The causes and effects of climate change have been widely discussed and debated for decades. Most scientists agree that increased carbon dioxide (CO2) in the atmosphere resulting from the burning of fossil fuels is causing global warming, at least in part. However, global warming is not the only effect of CO2 emissions.
In recent months, the media have begun to focus on another effect of rising atmospheric CO2 concentrations: the effect of increasing CO2 levels on marine life. The oceans absorb 22 million tons of carbon dioxide every day, writes Richard Feely, a scientist with the National Oceanic and Atmospheric Administration (NOAA) in a 2006 science brief, "Carbon Dioxide and Our Ocean Legacy" (100 KB PDF). Although this absorption is said to significantly reduce atmospheric greenhouse-gas levels, some scientists have observed that such an excess of CO2 may be altering the chemistry and biology of the world's oceans.
When the oceans absorb CO2, the chemical reaction that takes place produces carbonic acid (H2CO3), which increases the acidity (lowers the pH) of seawater. Many scientists believe that decreasing pH in the oceans interferes with the ability of certain marine animals, such as corals and other calcifying marine organisms, to make their skeletons and shells from calcium carbonate minerals. Other marine species that may be affected include lobsters, snails, starfish, oysters, clams, and various species of phytoplankton, which are all species that occupy vital spots in the global-ocean food web. These environmental impacts would reverberate through economies everywhere; various industries, including tourism and fisheries, would likely suffer if the ecology of our oceans were to be altered.
In the past year, numerous agencies and organizations have contributed time, money, research, and insight with the aim of better understanding the impact of carbon dioxide on life in our oceans. The U.S. Geological Survey (USGS) contributes substantially to this effort by conducting research and sharing knowledge with such partners as NOAA, the National Science Foundation (NSF), and the National Center for Atmospheric Research (NCAR).
In April 2005, the USGS, NOAA, and NSF cohosted a scientific workshop at the USGS Florida Integrated Science Center (FISC) office in St. Petersburg, Fla., which examined carbon dioxide's effects on marine life. "The workshop convened international experts to compare their research and propose courses of action to further investigate the impacts of rising levels of CO2 on marine life," said USGS scientist Lisa Robbins, cohost and coorganizer of the workshop. Panelists for the workshop included scientists from government agencies and academic institutions. Topics addressed included the background and importance of the issue, major gaps in collective scientific knowledge, emerging technologies, responses of ecosystems and calcifying organisms, and the design of experimental and monitoring systems.
Working with the workshop organizers, Joanie Kleypas of NCAR compiled the findings and conclusions of the 2005 workshop in a report released in summer 2006, entitled "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research." The report provides a collection of discoveries shared and facts established at the workshop, future predictions, and recommendations in regard to the fulfillment of pressing needs within the field and collaboration among scientists at the international level. The authors of the report are Joanie Kleypas (NCAR), Richard Feely (NOAA), Victoria Fabry (California State University, San Marcos), Chris Langdon (University of Miami), Christopher Sabine (NOAA), and Lisa Robbins (USGS).
In addition to hosting and participating in such collaborations, the USGS also is carrying out extensive research on carbon dioxide levels and their effect on rates of calcification by marine calcifiers. In the 1990s, Lisa Robbins and Kim Yates, both then at the University of South Florida, studied marine calcifying microbes and their role in sequestering atmospheric CO2, with support from the U.S. Department of Energy and the Electric Power Research Institute. Now at the USGS, both have continued to contribute to our understanding of the effects of CO2 on coral reefs, funded by the USGS Coastal and Marine Geology Program.
In March 2006, Yates and USGS scientist Bob Halley published a paper titled "CO3-2 Concentration and pCO2 Thresholds for Calcification and Dissolution on the Molokai Reef Flat, Hawaii" in the journal Biogeosciences. Yates and Halley used the Submersible Habitat for Analyzing Reef Quality (SHARQ) to measure, in situ, CO2's impact on rates of calcification and dissolution in Molokai coral-reef habitats. "We want to look at how increases in atmospheric CO2 are affecting the ability of reefs to grow," said Yates. Increases in pCO2 cause a decrease in CO3-2 concentration, which in turn can decrease rates of skeletal production in corals and increase rates of carbonate sediment dissolution. "The question is whether or not reefs will be able to continue to grow enough to keep up with rising sea levels, or if they will begin to erode away or become dominated by more opportunistic non-calcifying species, such as algae and sponges," said Yates. Yates and Halley have used the SHARQ in similar research in Florida Bay, Biscayne National Park, the U.S. Virgin Islands, and the Bahamas (for example, see Sound Waves article, "USGS Collaborates with Biscayne National Park on Coral-Reef Research."
"What we measure," continued Yates, "is the net result of the calcification of corals and other marine calcifiers and the dissolution of sediments at different levels of pCO2. This lets us determine what we call CO2 thresholds." These thresholds are the amount of CO2 that needs to be present before the rates at which sediments are dissolving exceed the rates at which calcifying marine organisms produce the calcium carbonate needed to make shells or skeletons. At the Molokai reef site, Yates and Halley found that the average threshold value for carbon dioxide in seawater was 654 parts per million (ppm), while the present-day level of atmospheric CO2 is 380 ppm. Some scientists predict that atmospheric CO2 levels will reach 560 ppm by 2065 and 700 ppm by 2100; this increase may prove to be a major threat to marine habitats. Yates and Halley found that CO2 in seawater on the Molokai reef flat is already exceeding the 654-ppm threshold approximately 18 percent of the time.
The results of Yates and Halley's study, in combination with the work of other scientists, can help resource managers predict future impacts on marine habitats. For example, Yates and Halley are working with USGS research oceanographers John Brock and David Zawada to combine their data on reef processes with benthic-habitat maps produced by Brock and Zawada. Combining the datasets will allow scientists to develop predictive capabilities for assessing the impact of environmental stress on coral reefs. According to Zawada, the georeferenced imagery that he and Brock gather using the USGS Along-Track Reef Imaging System (ATRIS) can be combined with Yates and Halley's data in a Geographic Information System (GIS) database for use by people ranging from scientists to politicians. (ATRIS produces video footage and photographs of the sea floor keyed to precise geographic locations and water depths; see URL http://coastal.er.usgs.gov/remote-sensing/advancedmethods/atris.html).
Also involved in studying ocean acidification is USGS scientist Ilsa Kuffner, currently nearing the conclusion of an experiment she is conducting with colleagues Paul Jokiel and Ku'ulei Rodgers at the Hawaiʻi Institute of Marine Biology and Andreas Andersson and Fred Mackenzie at the University of Hawaiʻi Department of Oceanography. This experiment, jointly funded by the USGS Geologic Discipline's Coastal and Marine Geology Program and the USGS Biological Resources Discipline's Terrestrial Freshwater and Marine Ecosystems Program, is an "open seawater experiment" in which seawater continually gets pumped from coral-reef flats into tanks. It is similar to Yates and Halley's work with the SHARQ in that some parameters are controlled while others are left to reflect natural cycles and processes.
The outdoor tanks, which are 1 m2 in area by 0.5 m deep, are exposed to full sun so that they undergo realistic diurnal, or daily, fluctuations in calcification and dissolution. There are six tanks: three control tanks, and three "treatment tanks" with identical acid treatments. In addition to the coral colonies that the researchers added to the tanks, organisms that arrive as larvae in the circulating seawater "recruit," or settle, in the tanks over time, resulting in dynamic, naturally established biological communities. The chemistry is altered in the treatment tanks by slowly dripping dilute hydrochloric acid as the seawater passes through to simulate CO2 levels predicted for the year 2100. Marine organisms established in the tanks include various types of algae, cyanobacteria, mollusks, and crustaceans.
In agreement with other recent studies, Kuffner's group observed a 15- to 20-percent reduction in coral-calcification rates in the treatment tanks, and Kuffner said that she has seen some additional, surprising results, especially for the encrusting community. In the control tanks, the presence of crustose coralline algae was visually apparent, owing to its red hue. In the treatment tanks, however, this calcifying red alga, which is known to attract coral larvae for permanent settlement, was extremely sparse. "You can plainly see which tanks are treatments and which are controls," Kuffner said. "It's really striking."
According to Kuffner, this research suggests that ocean acidification "could affect the ecology of coral-recruitment processes," which means that young coral may have difficulty finding places to settle in the absence of such calcifiers. "It's been a very interesting experiment to watch," she added.
Understanding ocean acidification also may help scientists discover whether and how lower pH values interact with other environmental factors within an ecosystem, and the resulting effects on wildlife. For example, USGS coral-reef ecologist Ginger Garrison researches various aspects of African and Asian dust transported by global wind patterns. She is especially focused on potential impacts on human health and coral reefs due to the microorganisms and synthetic organic chemicals that may travel with the dust (for example, see Sound Waves, article "Researcher Follows Trail of Dust to Investigate Its Effects on Human and Coral Health"). Garrison hopes to examine whether or not a more acidic ocean will affect the availability of dust-derived nutrients to marine organisms and the solubility and bioavailability of toxins transported with African and Asian dust. "The question is, does it affect critters on the reef, and how?" Garrison said.
All of this information plays a vital role in the larger context of understanding and predicting the impacts of climate change. "The USGS is conducting a wide range of studies that assess the impacts of climate change on ecosystems," said Virginia Burkett, Global Change Science Coordinator for the USGS and one of the authors of "Climate Change 2001," the third assessment report by the United Nations Intergovernmental Panel on Climate Change (IPCC) (see "Climate Change 2001: Impacts, Adaptation and Vulnerability"). Burkett is also an author of the IPCC's fourth assessment report, to be released in 2007. "The IPCC is appointed to assess and synthesize the peer-reviewed literature about what the science is telling us about climate change and its impacts, and to develop a consensus on the rates and impact of increasing CO2 and other greenhouse-gas emissions," Burkett said. Other data the panel uses include findings from USGS studies of the impacts of CO2 on estuarine seagrass beds and coastal marshes. "This information is used to inform policy," Burkett added.
Tom Armstrong, Program Coordinator for the USGS Earth Surface Dynamics Program, uses the results of such research to inform policymakers about the effects of global climate change. He has briefed the White House science advisor and Congress on global warming. Recently, he addressed a U.S. Senate Subcommittee on the past and projected effects of climate change, using several striking results from scientific research conducted by the USGS and other agencies.
According to Armstrong, the Earth Surface Dynamics Program deals mainly with climate change and forecasting changes in CO2 levels. This work often involves tracing patterns of atmospheric CO2 concentrations over very long spans of geologic time, using such methods as ice-core and tree-ring studies. "What we work on is establishing the broader context of climate change," Armstrong said. "[Scientists working within the Earth Surface Dynamics Program] collect data in the paleorecord and build those data into predictive models."
He believes that researching the effects of increasing CO2 on marine ecosystems is important for understanding the environmental impacts of global warming. "As the atmosphere goes, so go the oceans," he said. The Kleypas workshop report (2006) supports this view, citing a 2004 paper by Sabine and others in the journal Science, titled "The Oceanic Sink for Atmospheric CO2," which reports that about a third of the total amount of CO2 released into the atmosphere by human activities has been taken up by the oceans. This figure is expected to climb to 90 percent over the long term.
While understanding the impacts of increasing atmospheric CO2 concentrations is essential, also vital to our understanding of climate change is determining how much of the increase is anthropogenic (human caused) and not simply due to a natural, cyclical rise in CO2 levels, according to Armstrong. Anthropogenic increases in atmospheric CO2 concentrations can be somewhat mitigated, he says, but the effects of natural CO2-level change can be compensated for only by adaptation. For example, in 2003, an estimated 53 percent of the U.S. population lived in coastal communities; many of these people would be forced to move inland in the face of unpreventable sea-level rise caused by a natural cycle of global warming.
Whether increases in atmospheric CO2 concentrations are anthropogenic or not, Armstrong says that with glacial melting factored in, sea levels are expected to rise by 2 to 9 m over the next century. "The best science today is telling us that we have got to start planning," Armstrong said. The more scientific research that the USGS, NOAA, and others conduct on the numerous impacts of rising CO2 levels, Armstrong believes, the better scientists will be able to quantify the critical nature of the situation for policymakers.
"The more science we do, the more we find that [CO2 level] changes were far greater than expected," Armstrong added. "[The research] means everything."
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