Ocean acidification: Key concepts from the past

Past environmental change, based on Earth’s natural variability, can inform scientists how humans have influenced climate and the environment on a global scale. One of the most well-publicized facets of human-induced climate change is the release of carbon dioxide into the atmosphere, which has sharply increased since the Industrial Revolution. Only within the last decade have scientists begun to characterize the ecological consequences of increased carbon dioxide absorption by the oceans.

When carbon dioxide from the atmosphere dissolves in the oceans, a number of chemical changes occur and seawater becomes more acidic. This process is called ocean acidification (OA). This shift creates conditions that make it difficult for marine animals with shells (e.g. corals, clams, oysters, and snails) to build their skeletons. While there are many studies that demonstrate the negative effects of ocean acidification on individual species, it is much more difficult, time consuming, and costly to conduct experiments on an entire marine ecosystem scale. However, we can look to the past: the fossil record serves as an ideal natural laboratory for studying large-scale impacts of ocean acidification on ecosystems over time.

Our class read a paper on ocean acidification events through geologic time (Hönisch et al. 2012).  The authors review several important concepts to contextualize modern ocean acidification in the frame of past acidification events (within the last ~300 million years). The authors highlight that the rate of change of 1) carbon dioxide uptake by the ocean, 2) decrease in seawater pH (how acidic a solution is, with a low pH being high acidity), and 3) saturation of calcium carbonate (a key indicator of the ability of organisms to make shells) must be fairly rapid to produce a rapid acidification event (10,000 years or less). Signs of biotic change and distress during past acidification events includes thinner or deformed shells, which scientists can use to understand how animals reacted to these changes.

We identified two key concepts that highlight the significance of past ocean acidification for informing policy:When trying to predict ecosystem changes based on OA in the paleo record, some past OA events are more analogous to today’s OA perturbation than others. By analyzing past OA events, we are able to understand the control of specific variables (e.g. time, temperature, ocean productivity). In turn, understanding the parameters and outcomes of past ocean acidification events will help us determine which ecosystems or organisms might be most vulnerable to modern OA changes.

The rate and duration of carbon dioxide influx into an ocean basin greatly impacts the severity of an ocean acidification event.  For example, an influx of carbon dioxide over 150,000 years will not produce an ocean acidification event, as the influx will be buffered by preexisting calcium carbonate deposits in ocean basins.  In contrast, a rapid influx of carbon dioxide will likely cause the collapse of coral reefs and associated biodiversity (as it did during the Paleocene Eocene Thermal Maximum).

Understanding past acidification events allows scientists to better understand how ecosystems and organism might be most vulnerable to present changes. The most analogous past OA event (the PETM) shows that this rapid influx will damage coral reef ecosystems amongst other carbonate dependent species like oysters, clams, and mussels. While many ocean parameters are changing at once (like oxygen, temperature, and OA), Hönisch et al. (2012) argue that isolating the distinct contributions of ocean acidification may not be necessary since the parameters are not independent. Instead, they argue that the integrated nature of large carbon releases and associated warming temperatures is actually a strength for investigating ocean acidification in the fossil record. Using the paleo record to predict ecosystem changes, whether from OA or not, is important to managing our calcareous resources (shellfish and corals).

To address how this information can be used in the policy and management realm, we contacted Jennifer Phillips at the California Ocean Protection Council, who noted: “Using the paleo record to show impacts from ocean acidification can be a powerful tool for highlighting how important it is to act swiftly on ocean acidification – especially when considering we don’t have a historical analog that is as matched as the present day rate of change of our ocean’s chemistry. Given the rate of CO2 emissions, we can’t look at ocean acidification independent of other changes in our oceans due to the increased uptake of greenhouse gases so it is important to look at historical scenarios of coupled changes. As a policymaker, I must address ocean acidification changes in tandem with other changes which is a critical and urgent challenge for our community.“

K. Barclay, R. Banker, P. Edwards, C. Fish, K. Hewett, T. Hill, G. Hollyday, C. Livsey, H. Palmer, P. Shukla & D. Vasey, Paleoclimate into Policy, 14 June 2017. Article.

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