Troubled oceans at the bottom of the world

Today we visit the Weddell Sea, which lies to the east of the Antarctic Peninsula. Like the rest of the Southern Ocean, the body of water that surrounds Antarctica, it is home to diverse ecosystems that are filled with unique and astonishing wildlife. And like oceans everywhere, the impacts of climate change are becoming more apparent.

As the world warms, its waters are warming, too. Increased temperatures have already led to changes in ocean life. For example, on the other side of the Peninsula, king crabs have invaded an area previously considered far too cold for their survival. The impact of the arrival of these predators, for the first time in millions of years, could be catastrophic for the surrounding ecosystem, which has evolved exotic and unique life forms that have no defenses against crabs.

Unfortunately, scientists are observing not only changes to the oceans’ temperature but also to its chemistry. The Weddell Sea — and the rest of the Southern Ocean — is experiencing what scientists call ocean acidification. Currently, about a quarter of the carbon dioxide released each year by human activities is absorbed by the world’s oceans. The Southern Ocean alone absorbs more than 40% of that due to the frigid temperatures of its waters. As the concentration of carbon dioxide increases, the water becomes more acidic.

As one scientist described it to me, ocean acidification is the “osteoporosis” of the world’s oceans. As the ocean acidifies, the exoskeletons of marine animals become brittle and frail, just as osteoporosis weakens the bones of humans. Acidification can also affect the nervous systems, blood circulation, and breathing of fish and other animals in the sea. In other parts of the world, acidification may cause tissue damage in economically important species of fish, threaten the survival of rare or endangered shellfish, and reduce the number of species in coral reefs. If left unchecked, this fundamental alteration to ocean chemistry has the potential to threaten the livelihood and food security of millions, if not billions, of people worldwide.

And, what does this mean for us? About 1 billion people in the world rely on fish and shellfish as their primary source of dietary protein. By one estimate, the effect of acidification on mollusks alone (animals such as oysters and clams) could cost the world tens of billions of dollars by the end of the century. Some researchers have called acidification “one of the most critical anthropogenic threats to marine life.”

The climate crisis is a problem of multiple dimensions. Rising ocean temperatures alone have the potential to disrupt the web of life in the ocean. Acidifying oceans, a result of the same carbon dioxide pollution that is warming our planet, are magnifying the problem even further.

Unprecedented, man-made trends in ocean’s acidity (movie)

The animation shows how aragonite saturation at the ocean’s surface is projected to decrease towards the end of the 21st century as man-made carbon dioxide accumulation in the atmosphere continues to rise. Recent carbon dioxide emissions have pushed the level of seawater acidity far above the range of the natural variability that existed for thousands of years and are affecting the calcification rates of shell-forming organisms, according to a study of an international team of scientists led by IPRC’s Tobias Friedrich and Axel Timmermann published in Nature Climate Change.

The availability of carbonate ions is crucial for marine calcifying organisms to form their skeletons or shells that are made of different crystalline forms of calcium carbonate, such as calcite and aragonite. Aragonite is more soluble than calcite and organisms forming aragonite; thus, the saturation state of aragonite can be taken as an indicator for ocean acidification.

This animation was generated as part of a project funded by The Nature Conservancy, the National Science Foundation and JAMSTEC.

YouTube, 25 January 2012. Movie.

End-Cretaceous marine mass extinction not caused by productivity collapse

An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms are not well-understood. The collapse of sea surface to sea floor carbon isotope gradients has been interpreted as reflecting a global collapse of primary productivity (Strangelove Ocean) or export productivity (Living Ocean), which caused mass extinction higher in the marine food chain. Phytoplankton-dependent benthic foraminifera on the deep-sea floor, however, did not suffer significant extinction, suggesting that export productivity persisted at a level sufficient to support their populations. We compare benthic foraminiferal records with benthic and bulk stable carbon isotope records from the Pacific, Southeast Atlantic, and Southern Oceans. We conclude that end-Cretaceous decrease in export productivity was moderate, regional, and insufficient to explain marine mass extinction. A transient episode of surface ocean acidification may have been the main cause of extinction of calcifying plankton and ammonites, and recovery of productivity may have been as fast in the oceans as on land.

A decade of climate change experiments on marine organisms: procedures, patterns and problems

The first decade of the new millennium saw a flurry of experiments to establish a mechanistic understanding of how climate change might transform the global biota, including marine organisms. However, the biophysical properties of the marine environment impose challenges to experiments which can weaken their inference space. To facilitate strengthening the experimental evidence for possible ecological consequences of climate change, we reviewed the physical, biological and procedural scope of 110 marine climate change experiments published between 2000 and 2009. We found that 65% of these experiments only tested a single climate change factor (warming or acidification), 54% targeted temperate organisms, 58% were restricted to a single species and 73% to benthic invertebrates. In addition, 49% of the reviewed experiments had issues with the experimental design, principally related to inappropriate replication of the main test-factors (temperature or pH), and only 11% included field assessments of processes or associated patterns. Guiding future research by this inventory of current strengths and weaknesses will expand the overall inference space of marine climate change experiments. Specifically, increased effort is required in five areas: (i) the combined effects of concurrent climate and non-climate stressors; (ii) responses of a broader range of species, particularly from tropical and polar regions as well as primary producers, pelagic invertebrates, and fish; (iii) species interactions and responses of species assemblages, (iv) reducing pseudo-replication in controlled experiments; and (v) increasing realism in experiments through broad-scale observations and field experiments. Attention in these areas will improve the generality and accuracy of our understanding of climate change as a driver of biological change in marine ecosystems.