Marine ecosystem risk in the face of climate change

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In this excerpt from Ocean Recovery: A Sustainable Future for Global Fisheries?, authors Ray and Ulrike Hilborn explore what rising ocean temperatures will mean for marine ecosystems.

The oceans are changing. We know that they are getting warmer and more acidic, and we know that this will greatly change the established patterns of how we get food from the sea and, consequently, how we manage fisheries.

We also know that we, the people, with our stupendously fertile minds, are the cause of the profound changes happening to our planet, and we can only hope that our collective minds will be nimble enough to adapt in good time to avoid or mitigate the worst. Over the last several decades, intense scientific efforts have both documented historical changes in climate and attempted to forecast what that means for our future. I admit to being wary about our ability to predict the future. Our group at the University of Washington has been forecasting salmon returns for Bristol Bay for the next year only. We have been right within 20 percent in our 1-year forecasts, but we would never presume to attempt a 5-year prediction. This is not something we can do.

Weather and climate are relatively simple physical processes. Forecasts have become very reliable in recent years owing to better data collection and computational power. Marine ecosystems, however, are much more complex. Our forecasts concerning marine ecosystem changes for one relatively simple process like the salmon returns to Bristol Bay are much less reliable. But we can say with a great deal of certainty that the oceans have been getting warmer and more acidic. Right now, warming happens at warp speed in the Arctic, but since so few people live there, the alarm bells are not being heard by all yet. Even in the tropics, warming is evident. The physics and chemistry of ocean acidification, on the other hand, are very simple and highly predictable. More CO2 in the atmosphere means more acidic oceans.

Right now, warming happens at warp speed in the Arctic, but since so few people live there, the alarm bells are not being heard by all yet.

The most publicized changes in marine ecosystems associated with ocean warming have been coral-bleaching events. Bleaching happens when the corals expel the symbiotic algae as the water gets too hot, although other stressors can contribute. The corals rely on their algae for most of their energy, and if the algae do not manage to re-establish themselves, the corals die. The future is pretty dire for corals. There are estimates from 2016 that 31 percent of tropical corals worldwide were already bleached, and that at current rates, most corals will be dead within decades.

The biggest potential threat to marine fisheries is acidification due to ever- increasing CO2 in the atmosphere. Acid dissolves calcium carbonate. This is why you can dissolve an eggshell in vinegar. Calcium carbonate is essential to making corals and shells precisely because it is insoluble in water — but not in acid.

Consequently, many marine organisms, from corals to oysters to tiny coccolithophores cannot make their shells when ocean acidity gets too high. Oyster growers in Washington State have already found the acidity of the ocean too high to successfully hatch their oysters and have had to move their operations to less acidic waters. We had better start imagining an ocean without the calcium-based shells that so many marine creatures depend on. Because coccolithophores and their photosynthesis form an important part of marine food chains, we may have to contemplate the entire food web of the oceans’ fundamental transformation.

Ocean warming has already noticeably changed the life histories of marine fish. Quite a few species have found their usual habitat too warm and have moved toward the cooler polar regions.

Ocean warming has already noticeably changed the life histories of marine fish. Quite a few species have found their usual habitat too warm and have moved toward the cooler polar regions. Taking data from scientific surveys of distribution of fish species from 1968 to 2011, a 2013 study showed that fish are steadily moving toward the poles and to deeper and cooler water at rates roughly proportional to the increase in temperature. In the Northern Hemisphere, this is fairly evident already. Lobster in southern New England, salmon in California, and cod in the Gulf of Maine, all at the southern end of their historic ranges, are getting scarce. In Europe, movement of many stocks north of their traditional range is upsetting traditional allocation of catch between countries. Mackerel have moved to Iceland, but Iceland has little quota to catch them.

All scientific studies suggest that the oceans will continue to grow warmer and more acidic in the next decades. Moreover, there are estimates that there will be more severe storms, ocean levels will rise, and ocean oxygen concentration will decline. Even more alarming is the possibility that major ocean circulation patterns may disrupt the highly productive regions where upwelling brings nutrients from the deep waters of the ocean, and that large-scale phenomena like the Gulf Stream, that warms the northern parts of Europe, may shift. The range of possible changes is terrifying.

If we look at the range in climate forecasts that come from the Intergovernmental Panel for Climate Change, the uncertainty is large, because we do not know what our future emissions of greenhouse gases will be. If we add the even greater uncertainty of how ocean and freshwater ecosystems will respond, I conclude that forecasting changes provides at best a range of what the future might hold and that it is far more important to understand how societies can adapt to whatever changes will occur.

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