If you’re a fan of seafood and you’ve spent any time here in New England, you’ve probably enjoyed your fair share of lobsters, scallops and oysters. Despite differences in appearance, these animals have something major in common: they’re all at risk from “the other CO2 problem,” or ocean acidification. Together with warming ocean temperatures, ocean acidification threatens to alter ocean ecosystems in unprecedented ways.
In March of this year, I blogged about the need to look toward the ocean for better indicators of climate change. This week, the Yale School of Forestry & Environmental Studies hosted Dr. Scott Doney from the Woods Hole Oceanographic Institute, who underscored this message by providing an insightful overview of the status of climate change impacts on the world’s oceans. His takeaways: (1) rising CO2 levels are already affecting oceans by warming water temperatures and increasing CO2 uptake in seawater; (2) these effects are expected to increase and possibly accelerate in the future; and (3) the on-the-ground impacts of these changes will be highly variable across different parts of the world.
Over the last hundred years, looking past natural decadal variability, there has been an ocean warming trend that can likely be attributed to human activities (See Figure 1). Atmospheric CO2 is higher than it’s been in the last 800,000 years – possibly than in the last 3-4 million years – and the rate of CO2 increase over the last two centuries is roughly 30 to 50 times faster than what’s recorded in the geologic record. Most of the extra heat generated as a result of the CO2-mediated greenhouse effect ends up in the ocean, where this warming can perpetuate ecological changes as well as contribute to sea level rise (because water expands as it warms, and warmer sea surface temperatures also accelerate the melting of polar ice sheets in Greenland and Antarctica).
Figure 1: This graph depicts rising sea surface temperatures from 1880-present as compared to the average sea surface temperature of the latter 20th century (Source: EPA)
The ecological effects of a warmer ocean are still being teased apart, but are likely to be complex. The most productive zones of the open ocean are found in the top 100-200 meters of surface water, which is also the part of the ocean experiencing the most substantial rise in temperatures (specifically, temperatures near the ocean’s surface). This area is known as the “photic zone” since this is where tiny sun-dependent phytoplankton live; these photosynthesizing organisms form the base of a complicated ocean food web and are a critical foundation of the entire ocean ecosystem. Dr. Doney explained that phytoplankton productivity actually increases in slightly warmer water, but this is where the story gets more complicated: Even if warmer sea surface temperatures increase productivity, these warmer temperatures also increase a phenomenon known as “stratification” – a warmer ocean surface layer is less likely to mix with the colder water further below the surface. This is important because most of the nutrients that phytoplankton require are actually supplied from the deep ocean. Increasing ocean stratification means less ocean mixing, which means less transport of nutrients up to the photic zone. Taken together, these changes are expected to decrease net primary productivity in most regions of the ocean (with the exception of ice-limited regions, where decreasing ice cover will likely boost planktonic productivity). These changes will likely have reverberating effects throughout the entire marine ecosystem.
Besides ocean warming, the other major impact of rising CO2 levels is increased ocean acidification, a result of chemical reactions between CO2 and water. Though long-term data about historical ocean acidity levels is lacking, the best data set from Mauna Loa, HI reveals a tight correlation between atmospheric CO2 and ocean CO2 levels since the late 1980s and other datasets reveal a small but noticeable rise in ocean acidity. Unlike warming sea surface temperatures, for which researchers are still trying to tease apart impacts on species, ocean acidification has a fairly straightforward implication for a cohort of species known as “calcareous organisms” – commercially-important animals like lobsters, crabs, shrimp, oysters and clams that use calcium carbonate to build their outer shells. Because acidic water reacts with carbonate, these organisms have a difficult time building the shells on which they depend for survival. In the Pacific Northwest region of the United States, oyster farmers have already begun to feel the impacts of ocean acidification as their harvests fall. More broadly, roughly three quarters of the most commercially-valuable species in the United States are either shellfish, or finfish that prey primarily on shell-building organisms.
At many points in his talk, Dr. Doney took the time to acknowledge the importance of taking good measurements to gather all of this information. Everything that we know about future impacts of climate change and ocean acidification relies on modelling, and a model is only as accurate as the data that underpins it. Sea surface temperature used to be measured by hauling buckets of water onto a ship and taking measurements from onboard. Unsurprisingly, our techniques have come a long way; today, these measurements are generally made by robotic floats in the water. Having historical baseline data is critical to understanding how environmental systems are changing today, but also creates unique challenges in terms of integrating data from different sources to paint a complete picture across time. Luckily, across the world, there are many teams of dedicated researchers focusing on just such a task.