Air Pollution Sinks: What Are They?

A sink in the context of air pollution refers to any process, activity, or mechanism that removes a greenhouse gas from the atmosphere. Natural carbon sinks include oceans, soil, and forests, which absorb more carbon from the atmosphere than they release. Oceans, for example, have absorbed about a quarter of the carbon dioxide released since the Industrial Revolution, largely due to phytoplankton. Soil is another important carbon sink, absorbing roughly a quarter of all human emissions annually. Artificial carbon sinks, such as those that store carbon in building materials or deep underground, are also being explored.

The ocean, the world's biggest carbon sink

A carbon sink is a natural or artificial reservoir that absorbs and stores carbon from the atmosphere. Natural carbon sinks capture about half of the human-created carbon dioxide emissions. Oceans, forests, large mammals like elephants, and fungi are some examples of natural carbon sinks. The ocean is the world's largest carbon sink, absorbing about 31% of the carbon dioxide emissions released into the atmosphere. This is due to the presence of phytoplankton, microscopic marine algae, and bacteria that play a crucial role in the carbon cycle, absorbing as much carbon as all the plants and trees on land combined.

The ocean's capacity as a carbon sink is influenced by various factors. In polar regions, denser water flows towards the deep sea, dragging down dissolved carbon. The cold temperatures in these regions facilitate the absorption of carbon dioxide. Additionally, healthy coastal ecosystems, such as mangroves, seagrass beds, and salt marshes, contribute significantly to carbon sequestration. These ecosystems can store up to ten times more carbon than continental forests.

However, the effectiveness of the ocean as a carbon sink is under threat. Ocean acidification, caused by increasing carbon dioxide levels, is altering the chemical equilibriums in seawater. This process may reduce the ocean's ability to act as a carbon sink over time. Moreover, plastic pollution in the ocean impacts the rate at which plankton can trap carbon. The destruction of natural carbon sinks releases stored carbon back into the atmosphere, emphasizing the importance of their preservation.

To address these challenges, various initiatives are being undertaken. The Surface Ocean CO₂ Atlas (SOCAT) is a key tool for quantifying the ocean carbon sink and evaluating ocean biogeochemical models. Additionally, the World Economic Forum's 2030 Water Resources Group has facilitated almost $1 billion in financing for water-related programs, operating in 14 countries. Legal efforts are also being made to end plastic pollution, recognizing its impact on the ocean's carbon sequestration capacity.

In conclusion, the ocean is the world's largest carbon sink, playing a vital role in mitigating climate change. However, it faces threats from ocean acidification and plastic pollution. Protecting and preserving marine ecosystems are crucial steps in ensuring the continued effectiveness of the ocean as a carbon sink and combating the climate crisis.

Soil, an important carbon storage medium

A carbon sink is anything that absorbs more carbon from the atmosphere than it releases. The ocean, soil and forests are the world’s largest carbon sinks. The Earth’s soil absorbs roughly a quarter of all human emissions each year. This is known as soil-based carbon sequestration.

Soils are made in part of broken-down plant matter, which contains carbon that was taken in from the atmosphere. In colder climates, where decomposition is slow, soils can store or sequester carbon for a long time. Soil organic carbon tends to be concentrated in the topsoil. In some soils, like those in wetlands or peat forests, carbon can make up 10% or more.

However, converting natural ecosystems like forests and grasslands to farmland disturbs soil structure, releasing stored carbon and contributing to climate change. Over the past 12,000 years, the growth of farmland has released about 110 billion metric tons of carbon from the top layer of soil. Climate change is also making it harder for soils to naturally store carbon. The warming of the planet could lead to widespread soil carbon losses by speeding up the decay of soil organic matter.

Scientists have estimated that soils, especially agricultural ones, could sequester over a billion additional tons of carbon annually. This has led policymakers to turn to soil-based carbon sequestration as a "negative emissions" technology. Farmers can add more carbon to agricultural soils by planting certain kinds of crops. For example, perennial crops grow deep roots that help soils store more carbon. "Cover crops" like clover, beans and peas, planted after the main crop is harvested, help soils take in carbon year-round.

Other management practices such as irrigation of pasture or rangelands may also increase carbon levels. The effect of land management on SOC levels, especially the impacts of management in agricultural settings, is the subject of much current research.

Photosynthesis, a natural carbon sink

A carbon sink is anything that absorbs more carbon from the atmosphere than it releases. The ocean, soil, and forests are the world's largest carbon sinks. Photosynthesis is a natural process through which carbon sinks absorb carbon from the atmosphere.

Photosynthesis is a biological process that forms the foundation of the fast carbon cycle. During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. Plants use energy from the sun to combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen.

The process of photosynthesis enables plants to grow more. This increased growth is referred to as carbon fertilization. Models predict that plants might grow anywhere from 12 to 76 percent more if atmospheric carbon dioxide is doubled, as long as other factors such as water availability do not limit their growth.

Forests, which are carbon sinks, continually take carbon out of the atmosphere through photosynthesis. Similarly, large mammals like elephants can play a role in speeding up natural carbon capture. Elephants inhibit wildfires by eating flammable vegetation, and by trampling plants underfoot, they help force the carbon in them into the soil.

However, human activities such as deforestation are depleting Earth's supply of carbon sinks. As a result, the amount of carbon in the atmosphere is rising.

Artificial carbon sinks, storing carbon underground

A carbon sink is anything that absorbs more carbon from the atmosphere than it releases. Natural carbon sinks include forests, oceans, and soil. Oceans are the biggest carbon sinks on the planet, absorbing about a quarter of the carbon dioxide released into the atmosphere since the Industrial Revolution.

Artificial carbon sinks are those that store carbon in building materials or deep underground (geologic carbon sequestration). Geological carbon sequestration happens when carbon dioxide is stored in underground geological formations or rocks. This process is largely artificial and is an effective way of neutralizing emissions from human practices such as manufacturing or construction.

One example of a man-made carbon sink is spraying artificial trees with special chemical compounds to help them absorb carbon dioxide. Fertilizing the surface of the seas with iron also helps with carbon capture by stimulating the growth of carbon dioxide-absorbing microorganisms. However, there are concerns about how effective this method is and whether it could damage the oceans' ecological balance by creating too much phytoplankton, blocking out sunlight, and taking nutrients from other creatures.

Another artificial carbon-trapping technology is carbon capture and storage (CCS). This involves capturing carbon dioxide produced by power generation or industrial activity, compressing it, and transporting it to deep underground facilities, where it is injected into rock formations for permanent storage.

While public awareness of the significance of carbon sinks has grown since the passage of the 1997 Kyoto Protocol, no major artificial systems have yet been able to remove carbon from the atmosphere on a large scale.

The role of large mammals in speeding up natural carbon capture

Carbon sequestration is the process by which atmospheric carbon is captured and stored. This removes carbon dioxide (CO2) from the atmosphere and helps mitigate the greenhouse effect. There are two main types of carbon sequestration: biological and geological. While plants have traditionally been the focus of investigations into carbon sequestration, animals also play a crucial role in the capture and storage of carbon from the atmosphere.

Large mammals, in particular, can play a role in speeding up natural carbon capture. For example, elephants inhibit wildfires by eating flammable vegetation, and by trampling plants underfoot, they help force the carbon in them into the soil where it is much more likely to stay. In grassland ecosystems, large animals can reduce forest and bush fires, increase albedo (the ability of surfaces to reflect solar radiation), and help retain carbon in the vegetation and the soil. In marine ecosystems, whales and other large animals fertilise phytoplankton, which capture billions of tonnes of CO2 each year.

A study from Stanford University published in Nature: Ecology and Evolution found that large animals in the wild play an important role in sequestering carbon and that the more species in an area, the better. The study estimated that a forest with 30 mammal species probably sequesters an extra 10,000 kilograms per hectare in aboveground tree biomass alone. Another study published in Current Biology explored the role of large wild animals in restoring ecosystems and battling climate change, highlighting the potential of large animals such as elephants, rhinoceroses, giraffes, whales, bison, and moose to mitigate climate change through their impact on carbon stocks, albedo, and fire regimes.

Rewilding certain animal species can help boost carbon storage, making it a powerful nature-based solution to meet climate targets. Protecting and restoring wild animals and their functional roles can enhance natural carbon capture and storage. For example, when the wildebeest population in Africa dropped due to an epidemic, there were too few animals to graze the Serengeti, leading to an increase in wildfires and the release of large amounts of CO2. By contrast, strong, healthy populations of just nine different types of animals, including sea otters, grey wolves, and whales, could lead to the capture of 6.41 gigatons of CO2 every year, representing 95% of the CO2 that needs to be removed from the atmosphere to keep global warming below 1.5°C.

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