How does climate change affect ocean waters and ecosystems?
Biological oceanographer Hugh Ducklow studies the marine food web and its interaction with the physical properties of the oceans. Much of his work is done through the US Long-Term Ecological Research (LTER) program, in which researchers have studied trends for decades in 28 land and sea regions of the United States, as well as a few sites elsewhere. In addition to the open ocean, studies encompass deserts, coasts, rivers, forests, and grasslands. From 2012 to 2018, while based at Columbia University’s Lamont-Doherty Earth Observatory, Ducklow ran the Palmer Station LTER site, the base for annual cruises through 800 miles of icy waters. off the Antarctic Peninsula.
On the occasion of the 40th anniversary of the LTER program, researchers have just published a series of articles on the impact of climate change on their sites. Ducklow led the Deep Sea Environments section which, in addition to Antarctica, spans the waters off Alaska, California and the northeastern United States. We discussed with him the work, his observations and those of his colleagues and the prospects for the future.
Why should we care what climate change is doing to the oceans?
Besides the fact that seafood is the main source of protein for around 3 billion people, the ocean absorbs much of the excess heat and carbon dioxide generated by humans. About 90% of all the excess heat produced by the greenhouse effect since the industrial revolution is in the ocean. The global ocean also absorbed about a quarter to a third of our carbon dioxide emissions. Both of these processes keep air temperatures cooler than they otherwise would be. But they both come with costs. The ocean is warming due to the added heat. The human-caused warming signal is even detectable in the depths of the Southern Ocean. The increased uptake of carbon dioxide causes ocean acidification. The ecological consequences of warming and acidification are only just beginning to be understood, and the future ability to continue to store heat and CO2 is uncertain.
What are some of the physical effects of climate on ocean waters, and where do we see them most strongly?
As I said, the oceans are warming, but the warming and its effects are not uniform across space or time. Responses to climate change by the physical system are strongest and most evident on the surface. This is important because heat and CO2 are exchanged there and because phytoplankton grow there. Depending on winds, storms and currents, the surface layer will vary in depth from almost zero in summer to over 1,000 meters in winter. Temperature affects the depth of the surface layer and, in the case of polar sites, the sea ice as well. Near the poles in winter, there is little or no solar irradiation and sea ice covers the ocean. In spring, when the sun rises, the surface of the ocean heats up and the sea ice melts, adding fresh water to the surface. Warmer, cooler waters are less dense than colder, saltier waters, and therefore the surface layer is shallow.
The depth of the surface mixed layer is decreasing at most LTER sites – Palmer Antarctica, the continental shelf of the northeastern United States, and the northern Gulf of Alaska. However, no change is evident in the California Current, despite an unbroken record of sightings since 1950 and warming water temperatures.
What biological changes are taking place? Can we clearly link them to climate trends?
The depth of the surface layer of the ocean controls the growth rate of phytoplankton. When the surface layer is shallow, phytoplankton are retained by sunlight, but do not have access to nutrients. When the surface layer is deep, phytoplankton can access nutrients, but sunlight is low or absent. Phytoplankton trends have been documented at some LTER sites, but not all. Phytoplankton is the only organism that can be detected by satellite, but the trends in its abundance are not as clear as the physical changes I have just described. Phytoplankton evidence increases in Antarctica, as expected in a shallow surface layer, but decreases over the northeast US continental shelf, despite the shallow depth. No changes are visible on other sites. Zooplankton shows increasing trends in Antarctica, as predicted by the increase in phytoplankton. They also increase in the California Current system, even though the phytoplankton do not.
Even though there are long records of changes in the California Current (70 years), the US Northeast Shelf (40 years), and Palmer’s Antarctica (30 years), it is still difficult to say with certainty that they are caused by climate change. Numerical simulations of satellite imagery suggest that around 50 years is the minimum time needed to attribute the observed trends to climate change. Some changes may take a century or more.
Are there things going on in Antarctica that set it apart from other areas?
A simple feature of the Arctic and Antarctic seas is that they are covered in sea ice. But the duration and extent of the ice cover is decreasing as the polar oceans warm. The life cycles of Arctic and Antarctic organisms such as krill and seabirds are adapted to seasonal ice cover and can be disrupted as the cover decreases. Sea ice blocks sunlight, influencing the timing of phytoplankton blooms. Although sea ice is rapidly shrinking at both poles, the effects are uncertain. As sea ice shrinks, new areas that were once ice-covered open up for phytoplankton growth, expanding the polar marine ecosystem. But as the cover disappears, its contribution of fresh water will decrease and decrease the fresh layer on the surface of the ocean. The net impact for the future ecosystem is unclear.
Another distinguishing feature of Antarctic ecosystems appears to be the diversity and rate of ecological change. We hypothesize that climate variability and change first affect physical properties and then physical changes cause ecological responses. Ecological responses can be organized into those that begin with phytoplankton at the base of the food web, i.e. bottom-up responses; and those that affect top predators like penguins with changes that ripple through the food web or top-down responses. In Antarctica, we see changes in climate and physical systems and throughout the food web, from diatoms to krill to penguins. These processes meet in the middle, converging on the krill.
Have we watched these sites long enough to get a good idea of where things will go in the future?
The time it takes to know where ecosystems are heading depends on the changes you are interested in. It is easier to observe and document physical changes, because the system consists only of heat, salinity, currents and mixing, and because we have good instruments to make precision measurements of these variables . In contrast, tens to hundreds of different measurements are required to characterize the variability of multi-species biological responses, and only a few can be sampled and measured remotely. With few exceptions, change detection for many groups of organisms still depends on individual scientists and students performing simple, time-consuming and tedious one-by-one visual counts. These measures are slowly being automated. Drones, on-board acoustics, submersible digital video cameras and instrumented ocean gliders are beginning to provide comprehensive real-time views of the oceans. Sea ice cover and icebergs are always big obstacles to leaving instruments unattended during the winter, so many measurements are limited to the ice-free summer months.
What have been some of the challenges of working offshore Antarctica?
There are obvious challenges: planning work in a remote location (the trip takes seven days door-to-door each way) and anticipating anything you might need. There are storms, high seas, ice cover. We were stuck in the ice for two weeks in September 2001. Then came problems with the supply chain, recruiting staff, and maintaining a decades-long time series of high-quality observations and measurements. The preparatory work for next year literally begins before you leave for the ship this year. The project is not simply the time series, but living, evolving scientific research with erroneous detours, dead ends, and unexpected discoveries. Despite the challenges, it is a beautiful and exciting place to work.