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Global warming in context
We be looking at the following
The natural greenhouse effect
The fate of solar radiation
The human effects on global warming
Carbon dioxide in the atmosphere and its effects
Some urban myths
A Orme
The Natural Greenhouse Effect
In 1862 Tyndall discovered that
longwave infrared radiation emitted
from the Earth’s surface is
repeatedly absorbed and re-emitted
by greenhouse gases naturally
present in the Earth’s atmosphere.
(CO2, CH4, N2 and O3)
John Tyndall (1820-1893)
Eventually some of this re-emitted
radiation goes back out into space.
Back radiation from the atmosphere
keeps the Earth’s surface warmer
than it naturally would be.
Greenhouse effect- the warming of the surface by back radiation from the
atmosphere
Exciting molecular vibrations
The main way these molecules absorb infrared radiation is
through the vibrations of their molecules.
Key Points
• Chemical bonds that hold molecules together can stretch and
flex causing the molecules to vibrate. Molecular vibrations have
a characteristic frequency and absorbs its corresponding
radiation (infrared). This causes the molecules to vibrate more
quickly.
• A molecule must contain more than 2 atoms or 2 atoms of
different elements. Greenhouse gases can vibrate in several ways
allowing it to absorb a range of infrared radiation.
• The excited molecule loses energy re-emitting radiation or
transfer energy as they bump into other molecules increasing
‘energy’ in the air equalling warming.
The fate of solar radiation
Some radiation is absorbed in the atmosphere
Ozone layer acts as a green house gas and absorbs long and short-wave radiation
Water vapour contributes to 60% of global warming and absorbs near-infrared
radiation
Radiative forcing as an agent to climate change
Radiative forcing- anything that disturbs the radiation balance in the upper
atmosphere has the ability to force the global climate change.
Positive forcing- inputs that increase the temperature
Changes in solar constant, albedo or longwave emission to space. One theory is the
sun’s output varies through time and humans have little effect on global warming.
Negative forcing- inputs that decrease temperature
Volcanic eruptions increase sulphur aerosols that reflect incoming radiation causing
cooling. They last 1-3yrs and are washed out as acid rain.
Natural Carbon Dioxide in the Atmosphere
Terrestrial- Carbon is stored in living
things. Plants and trees have the ability
to fix carbon. They use carbon for cellular
processes and making organic compounds.
(Gross Primary Product GPP) Little is
transferred back to the atmosphere during
respiration.(Net Primary Product
NPP)When they die their debris is
transferred to the soil where they are
broken down by decomposers. The
residence time of fixed carbon is dependant
on plant longevity and rate of
decomposition. Human activity will lower
NPP.
Oceanic- Carbon is dissolved in water and transported around the globe. Carbon can only be
dissolved in limited amounts leading to saturation and then out gassing of carbon back into the
atmosphere. Phytoplankton use dissolved carbon for photosynthesis and will store carbon the
same as plants. Phytoplankton live for up to 2 weeks. When they die they fall to the sea bed
carrying the fixed carbon with them. If the plankton are consumed by zooplankton their carbon
is too released back into the atmosphere.
Effects of Carbon Dioxide in the Atmosphere
Benefits- CO2 fertilisation effect occurs as carbon dioxide in the atmosphere
increases stimulating plant growth. Increased plant growth increases the
amount of carbon that is fixed. Scientists have noticed an increase of 20-40%
of plant growth from doubling the amount of CO2 in the atmosphere.
Recent climate change effects- temperate/boreal forests are more productive
with the increased rainfall caused by climate changed initiated by global
warming. In the last 20yrs changes in rainfall have increased growing time
and productivity in arid and semi arid area, therefore, increasing NPP.
Increase nitrogen decomposition effects- enhances plant growth. Combustion
of fuels increase nitric acid that rapidly converts into soluble nitrogen that are
washed out by rain stimulating plant growth and increasing NPP.
Land use management- in northern and mid-latitudes forests where logging
occurs there is evidence of rapid re growth. Although this year-year increase
increases the amount of organic material to the soil, decomposition in these
areas have a slow time lag before decomposition release of carbon catches up
There is no evidence of an increased uptake by oceans.
An Urban Myth
Glaciers- Glaciers are sensitive to
ablation caused by warming. Where
Greenland is experiencing large
amounts of albation, they are
counterbalanced by areas like
Antarctica that are experiencing
increased precipitation.
Large ice bodies- Large ice bodies
suffer from a lag time between
changes and effects. Response times
range between 50-1000yrs
depending on their surface area and
thickness. Melting could be due to
changes in climate during the last
glacial/interglacial transition. West
Antarctic ice sheet is continuously
fed by ice streams.