Five
Carbon in the Water
The oceans play a big role in the carbon cycle. Water absorbs carbon dioxide from the atmosphere, and living things in the ocean also take in carbon. For instance, microscopic sea creatures called phytoplankton pull carbon dioxide from the air during photosynthesis. Phytoplankton are not plants, but they make food the way plants do. They use a green pigment called chlorophyll to absorb sunlight. Then they combine sunlight with water and carbon dioxide to make food. The sun’s rays can’t penetrate deep into the ocean, so phytoplankton grow near the surface, where sunlight can reach them.
Phytoplankton use carbon in another way. They absorb carbon compounds from the water to make their protective outer shells. Other animals, including clams, oysters, and corals, also use carbon from the water to build their shells. When such creatures die, they fall to the seafloor. Over time, their carbon-filled shells turn into limestone. Trapped inside limestone, carbon can be sidetracked in the geological carbon cycle for millions of years. However, at any time, shifting tectonic plates might heat up the limestone and send the carbon it contains back into the air through volcanic eruptions. This is just one step in the ongoing geological carbon cycle.
Sea creatures such as clams, oysters, corals, and phytoplankton are part of the geological carbon cycle. They use carbon compounds from the water to build their protective shells. After the creatures die and fall to the seafloor, the carbon in their shells turns into limestone.
Acid Tests
The oceans are already enormous carbon sinks. They are responsible for absorbing almost 26 percent of the carbon that the burning of fossil fuels adds to the atmosphere. According to a study published in the journal Earth System Science Data, between 2002 and 2011, humans added about 9.3 billion tons (8.4 billion metric tons) of carbon to the atmosphere per year, and each year, the oceans absorbed almost 2.5 billion tons (2.3 billion metric tons) of that carbon.
Oceans not only absorb carbon dioxide, but they also release it. Before people began burning vast amounts of fossil fuels, the amounts being absorbed and released were equal. Seawater normally contains a certain amount of acid, and before industrialization, that level was healthy for sea life. But the burning of fossil fuels has released large amounts of nitrogen and sulfur into the air. These chemicals return to the oceans with rainwater, increasing ocean acidity and threatening the health of plants and animals in the water. Excess amounts of carbon dioxide, also from the burning of fossil fuels, increase ocean acidity even further.
So while the absorption of carbon dioxide by the oceans helps to offset carbon emissions from fossil fuels, it is also making the oceans more acidic. Since the Industrial Revolution, the acidity of the oceans has gone from 8.2 pH to 8.1 pH (pH stands for potential hydrogen, with a lower number equaling more acid). While a 0.1-pH drop might not seem like a lot, the pH scale is logarithmic, which means that pH 4 is ten times more acidic than pH 5 and one hundred times more acidic than pH 6. So a 0.1-pH drop represents an increase in acidity of about 25 percent. The extra acidity is harming ocean ecosystems. Along with higher ocean temperatures, it is causing coral bleaching and killing fish, plants, and other sea life.
Coral in this section of the Great Barrier Reef has bleached and died due to increased ocean temperatures and excess carbon dioxide in the water. The excess carbon makes the ocean too acidic for many plants and animals to survive.
Algal Blooms: Harmful and Helpful
Carbon dioxide isn’t the only substance building up in Earth’s waters. People are also adding vast amounts of nitrogen and phosphorus to rivers, lakes, and seas. These substances come from agricultural fertilizers that run off farm fields in the rain, wash into rivers, and eventually reach the ocean. Nitrogen and phosphorus, combined with sunlight and the excess heat brought on by global warming, nourish phytoplankton, causing phytoplankton populations to boom. These booming populations are called algal blooms. They can be small or enormous. The biggest ones can cover hundreds of square miles.
One of the largest algal blooms in recent history occurred in November 2015 in Lake Erie, between the United States and Canada. Caused by the runoff of phosphorous fertilizers, the sticky green bloom covered more than 300 square miles (777 sq. km) of the lake, an area about the size of the state of New York. Algal blooms such as this one harm the environment. Here’s how it works: When the phytoplankton in an algal bloom die, they fall to the seafloor, where bacteria break them down. The bacteria consume most of the oxygen from the water, creating a dead zone where fish and other animals cannot survive. Algal blooms can also produce dangerous toxins that can kill fish and sicken humans who eat the contaminated fish.
Yet algal blooms are not all bad, at least when it comes to climate change. Since phytoplankton absorb carbon dioxide during photosynthesis and also to build their skeletons and since algal blooms consist of vast numbers of phytoplankton, algal blooms pull large amounts of carbon dioxide from the air. If biologists can figure out how to create algal blooms without harming ecosystems, they might be used as a weapon to fight climate change.
The plan is fairly simple. Scientists would fertilize the ocean with iron, a much needed nutrient for phytoplankton growth. The extra iron would trigger algal blooms. The science seems logical, but as is typical when humans interfere with nature, the outcomes are not completely predictable.
Marine biologists have conducted multiple experiments to test the theory of ocean fertilization. Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany, conducted one such experiment in 2012. His team added iron fertilizer, which is normally used to make lawns greener, to an eddy (a circular water current) in the Southern Ocean near Antarctica. The goal was to create a human-made algal bloom.
The experiment worked. A bloom appeared, turning the water from blue to turquoise, which indicated the large growth of phytoplankton. The bloom lasted for twenty-four days, during which Smetacek’s team noticed a sharp decrease in dissolved carbon in the water and the atmosphere. The team assumed that the phytoplankton were absorbing carbon from the ocean and the air.
Algal blooms, such as this one in Kennedy Bay on New Zealand’s North Island, threaten sea life by depleting oxygen supplies and sometimes producing toxins. But some scientists propose deliberately creating algal blooms because the phytoplankton they contain absorb large amounts of carbon dioxide during photosynthesis.
When the bloom died off, the phytoplankton fell to the bottom of the ocean, taking the carbon with them. Smetacek described the dead phytoplankton as “bits [that] settled on the seafloor as ‘fluff’ . . . like a layer of fluff that you would find under your bed if you did not vacuum it for a long time.” He explained, “Eventually, this loose matter flattens into the sediments and a part gets buried; this stuff is sequestered [inside limestone] for geological time scales [hundreds of thousands to millions of years].”
The experiment seemed to be successful, since it showed that ocean fertilization is possible. However, Smetacek believes more research is needed before humans can use ocean fertilization on a grand scale. And even large-scale fertilization might not produce enough carbon absorption to offset the amount of carbon released by the burning of fossil fuels.
Upsetting the Balance
While ocean fertilization to increase algal blooms might sound like a promising idea, environmentalists warn about the repercussions of interfering with ocean ecosystems. Marine biologists say that phytoplankton are a key indicator of ocean health, and any shift in their populations can signal big environmental changes.
Climatologists know that Earth’s oceans are warming due to climate change, but they don’t know how warmer oceans will affect phytoplankton. One 2011 study published in a biological journal of the Royal Society in Great Britain found that phytoplankton in temperate zones of the ocean can adapt to warmer temperatures. But those in colder climates, near the North and South Poles, cannot adapt. This could mean that some phytoplankton will survive climate change while others will die.
Biologists are urgently trying to understand how climate change will affect phytoplankton because they are the food for many different ocean animals, including whales, snails, shrimp, jellyfish, and tiny zooplankton. Without phytoplankton, Earth’s food chain—in which certain living things are eaten by other types of living things, which are themselves eaten by different types of living things—could break down. For example, without phytoplankton, zooplankton might starve. Without zooplankton, herring might starve. Without herring, tuna fish might starve. That would mean no tuna fish for humans. While humans might not starve if they couldn’t eat tuna, other creatures in the food chain would starve. If the food chain suffers enough disruption, eventually humans would find their food sources greatly reduced.
Many biologists urge caution when considering making changes to the lowest member of the food chain. The relationships between the carbon cycle, the oceans, and ocean life are extremely complex. Scientists don’t know how making alterations to these relationships will affect Earth and its inhabitants.