Environmental impacts of climate change: Carbon

By Matt Burdett, 16 March 2018

On this page, we look at the impacts of climate change on carbon cycles, including carbon stored in ice, oceans and the biosphere.

Introduction to carbon cycles

The carbon cycle is a hugely important aspect of climate change. The carbon cycle describes where carbon is stored, how carbon is added to the atmosphere through natural processes such as volcanic eruptions and decomposition of plant material, and how it is removed from the atmosphere through plant growth and other means. For climate to remain stable, the incoming and outgoing atmospheric carbon should be in balance.

The diagram below shows the ways in which carbon moves between carbon stores, and the relative size of the stores. The unit of measurement for global carbon is the petagram, shortened to Pg. One petagram is equivalent to one trillion kilograms. A store that absorbs carbon from the atmosphere is called a carbon sink. The largest carbon sink is the ocean, and forests are also an important sink. But significant stores are trapped in soil that is currently permafrost in northern Canada and Russia.

  • The global carbon cycle. The blue values are for stores of carbon, while the transfers of carbon between different areas are shown in red. All measurements are in petagrams. Source: Global Carbon Cycle Project, 2012.

There are actually two types of carbon cycle, which operate at different speeds and through different mechanisms. There is the fast carbon cycle, which takes place over months and years and generally involves the processing of carbon by living things. There is also the slow carbon cycle, which moves through the lithosphere (rock in the Earth’s crust and upper mantle). The diagram above includes both, but it doesn’t identify the key difference which is the speed at which the movements occur.

The slow carbon cycle

Carbon from the atmosphere enters the slow carbon cycle through rain, eventually moving into rock which fixes the carbon in place. This rock is eventually destroyed at tectonic plate boundaries and releases the carbon through volcanoes. This process takes between 100 and 200 million years to go through the full cycle (Riebeek and Simmon, 2011).

Although the total amount of carbon moved in the slow carbon cycle is immense overall, it is not very significant year-to-year compared to human activities. Humans add about 30 billion tons of carbon dioxide to the atmosphere per year, while volcanoes add just 130-380 million tons (Riebeek and Simmon, 2011). It is therefore clear that the slow carbon cycle cannot be a cause of the current period of warming as it is moving too slowly.

However, it is one of the causes of long term climate change. Fifty million years ago the formation of the Himalayas created a carbon sink, because carbon was absorbed into the rock and held in the newly formed mountains. This has contributed to the reduction in carbon in the atmosphere which has cooled down the Earth to present levels over the millions of years since, as shown on the graph below.

Graph of oxygen isotope ratios in the deep ocean for the past 67 million years.

  • The impact of the Himalayas on temperatures due to the carbon being removed from the atmosphere. Source: Riebeek and Simmon, 2011.

What is certain is that there is no significant short term impact of the current level of carbon in the atmosphere on the slow carbon cycle because it moves too slowly. In other words, the current period of climate change has almost no consequence on the slow carbon cycle.

The fast carbon cycle

The fast carbon cycle is much more important in the current period of climate change. The growth of plants that occurs in spring and summer absorbs carbon dioxide from the atmosphere, while the slow in growth in the autumn and winter results in carbon being released back as plant matter (such as leaves) decays. This makes a seasonal variation in the carbon cycle occur.

(Note: Because there is much more land in the northern hemisphere, there is more plant life there. This means that the global CO2 levels decrease in the northern hemisphere spring and summer between March and September.)

Maps of global net primary productivity in the land and ocean.

  • Graph and maps showing the link between carbon dioxide in the atmosphere, measured in parts per million (ppm), and the growth of vegetation, measured in kilograms of carbon per square meter per year. Source: Graph by Jentoft-Nilsen and Simmon; maps by Simmon and StöckliRiebeek, in Riebeek and Simmon, 2011.

The overall impact of this seasonal variation on average global temperatures is very small. However, humans are adding large amounts of carbon to the atmosphere by burning fossil fuels that would previously have remained locked up for millions of years as part of the slow carbon cycle. The result is that the fast carbon cycle now has more available carbon, with consequences for the amount of carbon stored in ice, oceans and the biosphere.

Carbon stored in ice

Ice is a hugely important store of carbon. Although ice itself doesn’t contain carbon (the chemical composition is H20 – no carbon there!), ice slows down the process of chemical decomposition of organic material that has been trapped in the ice.

There are two main sources of carbon stored in ice:

  • Organic matter stored in permafrost, which is frozen soil
  • Organic matter stored in the Antarctic ice sheet

While the atmosphere contains around 850 petagrams of carbon, frozen soil contains around 1400 petagrams of carbon (Schaefer, n.d.). These frozen areas do have some plant growth in the summer, which removes carbon from the atmosphere. However temperatures are too low for the plant life to decompose, so the regular fast carbon cycle is paused. If these frozen soils warm up and thaw, the organic matter will decay and some of the carbon will be returned to the atmosphere (NSIDC, n.d.). This means that ice is currently a carbon sink but this might change in the future.

The other main store of carbon in ice is the Antarctic ice sheet. Some estimates suggest that most of the 6 petagrams of carbon stored in glaciers worldwide are stored here (Hood et al., 2015). However, whereas most glaciers regularly release their carbon thanks to the melting of ice at the bottom of the glacier, the Antarctic ice sheet holds on to the carbon for much longer. The impact of melting ice in the Antarctic on carbon release is likely to increase in the future.

Carbon stored in oceans

There is a vast amount of carbon stored in ocean water. The sea contains about 38,000 petagrams of carbon (Global Carbon Cycle Project, 2012) which is about 60 times more carbon than in the atmosphere (Martinez-Boti and Marino, 2015). Most of this carbon is stored at great depths, but around 1000 petagrams is stored near the surface. This is important because this surface level carbon can be exchanged with the atmosphere, enabling the oceans to become a carbon sink.

This exchange of carbon between the atmosphere and the ocean is shown on the diagram below. The diagram shows that there have been changes in the amount of carbon exchanged since the beginning of the industrial revolution. These ‘new’ changes are shown in red. The net result is that less carbon is being taken from the atmosphere into the oceans. It is the surface ocean exchanges that are most important for climate change because roughly 30% of the carbon dioxide added to the atmosphere by humans has been absorbed by the ocean (Riebeek and Simmon, 2011).

Carbon Cycle

  • Global carbon cycle for the 1990s, showing pre-industrial ‘natural’ fluxes in black, and ‘anthropogenic’ fluxes in red. Source: IPCC, 2007.

This atmosphere-ocean carbon exchange happens in two main ways:

  • Organic exchange: plankton and other life in the oceans absorb carbon as part of the fast carbon cycle; when they die, they decay and some is returned to the water and the atmosphere but some is carried deeper and trapped for a long time
  • Inorganic exchange: water directly absorbs and releases gas from the atmosphere

The consequence of the increasing temperatures is that the oceans are getting warmer. This could create extra growth among some marine species which absorb carbon directly from water. However, most species do not do this. Instead, plankton which grow best in cool, nutrient-rich waters are likely to reproduce less, and reduce the amount of carbon taken in to the ocean (Riebeek and Simmon, 2011).

Carbon stored in the biosphere

The biosphere is a significant store of carbon, with around 560 petagrams of carbon stored in trees and other organic material. (For this discussion, organic refers to any product of living material including decaying leaves etc.) However, this is dwarfed by the amount of carbon stored in soil which is up to 1500 petagrams (Global Carbon Cycle Project, 2012). The biosphere is an important carbon sink, having absorbed around 25% of the extra carbon from human activities so far (Riebeek and Simmon, 2011).

Organic material makes up most of the fast carbon cycle. It does this through four main mechanisms which absorb carbon from, and release carbon to, the atmosphere in different amounts (Global Carbon Cycle Project, 2012):

  • Photosynthesis – 120 petagrams of carbon absorbed per year
  • Respiration – 60 petagrams of carbon released per year
  • Decomposition (including fire) – 60 petagrams released
  • Harvesting of plant material (by humans)

  • Carbon continuously moves between the atmosphere, plants and soils through photosynthesis, plant respiration, harvesting, fire and decomposition. Source: Global Carbon Cycle Project, 2012.

Because these processes take time, they temporarily remove carbon from the atmosphere. It is not necessarily a clear picture of how plant growth and the carbon cycle are affected by climate change, as shown in the table below. (List of human activities based on Riebeek and Simmon, 2011.)

Human activity

How it might help reduce the impact of carbon

Rebuttal point

Release of carbon into atmosphere from fossil fuels

More carbon means plants have more carbon available for growth. They might grow faster – anywhere from 12% to 76% – and remove more carbon from the atmosphere. This is known as ‘carbon fertilization’.

The extra plant growth will be limited by other factors including the availability of nitrogen. The long term growth will be limited, leading to more decay in the future.

Change in agricultural land use

More intensive agriculture means more food grown on less land. High and mid-latitude farmland is being abandoned and returning to forest, which absorbs more carbon.

The intensive agriculture is relatively small-scale, and leads to reduction in biomass in the areas it is practiced.

Attitudes towards woodland

Fires are often put out by humans, so build up of trees and other organic material can continue to remove carbon from the atmosphere

Global deforestation is larger than afforestation; around 12% of carbon emissions are from deforestation that is especially focused in the tropics

Climate change causes changes in growing seasons

As temperatures go up, the growing season becomes longer, leading to more plant growth.

Plants need water to grow which is not always available, leading to overall reduction in growth due to aridity and drought


Global Carbon Cycle Project, 2012. An Introduction To The Global Carbon Cycle. http://globecarboncycle.unh.edu/DownloadActivities/TeacherPrep/GlobalCarbonCycleBackground/CarbonCycleBackground.pdf via http://globecarboncycle.unh.edu/cmapTP.shtml Accessed 12 February 2018.

Hood et al., 2015. Storage and release of organic carbon from glaciers and ice sheets. In Nature Geoscience, Volume 8, pages 91–96. Doi:10.1038/ngeo2331. https://www.nature.com/articles/ngeo2331 Accessed 22 March 2018.

IPCC, 2007. The global carbon cycle for the 1990s. Via McKinley, n.d. Carbon Cycle. University of Wisconsin-Madison http://carboncycle.aos.wisc.edu/global-carbon-cycle/ Accessed 25 March 2018.

Martinez-Boti and Marino, 2015. Carbon Stored Deep In Antarctic Waters Ended The Last Ice Age. https://theconversation.com/carbon-stored-deep-in-antarctic-waters-ended-the-last-ice-age-37488

NSIDC, n.d. Climate and Frozen Ground. https://nsidc.org/cryosphere/frozenground/climate.html

Riebeek and Simmon, 2011. The Slow Carbon Cycle. NASA Earth Observatory. https://earthobservatory.nasa.gov/Features/CarbonCycle/page1.php Accessed 15 February 2018.

Schaefer, n.d. Methane and Frozen Ground. NSIDC [National Snow and Ice Data Centre]. https://nsidc.org/cryosphere/frozenground/methane.html

Impacts of climate change: Carbon: Learning activities


  1. Distinguish between the fast and slow carbon cycles. [4]
  2. Explain how the slow carbon cycle may affect climate change over the very long term. [4]
  3. Explain how the fast carbon cycle is a factor in the current period of global climate change. [4]
  4. How much carbon is stored in ice, and in what forms? [4]
  5. How much carbon is stored in the oceans? [1]
  6. Describe how carbon moves between the oceans and the atmosphere. [3]
  7. State the percentage of carbon that has been removed from the atmosphere by oceans since the start of the industrial revolution. [1]
  8. How much carbon is stored in the biosphere? [1]
  9. Outline the amounts of carbon transferred in different ways between the atmosphere and the biosphere. [3]
  10. Examine the arguments that human activities are benefitting the ability of the biosphere to absorb carbon from the atmosphere. [6]

Other tasks

Using the diagram below, make an annotated copy to explain the relationships between the carbon in the atmosphere, cryosphere (ice), hydrosphere (ocean) and biosphere.

  • The global carbon cycle. The blue values are for stores of carbon, while the transfers of carbon between different areas are shown in red. All measurements are in petagrams. Source: Global Carbon Cycle Project, 2012.

© Matthew Burdett, 2018. All rights reserved.

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