How it all began ...
Since the start of the industrial age, the Earth has been experiencing rising temperatures. This has mainly been due to a rise in greenhouse gas (GHG) emissions from buring fossil fuels, deforestation, intensification of industry, etc. While GHGs such as carbon dioxide are part of a natural cycle of life on Earth, it is the sharp increase in such GHGs that is a major cause of concern with regards to climate change. For example, greenhouse gases make up less than 1% of the atmospheric composition, but without them the Earth's temperature would be -18°C, rather than the present average of 15°C.
This section introduces the natural carbon cycle, the greenhouse effect, GHGs and how they affect our climate.
The Carbon Cycle
Carbon is continually cycled within a closed system made up of the atmosphere, the oceans and the land, where carbon is stored and exchanged in various forms. This cycle has been naturally occurring for thousands of years with the exchange between “sources” (where carbon comes from) and “sinks” (where carbon is absorbed, for example, the oceans and trees), balancing carbon concentrations across the cycle.
There are a number of natural cycles within the overall exchange, including the cycle between plants and animals with the atmosphere through the natural processes of respiration and photosynthesis. Similarly, the surface of the oceans continually exchange carbon with the atmosphere on a cyclical basis and carbon is continually cycled between the deep and surface oceans.
The main natural “sources” of carbon include:
- Respiration by plants and animals
- Decay and decomposition of plant and animal organic matter
- Natural fires and volcanoes
- Natural warming of ocean surfaces releasing CO2 to atmosphere
The main natural “sinks” of carbon include:
- Photosynthesis by plants
- Sedimentation of dead organic matter and transformation to form fossil fuels
- Natural cooling of ocean surfaces to form carbonic acid
- Metabolism of carbon by ocean life to form shells which eventually form carbon sediments on the ocean floor
These cycles have been in natural balance for thousands of years with no net change in the concentrations of carbon dioxide in the atmosphere. However, there is mounting scientific evidence that this balance is changing dramatically, with carbon dioxide concentrations increasing significantly.
Humans have always contributed to the carbon cycle through minor emissions to the atmosphere (through respiration, fires, etc) but in the19th century with the widespread use of combustion to generate heat and power and the onset of industrial times, the influence of human activity became significantly more profound. The carbon that had been naturally stored in fossil fuels for millions of years was now used for combustion to generate electricity leading to a significant increase in the volume of CO2 emitted to the atmosphere.
DeforestationCourtesy of the EPA
DeforestationCourtesy of the EPA
In addition, land use change led to the removal of forests and other green areas, eliminating these natural sinks and exacerbating the problems caused by increased emissions.
The large scale combustion of fossil fuels with no corresponding sink to balance these emissions has continued and worsened to this day, leading to global atmospheric CO2 concentrations of 407 ppm in May 2015 at Mauna Loa: that is a 40% increase on pre-industrial times.
This human influence has led to a dramatic shift in the natural carbon cycle, whereby emissions to the atmosphere cannot be effectively balanced through the existing sinks. This is primarily due to ongoing deforestation and other land use issues.
Around 10 Gigatonnes of carbon, as carbon dioxide, are added to the atmosphere each year. Action on climate change is required immediately to both reduce emissions from fossil fuels and to increase natural sinks such as forests to redress the balance and stabilise the carbon cycle.
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