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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 (N2O), 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).
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Anthropogenic greenhouse gases
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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
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It's sometimes thought that farming practices could remove substantial amounts of CO 2
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 CO 2.
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.
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 CO 2 and the eutrophication of waters by reactive
nitrogen. Further studies are needed "to understand what's happening in
the soils under NZ's pastures."
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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.
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Climate change, ultraviolet radiation, and biogeochemical cycling - what links these together?
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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 18 th 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 CO 2 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 O 3 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 (O 3) molecules. Most of the UV radiation that does reach the surface is UV-A, because O 3
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 CO 2,
through respiration. Of course, they also release quantities of oxygen (O 2)
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 (CO 2), methane (CH 4) and nitrous oxide (NO 2),
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 O 3
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, O 3 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.
Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European
Society for Photobiology and the European Photochemistry
Association
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 CO 2. 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
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"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/b211466f. Reproduced 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 &
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potential of agricultural practices in northwestern USA and western
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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
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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.
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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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