As the body of research on the impacts of global climate change continues to expand, marine resource managers - including those in the National Marine Sanctuary System - have found themselves faced with a growing list of causes for concern.
Climate change's effects on the marine environment, including warming seawater temperatures, ocean acidification, sea level rise, and changes in currents, upwelling and weather patterns, have the potential to cause fundamental changes in the nature and character of marine and coastal ecosystems.
One of the immediate concerns of climate change from a marine resource manager's standpoint is its potential for intensifying the effects of "traditional" threats to marine resources. The added stress from global warming can worsen already severe impacts on marine ecosystems from factors like pollution, overfishing, habitat destruction, invasive species and disease.
Marine protected areas like NOAA's national marine sanctuaries are home to some of the richest and most diverse collections of marine life and habitats on the planet, providing a natural venue for researchers to study the impacts of climate change. Sanctuaries are also important places for communicating the marine effects of climate change to the public.
It has become clear that the ocean plays a critical role in regulating the amount of carbon dioxide (CO2) in Earth's atmosphere, absorbing nearly a third of the CO2 produced by humans since the Industrial Revolution. In 2007, the International Panel on Climate Change (IPCC) reported that atmospheric CO2 has increased by 31 percent since 1750, primarily due to human activities like burning fossil fuels and deforestation.
As the ocean is absorbing increasing amounts of CO2 from the atmosphere, a related reduction in seawater pH is taking place. The IPCC 2007 report stated that this process, known as ocean acidification, can hurt the ability of corals, plankton, shellfish and other invertebrates to build the calcium carbonate shells or skeletons they need to survive. Across the National Marine Sanctuary System, countless ecologically and economically important species like corals, coralline algae, sea urchins, starfish, lobsters, crabs, oysters, mussels, clams and scallops are directly threatened by ocean acidification.
Declines in certain types of plankton due to decreased ocean pH could have a cascading effect in marine ecosystems throughout all levels of the food web - including affecting higher-level species like fish, seabirds, and large marine mammals such as whales. The resulting losses in biodiversity could have disastrous consequences for coastal economies and communities. It has been proposed that national marine sanctuaries can serve as "sentinel sites" for monitoring and communicating information on changes caused by ocean acidification.
Warming Seawater Temperatures
In the past several decades, the global ocean temperature from the surface to a depth of 700 meters has been rising steadily - by about 0.1oC from 1961 to 2003, according to the IPCC 2007 report - and is expected to increase further in the coming centuries.
It might seem like a minor increase, but even small changes in water temperature can affect the growth, feeding behavior and reproduction of marine organisms, many of which are sensitive to thermal increases of just a degree or two above normal. Confronted with continued ocean warming, these species may be forced to adaptor relocate to cooler waters, or even face possible extinction. Conversely, rising ocean temperature can also make it easier for invasive species that favor warmer waters to expand into new areas, displacing native marine life and disrupting ecosystem structure.
Ocean warming has also been linked with outbreaks of marine disease. According to a 2002 study by University of California researchers, populations of black abalone in the warmer southern waters of Channel Islands National Marine Sanctuary were decimated by a bacterial disease known as "withering syndrome" between 1986 and 2001, while cooler waters appear to have suppressed the disease in abalone populations farther north along the California coast.
Corals have long been viewed as "canaries in the coal mine," alerting us to changes in our oceans. For example, in places like Florida Keys National Marine Sanctuary and the Papahanaumokuakea Marine National Monument, studies by sanctuary staff and partners have seen rising seawater temperatures coincide with increased coral bleaching. Bleaching causes corals to lose their symbiotic algae, drastically slowing their growth, and can make them more susceptible to disease.
Historically, corals stricken by bleaching during natural, temporary periods of warming have been able to recover once temperatures return to normal, but the combined pressures of global warming and other factors like pollution and habitat degradation can stress coral populations to the point that they experience reproductive failure or die-offs.
Storms are common throughout the sanctuary system. Storm intensity and frequency is likely to increase in some places with climate change, according to the IPCC, and although ecosystems are naturally resilient to damage from these events, the recovery process can be slow. Key ecological components like corals and kelp forests are particularly vulnerable to frequent, severe storms, which can overwhelm their ability to regenerate. As recently as September 2008, Hurricane Ike caused severe damage to portions of Flower Garden Banks National Marine Sanctuary's coral reefs, which were still in the process of recovering from Hurricane Rita in 2005.
Sea Level Rise
Global sea level has risen at an average rate of 0.18 centimeters per year from 1961 to 2003, with this rate increasing between the years 1993 to 2003 to about 0.31 centimeters per year, according to the IPCC 2007 report.
For the national marine sanctuaries, sea level rise has several potential consequences. In coastal sanctuaries, marine life in the intertidal zone is likely to experience shifts in abundance and distribution, and possibly widespread habitat loss as the shoreline moves inland. Animals like sea turtles could suffer serious population declines if rising sea level causes the sandy beaches where they nest to shrink or even disappear.
The tiny atolls and islets of the Northwestern Hawaiian Islands, encompassed by the Papahanaumokuakea Marine National Monument, are particularly at risk from sea level rise. A 2006 study published in the journal "Endangered Species Research" predicted that these small volcanic islands may shrink by as much as 65 percent with a 48-centimeter rise in sea level, threatening the survival of the endangered Hawaiian monk seal, which requires sandy beach areas to rest and recuperate, and numerous other species found nowhere else on Earth.
Changes in Currents and Upwelling
Scientists have found that ocean temperature has a major influence on currents and upwelling activity, which are fundamentally important to marine ecosystems around the world. Upwelling - circulation of cool water from the deep ocean to warmer areas near the surface - is especially crucial in places such as Cordell Bank National Marine Sanctuary off the Northern California coast and Stellwagen Bank National Marine Sanctuary off Massachusetts, where thriving communities of marine life depend on the nutrients it pulls up from the depths.
It is difficult to predict how warming sea surface temperatures will alter upwelling and currents, but any changes in ocean circulation are likely to have serious consequences for marine life. Without the influx of nutrients from deep water, sites like Cordell Bank and Stellwagen Bank could see devastating collapses in marine communities, with declines in tiny organisms like plankton and krill at the bottom of the food web leading to starvation for fish, seabirds, whales and other high-level consumers.
Looking to the Future
The Office of National Marine Sanctuaries is looking ahead as climate change poses an increasingly grave threat to the health of the world's oceans. Through sound science, public outreach and partnerships, sanctuary staff are working to develop management strategies that will help us to adapt to the changing marine environment, and perhaps more importantly, will help all Americans better understand the consequences of climate change.