1B ATMOSPHERIC CYCLES and ANCIENT ATMOSPHERES

Doc Brown's Chemistry - Earth Science & Geology Revision Notes for KS4 Science, GCSE, IGCSE & O Level Courses

INDEX OF EARTH SCIENCE PAGES

All my GCSE/IGCSE level chemistry revision notes

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) + 6CO_2(g) ==> C₆H₁₂O_6(aq) + 6O_2(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)

C₆H₁₂O_6(aq) + 6O_2(g) ==> 6H₂O_(l) + 6CO_2(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

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• INDEX OF EARTH SCIENCE PAGES

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above

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