concentrations have geologically oscillated between ~150 and 300 ppm for the last
800,000 years until the Industrial Revolution, which started to generate an unprecedented quantity of carbon dioxide into the atmosphere by burning fossil fuels and
changing land cover and use. Since reaching 400 ppm on May 7, 2013, at Mauna
Loa Observatory, Hawaii, whose data was used for the Keeling curve, carbon
dioxide concentration has continued on an upward trend (415 ppm, February
2020) and is projected to be between 550 and 900 ppm by the end of this century.
The global mean temperature has increased almost 1
C since the Industrial Revolution, and many climate models predict over a 2
C increase by the next century.
The Intergovernmental Panel on Climate Change (IPCC) published the 5th Assessment Report on 2013–2014 putting 2
C as a goal; however, with an updated
understanding of the devastating effect on the global ecosystem and human civilization, a Special Report was issued with a new goal of 1.5
C in 2017.
11.2 Contribution
The natural cycle of carbon is simply an exchange between autotrophy and heterotrophy, i.e., carbon dioxide in the atmosphere is fixed (reduced) into organic matter
by autotrophs, and heterotrophs break down (oxidize) the fixed carbon back to
carbon dioxide. These redox reactions support both the autotrophs and heterotrophs
that perform them, since the organisms gain energy from these chemical reactions.
Autotrophic microbes are divided into two general categories of photoautotrophs
and chemoautotrophs. The photoautotrophs use energy from the sun’s electromagnetic radiation and these organisms include algae, diatoms, Cyanobacteria, and
anoxygenic phototrophic bacteria (e.g., purple sulfur bacteria and green sulfur
bacteria). The chemoautotrophs derive their energy from chemical sources, and
these organisms include nitrifying bacteria and iron-oxidizing bacteria. Heterotrophic microbes are the majority of aerobic and anaerobic organisms prevalent in the
earth ecosystem.
While human activities are the ultimate reason for the imbalance of the carbon
cycle—which increases carbon dioxide concentration in the atmosphere—it is
microbes that facilitate the actual processes. Fossil fuels are stably stored organic
matter from geological processes; thus the excessive removal of them from the
ground and seafloor, and then burning them, is an obvious contribution to raising
the atmospheric carbon dioxide concentration. Through the Green Revolution,
humans have produced a tremendous quantity of crops from the agricultural fields,
of which a significant portion was converted from forests or other natural ecosystems. The conversion reduces long-term carbon storage capacity of most natural
ecosystems, as agricultural fields have shorter-term carbon storage in their crop
biomass through conventional agricultural practices, such as tilling. Forests can
store some of their carbon in cycles that last hundreds of years, contrasted with a
few types of crops that may store carbon in cycles lasting less than 1 year.
188
S. Kang
800,000 years until the Industrial Revolution, which started to generate an unprecedented quantity of carbon dioxide into the atmosphere by burning fossil fuels and
changing land cover and use. Since reaching 400 ppm on May 7, 2013, at Mauna
Loa Observatory, Hawaii, whose data was used for the Keeling curve, carbon
dioxide concentration has continued on an upward trend (415 ppm, February
2020) and is projected to be between 550 and 900 ppm by the end of this century.
The global mean temperature has increased almost 1
C since the Industrial Revolution, and many climate models predict over a 2
C increase by the next century.
The Intergovernmental Panel on Climate Change (IPCC) published the 5th Assessment Report on 2013–2014 putting 2
C as a goal; however, with an updated
understanding of the devastating effect on the global ecosystem and human civilization, a Special Report was issued with a new goal of 1.5
C in 2017.
11.2 Contribution
The natural cycle of carbon is simply an exchange between autotrophy and heterotrophy, i.e., carbon dioxide in the atmosphere is fixed (reduced) into organic matter
by autotrophs, and heterotrophs break down (oxidize) the fixed carbon back to
carbon dioxide. These redox reactions support both the autotrophs and heterotrophs
that perform them, since the organisms gain energy from these chemical reactions.
Autotrophic microbes are divided into two general categories of photoautotrophs
and chemoautotrophs. The photoautotrophs use energy from the sun’s electromagnetic radiation and these organisms include algae, diatoms, Cyanobacteria, and
anoxygenic phototrophic bacteria (e.g., purple sulfur bacteria and green sulfur
bacteria). The chemoautotrophs derive their energy from chemical sources, and
these organisms include nitrifying bacteria and iron-oxidizing bacteria. Heterotrophic microbes are the majority of aerobic and anaerobic organisms prevalent in the
earth ecosystem.
While human activities are the ultimate reason for the imbalance of the carbon
cycle—which increases carbon dioxide concentration in the atmosphere—it is
microbes that facilitate the actual processes. Fossil fuels are stably stored organic
matter from geological processes; thus the excessive removal of them from the
ground and seafloor, and then burning them, is an obvious contribution to raising
the atmospheric carbon dioxide concentration. Through the Green Revolution,
humans have produced a tremendous quantity of crops from the agricultural fields,
of which a significant portion was converted from forests or other natural ecosystems. The conversion reduces long-term carbon storage capacity of most natural
ecosystems, as agricultural fields have shorter-term carbon storage in their crop
biomass through conventional agricultural practices, such as tilling. Forests can
store some of their carbon in cycles that last hundreds of years, contrasted with a
few types of crops that may store carbon in cycles lasting less than 1 year.
188
S. Kang