1B ATMOSPHERIC CYCLES and ANCIENT ATMOSPHERES
Doc Brown's Chemistry - Earth Science & Geology Revision Notes
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1B
The carbon cycle, global warming, oxygen balance and photosynthesis, past
ancient atmospheres, changes due to man's activities
See also Section 1A
OUR
ATMOSPHERE - composition, determining % oxygen, uses of gases in air
Section 1C
EVOLUTION
OF EARTH'S ATMOSPHERE - changes over billions of years,
origin of life
More detailed pages on the chemistry of the atmosphere
(i)
Air pollution,
incomplete combustion, carbon monoxide & soot
(ii)
Greenhouse
effect, global warming, climate change,
carbon footprint from fossil fuel burning
(iii)
Air pollution, sulfur oxides, nitrogen oxides,
acid rain
1B The
Carbon Cycle, Global warming, Oxygen Balance and Photosynthesis

1b(i)
Removal of carbon dioxide
(CO2)
Photosynthesis in
green land plants absorbing
carbon dioxide to form biomass (and oxygen), then some plant biomass is
converted to animal biomass. Some of the CO2 will dissolve in the seas/oceans => where it may be further
changed in photosynthesising marine organisms (plankton) to produce
biomass, forming soluble carbonates and insoluble minerals e.g. calcium carbonate
(sedimentary limestone rock) as the shelly remains of creatures and coral etc.,
decay (without oxygen) of any organic material from dead plant and animal
remains to form the fossil fuels coal, oil and gas over millions of years.
Photosynthesis
water + carbon
dioxide == sunlight energy ==> glucose sugar + oxygen
6H2O(l)
+ 6CO2(g) ==> C6H12O6(aq) + 6O2(g)
1b(ii)
Production of carbon
dioxide
Natural burning of biomass like forests, plant and animal
respiration**, biological decay of plant and animal material, 'mans' burning/combustion of
fossil fuel, volcanic activity e.g. the thermal decomposition of minerals like
carbonates in
magma/lava. See Oil notes for more on
fossil fuels.
Respiration
glucose sugar +
oxygen ==> water + carbon dioxide + energy (exothermic, energy given out)
C6H12O6(aq)
+ 6O2(g) ==> 6H2O(l) + 6CO2(g)
Environmental note: We know the
oceans act as a reservoir for carbon dioxide, which is quite soluble in water,
but increased amounts of carbon dioxide absorbed by the oceans has an impact on
the marine environment. Carbon dioxide is a weakly acidic gas and as more
dissolves in the seas and oceans from fossil fuel burning, they are becoming
more acidic (a small fall in the pH). However, even a small decrease in pH, can
lead to disruption of previously stable ecosystems, so in terms of the flora
(plants) and fauna (animals), 'resident' species can suffer and 'foreign'
species move in!
For more details see
Carbon cycle, nitrogen cycle, water cycle
and decomposition
gcse biology revision notes
and
Limestone and lime
- their chemistry and uses
gcse chemistry revision notes
1b(iii)
Oxygen balance:
The main
process of CO2 removal by photosynthesis also produces oxygen.
Respiration and combustion (natural/man) mainly remove the oxygen from our
atmosphere.
So this means the Carbon-Cycle effectively maintains a constant
percentage of
oxygen in the atmosphere as well as controlling the carbon dioxide levels.
Note that as far as we know the Greenhouse Effect will not significantly change the
oxygen level in the Earth's atmosphere).
There is no evidence to suggest
that the increase in the world's population (respiration!) or the burning
of forests (deforestation by combustion) is having any effect on the oxygen
level BUT increase in 'man's' industrial and domestic activity by burning fossil
fuels is causing the carbon dioxide concentration to rise.
1b(iv)
Global warming, temperature and CO2
imbalance
The average temperature of the Earth depends on the net effect of
the Sun's input and the Earth's output of energy [mainly by heat/infrared (IR) radiation].
However, the relatively rapid rate of burning massive amounts of fossil
fuels over the last few hundred years is threatening this and the CO2
balance and may be leading to significant climate change.
The CO2 in the atmosphere absorbs some of the re-radiated
IR
to keep the Earth warm and a constant CO2 concentration, also means a steady
temperature.
The increasing CO2 levels means more
IR is
absorbed and the global temperatures are rising - the Greenhouse Effect.
This
global warming is predicted (maybe happening?) to: affect sea levels by melting
glaciers, change in weather patters e.g. more drought in Africa, more rain and
storms in other parts of the world, forcing change in agriculture management
with weather/temperature changes etc. etc. but its all a bit uncertain!
For more on pollution, environmental
problems, global warming and climate change see ...
...
Greenhouse
effect explained, global warming, climate change,
carbon footprint from fossil fuel burning
1B(v)
Is the
Earth's atmosphere still changing?
1b(v) The answer is quit simply YES! and not all for the better!
The Earth's atmosphere is changing
slowly all the time, though concentrations of nitrogen, oxygen and argon seem
pretty constant.
Some
changes are happening quite naturally but many other small, but NOT
insignificant changes, are happening all the time due to man's industrial and domestic
activity. We are having an effect on the atmosphere!
One example, not due to human
activity, is the continuing eruption of volcanoes which are constantly
giving out sulfur dioxide and dust high into the atmosphere. With the help
of sunlight energy, the sulfur dioxide gas can react with water, oxygen and
dust to form a volcanic smog. Volcanoes also give out carbon dioxide.
Some examples of how we are
changing the atmosphere (with links to more detailed notes on human
activity)
1. Burning fossil fuels
is increasing the concentration of carbon dioxide potentially causing global
warming and climate change.
burning fossil fuels in road
vehicles produces poisonous carbon monoxide, oxides of nitrogen and
particulates of carbon. Sulfur dioxide from the emissions of fossil fuel
power stations is a lung irritant and cause acid rain formation, leading
to excessive building corrosion and damaging the ecology of aquatic life
e.g. poisoning fish in rivers and lakes.
Global warming and climate
change from e.g. increasing carbon dioxide levels, has all sorts of
consequences e.g. rising sea levels, increased acidity of oceans,
disrupting agricultural patterns of food production.
For more on pollution from
using fossil fuels, environmental
problems and climate change see ...
...
Detailed notes on air pollution form
using fossil fuels and global warming and climate change
2. Deforestation is
adding to the rising concentration of carbon dioxide. The slashing and
burning of prime forests in e.g. in South America and Asia is not only
producing carbon dioxide, it is removing dense concentrations of plants like
trees that are absorbing carbon dioxide via photosynthesis.
3. Livestock farming is
growth industry in parts of the world as populations increase and demand for
meat and milk is rising all the time. Unfortunately all these beef and dairy
cattle release methane gas from their digestion systems. Methane is a much
more powerful greenhouse gas than carbon dioxide!
4. The use of CFC liquids
and gases in refrigerators and aerosols etc. has had a very negative effect
on the ozone concentration in the upper atmosphere. Ozone absorbs the most
harmful high energy uv radiation that can cause skin cancer. BUT, here we
have a success story, by stricter regulation and using alternative
chemicals, the situation is being reversed.
For more on CFCs and ozone
see
Ozone depletion, CFC's and free radicals
1B(vi)
How do we know about the past
atmosphere? - prehistoric atmospheres!
1b(vi) How do we know about the Earth's
atmosphere in the past?
With a special interest in the
concentration of carbon dioxide in the Earth's atmosphere.
Accurate average temperatures and CO2
levels from data sources around the world give us a reasonably accurate
picture of the things.
BUT, historic data beyond the last
100-200 years is not as accurate - less data, less measurement
locations.
However, there are sources of data
which us some idea of how the Earth's temperature and atmosphere (e.g.CO2
level) has changed over the past few thousand years.
One of the most important
sources of scientific data come from ice cores drilled out of the Antarctic
Continent in the Southern Hemisphere.
For thousands of years, year
after year, each snowfall gets compressed into a new layer of ice.
Bubbles of gas from the
Earth's atmosphere get trapped in each ice layer and can be very
accurately analysed to deduce the composition of the atmosphere at that
time i.e that particular year!
You can literally count back
from layer to layer to get a pretty exact date for each layer as each
layer is automatically buried by the next layer.
Any tiny changes in the
atmosphere can be detected and even trace specks of mineral dust from
huge volcanic explosions can analysed.
So each layer is never
exactly identical to another, the differences might be minute, but
they can be measured and it also helps and confirms the year count.
However, you can never be
sure that the gas bubble is a complete and accurate representation of
the atmosphere at the time, because we haven't sampled prehistoric
atmospheres directly.
So the deductions of how
the Earth's atmosphere has changed over time is still a bit
dependant on guesswork and speculation, but it's progress and part
of honest science!
Tree rings can be dated back 100's of
years in the oldest trees to provide data on climate - temperature - wet or
dry periods.
BUT, non of this data can be as
accurate as what we can measure now.
However, as you go back millions of
years, evidence becomes more difficult to interpret e.g.
geological evidence
from the chemical composition of rocks, which can be dated from radioisotope
analysis,
sequences of fossils and what environment they might have
lived in,
all play their part to help us interpret the history of the Earth.
For further reading see
evolution of the Earth's VERY early atmosphere described on a separate page
More detailed pages on the chemistry of the atmosphere
(i)
Air pollution,
incomplete combustion, carbon monoxide & soot
(ii)
Greenhouse
effect, global warming, climate change,
carbon footprint
(iii)
Air pollution, sulfur oxides, nitrogen oxides,
acid rain
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