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Changes in the carbon cycle over time

Changes in the carbon cycle over time, to include natural variation (including wild fires, volcanic activity) and human impact (including hydrocarbon fuel extraction and burning, farming practices, deforestation, land use changes).

The Carbon cycle and the location of carbon has changed over vast periods of time.  For example, with regards to  concentrations of CO2 in the atmosphere;
• They were as high as 4,000 parts per million (ppm) during the Cambrian period about 500 million years ago and;
• As low as 180 ppm during the Quaternary glaciation of the last two million years.
Current concentrations are over 400 ppm and on the 15th of December 2018 the concentration was 409 ppm.2 This is despite the fact that for the 400,000 years prior to the industrial revolution CO2 never went above 300ppm. 

Carbon can change locations from other stores too, as climate changes the amount of vegetation can change which either locks up carbon in the biosphere or releases it.  Some carbon can be locked up over long periods of time during Geological sequestration as hydrocarbons and sedimentary rocks are formed on the sea bed only for that carbon to be released later by weathering after tectonic uplift of those rocks.

The exam board would like you to consider change in the carbon cycle because of 2 major broad groups of factors;
Variation in natural factors that influence the stores of carbon including wild fires and volcanic activity and
Human factors (such as hydrocarbon fuel extraction and burning, farming practices, deforestation and land use changes) that are increasingly changing the balance within the Carbon Budget.

A summary of the size of these changes can be seen in the table below.  Many of these are human induced or Anthropogenic changes resulting from human activity since the industrial revolution.

Fluxes in Carbon Budget

n.b. not all fluxes considered, figures are to illustrate changes.  Source – IPCC3

Natural Variation

Wild fires
Wild fires link to combustion from the previous section.  Wildfires play a significant natural role within the carbon cycle and our natural systems.  Wild fires tend to occur when large amounts of leaf and plant litter builds up within an ecosystem (such as a grassland or forest) over time.  If this matter is dry enough then it can be ignited by lightning strikes and create a fire.  In this context wild fires are natural, but people can set fires deliberately or by accident too.  These wildfires release carbon into the atmosphere and clear a lot of vegetation.  However, over time plants will grow back and the ecosystem will re-establish itself as a secondary succession.   This new ecosystem takes carbon out of the atmosphere over time and returns it to plant and animal organic matter and into the soils.

It has been noted that wildfires in vary ecosystems across the world are increasing in intensity, duration and frequency.  The wild fire season in many places is also increasing.  Recent major events include;
2015 - Unusually large wildfires ravaged Alaska and Indonesia
2016 - Canada, California and Spain were devastated by uncontrolled flames.
2017 - massive fires devastated regions of Chile and in Portugal a blaze claimed dozens of lives. 4
2018 -Huge and devastating wildfires hit California 5
Rim Fire California
The Rim Fire in the Stanislaus National Forest near in California began on Aug. 17, 2013.  7

With reference to change over time, over the past few decades;
1. The number of wildfires has indeed increased, especially in the western United States. According to the Union of Concerned Scientists (UCS), every state in the western US has experienced an increase in the average annual number of large wildfires over past decades.
2. Some studies have found that large forest fires in the western US have been occurring nearly five times more often since the 1970s and 80s. Such fires are burning more than six times the land area as before, and lasting almost five times longer. 4
3. Forest fires are also occurring in areas that used to rarely get them, like Siberia

The reasons for the increase in wildfires are linked to global warming;
• Warmer local and global temperatures increase evaporation, which means the atmosphere draws more moisture from soils, making the land drier.
• A warmer climate also leads to earlier snowmelt, which causes soils to be drier for longer. And dry soils become more susceptible to fire.
This means that for the Carbon Budget:
Wildfires emitted about 8 billion tons of CO2 per year for the past 20 years. In 2017, total global CO2 emissions reached 32.5 billion tons, according to the International Energy Agency.6

Wild fires cycle

Not all of wildfire emissions as net emissions, though, because some of the CO2 is offset by renewed forest growth in the burned areas. As a result, they estimate that wildfires make up 5 to 10 percent of annual global CO2 emissions each year. 6

Volcanic activity
Carbon can be transferred from the lithosphere to the atmosphere through volcanoes. The Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.

When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. According to Earth Observatory “At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.” 8  This means that the amount of carbon emitted by volcanoes is minimal, and under normal natural conditions would follow the slow carbon cycle and slowly be sequestered back into rocks at the bottom of oceans. 

In addition, volcanoes can ironically cause COOLING of temperatures despite emitting carbon.  This is because during volcanic eruptions Sulphur Dioxide and huge quantities of dust are also emitted.  The Sulphur Dioxide combines with water in the atmosphere to create sulphuric acid.  This acid, together with the dust, act to reflect radiation from the sun back to space and actually cool the Earth.  Following the eruption of Mount Pinatubo in 1991, temperatures dropped by 0.6°C. 9

Mount Pinatubo Impact on climate

Impact of Mount Pinatubo on global climate. Source NASA 9

Human Impacts on the Carbon Cycle

Agriculture or farming can have a significant impact upon the carbon budget.  Farming is essential for life on earth as it provides people with the food they need, but with increasing numbers of people on the planet and modern agricultural methods the impact upon the environment can be large.  Farming impacts upon the carbon budget in the following ways;
1. People have cleared vast areas of natural biomes and replaced them with crops and pasture.  Although farmed plants do take in carbon seasonally, it tends to be far less that the storage in a natural biome. In Tropical forests the process of slash and burn releases huge amounts of stored carbon, and the replacement farming of soya and pasture for cattle takes in small amounts.  Palm oil plantations take in more carbon than soya, but not as much as tropical forests.  In the UK, the same can be said of our clearing of vast areas of deciduous forest to make way for our farms.
2. Peat lands and wetlands have been drained and used for farmland.  In doing so, methane is released as organic matter that was previously stored and in anaerobic conditions can now decay.
3. There has been a huge increase in stock densities of animals such as cattle and chickens as global demand for animal products increases.  These animals produce huge amounts of methane during their digestive processes and this is released into the atmosphere.  Up to 60% of all agricultural emissions of carbon come from this pastoral farming.  Researchers at the University of Oxford found that cutting meat and dairy products from your diet could reduce an individual's carbon footprint from food by up to 73 per cent. 10
4. Rice paddies produce methane.  This potent greenhouse gas is emitted from flooded rice fields as bacteria in the waterlogged soil produce it in large quantities. Nitrous oxide, commonly known as laughing gas, is also produced by soil microbes in rice fields. This can also have a harmful climate effect.
5. Ploughing of fields releases carbon.

According to the United Nations Food and Agriculture Organisation;
• Carbon emissions from agriculture have increased from  4.6 to 5.0 Gt CO2 eq. yr-1 in 1990s and 2000s to 5.3 Gt CO2 eq. yr-1 in 2011)11
• Agriculture, forestry and other land uses emitted 10Gt CO2 eq. yr-1  in 2010 but only stored 2Gt CO2 eq. yr-1  12

Hydrocarbon fuel extraction and burning
Hydrocarbons are a compound of hydrogen and carbon, for example any of those which are the chief components of petroleum and natural gas. Fossil fuels are types of hydrocarbon and are created from the compressed remains of organic matter (plant or animal) at the bottom of oceans over millions of years.  They effectively lock up carbon within the lithosphere over long periods of time as carbon enters the oceans from the atmosphere and enters into phytoplankton or animal life.  Those living things die and sink to the oceans where they are compressed into oil and gas. 

Humans have been extracting hydrocarbons to produce energy and heat for centuries.  The rate of extraction and burning has increased over time shifting carbon stores from the lithosphere to the atmosphere and this poses significant problems for the delicate climate balance of our planet.  This is because carbon dioxide is released in the burning of hydrocarbons which contributes to climate change and global warming.  The extra carbon dioxide in the air acts as a greenhouse gas and effectively traps heat within the Earth’s atmosphere. Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years. 13

Global carbon (C) emissions from fossil fuel use were 9.795 gigatonnes (Gt) in 2014 (or 35.9 GtCO2 of carbon dioxide). 14 Fossil fuel emissions (including cement production) accounted for about 91% of total CO2 emissions from human sources in 2014. This portion of emissions originates from coal (42%), oil (33%), gas (19%), cement (6%) and gas flaring (1%). 14  Countries such as China and the USA dominate CO2 emissions from these sources.

Cement manufacture
Cement is a vital material used to bind building materials together. China alone makes and uses 45 percent of worldwide output. In places like Ukraine, production is doubling every four years. 15
However, the manufacture of cement creates greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide, and also through the use of the energy needed to make cement, particularly from the combustion of fossil fuels.  Cement plants account for 5 percent of global emissions of carbon dioxide, the main cause of global warming.

Land use changes
Changes in land use are responsible for about 9% of all global CO2 emissions. 14
Deforestation is the widespread removal of forest cover for other land uses.  This happens globally and much attention is currently on tropical forests, but deciduous and boreal forests also store a lot of carbon and are used as a wood stock.  In tropical regions there is particular focus because of the rates of forest clearance and the abundance of carbon stored per hectare.  For example, the Amazon helps a Newly Emerging Economy(NEE), Brazil, to make money. They build roads into the forest, logging firms then go in and take out valuable hard woods such as mahogany and cedar, worth thousands of pounds in richer economies like Europe. Then farmers, often cattle ranchers from big companies, burn the rest to make way for cattle pasture. 75% of cleared areas are used in this way.
Similarly, small scale farmers can use a process known as slash and burn to remove an area of forest to create a small holding to be farmed by themselves and their family. 

Imapct deforestation on carbon cycle

Deforestation impacts the carbon budget because;
1. The biomass store of trees and plants is removed and replaced by something inferior in terms of carbon storage
2. Waste from the deforestation process such as any woods not valuable enough to sell, small plants etc. are burned, this releases Carbon directly into the atmosphere as part of the combustion process.
3. Clearing forests speeds up the decay of leaf litter on the forest floor, releasing even more carbon to the atmosphere
4. Soil carbon is exposed to the atmosphere, speeding up soil erosion, removal of carbon into the hydrosphere by rainwater and rivers and even the release of stored soil carbon through decomposition into the atmosphere.
Recall from the water cycle that this deforestation process also damages the forest water cycle and can result in these environments being significantly damaged.  Some dry out so much that they are more at risk of wild fires which alters the carbon budget further.

According to “Around 13 Million hectares of forests lost per annum between 2000 and 2010 and as well as the ecological impact, rainforest deforestation jeopardises people’s livelihoods.”  Deforestation accounts for a staggering 15% of Global Greenhouse Gas (GHG) emissions plus there is the loss of the carbon sink in the biosphere.  15

In 1990 forests made up 31.6 percent of the word's land areas, or some 4,128 million hectares, this has changed to 30.6 percent in 2015, or some 3,999 million hectares. 17 The rate of forest destruction is slowing down however.

Urban growth
We are increasingly interconnected—no city can wall itself off from the consequences of climate change, and no city can prevent catastrophic climate change on its own.
—KEN LIVINGSTONE Former Mayor of London (2007)

More and more people are living in the World’s urban areas.  Half of the world’s population lives in cities, a share that is likely to reach 70 percent in 2050. 18  Cities are major contributors to greenhouse gas emissions. This is because;
1. Cities consume as much as 80 percent of energy production worldwide, due to the many industrial activities that take place within them.
2. As development proceeds, greenhouse gas emissions are driven less by industrial activities and more by the energy services required for lighting, heating, and cooling.
3. A lot of the cement produced creates greenhouse gasses
It is not surprising that rich cities use more energy than poor cities and therefore emit more greenhouse gas emissions.

• The International Energy Agency (IEA) estimates that cities in 2006 emitted 19.8 gigatonnes of CO2e (GtCO2e) from energy use, which was 71 percent of global energy-related GHG emissions.
• By 2030, this number is expected to increase to 30.8 GtCO2e, or 76 percent of global energy-related emissions.
• The 50 largest cities, with more than 500 million urban citizens, generate about 2.6 GtC of CO2 equivalent GHG emissions, more than all countries apart from the United States and China.
The table below signifies the importance of cities to carbon emissions compared to countries.  Note, C40 cities belong to a network of the world’s megacities committed to addressing climate change and Top 10 GHG cities means the top 10 greenhouse gas emitting cities in the world combined.

Top 10 Cities, Countries, C40 Cities (Combined), and the 50 Largest Cities (Combined), in Terms of Population Size, GHG Emissions, and GDP

Cities and carbon emissions

NEXT TOPIC - Carbon budget & Impacts of Carbon Cycle


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7 – Image from,_2013-0004.jpg
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Climate, Energy and Tenure Division, FAO Accessed the 15th December 2018 retrieved from

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14 – (2018), Annual Global Carbon Emissions.  Accessed the 16th December 2018 retrieved from
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16 – Carbon footprint ltd (2018) Deforestation. Accessed the 16th December 2018 retrieved from

17 – FAO (2015).  World deforestation slows down as more forests are better managed
18 -  World Bank (2015) Cities and climate change: an urgent agenda
19 -   Axel Baeumler, Ede Ijjasz-Vasquez, Shomik Mehndiratta, Editors (2012). Sustainable Low-Carbon City Development in China. World Bank group. Retrieved 9th December 2018 from

Written by Rob Gamesby



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