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2 Energy from plants and climate change

Plants capture carbon dioxide (CO2) during photosynthesis and store it in their tissues in a variety of carbon compounds (including both carbohydrates and oils). These are often energy stores for the plant but they can also be harvested and processed to provide energy for human use.

Table 1 Comparison of the energy content of plant products
Energy storeEnergy (megajoules per kilogram)
Plant oil37
Carbohydrates (including sugars)17
Wood (dry)16

1 megajoule is roughly the amount of energy that a one-bar electric fire would emit in 15 minutes. (Note that if wood has not been seasoned to remove excess water, less energy is obtained because some of the energy is used to heat and evaporate the water.)

Question 5

From Table 1, which of the energy stores releases the most energy and which the least energy per kilogramme?


Most: Plant oil (37 megajoules per kilogram)

Least: Wood (dry) (16 megajoules per kilogram)

At present, most of our energy comes from fossil fuels which originate from CO2 'locked up' in plants by photosynthesis millions of years ago. When fossil fuels are burnt, CO2 is released into the atmosphere adding to the levels already present. Since the industrial revolution, there has been an acceleration in the burning of fossil fuels. The levels of CO2 in the atmosphere are monitored by the Mauna Loa Observatory in Hawaii and are estimated to have risen from 280 parts per million (p.p.m.) in 1800 to 387 p.p.m. today (i.e. in 2009). The increase in CO2 in the atmosphere has been linked to global warming (the increase, of around 0.5°C, in the average temperature of the Earth since around 1920). CO2 is known as a 'greenhouse' gas and it acts with other greenhouses gases in the atmosphere (water vapour, ozone, nitrous oxide and methane) to insulate the Earth. Increases in these greenhouse gases increase this insulation and result in rises in the Earth's temperature. The predicted dire consequences include extreme weather conditions and melting of ice sheets and glaciers, resulting in rising sea levels and hence coastal flooding.

Figure 2 Simple representation of the carbon cycle. All living organisms, not just plants, have structural components that are based on carbon. The pictorial presentation of a carbon cycle illustrates how carbon moves between the reserves found in living things (plants and animals), soil (decomposing organic matter), rocks (including fossil fuels), the atmosphere and the oceans.

Question 6

In Figure 2 there is an arrow pointing from the atmosphere to the tree (living things). What carbon-containing compound do you think this arrow relates to?


The compound is gaseous carbon dioxide and it is used by the tree for photosynthesis.

In Figure 2 there are many arrows depicting movement of carbon compounds from one place to another. For instance, when plants and animals die they may get buried in soil and eventually as more material is deposited on top of them, they are buried deep enough to be considered part of the rocks beneath our feet. Thus the carbon has been transferred from the atmosphere to the rock, via animals, plants and soil. This is exactly what happened when fossil fuels such as oil, coal and gas were formed millions of years ago.

The Kyoto Protocol from the United Nations Framework Convention on Climate Change, which came into force in 2005, legally commits countries that signed the protocol to reduce emissions of four greenhouse gases, including CO2. One of the ways in which this could occur is by a shift away from fossil fuels to biofuels, although this view is contentious for reasons indicated shortly. Another is by the process known as carbon offsetting, whereby the amount of carbon released into the atmosphere by burning fossil fuels is balanced, for example by planting a certain number of trees to take up the equivalent amount of CO2 by photosynthesis - so-called carbon sequestration. This has led to the currently used terms 'carbon neutral' (where the amount of CO2 that is produced by an individual or a population is balanced by the amount of CO2 that can be absorbed through various measures taken) and 'achieving a zero carbon footprint' (where the net amount of CO2 released by a person, or by an object such a house, is zero).

Activity 1

Timing: 20 minutes

In this activity you should listen to the nine-minute audio clip in which Professor Chris Somerville, a global leader in biofuel research, discusses the potential of biofuels, and then answer the following questions.

[You may find it helpful to listen to the clip once, then read the questions before going back to listen to the clip again, pausing where appropriate to make notes.]

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  1. What crops did Prof. Somerville think would have the biggest potential for acting as biofuels?
  2. In his view what are the biggest misconceptions about biofuels?
  3. What percentage of land worldwide is currently (as of 2010) used for agriculture and what percentage for biofuel production?
  4. What percentage of their transportation fuel do Brazilians obtain from biofuel sources?
  5. If the Brazilians scaled up their biofuel production, by intensifying the land used for cattle production, what percentage of the world's transportation fuels does Prof. Somerville estimate that Brazil could produce?
  6. What is his 'take home' message about the advantages of biofuels over fossil fuels?
  7. Did Prof. Somerville feel that biofuels are the single answer to the world's diminishing fuel availability, and if not, why not?


  1. Perennial crops such as Miscanthus or switchgrass which can produce a large amount of biomass but in a sustainable way. He also has an interest in agave species that are very water efficient, can produce large amounts of sugar and can be grown on land that is too dry for agricultural crops.
  2. The biggest misconceptions are:
    • The net energy return (amount of energy required to grow and process the crop) is greater than the energy that is returned when the fuel is burned.
    • Growing some biofuels might actually increase the net amount or carbon dioxide (a greenhouse gas) being released to the environment whilst growing other biofuels might help reduce the amount.
    • Also the amount of land that is available and which might be used for growing biofuel crops could threaten food production (which Professor Somerville thinks is entirely wrong).
  3. About 12% for agricultural use and a tiny (fraction) percentage '0.00 something' % for biofuels.
  4. About 40%.
  5. About one third of the transportation fuel (33%).
  6. They are sustainable and they can have a net positive greenhouse gas effect (take in more carbon dioxide gas than is released when they are burned).
  7. No. He felt that the world could perhaps get 30% of their transportation fuel through biofuels. However, he felt that a number of other renewable energy sources needed to be used alongside biofuels.

If you are interested in how Professor Somerville became interested in plants and what he does now, you can listen to another (four-minute) audio clip.

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