Global Cycles

Carbon Cycle:

The Natural Carbon Cycle:

Most of the carbon on Earth is stored in sedimentary rocks and does not play a significant role in the carbon cycle on the timescale of decades to centuries. The atmospheric pool of CO2 is smaller but is very important because it is a greenhouse gas. The sun emits short-wave radiation that passes through the atmosphere, is absorbed by the Earth, and re-emitted as long-wave radiation. Greenhouse gases in the atmosphere absorb this long-wave radiation causing them, and the atmosphere, to warm. The retention of heat in the atmosphere increases and stabilizes the average temperature, making Earth habitable for life.

More than a quarter of the atmospheric CO2 pool is absorbed each year through the process of photosynthesis by a combination of plants on land and at sea. Photosynthesis is the process in which plants use energy from sunlight to combine CO2 from the atmosphere with water to make sugars, and in turn build biomass. At sea, the phytoplankton that perform photosynthesis sink after they die, transporting organic carbon to deeper layers that then either are preserved in ocean sediments or decomposed into a very large dissolved inorganic carbon pool.

Plants are called primary producers because they are the primary entry point of carbon into the biosphere. In other words, almost all animals and microbes depend either directly or indirectly on plants as a source of carbon for energy and growth. All organisms, including plants, release CO2 to the atmosphere as a by-product of generating energy and synthesizing biomass through the process of respiration. The natural carbon cycle is balanced on both land and at sea, with plant respiration and microbial respiration (much of it associated with decomposition, or rotting of dead organisms) releasing the same amount of CO2 as is removed from the atmosphere through photosynthesis.

Human Interactions with The Carbon Cycle:

The global carbon cycle contributes substantially to the provisioning of ecosystem services upon which humans depend. We harvest approximately 25% of the total plant biomass that is produced each year on the land surface to supply food, fuel wood and fiber from croplands, pastures and forests. In addition, the global carbon cycle plays a key role in regulating ecosystem services because it significantly influences climate via its effects on atmospheric CO2 concentrations. Atmospheric CO2 concentration increased from 280 parts per million (ppm) to 390 ppm between the start of industrial revolution in the late eighteenth century and 2010. This reflected a new flux in the global carbon cycle —anthropogenic CO2 emissions— where humans release CO2 into the atmosphere by burning fossil fuels and changing land use.

Fossil fuel burning takes carbon from coal, gas, and oil reserves, where it would be otherwise stored on very long time scales, and introduces it into the active carbon cycle. Land use change releases carbon from soil and plant biomass pools into the atmosphere, particularly through the process of deforestation for wood extraction or conversion of land to agriculture. Slightly more than half of this anthropogenic CO2 is currently being absorbed by increased photosynthesis on land and at sea. However, total emissions of CO2 are increasing, while the proportion absorbed by photosynthesis and stored on land and in the oceans is declining (Le Quere et al., 2009). Rising atmospheric CO2 concentrations in the twentieth century caused increases in temperature and started to alter other aspects of the global environment. The scale and range of impacts from global environmental change of natural and agricultural ecosystems is projected to increase over the twenty-first century, and will pose a major challenge to human well-being.

Nitrogen Cycle:

The Nitrogen Cycle:

The vast majority of nitrogen on Earth is held in rocks and plays a minor role in the nitrogen cycle. The second largest pool of nitrogen is in the atmosphere. Most atmospheric nitrogen is in the form of N2 gas, and most organisms are unable to access it. This is significant because nitrogen is an essential component of all cells—for instance, in protein, RNA, and DNA—and nitrogen availability frequently limits the productivity of crops and natural vegetation.

Atmospheric nitrogen is made available to plants in two ways. Certain microbes are capable of biological nitrogen fixation, whereby N2 is converted into ammonium, a form of nitrogen that plants can access. Many of these microbes have formed symbiotic relationships with plants—they live within the plant tissue and use carbon supplied by the plant as an energy source, and in return they share ammonia produced by nitrogen fixation. Well-known examples of plants that do this are peas and beans.  Additionally, lightning causes nitrogen and oxygen in the atmosphere to react and produce nitrous oxides that fall or are washed out of the atmosphere by rain and into the soil, but the flux is much smaller than biological nitrogen fixation.

Fossil fuel burning takes carbon from coal, gas, and oil reserves, where it would be otherwise stored on very long time scales, and introduces it into the active carbon cycle. Land use change releases carbon from soil and plant biomass pools into the atmosphere, particularly through the process of deforestation for wood extraction or conversion of land to agriculture. Slightly more than half of this anthropogenic CO2 is currently being absorbed by increased photosynthesis on land and at sea. However, total emissions of CO2 are increasing, while the proportion absorbed by photosynthesis and stored on land and in the oceans is declining (Le Quere et al., 2009). Rising atmospheric CO2 concentrations in the twentieth century caused increases in temperature and started to alter other aspects of the global environment. The scale and range of impacts from global environmental change of natural and agricultural ecosystems is projected to increase over the twenty-first century, and will pose a major challenge to human well-being.