolved in the oceans to be taken up by marine plants or, in combination with seawater, to become carbonic acid. As a result, the world’s naturally alkaline oceans have already become 30% more acidic in a process known as “ocean acidification.” The twin threats of climate change and ocean acidification pose major problems not only for marine species but for life on Earth.
According to the 2007 Intergovernmental Panel on Climate Change, the average temperature of the Earth has risen .76 ° C in the last 150 years. Covering nearly 70% of the Earth’s surface, the oceans have a profound influence on global climate and the atmosphere. In a continuous cycle, oxygen produced by marine plants through photosynthesis is released into the atmosphere and atmospheric carbon dioxide is absorbed by the oceans.
Temperature is a major driver in ocean ecosystems. As ocean waters warm, they expand. Some low lying and coastal areas are already being inundated as a result of climate change. Warmer waters have also increased the melting of glaciers, adding fresh water to the oceans. As surface waters warm and mix less with deeper more productive waters, circulation patterns and ocean productivity will change. Because many species exist only within a very narrow temperature range, changes in ocean temperature are expected to affect marine habitats and biodiversity, resulting in shifts in geographic range and abundance for algae, plankton, fish, and other species. This is a particularly critical issue for microscopic plankton at the base of the food web, creating a chain reaction throughout marine ecosystems.
Ocean chemistry is complex, and not surprisingly, the oceans are not chemically uniform. Through the addition of CO2 and subsequent creation of carbonic acid, the oceans have become measurably more acidic in the last several hundred years. Ocean acidification will alter the oceans significantly, but this process is poorly understood today. Unfortunately, it was not addressed by governments at the global climate talks in Copenhagen in December 2009. Despite our ignorance, however, ocean acidification poses a huge threat to marine species and will continue to worsen for some years to come. The effects will be widespread, but the species most at risk are those which use calcium carbonate to build shells and reefs.
Sea water has a pH of 7.8 to 8.5 (for pH, the most acidic measure is 1 and the most alkaline measure is 14). Like the Richter scale, which monitors the intensity of earthquakes, the pH scale is a logarithmic measure of how alkaline or acidic a solution is; small numerical changes in pH represent major chemical changes. Thus far, the pH of the oceans has decreased by .1, which represents a 30% increase in acidity. But the actual pH of the oceans is less important than the chemical reactions that occur as a result of more acidity. Like changes in water temperature, these chemical changes will have adverse effects on species at the base of the marine food.
Additional threats to marine ecosystems from climate change and ocean acidification separately or in tandem include the following issues:
How will climate change and ocean acidification affect sea turtles?
Sea turtle biologists are beginning to assess how changes in major ocean currents, key habitats, weather patterns, and prey abundance and distribution resulting from climate change and ocean acidification will affect sea turtles and their reproductive fitness. Numerous habitats utilized by the world’s seven species of sea turtles, including beaches, sea grass beds, coral reefs, near-shore bottom areas, and the waters of the open ocean will be altered.
Nesting habitat will be lost with more severe storms, rising sea levels, and eroding beaches; nests will be subject to increased precipitation or drought, inundation, and changing temperature. Hotter beaches will put sea turtle embryos at risk. During development, the sex of an embryo is determined by the heat of incubation, with warmer nests producing females and cooler nests producing males, but temperatures above 33° C are lethal.
The seagrasses on which green turtles depend will become less productive as a result of warmer water, increased sedimentation and runoff from coastal flooding, and decreased visibility and light penetration. Near-shore habitats where some species forage will also be destroyed by coastal flooding and decreased visibility; for prey such as crabs which are dependent on their ability to make shells, acidification will have a marked effect. In the coral reefs with which hawksbills are closely associated, increased water temperatures and acidification will undermine the ability of coral polyps to survive and build reefs. Because some species of fish are losing their olfactory sensibility and homing ability in more acidic waters, the potential exists for sea turtles to suffer the same fate. Other threats include changes in the timing of nesting and variation in prey abundance and range.
Global temperature increases of 1.5 to 2.5° C above pre-industrial levels will expose a third of the world’s species to a greater risk of extinction. If we are to limit global temperature increases to just 2° C, within the next 10 years emissions of CO2 and other heat-trapping gases must be reduced by 25% to 40% from 1990 levels. Some prominent scientists go even further to suggest that global CO2 levels need to be reduced from the current 387 ppm (parts per million) to 350 ppm. Each year the world’s seas are absorbing an additional two billion metric tonnes of carbon. On average, each American adds about 40 pounds of carbon dioxide to the oceans every day. Each of us should be taking steps to reduce our carbon footprint. Most importantly, we need to become better informed about CO2, climate change and ocean acidification. Al Gore’s new book “Our Choice” provides excellent information and solutions. Several worthwhile Internet resources include a New Yorker article by Elizabeth Kolbert entitled “A Darkening Sea”; a carbon calculator at www.soprisfoundation.org (found in the “Projects” section); and an “Ocean Acidification Summary for Policy Makers 2009” available at www.ocean-acidification.net. On PBS, look for the film “A Sea Change” directed and co-produced by Barbara Ettinger.
As STC continues to focus on the substantial threats to sea turtles and the habitats on which they depend, our activities are protecting countless other marine and coastal species. An underlying goal of our work is to ensure that sea turtles are as resilient as possible so they can withstand future survival challenges. With this in mind, STC is working hard to reduce existing threats over which we can exert some control, such as incidental capture in fisheries and disturbance of nesting habitats, while at the same time preparing for the future impacts of climate change through education and policy initiatives.