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Greenhouse Gases
The Greenhouse effect | Anthropogenic greenhouse gases | Farming & soil carbon stores | Climate change, UV radiation & nutrient cycles | Questions about climate change | Bibliography | Useful websites |

The 'Greenhouse' Effect

We hear a great deal about the 'greenhouse' effect and global warming - which could be more accurately referred to as global climate change. But it's important to distinguish between the natural greenhouse effect and potential human impacts on it (the 'anthropogenic' greenhouse effect). Remember that, in the absence of the natural greenhouse effect, global temperatures would be too low to sustain life as we know it.

The naturally-occurring greenhouse effect is due to the fact that a number of gases in the atmosphere absorb infra-red radiation (heat) emitted from the Earth's surface: instead of being radiated into space, this heat warms the atmosphere. These gases include water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N
2O), and ozone (O3) .

Methane makes up just 0.00017% of the Earth's atmosphere. However, it is an important greenhouse gas, with a much greater warming potential than CO2. Methane is generated through anaerobic decay of organic material

The amount of methane in the atmosphere is the result of a balance between production on the surface and destruction in the atmosphere. CH4 remains in the atmosphere for between 8 and 12 years. It's removed by being oxidised in the troposphere, first to carbon monoxide (CO) and finally to CO2 and hydrogen gas (H2).


Anthropogenic greenhouse gases

However, human activities generate additional 'greenhouse gases'. Globally, atmospheric concentrations of two of the main greenhouse gases, carbon dioxide (CO2) and methane (CH4) have increased over the last 150 years. Agriculture has played a significant role in this - up to 35% of anthropogenic CH4 comes from animals and their wastes (Monteny et al. 2006). For example, dairy cows produce between 84 and 123kg of CH4 per year, per animal, as a result of rumen fermentation. More methane is released from animal manure, either collected under animal housing or stored in heaps. This is because these conditions encourage the growth of methane-producing bacteria. Around 70% of the CH4 generated on pig and poultry farms comes from manure.

Attempts to reduce the amount of CH4 released by farming involve a number of approaches: changing the animals' diets, reducing the proportion of methane-generating bacteria in their guts, removing manure from animal housing, and generating biogas from animal wastes. For example, in animals kept indoors and fed food concentrates, changing the carbohydrate source from sugar to starch reduced methane emissions by nearly 15% (Monteny et al. 2006). This may have worked by altering the microbial community living in the rumen, possibly allowing a group of bacteria called acetogens to compete more strongly for H2 than the methane-producing bacteria (Joblin, 1999). In addition to reducing greenhouse gases, a cut in methane production by dairy cows may also result in increased milk output.

Farming is also a source of anthropogenic nitrous oxide (N2O).  N2O is produced during the decay of animal manure in paddocks (as part of the nitrogen cycle) and from the use of nitrate-based fertilisers (Monteny et al., 2006). Use of slow-release urea-based fertilisers, and of nitrification inhibitors, may reduce agricultural N2O production.

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Farming & soil carbon stores

It's sometimes thought that farming practices could remove substantial amounts of CO2 from the atmosphere, storing this carbon in soil as organic matter.  "Small changes in carbon stored in soil can result in large changes in global carbon cycling because a large proportion of terrestrial carbon is stored in soil" (Schipper et al., 2007).  So an increase in soil carbon could help to offset other human contributions to increasing atmospheric CO2.

But what is actually happening in New Zealand soils? We often hear of farms growing topsoil, but is this actually the case?

We already know, from earlier research projects, that intensive cropping can lead to a loss of carbon and nitrogen from soil. Conversely, conversion of forest to pasture may lead to slight increases in soil carbon and nitrogen. In New Zealand, however, pastures originally converted from forest are now subject to more intensive stocking and increasing fertiliser use. And we know very little about how this intensification of land use has altered the amounts of carbon and nitrogen stored in the soil.

Louis Schipper and his colleagues (from the University of Waikato, Landcare Research, and GNS Science) recently reported on a research project that set out to answer this question, by looking at changes in pasture soil carbon and nitrogen levels over a period of more than 20 years.

They were able to do this by taking measurements of carbon and nitrogen in a number of different pastures, and comparing their information with data recorded from the same pastures around 20 years ago. In all the sites studied, the major land use over this time period had been pastoral farming.

pastoral farming - dairying
Pastoral farming. Image courtesy of David J. Lowe.
The team found an average loss of around  -21t.ha-1 of soil organic carbon. Nitrogen losses - of about -1.8t.ha-1 on average - also occurred. These results were similar to those from studies in the UK. The authors comment that their results give a good idea of what was going on in the study areas, but not necessarily at regional or national scales - for that we need a lot more data, which they are currently collecting. Their latest unpublished findings suggest that dairy soils are behaving differently from soils used for drystock.

The reasons behind these losses aren't clear - a number of different factors could be involved. Any changes in soil carbon and nitrogen are the result of differences between inputs (from photosynthesis, nitrogen fixation, and fertilisers - including animal wastes) and exports (of crops; and through mineralisation, erosion and leaching).

The scientists concluded (Schipper et al. 2007) that "large losses of soil carbon and nitrogen cause concern because of the likelihood that they contribute to increases in atmospheric CO2 and the eutrophication of waters by reactive nitrogen. Further studies are needed "to understand what's happening in the soils under NZ's pastures."

Key messages from this study

  • Soil organic matter is the store of carbon in soil, and has a complex amorphous structure.
  • Soil organic matter plays an important role in maintaining soil quality (structure of soil; habitat for living things; nutrient store; water storage).
  • Loss of soil organic matter to CO2 will contribute to CO2 concentration in the air. Conversely, trapping CO2 as soil carbon will reduce atmospheric CO2 levels.
  • It's not completely clear what management practices and land uses contribute to changes (losses/gains) of soil organic matter.
  • Different soils can store different amounts of organic material.

Climate change, ultraviolet radiation, and biogeochemical cycling - what links these together?

Material in this section was kindly provided by Janet Bornman.

Climate change | Ultraviolet radiation | Biogeochemical cycles |
Link between climate, UV, & nutrient cycling

First, we need to know what all these terms mean. You'll find many definitions of climate change, and sometimes these don't distinguish between 'climate change' and 'global warming'. However, 'climate change'  involves more than just increasing temperatures.

The Earth's climate has always been changing, just as life on Earth constantly changes. What we are concerned about is that human actions have added to natural changes ever since the Industrial Revolution in the 18th century, and during the last few decades this contribution has escalated.

Much of this human influence on our climate has to do with economic development and increasing quality of life in many countries. This is reflected in higher  demands for energy to heat homes and fuel our cars, and increased consumption of a range of products, most of which have used fossil fuels in their production.

In other words, climate change is not new, not even human-influenced climate change. What is new is the rate at which we are now contributing to changes in climate through our actions. Two major consequences of human activity have been the increase in atmospheric CO2 and the decrease in the stratospheric ozone layer.  This declining ozone layer is linked to climate change, because many of the man-made chemicals responsible for the decrease in O3 are also gases that absorb infrared (heat) radiation and so have contributed to the greenhouse effect.  And of course, the 'ozone hole' allows more ultraviolet (UV) radiation to reach the Earth's surface.

What do we know about ultraviolet radiation? 'Ultra' is a Latin word meaning 'beyond', so ultraviolet lies beyond violet in the spectrum. It's not usually called 'light' because it's invisible to human eyes, athough some birds, reptiles and insects can see in the ultraviolet.

Ultraviolet radiation is part of the electromagnetic spectrum (the whole range of radiation that we get from the sun), and is defined as the electromagnetic wavelengths between ~ 100nm to 400nm. (Remember that nm = nanometre, 10-9m.)  This is further subdivided into UV-A (315-400nm), UV-B (280-315nm), and UV-C (100-280nm). The shorter the wavelength, the more energy it has: UV-C is a very damaging form of UV radiation and most forms of life on Earth would be killed by exposure to it. Fortunately for us no UV-C reaches the Earth's surface because it is all absorbed by ozone (O3) molecules. Most of the UV radiation that does reach the surface is UV-A, because O3 is less efficient at absorbing these longer wavelengths of radiation. A little UV-B also makes it through - UV-B makes up 2-5% of the total UV radiation arriving at the Earth's surface. However, it's the UV-B that does most damage, causing sunburn and skin cancer.

Biogeochemical cycles are the movement of compounds or elements through the physical environment and through living organisms. You could also define them as the process of biological, chemical, and physical interactions that results in the transport and recycling of energy and matter.

The 'carbon cycle' is a good example. Plants use carbon to synthesise carbohydrates and - like all other living things - they also release carbon in the form of CO2, through respiration. Of course, they also release quantities of oxygen (O2) into the atmosphere through photosynthesis. (In other words, our current oxygen-rich atmosphere was generated as a by-product of photosynthesis, and without it life as we know it would not be possible.)

When a plant is eaten the carbon it contains passes to the consumer e.g a cow grazes on grass, and as we enjoy our steak so the carbon is transferred to us. Along with other elements and compounds, carbon also passes into the soil from plants & animals and may move into rivers, streams, and the atmosphere. In other words, the cycling of nutrients involves their transport from one 'carrier' to another. With our growing interest in climate change and the impacts of carbon dioxide (CO2), methane (CH4) and nitrous oxide (NO2), we are concerned about how much of these substances is emitted into the atmosphere, how much is absorbed on land and in water, and how all this affects our climate.

We are now beginning to see the enormous complexity of different interacting processes and feedback loops on climate change. Humans, through energy-consuming activities such as driving cars, travelling in aeroplanes, and using heating or cooling systems, are burning (mostly) fossil fuels and releasing greenhouse gases into the atmosphere. Much research is now focused on renewable energy sources, but these are unlikely to meet the growing global demand for energy.

So how are climate change, ultraviolet radiation, and biogeochemical cycling linked? Let's look at a few examples. We'll concentrate mainly on UV-B radiation (280-315nm), since this is the part of the electromagnetic spectrum that's most affected by changes in the ozone layer. This stratospheric ozone layer  is found 10-40km above the Earth's surface. The concentration of O3 molecules in this layer has been decreased by man-made chemicals such as the chlorofluorocarbons (CFCs). This effect is most apparent over the Arctic and Antarctic, where it has resulted in the 'ozone hole'. The stratospheric ozone layer protects life on Earth by absorbing the damaging, high-energy UV-C radiation. However, O3 itself acts as a greenhouse gas, and  this is important for ozone in the layer of the atmosphere called the troposphere, where it forms smog and, as a pollutant, has a harmful effect on animals and plants.

Back to the stratospheric ozone layer. When this becomes thinner, more UV-B radiation reaches the Earth's surface - and this can affect the biogeochemical cycles. Just how does this work?

An increase in UV-B can promote production of carbon monoxide (CO) from dead plant matter, and release of nitrogen oxides from snow in the Arctic and Antarctic. UV-B can also help break down dead plant material, and thus contributes to the release and cycling of plant nutrients, including nitrogen. Since the efficiency of nitrogen cycling also depends on temperature, any warming of the Earth will also affect the availabiltiy of nitrogen for plants. UV (both A and B) can also make metals such as iron and copper more readily available for uptake by plants. For example, copper can form complexes (linkages) with other materials and thus is not available for plants and animals. However, UV radiation can break up these complexes, freeing up the copper (sometimes at toxic levels).

UV radiation is a key factor in the chemistry of iron and copper in aquatic systems, including their interactions with dissolved organic matter (DOM) and microorganisms..This diagram shows the UV-induced reduction-oxidation cycling of iron & copper and the production of free radicals that can adversely affect bacterio- & phytoplankton and react with pollutants such as dissolved gaseous mercury.

Image source: Fig. 4 in R.G. Zepp, D.J. Erickson III, N.D. Paul & B. Sulzberger (2007) Photochem. Photobiol. Sci. 6: 286-300. DOI: 10.1039/b700021a
Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology and the European Photochemistry Association.

Many events related to climate change, such as increased drought or snowfall, increased numbers or outbreaks of pests, thawing of snow or ice, will all change the pattern and rate of biogeochemical cycling.
At the same time, changes in UV-B radiation will have an effect on the amount of plant material available for recycling, and  the growth of aquatic organisms.  And the effects of climate change and UV radiation are linked. For example, with the thawing of snow and ice, organisms previously protected from UV-A and -B radiation  will be more exposed, and this can have a damaging effect on their productivity. With prolonged periods of drought, the amount of carbon in soils will decrease , especially in peat bogs and wetlands where large stores of carbon are found. At the same time, UV-B radiaton often increases the breakdown of dead plant material, returning more nutrients (including C and N) to the soil. A higher frequency of forest fires - due to drought - and increasing temperatures will decrease the carbon stored in the short term, but increase it through the production of charcoal through incomplete combustion, while N will be lost from forests and escape to the atmosphere.


Modelling the potential effects of enhanced UV  radiation and climate change on biogeochemical cycles in terrestrial ecosystems. Key: CO = carbon monoxide, NOx = oxides of nitrogen, CH4 = methane, CO2 = carbon dioxide, VOC = volatile organic compounds.

Image source: Fig. 1 from Richard G. Zepp, Terry V. Callaghan & David J. Erickson III (2003), Photochem. Photobiol. Sci. 2: 51-61, DOI:: 10.1039/b211154n.

Thus both land and water provide sinks and sources for gases, nutrients, and other compounds. And climate-change factors such as increasing temperatures, increasing UV-B radiation, and changes in rainfall will all contribute to changing the cycling of nutrients in so many ways that it's hard to predict the outcomes. Scientists are becoming more aware that tey have to focus their research on the many interactive processes that are occurring, rather than on just one event. This makes the science both exciting, and a challenge for future generations, and opens up many opportunities to apply knowledge from many subject areas to solve a particular problem.

Fortunately most countries have signed an international agreement, the Montreal Protocol,  to stop any further decline in the stratospheric ozone layer. The next big challenge is to slow down the warming of the Earth by reducing other greenhouse gas emissions, particularly CO2. The Kyoto Protocol was written for this purpose, but it has a tough road ahead of it. Through participating in the meetings of the Environmental Effects Assessment Panel of the United Nations Environmental Programme, as well as the Montreal Protocol, Waikato University's Janet Bornman and scientists from around the world are trying to get a better understanding of the complexity behind the interactions and impacts of climate change and a declining ozone layer.

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Questions about climate change

"What are the effects of stratospheric ozone depletion on ... processes and cycles in the environment?"

"Ozone depletion results in greater amounts of UV-B radiation that will have an impact on terrestrial and aquatic biogeochemical systems. Biogeochemical cycles are the complex interactions of physical, chemical, geological and biological processes that control the transport and transformation of substances in the natural environment and therefore the conditions that humans experience in teh Earth's system. The increased UV-B radiation impinging on terrestrial and aquatic systems, due to ozone depletion, results in changes in the trace gas exchange between the continents, oceans and the atmosphere. This results in complex alterations to atmospheric chemistry, the global elemental cycles such as the carbon cycle, and may have an impact on the survival and health of all organisms on Earth, including humans."

"Will stratospheric ozone depletion change air quality, and how does this relate to global warming?"

"Stratospheric ozone depletion normally increases the ozone concentration at ground level. In general the impact of stratospheric ozone dpeletion is smaller than that of local and regional air pollution sources. Increases in the particulates in the atmosphere related to global warming may reduce tropospheric ozone production...

"Climate change can alter air qualtiy in many ways. Changes in temperature, winds and cloudiness can all be important. Some of these changes will also alter the impact of stratospheric ozone depletion.

"As an example, an increase in atmospheric CO2 concentration would accelerate photosynthesis, which might enhance the emissions of biological volatile organic compounds from forests and other natural ecological systems. otehr sources of tropospheric air pollutants may be affected by globalwarming. It is known that local and large-scale biomass fires, such as are used for land-clearning, are fich sources of nitrogen oxides, carbon monoxide, methane, and other non-methane hydrocarbons, that can lead to enhanced tropospheric ozone production. Climate changes resulting from global warming may increase the risk of large-scale forest and brush fires and so affect concentrations of tropospheric air pollutants. The resulting particulates in teh atmosphere can scatter sunlight, thus improving the efficiency of UV-B absorption of the boundary layer ozone and contributing to global warming."

Q&As  from: Pieter J. Aucamp (coordinator), Photochem. Photobiol. Sci., 2003, 2, 1-ix-1-xxiv, DOI: 10.1039/b211466fReproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology and the European Photochemistry Association

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A.T. Austin & L. Vivanco (2006) Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation. Nature  442: 555-558

H. Dalton & R. Brand-Hardy (2003) Nitrogen: the essential public enemy. Journal of Applied Ecology 40: 771-781

S.C. Doney (2006) The dangers of ocean acidification. Scientific American 294: 58-65

M.E. Gallo, R.L. Sinsabaugh & S.E. Cabaniss (2006) The role of ultraviolet radiation in litter decomposition in ecosystems. Applied Soil Ecology 34: 82-91

J. Grace (2004) Understanding and managing the global carbon cycle. Journal of Ecology 92: 189-202

K. Joblin (1999) Ruminal acetogens and their potential to lower ruminant methane emissions. Australian Journal of Agricultural Research 50(8): 1307-1314

M.A. Liebig, J.A. Morgan, J.D. Reeder, B.H. Ellert, H.T.Gollany & G.E. Schuman (2005) Greenhouse gas contributions and mitigation potential of agricultural practices in northwestern USA and western Canada. Soil & Tillage Research 83: 25-52

G.H. Monteny, A. Bannink & D. Chadwick (2006) Greenhouse gas abatement strategies for animal husbandry. Agriculture, Ecosystems & Environment  112: 163-170

F.M.M. Morel & N.M. Price (2003) The biogeochemical cycles of trace metals in the oceans. Science 300: 944-947

V.A. Pancotto, O.E. Sala, T.M Robson, M.M. Caldwell & A.L. Scopel (2005) Direct and indirect effects of solar ultraviolet-B radiation on long-term decomposition. Global Change BIology 11: 1982-1989

L.A. Schipper, W.T. Baisden, R.L. Parfitt, C.Ross, J.J. Claydon & G. Arnold (2007) Large losses of soil C and N from soil profiles under pasture in New Zealand during the last 20 years. Global Change Biology 13: 1138-1144.

L.A. Schipper, R.L. Parfitt & C. Ross ( 2007) Are New Zealand pasture soils losing carbon? Soil Horizons issue 15: 1. Follow this link for a pdf of this article.

J.Takahashi & B.A. Young (eds) (2001) Greenhouse gases and animal agriculture. Proceedings of the 1st International Conference on Greenhouse Gases & Agriculture. Obihiro, Japan, 7-11 November 2001.

UN Environment Programme, Environmental Effects Assessment Panel (2008) Environmental effects of ozone depletion and its interactions with climate change: Progress report 2007. Photochemical & Photobiological Science 7: 15-27

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Useful websites

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