Rainforests and the Atmosphere

Rainforests, found worldwide at tropical latitudes, play a critical role in processing carbon dioxide. They also store carbon and aid in cloud formation. Rainforests are rapidly disappearing because of human interference.

The Rainforest and the Atmosphere

Rainforests are an integral part of the environment. Although they do not produce vast amounts of oxygen, as previously thought, they effectively sequester much carbon dioxide in biomass. They also aid in cycling the carbon dioxide from the atmosphere, though they release as much as they store. As deforestation has increased since the early twentieth century, scientists have worked to understand the relationship between the rainforests and the atmosphere and their global effects.

Rainforests

Rainforests are characterized by high rainfall. While most rainforests are in the tropics, they also exist in temperate zones, limited to narrow belts that get much rain or snow, and are typically near mountains (which tend to have moist seaward slopes because of the rain shadow effect). The California redwood forests and the mountains of Northeast Asia (formerly Manchuria) and Japan are examples of temperate rainforests.

Most other rainforests fall in a band around the tropics and mark the tropical rainforests as hot spots of biodiversity. Rainforests are characterized by poor soil quality, as rapid decay prevents soil buildup. Trees have root networks near the surface to catch the nutrients from the top layer of decomposing plants and animals. There are, however, exceptions. A few rainforests, typically young ones on volcanic land, have rich soils. Examples of such rainforests include those on volcanic islands, such as Sumatra or Jakarta, and those in the volcanic regions of Africa and Central America. Tropical rainforests, in general, can be found in the tropics around the world where it is wet enough, including Southeast Asia, Borneo and New Guinea, the Amazon, Central America, Central Africa, and many islands in that range, such as Madagascar and Hawaii.

Amazon

The Amazon rainforest is in the Amazon Basin, mostly in Brazil but also in parts of Colombia, Bolivia, Peru, and other countries. For many people, the Amazon is the iconic rainforest, the largest in the world, and it comprises the greatest biodiversity on the planet.

It is thought that a large population once inhabited the forest, as indicated by terra preta, a black soil not natural to the Amazon Basin and rich in nutrients. It is believed the soil was created by farmers who buried charcoal and biochar in it to enrich it. Terra preta has been shown to regenerate itself and is highly sustainable when used carefully. This system of soil enrichment collapsed when Spanish and Portuguese colonists introduced diseases that caused the demise of local societies.

The Amazon is being deforested at an alarming rate, with a loss of nearly 9,500 square kilometers in 2019. Much of the land is being used as cattle pasture and for agricultural crops such as soy. In 2022, an area equivalent to the size of Qatar, 11,568 square kilometers (4,466 square miles), was destroyed. Despite progress in previous years, this marked the worst destruction in six years. In 2023, the rate of deforestation dropped by 9 percent. However, the situation remained dire, as the carbon emissions generated by the loss still produced an amount almost equal to half of the annual fossil fuel emissions of the United States.

Central Africa

The Central African rainforest is the second largest in the world, contains the world's largest tropical carbon sink, and is home to many species, including bonobos and gorillas. Much of this rainforest is drier than other rainforests. It, too, is being deforested at an alarming rate. Much of the West African forest is gone, and scientists predicted in 2022 that at least 27 percent of the remaining rainforests would be gone by 2050 if destruction persists. The densest and most intact portion is in the Congo Basin. Countries with land in the Congo Basin have created a nature preserve that may help to stop deforestation.

Southeast Asia

The rainforests of Southeast Asia cover land from Bangladesh to New Guinea. However, New Guinea is technically part of the Australian continent and is home to organisms more closely related to those in Australia than those in Southeast Asia. The rainforests of Indonesia, particularly Borneo, are the oldest on this planet; they compose the second-highest biodiversity on Earth.

However, these forests are being deforested, and all are nearly gone, especially in Java. New Guinea’s rainforests, however, are mostly intact because of the inaccessibility of that country’s interior.

Deforestation

Rainforest destruction, occurring globally, is estimated to account for 20 percent of greenhouse gas emissions. Removal of such forests causes massive extinctions, as species lose their habitats. Additionally, deforestation causes mass erosion, which can sweep organic matter out to sea, where it can fuel algal blooms (which create greenhouse gases). Forests also can serve to filter the atmosphere and remove some pollutants, so their destruction eliminates that buffer.

The Food and Agriculture Organization of the United Nations notes large-scale commercial agriculture and local subsistence agriculture account for most rainforest clearing. Typically, subsistence agriculture takes the form of slash and burn. In slash and burn, the existing forest is cut down, and what is left is set ablaze. This temporarily enriches the soil, though much of the nutrients are lost as carbon dioxide. The land is then worked until it is depleted of nutrients, at which point the farmer moves on. Ideally, the land is left for new forest growth, and the farmer returns; however, when the surrounding population grows rapidly, as it often does now, reforestation fails. Additionally, extensive deforestation prevents reforestation of the exhausted land. As populations increase, the time the land can be allowed to lay fallow decreases. This means declining productivity, which makes the land more vulnerable to effects such as erosion, which causes problems elsewhere. While traditional (going back to the Neolithic revolution ten thousand years ago, when farming was invented), slash and burn is not sustainable in times such as these, with large population growth. Further environmental damage exacerbates the situation.

Slash and burn differs from slash and char, which uses biochar to enrich the soil by being buried along with other organic matter. It generates less loss and can be used to create terra preta. As a result, slash and char is being presented as a more sustainable alternative.

History

Rainforests have existed almost since large land plants evolved in the Late Devonian period, around 380 million years ago. The Carboniferous period (359 to 299 million years ago) is known for the coal formed from rainforests. Since then, rainforests have been a constant feature of the planet, with their range varying with the planet’s climate.

During the Eocene period, about 50 million years ago, rainforests grew as far north as Northern Europe and covered Antarctica. Twenty-first-century rainforests are thought to be parts of those of the Mesozoic eraGondwana. The current Borneo rainforest is thought to be 130 million years old, and the Amazon is 60 million years old. No matter the era, ancient rainforests provided an environment similar to the one of today and harbored great biodiversity.

Photosynthesis and Respiration

The rainforest is a center of turnover in the carbon-oxygen cycle because of the high density of biomass. The large number of plants leads to high levels of photosynthesis.

Photosynthesis is the process by which plants make oxygen and sugar from carbon and water. It occurs in two primary stages. The first stage is the light-dependent reaction, which involves the separation of hydrogen ions from water and preparing chemicals, such as adenosine triphosphate (ATP), for the second stage, which produces oxygen. Photosynthesis is an endothermic reaction, meaning that photosynthesis requires energy, so the efficiency of light absorption is critical. Scientists believe that plants may use quantum phenomena such as electron tunneling to increase efficiency.

The second stage of photosynthesis is the Calvin cycle, or dark cycle. This stage does not involve captured photons but uses the ATP from stage one to bind carbon dioxide to make sugars. Several variations have evolved, and the C4 variant is particularly efficient under proper conditions. Plants that use this mechanism include sedge, millet, corn, and sugar cane. Scientists are considering creating bioengineered plants that use this variant to help mitigate climate change.

The other process at work in rainforests is respiration. Respiration is how organisms extract energy from organic compounds. There are three forms of respiration, two of which—aerobic and anaerobic—will be discussed here.

Aerobic respiration is the form used by animals and plants. It requires oxygen. The other form is anaerobic respiration, which uses no oxygen and is used mainly by bacteria, such as bacteria involved in decomposition. Aerobic respiration is more efficient than anaerobic respiration and is developed in response to the oxygenation of the atmosphere. Aerobic respiration involves a chemical process called the Krebs or citric acid cycle to release the binding energy in the form of ATP. This accompanies the oxygenation of the carbon as carbon dioxide, which is released as a waste product. Plants can use this to photosynthesize. However, some bacteria use the older anaerobic respiration process, which does not produce as much ATP but works in anoxic environments, such as those inside a decomposing animal. Typical waste products, which include hydrogen sulfide, ammonia, and methane, are greenhouse gases. Both forms occur in the rainforests, where animals and plants employ aerobic respiration and where organic material decomposes after death.

It was once thought that rainforests produced an excess of oxygen. However, data analyzed in the twenty-first century suggests that before human influence on the rainforests, the net oxygen output of rainforests was low, if not zero. Rainforests have some of the highest concentrations of photosynthetic life, but the high biomass and turnover rate of material and the many animals that inhabit the rainforests effectively release carbon equal to the oxygen output and use the oxygen in the process. This process is ideal for trees, which use the carbon to make more oxygen. As a result, the rainforest is naturally more or less carbon neutral. What, then, is the significance of the rainforest to the atmosphere?

The primary impact of the rainforest is its storage of vast amounts of carbon. Carbon sequestration is the storage of carbon, typically due to human activity. Due to carbon sequestration, stored carbon has been removed and is less likely to harm the environment. Many carbon sequestration proposals involve carbon-soaking chemical reactions or pumping excess carbon dioxide into coal seams, old oil wells, or other similarly inaccessible locations. Other approaches, collectively called biosequestration, use living systems to capture and store carbon dioxide. Some, such as biochar systems, seal carbon by reducing the carbon to a stable state and burying it. Other methods are designed to store it in a living ecosystem through reforestation or better-managed forests. Lumber industries worldwide, in countries such as Canada and Finland, are planting trees to replace those cut down. Additionally, China has launched several massive reforestation projects, though it will be long before they counterbalance forest losses and emissions.

Because a rainforest sequesters carbon in trees and organisms, when a rainforest is destroyed, not only is it no longer producing oxygen, but all the stored carbon is also released, increasing atmospheric carbon. Additionally, evidence suggests that even old groups of trees still absorb carbon dioxide. It is thought that rainforests absorb one-fifth of the carbon dioxide output. This means they serve as a buffer against greenhouse gas increases. It is important to note that they are not producing oxygen but simply removing carbon dioxide from the atmosphere. Their capacity to do this, however, is not unlimited. Thus, it is imperative to stop emitting carbon dioxide beyond the environment’s carrying capacity.

Rainforests and the Water Cycle

In addition to their carbon-oxygen role, rainforests play a role in the water cycle, which also plays a significant role in the atmosphere. The water cycle is how water is cycled through the environment. Water evaporates from various bodies, such as lakes, rivers, and oceans. It then condenses into clouds and rain. The rainwater is caught and used by plants and animals or runs down the land into rivers and back to the ocean.

Plants need water for photosynthesis, and rainforests have a large effect on atmospheric water content through a phenomenon called evapotranspiration. Evapotranspiration is made of two phenomena: evaporation and transpiration. Transpiration is the process by which plants lose water to the atmosphere. Ninety percent of the water that enters a plant’s roots is lost in this manner. As a result, plants can contribute greatly to the humidity of a region and, thus, to rainfall and cloud production patterns. When the forest is lost and the climate is drier, additional water is lost by trees, which stresses them further, resulting in a negative feedback loop. Because of evapotranspiration, the climate of deforested rainforest regions often grows drier.

Protecting the Rainforests

As a result of the enormous impact the rainforests have on the atmosphere, protecting them is of the utmost importance. In the twenty-first century, much study is underway to determine the best way to do this, not only in the field of biology but also by economists and political scientists. This is because the factors influencing rainforest loss are often economic. As a result, the best means of protection for them is through international efforts. Since many rainforests are global, the countries with them are often developing, and the rainforests are the common heritage of the whole species. Shifts in agricultural practice and reforestation are the best methods to defend this vital bioregion.

There are positive signs that mitigation efforts have begun to work. Deforestation rates have fallen significantly in the Brazilian Amazon. Colombia, Peru, and Bolivia also saw significant declines in forest loss in 2023. These positive movements were seen as responses to effective policy changes. Still, rainforests remain threatened by climate change, and areas outside the Amazon continue to see forest loss. Many countries are also struggling to keep their commitments to mitigating deforestation in the face of social, economic, and political pressure. 

Principal Terms

anoxic: lacking oxygen

ATP: Adenosine triphosphate is a chemical that is the primary carrier of energy in all organisms

biochar: charcoal used for soil enrichment or carbon sequestration

carbon-oxygen cycle: cycle in which carbon dioxide, living organisms, and the atmosphere are related by the cycling of carbon in the atmosphere and in organisms through photosynthesis and respiration

carbon sequestration: long-term storage of carbon in a stable state to remove carbon from cycling in the environment

greenhouse effect: the process by which some gases trap heat on Earth, rather than allowing it to be reflected back to space

photosynthesis: a process by which plants convert carbon dioxide and water into sugar and oxygen

slash-and-burn agriculture: a paradigm of subsistence agriculture commonly practiced in the developing world

subsistence agriculture: the practice of growing to meet the needs of a self-sufficient farmer rather than the market

tropical climate: a climate characterized by high annual temperature

water cycle: the process by which water cycles through the environment

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