How Is Climate Change Affecting Ocean Waters and Ecosystems?
Biological oceanographer Hugh Ducklow studies the marine food web, and the way it interacts with the physical properties of the oceans. Much of his work is thru the U.S. Long Term Ecological Research Program (LTER), by which researchers have for a long time investigated trends across 28 land and marine regions in america, together with a number of sites elsewhere. Along with 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 led the Palmer Station LTER site, the bottom for yearly cruises through 800 kilometers of icy waters off the Antarctic Peninsula.
To mark the fortieth anniversary of the LTER program, researchers just published a series of articles on how climate change is affecting their sites. Ducklow led the section on open-ocean environments, which along with Antarctica spans waters off Alaska, California and the U.S. Northeast. We spoke with him in regards to the work, his and colleagues’ observations, and prospects for the longer term.
Ducklow at a glacial ice cave near Palmer Station, Antarctica, 2006. The glacier collapsed and wasted away a yr or two later. (Courtesy Hugh Ducklow)
Why should we care about what climate change does to the oceans?
Besides the incontrovertible fact that seafood constitutes the most important protein source for about 3 billion people, the ocean soaks up a serious amount of excess heat and human-generated carbon dioxide. Around 90 percent of all the surplus heat produced by the greenhouse effect for the reason that Industrial Revolution is within the ocean. The worldwide ocean has also taken up about one quarter to at least one third of our carbon dioxide emissions. Each these processes keep air temperatures cooler than they might be otherwise. But they each include costs. The ocean is warming in consequence of added heat. The human-caused warming signal is even detectable within the deep Southern Ocean. Enhanced carbon dioxide uptake is causing ocean acidification. The ecological consequences of warming and acidification are only starting to be understood, and the longer term capability to proceed to store heat and CO2 isn’t certain.
What are the among the physical effects of climate on ocean waters, and where are we seeing them most strongly?
Like I said, the oceans are warming, however the warming and its effects are usually not uniform in space or time. Responses to climate change by the physical system are strongest and most evident right on the surface. This is vital because heat and CO2 are exchanged there, and since phytoplankton grow there. Depending on winds, storms and currents, the surface layer will vary in depth from nearly zero in summer to over 1,000 meters in winter. Temperature affects the depth of the surface layer, and within the case of polar sites, so does sea ice. Near the poles in winter, there may be little or no solar irradiation, and sea ice covers the ocean. Within the spring, because the sun rises, the surface ocean warms and sea ice melts, adding freshwater to the surface. Warmer, brisker waters are less dense than cooler, saltier waters, and so the surface layer shallows.
Surface mixed layer depth is getting shallower at most sites within the LTER network—Palmer Antarctica, the Northeast U.S. continental shelf and the Northern Gulf of Alaska. Nonetheless, no change is clear within the California Current, regardless of an unbroken record of observations since 1950 and warming water temperature.
What biological changes are happening? Can we link them clearly to climate trends?
The depth of the ocean surface layer controls the speed of phytoplankton growth. When the surface layer is shallow, phytoplankton are retained in sunlight, but lack access to nutrients. When the surface layer is deep, phytoplankton can access nutrients, but sunlight is dim or absent. Trends in phytoplankton have been documented in some, but not all of the LTER sites. Phytoplankton are the one organisms that will be detected by satellite, but trends of their abundance are usually not so clear because the physical changes I just described. Evidence of phytoplankton is increasing in Antarctica, as expected in a shallowing surface layer, but decreasing over the Northeast U.S. continental shelf, regardless of shallowing. No changes are evident at the opposite sites. Zooplankton show increasing trends in Antarctica, as expected from increasing phytoplankton. Also they are increasing within the California Current system, although phytoplankton aren’t.
Although there are long records of changes within the California Current (70 years), Northeastern U.S. shelf (40 years) and Palmer Antarctica (30 years) it’s still difficult to say for certain that they’re attributable to climate change. Numerical simulations of satellite imagery suggest about 50 years is the minimum time needed to attribute observed trends to climate change. Some changes may take a century or longer.
Are there things happening in Antarctica that distinguish it from the opposite regions?
One easy distinguishing feature of Arctic and Antarctic seas is that they’re covered by sea ice. However the duration and extent of ice cover are declining because the polar oceans warm. The life cycles of Arctic and Antarctic organisms comparable to krill and seabirds are attuned to seasonal ice cover, and should be disturbed as the duvet decreases. Sea ice blocks sunlight, influencing the timing of phytoplankton blooms. Although sea ice is decreasing rapidly at each poles, the results are uncertain. As sea ice declines, recent, formerly ice-covered areas are opened up for phytoplankton growth, expanding the polar marine ecosystem. But as the duvet disappears, its contribution of freshwater will decline and reduce the fresh layer on the ocean surface. The web impact for the longer term ecosystem isn’t clear.
One other distinguishing feature of Antarctic ecosystems appears to be the variety and pace of ecological change. We assume that climate variability and alter first affect physical properties after which the physical changes cause ecological responses. The ecological responses will be organized into those who start with phytoplankton at the bottom of the food web, that’s, bottom-up responses; and those who affect the highest predators like penguins with changes rippling down through the food web, or top-down responses. In Antarctica, we’re seeing changes within the climate and physical systems and throughout the food web, from diatoms to krill to penguins. These processes meet in the center, converging on krill.
Have we been observing these sites long enough to get a superb idea of where things are headed in the longer term?
How long is required to know where ecosystems are headed relies on what changes you’re concerned with. It’s easier to watch and document physical changes, since the system only consists of warmth, salinity, currents and mixing—and since now we have good instruments to make precision measurements of those variables. In contrast, dozens to tons of of various measurements are needed to characterize variability in multispecies biological responses, and only a number of will be sampled and measured remotely. With a number of key exceptions, detecting changes for a lot of groups of organisms still relies on individual scientists and students making easy, time-consuming and tedious one-by-one visual counts. These measurements are slowly becoming automated. Drones, ship-mounted acoustics, submersible digital video cameras and instrumented ocean gliders are starting to make real-time, comprehensive views of the oceans. Sea ice cover and icebergs are still big obstacles to leaving instruments unattended over the winter, so quite a lot of measurements are restricted to ice-free summer months.
What have been among the challenges in working off Antarctica?
There are the plain challenges: planning work in a distant location—travel takes seven days door to door each way—and anticipating all the things you may need. There’s storms, high seas, ice cover. We got stuck within the ice for 2 weeks in September 2001. Then supply-chain issues, personnel recruiting, and maintaining a high-quality time series of observations and measurements over a long time. The work to organize for next yr literally starts before you depart for the ship this yr. The project isn’t simply the time series, but living, evolving scientific research with mistaken turns, blind alleys and unexpected discoveries. Regardless of the challenges, it’s a stupendous and exciting place to work.
