I live on a quiet tidal creek that feeds into the York River, a tributary to the Chesapeake Bay in the same county where Pocahontas was born four fateful centuries ago. My house is a short crow’s flight from the point where Englishmen first pulled up their tiny ships on the North American coast after their perilous TransAtlantic voyage and decided that this is the place. After everything that has happened across those ages it is still a beautiful place—waterbirds of all sorts haunt the tree-lined creek, bald eagles are seen occasionally.
But that is all changing, gradually. The carbon that we in the industrialized societies are relentlessly pumping into the atmosphere threatens to do what even four centuries of human occupation couldn’t. We are going to lose this place. Jamestown and the surrounding areas will very likely be underwater when my great-grandchildren, if I’m lucky enough to have some, are grown.
It’s now well documented that average global temperatures are steadily increasing, and the physical-chemical mechanisms responsible are pretty well understood. We’ve all heard the general story in An Inconvenient Truth, and the details can be found in the reports of the Intergovernmental Panel on Climate Change. What does it mean for the marine environment and for those of us who are linked to it by livelihood or by spiritual bonds? I will focus on what it means for people around where I live, on the Chesapeake Bay, but similar challenges face most any coastal area. Let’s consider sea level rise first.
The Rising Tide
Sea level has risen steadily in recent decades, indeed faster in the Chesapeake Bay region than the national average because the land here is still sinking gradually as the earth’s crust rebounds from the weight of the last ice age up north. Many places in the Bay have seen a one foot rise in sea level over the 20th century. At least 13 islands have disappeared entirely. Poplar Island, south of Baltimore, used to have a small town and even a sawmill, and was an exclusive resort for Presidents Roosevelt and Truman in the old days; now it is only a group of small marshy banks and would be gone entirely if various state and federal governments had not put $400 million into building it up. The figure of Virginia’s eastern shore shows projections from a model done by the National Wildlife Federation of what will happen to various habitat types (represented by different colors) as sea level rise over the coming century. Much of the shallow marshes and associated wetlands that are key habitats for migrating birds will shift to open water with very different communities.
Wildlife feels the heat
Average annual water temperatures in Chesapeake Bay have also increased steadily over the last 50 years, by roughly one degree C. Similarly, the extreme temperatures have also increased. This is important since many biological processes are determined not by average temps but by extremes.
Changing water and air temperatures have both direct effects on individual species, as well as indirect effects that arise from complex interactions among species. These arise from the fact that life cycles of animals and plants are closely tied to temperature—this determines when they emerge from dormant stages, when they reproduce, when they start seasonal migrations, and so on.
I’ll consider three general effects of climate change. First are changes in timing. In oceans and estuaries, one of the most critical time periods is the beginning of spring. This is when the water warms and stratifies so that microscopic plants are retained in sunlit surface waters and can grow. This is also a time when many animals, including commercially and recreationally important fishes, “wake up” from winter and begin to actively feed, reproduce, and migrate. The beginning of spring can be set roughly as the day when water temperatures first reach 15 C. This date has been pushed up by about three weeks since 1960 (Austin 2002).
What effect does this have on animals and plants? We can get a clear picture from a review of over 1400 species of all kinds of organisms—from weeds and trees to bugs and birds (see Root et al. 2003). This review shows that, on average, all kinds of living things have shifted the timing of these life events toward earlier seasons. The authors of that review refer to these shifts as a “fingerprint of global warming”, and the data show that this fingerprint
(1) occurs in a wide range of organisms
(2) occurs throughout the world, and (3) is in the direction predicted by rising global temperatures.
Although the average trend is clear, we will see shortly that these shifts differ among individual species, and that those difference change interactions among species that have important implications for how food chains and ecosystems work.
The second direct effect of rising temperature involves changes in the distribution of organisms in space. The easiest such changes to measure involve changes in geographic range, which are predicted to move poleward (northward in this hemisphere) with rising global temperatures. The North Sea has been especially well studied in this regard. A review of changing distributions of > 90 species of fishes there showed that nearly 2/3 of them shifted their mean range either northward or deeper into cooler waters or both over the last 25 years. This was true of both fished and non-fished species. Interestingly, shifts were slower for species with large bodies and slow life cycles, which are already under stronger fishing pressure. This suggests “double trouble” -- global warming may aggravate the effects of existing fishing pressure on these species.
Future range shifts will be much more pronounced and are likely to have major impacts on commercial fisheries, for example as species ranges move into or out of the territorial boundaries of particular nations.
In our neck o’ the woods, declines are likely for economically important blue crab, menhaden, and rockfish, largely due to effects of high water temperatures on habitat quality and interactions with diseases (see the story of the oyster below) . A majority of the East Coast’s striped bass, or rockfish, begin their lives in the Chesapeake Bay and its tributaries so this could be a significant loss. On the other hand, some commercially important southern species may increase or expand into our region. These include commercial shrimp, black drum, and grouper, none of which currently supports a substantial fishery.
A central effect of changing climate involves impacts on what ecologists call “foundation species”, that is, key species that support entire ecosystems. One of the major foundation species in the Chesapeake is the submerged plant eelgrass, which historically formed expansive meadows throughout the shallow waters of the Bay and supported large densities of fish and shellfish, primarily as a nursery for their young. Eelgrass is highly vulnerable to climate change both because it is near the southern end of its distribution in the Bay, and because it’s already under threat from poor water quality.
In summer 2005 the mid-Atlantic region experienced record high water temperatures. Eelgrass soon disappeared from large areas of the Bay. Although the grass rebounded from buried seeds and rhizomes in many areas, it has not returned to others, resulting in continuing loss of nursery area of crabs and fish. Experts on seagrasses are very concerned that we might lose eelgrass for good after a few more hot summers like 2005.
This would be a serious blow because a large number of species in the Bay ecosystem depend on eelgrass, and the algae and invertebrates it supports, either for food or shelter during some part of their lives. These include many commercially and recreationally important fishes as well as waterfowl.
Another set of foundation species are the plants that support salt and freshwater wetlands. These are important in literally holding the land together by trapping sediments and producing detritus that makes soil.
In the Chesapeake region, wetlands have been declining fast in recent decades. For example, the Blackwater National Wildlife Refuge in Maryland has been called the “Everglades of the North” because of the great abundance and diversity of wildlife it harbors, including the largest population of bald eagles north of Florida. Blackwater illustrates well how climate change interacts with other stresses. Over the last seventy years, it has lost a third of its marsh area to sea level rise, sinking of the land, and overgrazing by nutria, an alien rodent. The nutria is currently kept from spreading north largely by its intolerance of cold winters.
Tipping points
This provides a transition into a discussion of the more complex responses of ecosystems to climate change. We’ve seen that rising temperatures can lead to changes in both timing and distribution of coastal species. It’s important to recognize that changes in a condition—say water temperature—do not always produce a smooth change in the system’s response. Often the relationship is non-linear, meaning that a small change in conditions can produce a sudden or large change in the system.
In colloquial language we often refer to such changes as a tipping point. Moreover, the relationship between a stressor and the system’s behavior may be complex, such that the change is difficult to reverse. This means that even after you relax the stress (say, nutrient pollution to an estuary), the system may not return to its original state. A classic example involves submerged vegetation. Nutrient pollution may kill underwater grasses, which then releases the underlying mud and allows it to be mixed up into the water column. The mixed up mud in turn reduces light and so prevents the grass from becoming established again, even if nutrient loading is reduced well below where it originally was.
A common example of these disrupted interactions involves the seasonal match between animals and their food. In one Canadian lake, for example, the timing of algae and grazers has become mismatched. Algae are blooming earlier in recent years, but the waterfleas that eat them are blooming at the same time they always did. The result is that there is no food around when the waterfleas finally come out. And that means they starve, and there is in turn little food available to the fish (like salmon) that eat them.
No account of the Chesapeake Bay would be complete without considering the oyster, which was the backbone of the regional economy for many decades. Alas, the oyster has been in decline for many years and growing evidence suggests that it may have crossed a tipping point due to a combination of several stressors. In particular, two disease organisms arrived in Virginia in the later 40s and 50s and interacted with overfishing to crank up a steep decline in oysters.
The parasite Dermo is one of them. Dermo is a microscopic protozoan that proliferates at high water temperatures and high salinities. In Delaware Bay, epidemics of the disease followed extended periods of warm winter weather. In the Chesapeake, outbreaks have occurred after droughts allowed salty water to penetrate up the rivers. These trends in time are also reflected in the spread of Dermo in space, as shown by Ford et al (1996). From 1994 through the mid 80s it was confined to the region south of Chesapeake Bay. But beginning in the mid 80s, Dermo moved steadily up the East Coast in conjunction with a sharp increasing trend in winter water temperatures. Now it extends to Maine. This example shows that both the intensity and the geographic spread of an important interaction between species can be affected quickly by modest changes in climate.
Concluding thoughts
This account gives just a sketch of some of the changes likely in store ofr us in coastal marine ecosystems. Many of them will have direct and generally negative consequences on the well-being of coastal societies and economies. My view on climate change has been molded by my career studying the natural world and how we interact with it. But it’s also based on an intense concern for what kind of a world my eleven-year old son—and others of his generation—will inherit from us. What I want to say is this:
Whether we choose to recognize it or not, global warming will be the defining issue of the 21st century. Not terrorism. Not immigration. Not the economy. Not even overpopulation. All of these things are obviously centrally important to all of us. But all of them depend critically on what happens to the climate and ecosystems of planet earth. And that in turn depends critically on what governments and citizens all over the world are doing—or not doing—right now to deal with this challenge.
Climate change is not a future threat. It’s happening now. We may see it as a distraction from other things we’d prefer to focus on. I certainly do. It would be easier to turn back to the TV and forget it. But that would be very dangerous. Twenty years from now, or 30, or 50, our kids and their kids will look back on this time and see it as one of the two or three pivotal times in American and world history. And they will wonder what we did about it. I may be an inherently conservative scientist, skeptical about saying something with certainty. But I feel more sure of that than of almost anything else I know.
So what can we do about it?
Bookstores and websites are piled up with slick and inviting invitations for “10 easy steps you can take to save the world” and so on. That is nonsense. Nothing will be more challenging than saving the world from the complex of threats facing us. It will be hard, sustained work over the rest of our lifetimes and beyond. But at the same time, there is no challenge more worthy or more exciting. It is essential. Yes, there are many things we can do in our personal lives to reduce our footprints. We should of course all change our light bulbs, but it is more important to change our leaders.
In other words, the only way to make the massive changes necessary to deal with the global climate change already underway involves political action. Fortunately, the voters of the United States took exactly such a step last November in electing a new President who is clearly far more dedicated to doing something real about the threat of climate change than his predecessor. The citizens of Australia also changed leaders, partly because of the previous Prime Minister’s unwillingness to face the challenge of climate change. But we will need major, sustained public pressure to overcome the inertia that is inherent in large deliberative bodies like Congress, state legislatures, and so on. So if we want to change the world, we must write our congresspersons, state legislators, county officials, anybody with any authority or ability to influence decisions on this important topic. Get active, get networked, get educated, and get moving! It may not be glamorous but it gets results.
Sources
Austin, H.M. 2002. Decadal oscillations and regime shifts, a characterization of the Chesapeake Bay marine climate. American Fisheries Society Symposium 32:155-170.
Ford, S.E. 1996. Range extension by the oyster parasite Perkinsus marinus into the northeastern United States: response to climate change? Journal of Shellfish Research 15: 45-56.
Glick, P. et al. 2008. Sea level rise and coastal habitats of the Chesapeake Bay. National Wildlife Federation.
Perry, A.L. et al. 2005. Climate change and distribution shifts in marine fishes. Science 308:1912-1915.
Root, T.L. et al. 2003. Fingerprints of global warming on wild animals and plants. Nature 421:57-60. US Fish & Wildlife Service
J. Emmett Duffy is a professor of Marine Science at the Virginia Institute of Marine Science at William & Mary. He is also the creator of The Natural Patriot, a blog dedicated to cultivating a new, universal ideal of patriotism proper to our endangered homeland.
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