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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.]

Download this audio clip.Audio player: Audio 1
Skip transcript: Audio 1

Transcript: Audio 1

So in terms of say the basic person on the street that was coming into science with sort of very limited knowledge, how would you define what a biofuel is?
Chris Somerville
Well of course the term generally refers to two types of material. The most common type is woody biomass or woody materials that are generally just burned directly for heat. And the other type are gases or liquids such as biogas or ethanol, biodiesel that are made from biomaterials. So actually I think it's one of the difficulties of discussing the area, is that there's many types of biofuels and there's many different ways to produce them so there's a vigorous public discourse about biofuels but actually much of it is not very correct in some way because it - the discourse - tries to treat it as though it's a single subject but actually there's many different subjects. For example, there's many ways to produce ethanol. Some are environmentally positive. Some may be environmentally negative. And so inevitably in these discussions what happens is one side of the argument picks one scenario and the other side picks another scenario and they end up talking past each other.
That's an interesting point and one of the questions we've got down is what is the biggest misconception about biofuel production or usage?
The kinds of issues that are, I think, there's a lot of misunderstanding about is like the net energy return. Some types of biofuels, I think, don't have a positive net energy return, that is the amount of energy it takes to grow and process the material to a fuel. But certainly other types of biofuels look to us like they're very net energy positive. And so similarly the same things are true for greenhouse gas emissions. I think there are some types of so-called biofuels that could be very negative for greenhouse gas emissions. Partially through kind of indirect effects, that is, if you grow biofuels on a drained swamp in Malaysia that's gonna be very negative because as soon as you drain a swamp the greenhouse gas emissions out of that swamp will dwarf the rate at which the plants that you plant can possibly take up the CO2 for a century or more. And I think there is also a sense that there is not enough land for food production which is really strongly wrong. There is a tremendous amount of land. We use 12% of the land worldwide roughly for food production.
And what percentage do we currently use for biofuels?
Oh it's a tiny fraction. It doesn't show up. It's like point zero zero something per cent.
So I guess there is a lot of scope to increase significantly if people think that's a reasonable thing to do?
Yeah. Let me give you an example where I think the public discourse is not very clear on the matter. So in Brazil the Brazilians now get about 40% of their transportation fuels by fermenting sugars from sugar cane. And the amount of acreage required to do that is about four million hectares. Now that may seem like a lot if you're living in Great Britain but to put it in perspective there's 234 million hectares used to grow cattle, with about 1.4 cows per hectare, so way below the carrying capacity of the land. So the Brazilian government has calculated that with the small intensification of the cattle they could free up 57 million hectares for sugar cane production. So from the 4 that's currently used for fuels, there's 4 for food as well, so 4 for food, 4 for fuel. If they expanded it to 57 million hectares and used both the cellulosic material as well as the sugar, our estimate is that could produce almost a third of the fuel in the world - transportation fuel. So that's just in one country. So when we look at how much is possible worldwide, it certainly seems to us like quite a large amount. Around the world there is a billion acres that's been abandoned to agriculture and this is all land that at one time did grow crops.
And I guess that there is always an argument that some of the land that is being used for biofuel could be used for food production.
Yes and that's certainly true but of course you must be aware that in the last two generations the major problem in food production has been over production in the developed world so we pay farmers mostly not to produce. The numbers for the subsidies in Europe and North America to keep farming out of production, I believe it's more than 100 billion dollars a year. And in Africa the average crop yield is about one-third of what it does in the developed world? Because of lack of investment we live in a world economy now where I think we are going to have to use the regions of the world that do have abundant land to provide the food and fuel.
What crops in your view have the greatest potential for acting as biofuels?
We are particularly interested in perennial species that can be used to produce large amounts of total biomass in a sustainable way with net energy and greenhouse gas benefits. So the best species because of their high water use efficiency are types of grasses like miscanthus and switchgrass, so we are certainly investing in those. But we're also very interested in even more water efficient plants types of species called agaves - they're a kind of cactus like material - some agaves have higher sugar content than sugar cane. There's many species. So in Africa Agave sisalana has been grown on millions of hectares to produce sisal which is used for rope. Some of these species have ten times the water use efficiency of a plant like wheat or rice meaning that for one unit of water input they can produce ten times more dry biomass than a plant like wheat or rice and they're very, very drought hardy so we're interested in those for the semi-arid areas. There's a couple of billion acres worldwide that are too arid for agriculture but might be quite suitable for these kinds of highly water efficient, drought hardy species.
What is the biggest take home message about the potential benefits of biofuels compared to use of fossil fuels?
Oh well obviously they are sustainable and if done right they can have a net positive greenhouse gas effect, I think. We can dramatically reduce the amount of greenhouse gas emissions associated with producing fuels by doing it right. You know if you track the discoveries of new oilfields they're relatively infrequent since there's been any major find and there's every reason to believe that it will eventually run out. It's probably in the case of oil and gas in the 50-60 year time frame. Let me give you a number that's easy to understand to see what the challenge is. World energy use is currently expanding by about one billion watts, or so-called gigawatt, every 1.6 days. That's so you know I guess everybody knows what a watt is from a light bulb so think about a 100 watt light bulb well a billion watts is the size of a large nuclear power plant. One way of thinking about it is that energy use is expanding worldwide by about the output of one nuclear power plant every 1.6 days. So as an American - a profligate energy user, you know the United States uses 25% of world energy - my view is we need to get our country in order first. We've got to get a grip on our own energy production. We've got to clean it up and get energy efficient and then hopefully some of the technologies that we develop to accomplish that, they'll become available to other countries and hopefully those countries will feel the same imperative eventually to become energy efficient and put in place all the renewable systems.
So if I had to put you on the spot and give you a statement and just to see whether you would go along with it, so would you say that the future was and if not what is it?
Oh no. I don't believe in a single future. I think can make a component, you know. I believe we could get 30% of our transportation fuels from biomass so it maybe in 25 years roughly 6% of our energy use might come from. Long run we will probably see some improvements in solar technologies maybe solar thermal maybe some improvements in photovoltaic manufacturing and hopefully some big improvements in photoelectric chemistry. But they are pretty far away. I hope we will see more geothermal, wind of course is ready and I think we need to see a lot of wind. One of the challenges will be to also develop better storage so I hope we will see some battery improvements that would allow us to you know store energy better.
So I'm picking up that biofuels is part of the answer and the answer itself is quite complicated and it's a whole combination of different things working together?
Yes it's a combination of things. We need to use every - every renewable technology we possibly can to make change. Many students ask me how they can work on bioenergy. And while it's certainly possible there's many ways to work on plants in that context, I actually think it's important to remember that ultimately the land that we need to produce biofuels, you know, there's a finite supply and everything we can possibly do to improve agriculture will contribute ultimately to as well as the larger context of human well being. We need many other things besides energy. We need materials and fibres and fuels and feed and so I would say almost anything that anybody can do to improve plant productivity in a sustainable paradigm will contribute ultimately to the - the energy climate matter.
End transcript: Audio 1
Audio 1
Interactive feature not available in single page view (see it in standard view).
  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.

Download this audio clip.Audio player: Audio 2
Skip transcript: Audio 2

Transcript: Audio 2

Can you recall what your first memories of plants were and why you became interested in plants?
Chris Somerville
Yes well I grew up in a farming community in Northern Canada where my father is a veterinarian so I spent a lot of time on farms and I worked on farms during the school holidays and then when I went to university I worked in the forest service for five years, actually while I was an undergraduate, fighting fires.
So what did you do when you did your first degree - what subject was that in?
Well I started in physics because I was curious about the basis of many physical phenomena but I switched to mathematics in my first year because I was intrigued by the power of mathematics to provide formulae that provide concise representations of principles. And then after my maths degree I was recruited by a professor in genetics to work on a research project involving human population dynamics.
Was that the point at which you first started working on plants then?
No after getting several courses in chemistry and biology I became intrigued by bacterial molecular genetics so I did my PhD in the molecular genetics of bacteria. In the course of that I then married a plant breeder who had really introduced me to the issues in plants and she was very concerned and conveyed that to me that the world was unsustainable. So we sort of formulated the idea that we should collaborate and try and bring modern molecular methods to agriculture.
That's excellent. One of the first areas you worked on was photosynthesis and that plays a key role in terms of biofuels and I think it would be helpful if you could just say what your current job is and what does that actually entail.
Yes, well most of my career I've been interested in how plants build their bodies and the bodies of plants are made mostly of polysaccharides and lipids and lignins - the three main components and my group works on identifying the enzymes that make these major components and understanding how they're regulated. But a few years ago I realised that my knowledge of how plants build their bodies could be useful in sort of examining the possibility of developing a large-scale biofuels industry. So about 5 years ago I sort of turned my research towards understanding the biofuels option and that really led to an engagement with a new community for me that is the community that's trying to address renewable energy issues and I moved to Berkeley a few years ago so we had people working on photovoltaic's and wind and photoelectric chemistry and biofuels and renewable energies - everything all wrapped into a big programme and so we now have I estimated about 1000 scientists altogether working on those topics here. Anyway we eventually needed money to support our research and we were able to attract several large grants, one from the energy company BP and one from the US Department of Energy that allowed us to fund several large centres here that work on these topics. And now I lead one of those centres, that's funded by BP with a focus on what we call energy biosciences, that is the application of modern biological sciences to the energy sector. So we are trying examine where the opportunities are to do that. And biofuels - cellulosic biofuels are one of those areas but we are looking for other topics as well.
So I guess in terms of having 1000 people working within the area of plant science, that's a massive group of people working on related projects.
Actually they are not all working on plant science so in my institute, which is about 300 of the 1000, while we have economists, ecologists, environmental scientists, mechanical engineers, chemical engineers, chemists, biochemists, agronomists, meteorologists. These topics are very multidimensional. Cellulosic biofuels in particular is very multidimensional because it involves the intersection of chemical engineering and chemistry on the production side you may say with the environment and in land use and the impact on the environment. So we're trying to take a whole system approach to understanding the issues.
End transcript: Audio 2
Audio 2
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