October 6, 2022

How does climate change affect ocean waters and ecosystems?

How does climate change affect ocean waters and ecosystems?

Biological oceanographer Hugh Ducklow It studies the marine food web and how it interacts with the physical properties of the oceans. Much of his work is through the United States Long-term program of ecological research (LTER), where researchers have monitored trends for decades at 28 land and marine sites in the United States, along with a few sites elsewhere. In addition to the open ocean, studies include deserts, beaches, rivers, forests, and grasslands. From 2012 to 2018, while based at Columbia University Lamont-Doherty Earth ObservatoryDakhlo led Palmer Station LTER sitea base for annual cruises through 800 km of icy waters on the Antarctic Peninsula.

To mark the 40th anniversary of the LTER program, researchers recently published a paper Series of articles about how climate change affects their sites. Ducklow led Department of open ocean environments, In addition to the Antarctic, it includes the waters of Alaska, California, and the northeastern United States. We spoke with him about his work, his observations and his colleagues and future prospects.

Ducklow in an ice cave near Palmer Station, Antarctica, 2006. The glacier collapsed and disappeared a year or two later. (Courtesy of Hugh Ducklow)

Why should we care what climate change is doing to the oceans?

In addition to the fact that seafood is the main source of protein for about 3 billion people, the oceans absorb a lot of excess heat and carbon dioxide produced by humans. About 90% of all excess heat produced by the greenhouse effect since the industrial revolution is in the ocean. The global ocean also accounts for about a quarter to a third of our carbon dioxide emissions. Both of these processes keep the air temperature cooler than it would otherwise be. But both come with costs. The ocean is warming as a result of the extra heat. The human-induced warming signal is detectable even in the deep Southern Ocean. Increased absorption of carbon dioxide causes ocean acidification. The ecological consequences of warming and acidification are just beginning to be understood, and the future capacity for continued heat and CO2 storage 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 in space and time. Physical system responses to climate change are strongest and most obvious at the surface. This is important because heat and CO2 are exchanged there and phytoplankton grow there. Depending on wind, storms and currents, the depth of the surface layer varies from almost zero in summer to over 1000 meters in winter. Temperature affects the depth of the surface layer and, in the case of polar locations, sea ice. Near the poles in winter, there is little or no solar radiation, and sea ice covers the ocean. In spring, as the sun rises, the ocean surface warms and the sea ice melts, adding fresh water to the surface. Warmer, fresher waters are less dense than cooler, saltier waters, and therefore the surface layer is shallower.

The depth of surface mixed layers decreases over most of the LTER network locations—Palmer Antarctica, the continental shelf of the northeastern United States, and the northern Gulf of Alaska. However, despite an unbroken record of observations since 1950 and warming water temperatures, no change is evident in the California Current.

What biological changes are taking place? Can we clearly relate 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 remain exposed to sunlight, but do not have access to nutrients. When the surface layer is deep, phytoplankton can access nutrients, but there is little or no sunlight. Trends in phytoplankton have been documented at some, but not all, LTER sites. Phytoplankton are the only organisms that can be detected by satellite, but their trends in abundance are not as clear as the physical changes I have described. Phytoplankton evidence is increasing in Antarctica, as expected in a shallow surface layer, but decreasing in the continental shelf of the northeastern United States, despite its shallowness. No changes are evident on other sites. As expected from an increase in phytoplankton, it shows an increasing trend in Antarctica. They are also increasing in the current California system, even though phytoplankton are not.

Although there are long records of changes in the California Current (70 years), the US Northeast Shelf (40 years), and the Antarctic Palmer (30 years), it is not yet certain that these changes are caused by climate change. . Numerical simulations of satellite images show that about 50 years is the minimum time needed to attribute observed trends to climate change. Some changes may take a century or more.

Are there things happening in Antarctica that make it different from other regions?

A simple distinguishing feature of the Arctic and Antarctic seas is that they are covered by 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 synchronized with seasonal ice cover and may be disrupted by declining cover. Sea ice blocks sunlight and affects the timing of phytoplankton blooms. Although sea ice is rapidly shrinking at both poles, the effects are unclear. As sea ice recedes, new ice-covered areas open up for phytoplankton growth, expanding the polar marine ecosystem. But as the cover disappears, its share of fresh water decreases and the fresh layer at the surface of the ocean decreases. The net effect on the future ecosystem is not clear.

It seems that another distinctive feature of Antarctic ecosystems is the diversity and speed of ecological changes. We hypothesize that climate variability and change first affect physical characteristics, and then physical changes drive ecological responses. Ecological responses can be organized into those starting with phytoplankton at the base of the food web, i.e., bottom-up responses. And those that affect top predators like penguins and 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 and converge on the krill.

Have we watched enough of these sites to have a good idea of ​​where things are going in the future?

How long it takes to know where ecosystems are going depends on the changes you are interested in. Physical changes are easier to observe and record because the system consists only of heat, salinity, currents, and mixing—and because we have good tools to do it. Make accurate measurements of those variables. In contrast, tens to hundreds of different measurements are required to characterize variation in the biological responses of multiple species, and only a few can be sampled and measured remotely. With a few key exceptions, detection of change for many groups of organisms still depends on scientists and students making simple, time-consuming and tedious visual counts. These measurements are gradually becoming automated. Drones, shipboard acoustics, underwater digital video cameras, and equipped ocean gliders are beginning to provide real-time and comprehensive views of the oceans. Sea ice cover and icebergs remain major obstacles to releasing instruments in winter, so many measurements are limited to ice-free summer months.

What have been some of the challenges of working in Antarctica?

There are obvious challenges: planning to work in a remote location—the trip takes seven days 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 there are the issues of supply chain, staffing, and maintaining a high-quality time series of observations and measurements over several decades. The work to prepare for next year literally starts before you even set sail this year. This project is not just a time series, but a living and evolving scientific investigation with wrong turns, blind alleys and unexpected discoveries. Despite the challenges, it is a beautiful and exciting place to work.

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