How does climate change affect ocean waters and ecosystems?
biological oceanographer Hugh Ducklow Study the marine food web and how it interacts with the physical properties of the ocean.Most of his work is done through the U.S. Long Term Ecological Research Program (LTER), researchers have been investigating trends in 28 land and sea areas in the United States and several other locations for decades. In addition to the open ocean, the study includes deserts, coasts, rivers, forests and grasslands.From 2012 to 2018, at Columbia University Lamont-Doherty Earth ObservatoryDacromet leadership Palmer Station LTER Stationwhich annually sails through the base of icy waters 800 kilometers off the Antarctic Peninsula.
To mark the 40th anniversary of the LTER program, researchers have just published series of articles A website about how climate change is affecting them.led by daklo The section on the high seas environment, In addition to Antarctica, it also spans the waters of Alaska, California, and the northeastern United States. We talked with him about this work, his and colleagues’ observations, and vision for the future.
Dacromet in a glacier ice cave near Palmer Station, Antarctica, 2006. After a year or two, the glacier collapsed and disappeared. (Courtesy of Hugh Darklow)
Why should we care about the impact of climate change on the ocean?
In addition to seafood being the main source of protein for about 3 billion people, the ocean also absorbs a lot of excess heat and human-generated carbon dioxide. About 90 percent of all excess heat generated by the greenhouse effect since the Industrial Revolution is in the oceans. The global ocean also accounts for a quarter to a third of our carbon dioxide emissions. Both of these processes keep the air temperature cooler than the other processes. But they all have costs. The oceans are warming due to increased heat. Human-caused warming signals can even be detected deep in the Southern Ocean. Increased carbon dioxide uptake leads to ocean acidification. The ecological consequences of warming and acidification are only just beginning to be understood, and the ability to continue storing heat and carbon dioxide in the future is uncertain.
What are some of the physical effects of climate on ocean water, and where do we see them most strongly?
Like I said, the oceans are warming, but the warming and its effects are not uniform in space or time. The responses of physical systems to climate change are the strongest and most pronounced on the surface. This is important because the exchange of heat and carbon dioxide takes place there, and because phytoplankton grows there. Depending on winds, storms and ocean currents, the depth of the surface layer will vary from near zero in summer to over 1,000 meters in winter. Temperature affects the depth of the surface layer, and in polar regions, so does sea ice. Near the poles in winter, with little or no solar radiation, sea ice covers the ocean. In spring, as the sun rises, the surface oceans warm and sea ice melts, adding fresh water to the surface. Warmer, fresher water is less dense than colder, saltier water, so the surface layer is shallower.
At most locations in the LTER network—Palmer, Antarctica, the northeastern U.S. continental shelf, and the Gulf of North Alaska—the surface mixed layer is getting shallower. However, despite the observational records and rising water temperatures since 1950, the California current has not changed significantly.
What biological changes have occurred? Can we clearly link them to climate trends?
The depth of the ocean’s surface controls the growth rate of phytoplankton. When the surface layer is shallow, phytoplankton remain in sunlight but lack nutrients. When the surface layer is deep, phytoplankton can obtain nutrients, but sunlight is dim or absent. Phytoplankton trends were recorded at some but not all LTER sites. Phytoplankton are the only organisms that can be detected by satellites, but their abundance trends are not as pronounced as the physical changes I just described. Evidence of phytoplankton is increasing in Antarctica, as expected in the shallow surface layer, but decreasing, albeit shallower, on the continental shelf of the northeastern United States. Other sites have not changed significantly. Zooplankton showed an increasing trend in Antarctica, as expected from an increase in phytoplankton. They are also increasing in the California current system, although phytoplankton are not.
Although changes in the California Current (70 years), the Northeastern U.S. continental shelf (40 years), and Palmer’s Antarctica (30 years) are well documented, it remains difficult to determine whether they are caused by climate change. Numerical simulations of satellite imagery show that about 50 years is the minimum time required to attribute the observed trends to climate change. Some changes may take a century or more.
Is what’s happening in Antarctica different from other regions?
A simple distinguishing feature of Arctic and Antarctic seas is that they are covered by sea ice. But as polar oceans warm, the duration and extent of the ice sheets are declining. Arctic and Antarctic organisms, such as krill and seabirds, have life cycles adapted to seasonal ice caps and may be disrupted as ice caps shrink. Sea ice blocks sunlight and affects when phytoplankton blooms. Although sea ice is rapidly decreasing at both poles, its impact is uncertain. As sea ice declined, new, previously ice-covered areas were opened up for phytoplankton growth, expanding polar marine ecosystems. But as the cover disappears, its contribution to freshwater will decline and reduce the fresh layer on the ocean’s surface. The net impact on future ecosystems is unclear.
Another striking feature of Antarctic ecosystems appears to be the diversity and speed of ecological change. We assume that climate variability and change first affect physical properties, and then physical changes cause ecological responses. Ecological responses can be organized as responses that start with phytoplankton at the bottom of the food web, a bottom-up response; those that affect top predators, such as penguins, whose changes are driven by food web or top-down responses spread. In Antarctica, we see changes in climate and physical systems and entire food webs, from diatoms to krill to penguins. These processes meet in the middle and converge on the krill.
Have we been watching these sites long enough to know where the future is headed?
How long it takes to know where the ecosystem is going depends on which changes you’re interested in. It’s easier to observe and record physical changes because the system consists only of heat, salinity, water flow, and mixing — and because we have good instrumentation to make precise measurements of these variables. In contrast, tens to hundreds of different measurements are required to characterize the variability of multispecies biological responses, and only a few can be sampled and measured remotely. With a few key exceptions, detecting changes in many biological groups still depends on individual scientists and students performing simple, time-consuming and tedious visual counts one by one. These measurements are slowly becoming automated. Drones, onboard stereos, submersible digital cameras, and instrumented ocean gliders are beginning to make real-time, comprehensive observations of the ocean. Sea ice cover and icebergs remain huge obstacles to leaving instruments unattended in winter, so many measurements are limited to the ice-free summer months.
What are the challenges of working in Antarctica?
There are obvious challenges: planning work in remote areas (7 days per trip from door to door) and anticipating everything you might need. There are storms, high seas, ice caps. In September 2001, we were on ice for two weeks. Then there are supply chain issues, personnel recruitment, and decades of high-quality time-series observations and measurements. Preparations for next year actually start before your departure this year. More than a time series, the project is living, evolving scientific research, with false turns, dead ends, and unexpected discoveries. Despite the challenges, it’s a beautiful and exciting place to work.
