The first six months of 2006, according to the National Oceanic and Atmospheric Administration, were the warmest such stretch since climate record keeping began in 1895. In 37 states, the six-month period was in the top 10 hottest on record, and in 10 states new records were set. These six months fit in with a long-term trend of increasing temperatures that, according to one official at the NOAA's National Climatic Data Center, "cannot be explained without the influence of greenhouse gases."

While global warming skeptics question this direct link between greenhouse gases and climate change, there is undisputed consensus on the following facts: Recent years have seen rises in atmospheric greenhouse gas levels, rises in air temperatures, and rises in ocean temperatures. All three of these trends, if continued, pose potentially catastrophic threats to the life that inhabits the world's oceans. Here are a few of the reasons why:

When ocean temperatures rise, corals bleach, and entire marine populations can lose their only shelter and food source.

When ocean temperatures rise, mobile species relocate, and entire marine populations are exposed to new competitors, predators, parasites and diseases, while nonmobile species are left to die.

When sea levels rise, nursery habitats are flooded, and entire marine populations face extinction, with no place left to reproduce.

When upwelling patterns change, the food chain is dramatically altered, and entire marine populations can starve to death.

When carbon dioxide emissions rise, ocean acidity rises, and the calcium-based structure of entire marine populations – from plankton to coral to clams – crumbles.

Like underwater rainforests, marine ecosystems are intricately connected webs, and no effect is ever felt in just one place. The slightest alteration to the viability of the tiniest organism can have an accumulating effect right up the chain to the largest apex predators. When plankton disappear, so do the small fish who eat them, and the big fish who eat the small fish, and then the birds who eat the big fish, and finally the large predators who eat everything. Suddenly, a small shift in sea temperature, directly affecting only a few tiny creatures, manages to upset the delicate balance of life, and entire species are lost.

But what about the eons of climate change in our past? If animals can survive Ice Ages, surely they can survive a shift in temperature of just a degree or two, right? It's true that the earth's climate has been in a constant state of change since life first appeared, and organisms have adapted accordingly. But what is different now is the rate of change. Sudden increases in global temperatures and carbon dioxide levels are occurring too fast for marine life – used to a timescale of anywhere from centuries to millions of years – to adapt to the changing climate.

Warmer Oceans Lead to Coral Bleaching

One of the most clearly visible effects of rising ocean temperatures is the phenomenon called coral bleaching, whereby corals lose some or all of their color, in the worst cases bleaching completely white. The color usually observed in coral derives not from the coral animal itself but rather from the algae that live symbiotically within coral polyps. Through photosynthesis, the algae produce the food that feeds the coral. When water temperatures rise, this photosynthesis process is impeded, and coral tend to expel their algae. In doing so, the coral loses the organism that provides both its color and its food source; unless water temperatures drop to a level at which the algae can once again return and survive, the coral dies.

Bleached branching fire coral at East Flower Garden Bank. Photo taken in mid-October 2005 following hurricane Rita. Photo courtesy: Flower Garden Banks NMS.

Coral beaching is observed around the world and at all the National Marine Sanctuaries with coral reefs. In the Florida Keys, observations of coral bleaching began in the mid-1980s. The first events were relatively small, and most of the corals were able to recover. By the 1990s, however, the bleaching became considerably more extensive and more harmful to the reef. In the summer of 1997, a massive coral bleaching event occurred during a particularly long and hot spell. According to sanctuary manager Billy Causey, "It was a particularly noticeable episode for me. I received calls from long-time residents of the Keys, who were reporting they had never observed such an event. One very knowledgeable multigeneration [local] called me and reported he had observed massive coral colonies in the tidal passes west of Key West that were severely bleached. In all of his years of diving in the Keys, he had never witnessed such an event, nor had he heard of such extensive coral bleaching." Coral bleaching events continue in the Florida Keys and are getting larger and more global in scale, with similar events occurring simultaneously in oceans around the world.

The following year, 1998, saw the worst-ever worldwide coral bleaching event, as over 16 percent of the world's reefs were lost during the El Niño pattern of high ocean temperatures. The effects are still being felt today. In May 2006, the results of the first long-term study of the effects of warming-caused bleaching on coral reefs and fish where published in the Proceedings of the National Academy of Sciences. Researchers focusing on the reefs near the Seychelles, north of Madagascar, found that coral cover was less than 10 percent of what it had been a decade earlier, and that fish diversity had tumbled as their shelter died off. Loss of coral reefs has far-reaching effects in the marine environment. Coral is a slow-growing species and not very mobile, so it can be difficult, if not impossible, for reefs to grow back once lost. Reefs form the fundamental architecture of entire ecosystems; thus, when a reef disappears, so can the myriad diverse species that inhabit it.

Coral bleaching can have a cascade of side effects for both the overall reef life and the coral animal. One such side effect observed by officials at the Flower Garden Banks National Marine Sanctuary is that coral disease seems to increase significantly after a bleaching event, perhaps because of the weakened condition of the corals.

At Fagatale Bay National Marine Sanctuary, site of the only true tropical reef among the sanctuaries, coral bleaching events caused by increases in sea surface temperatures are on the rise. The sanctuary reports that bleaching has been observed nearly every summer in recent years, affecting corals mostly in shallow water but also in waters as deep as 130 feet.

With the exception of marine mammals and birds, most of the marine life in the ocean is, in general terms, cold-blooded, and therefore highly sensitive to water temperatures. A change in ocean temperatures as small as 1 degree Celsius in a geographic area can have an immense impact on what species are able to survive in that area. As ocean temperatures warm, more mobile species may be able to shift to cooler latitudes, but with the increasingly rapid pace of climate change, less mobile species may not be able to shift quickly enough and may simply die off. Rapid redistribution of marine life also can result in new combinations of species within geographic communities, which can expose all the species to new competitors, predators, parasites and diseases.

Within the Monterey Bay National Marine Sanctuary waters, researchers looking at trends over a 60-year period found that invertebrate animals in the rocky intertidal community shifted northward in a pattern consistent with the concurrent climate change. Between the 1930s and 1990s, shoreline temperatures increased by nearly 1 degree Celsius, and mean summertime maximum temperatures increased by over 2 degrees Celsius. In that same period, the composition of the community under study showed that northern species (that is, those species acclimated to cold water) decreased, while southern species (better adapted to warm waters) increased.

For fish, in addition to latitude changes, depth changes are also possible as an adaptation strategy. When surface temperatures change, fish can shift their main habitat to deeper, cooler waters. This shift, however, can have a devastating effect on seabird populations. In 1993, the World Wide Fund for Nature has reported, 120,000 seabirds starved to death in the Gulf of Alaska because their usual food source had moved to depths that placed them out of the birds' diving range.

Rising Sea Levels Lead to Loss of Habitat

According to the NOAA, over the past 100 years the mean global sea level has been rising at rates significantly faster than rates over the last several thousand years. The projected increase in sea level from 1990 to 2100 is up to 88cm, or close to 3 feet.

Chart shows increasing rise in global sea level according to satellite data. Image courtesy NASA/JPL-Caltech. Click to enlarge

Rising sea levels stand to have devastating effects on life around the world, both terrestrial and marine. One of the greatest dangers to sea life is the inevitable submerging of coastal salt marshes, sea grass meadows and mangroves, such as those that fringe the coastal areas in the Florida Keys National Marine Sanctuary. These environments serve as critical habitats for countless species, especially for the juveniles that are frequently spawned in their protective waters. Birds that use these areas for feeding and nesting would also be lost. With nowhere to reproduce, species do not last long before facing extinction.

Another threat of rising sea levels is that low-lying islands and atolls will completely disappear, eliminating entire habitats for land-dwelling species. A team of Hawai'ian researchers has found that the Northwestern Hawaiian Islands – recently declared a National Marine Monument – are at serious risk of being submerged by the end of this century. The researchers' findings show that up to 65 percent of some islands in the NWHI chain will be underwater if the sea rises by 48cm, the average projection of the Intergovernmental Panel on Climate Change. If the sea levels rise by the maximum forecast of 88cm, up to 75 percent of the wildlife habitat could disappear. Species that are most vulnerable include the endangered Hawaiian monk seal, whose main birth site may be completely submerged, leaving the species with virtually no chance of survival. The Hawaiian green sea turtle faces the same fate, as 90 percent of the species' females lay their eggs in one NWHI atoll. The island chain's endemic bird populations may also find themselves homeless in a vast ocean, with nowhere to live and breed.

Changes in Upwelling Patterns Lead to Altered Food Chains

The changing global climate is also affecting the circulation of ocean waters, which is influenced by ocean temperatures as well as by wind speed and direction. Changes in all these factors lead to changes in the upwelling patterns along the West Coast.

All the sanctuaries on the West Coast – Channel Islands, Monterey Bay, Gulf of the Farallones, Cordell Bank and Olympic Coast – rely on the Pacific Ocean upwelling patterns that carry cold, nutrient-rich water from offshore to areas near shore, where their vast quantities of plankton form the basis of the marine food chain. Every animal up the chain from these plankton – from tiny fish to giant blue whales – is dramatically impacted by a reduction in upwelling. When the base of the food chain disappears, every animal in the ecosystem starves, a situation that cannot last long without serious consequences.

The year 2004 saw one of the warmest surface temperatures in 50 years in the northern Pacific Ocean, according to Fisheries and Oceans Canada. All along the West Coast upwelling slackened. One of the most notable signs of the reduced upwelling is in the seabird population, which relies on fish that in turn rely on the rich upwelling waters for their own food. On the Farallon Islands in the Gulf of the Farallones National Marine Sanctuary, researchers observed an unprecedented decline in nesting among seabirds. They noted a steep decrease in nesting cormorants as well as a 90 percent drop in Cassin's auklets -- the worst in more than 35 years of monitoring.

Ocean Acidification Leads to Calcium Deprivation in Marine Life

Greenhouse gases in the Earth's atmosphere are on the rise, and on a rapid rise. The concentration of carbon dioxide in our air is currently about 32 percent greater than it was in pre-industrial times and is at the highest levels the Earth has seen in 420,000 years. By the year 2100, levels could rise to more than three times the current level. In addition to the effects that carbon dioxide has on life above the water, it also has significant effects on marine life. In the past two decades, about 30 percent of the carbon dioxide released by human activity has actually been absorbed by the world's oceans.

Increases in the levels of carbon dioxide absorbed in seawater lower the pH of the water, making it more acidic. The oceans are naturally alkaline, and while there are no predictions that the overall chemistry will tip to acidic (that is, fall below the neutral pH of 7), ocean water is decidedly dropping in pH, becoming less alkaline, or more acidic, with profound effects on marine life.

The most significant effect of lowered pH in the oceans is a reduction of the concentration of carbonate ions, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and create coral reef structures. For coral reefs, this means that coral cannot grow fast enough to keep up with the natural erosion that wears the reef away. For other organisms, this means that their skeletons become less dense, a phenomenon comparable to osteoporosis in humans. Decreased calcification also compromises the viability of calcifying marine plankton that serve as the critical food supply for certain species of fish.

This chemical change in seawater, which will only get worse unless current greenhouse gas trends are curbed, is "leading to the most dramatic changes in marine chemistry in at least the past 650,000 years," according to Richard Feely, oceanographer at NOAA's Pacific Marine Environmental Laboratory and co-author of a June 2006 report on ocean acidification.

What Now?

On May 9, for the first time, two species of Caribbean coral found in the Florida Keys National Marine Sanctuary – elkhorn and staghorn – were added to the list of threatened species under the federal Endangered Species Act. This status offers protection to the species, which have both suffered greatly due to coral bleaching and disease, but the protection may be coming too late. The underwater life in the sanctuaries and around the world stands a much better chance if protection is proactive, not reactive.

So what does that mean? If you believe that climate change is beyond human control, there's not much you can do but sit back and let it all happen. But if you believe that the rapid climate change currently being observed is due at least in part to increased levels of greenhouse gases in the atmosphere, there are simple steps you can take to do your part to prevent the loss of a wide range of marine species, from microscopic algae to majestic whales. Though it's the oceans that are at stake here, the actions of all people count, even if you live thousands of miles from a coast.

Here are some links that serve as a starting point. From here on, the sky's the limit.

Union of Concerned Scientists: What You Can Do about Global Warming (at ucsusa.org)

About.com Top 10 Things You Can Do to Reduce Global Warming (at about.com)

U.S. Environmental Protection Agency: Climate Smart Tips to Protect the Earth (at epa.gov)

Global Warming Has Devastating Effect on Coral Reefs, Study Shows

Dynamic fragility of oceanic coral reef ecosystems
Nicholas A. J. Graham , Shaun K. Wilson, Simon Jennings, Nicholas V. C. Polunin, Jude P. Bijoux and Jan Robinson

Rising Ocean Temperatures Threaten Florida's Coral Reef

Climate-Related, Long-Term Faunal Changes in a California Rocky Intertidal Community
J. P. Barry, C. H. Baxter, R. D. Sagarin and S. E. Gilman

Sea life in peril – plankton vanishing; Usual seasonal influx of cold water isn't happening

Maybe it's time to get serious about Global Warming?

National Oceanic and Atmospheric Administration
Global Warming: Frequently Asked Questions

U.S. Experienced Record Warm First Half of Year

Last 7 Months Were Warmest Stretch on Record

Global Warming May Swamp Hawaiian Wildlife, Study Warns

Report Warns About Carbon Dioxide Threats To Marine Life

Pew Reports
Coastal and Marine Ecosystems & Global Climate Change: Potential Effects on U.S. Resources

Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems