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Carbon dioxide emission limits can be set and determined, but the relationship between emissions per year, emission level in the atmosphere and temperature rise is complex and controversial. This is what we know:-
Delicate balance, suns rays and cooling
The effect of the sun warming the earth and the earths various cooling mechanisms are normally in balance retaining the earths temperature at a value which nicely sustains life on earth as we know it. Variations in the suns effect, as described below, and other variations happen and these can force a change in world climate of the order of say 50, 500 or 5000 years.
Fuel use now increasing CO2.
The extraction and burning of coal, oil and gas for electrical power generation, industry, domestic use and travel in the last 200 years is a man made effect. Oxygen in the air is consumed and in the process combines with carbon in these fuels to release CO2. Today the CO2 content has risen to 380 ppm in the atmosphere, and the rate of rise is increasing every year.
CO2 traps heat from sun and heats up earth.
The Suns energy in the form of light reaches earth and is absorbed, warming the earth. The warmed earth radiates energy, this time as heat (infra red) back to space. Heat radiation is absorbed by cloud, CO2 and other gases and reflects some heat energy back to earth. A greenhouse tends to trap heat also so these gases such as CO2 are referred to as greenhouse gases. Greenhouse gases are CO2, methane, nitrous oxide, ozone, CFCs and water (cloud.)
As greenhouse gases build up the earth warms up and does so at an increasing rate. Due to warming , glaciers, the ice cap on Greenland, the Arctic and parts of the Antarctic ice cap are now melting. Every time the CO2 blanket is increased, the effect of this extra thickness has an effect over many (100) years ahead.
Limit on fuel that can be burnt in the future - danger levels in red below.
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The atmosphere holds 600 billion tonnes of Carbon today (held as 2200 billion tonnes of CO2). There was before industrialisation a balance between the CO2 generated by animal life/surface oceans and that absorbed by land vegetation/surface oceans slightly in favor of absorption; so that a quarter of the CO2 generated now by man made by fuel burning and deforestation (itself 10%) is absorbed, the rest remains in the atmosphere. Transfer from surface water to deep oceans takes a long time to be significant.
CO2 level of 500 ppm could cause run away temperature.
A computer (simulation) prediction is that the earths temperature will rise by between 1.1 deg C and 6.4 deg. C by year 2100 if the emissions of CO2 are not limited and reduced (ref 147). The best estimates are that we need to contain the CO2 level to within 450 ppm in the atmosphere to retain the earths temperature within a further 2 deg C rise. If the level reaches 500 ppm, warming will continue to increase and due to the earths feedback processes it will accelerate or run away (ref 2.)
To limit the CO2 level to within 450 ppm, probably a maximum to prevent catastrophe we should stabilize the emissions now at the current level and then drop to zero after 60 years.
The director of Nasa's Goddard space science labs, Dr Jim Henson, warns that we should be aiming to reduce carbon in the air from 380 ppm to between 300 - 350 ppm, more toward the pre-industrial level in order to avoid a catastrophe. To do this coal burning would need to be phased out quickly (ST news review 10 2 08.)
CO2 level, Earth's temp. rise and consequences. Deserts spread, hunger increase, water shortages, tropical forests shrink, infertile seas (ref156)
The following table shows the effect of a range of CO2 levels on world temperature and resulting range of effect on planet and life sustaining systems.
Note Latest information suggests that (contrary to the table above) the west Antarctic ice melt could happen by 2100 due in large part to the undermining by sea of ice shelfs now breaking away. The above table shows that :-
At a sustained CO2 level of 450 ppm (parts per million) the eventual temperature rise compared to pre industrial level will be between 1 and 3.75 deg C.
Between 1.5 and 3.75 deg C rise, deserts will spread, crops fail, there will be a 25% to 80% increase in people affected by hunger in 2080s.
Between 1.5 and 3 deg C rise melting of greenland ice sheet will be irreversible.
Between 2 and 4 deg C rise glaciers disappear, one billion people suffer water shortages, increase in hurricane damage to US doubles.
At a sustained level of 550 ppm the eventual temperature rise above pre industrialized levels will be between 1.5 and 4.4 deg C.
Between 3 and 6 deg C rise the west Antarctic ice sheet melts, ecosystems collapse.
At 1 to 5 deg C rise the onset of collapse of Amazon rain forest will happen. The tropical forest areas will shrink or be lost due to higher temperature and less water retention. With a rise of sea temperature the area of infertile sea will increase; fertile areas will recede further towards the poles, with more storms, droughts & forest fires and possible natural methane releases.
Between 4 and 6 deg C rise sea level threatens world cities, Shanghai, Hong Kong, Calcutta, Tokyo, New York, London, Tokyo.
At a sustained level of 650ppm the eventual temperature rise above pre industrialized levels will be between 2 and 6 deg C.
At a temp rise of 4 deg C entire regions will experience crop failures.
Sea rise of 62 metres would flood capital cities.
If the Greenland ice melts, the sea will rise by 7 metres. If the west Antarctic ice shelf melts the sea will rise by 5 metres and if the East Antarctic ice eventually melts (probably a long way off now) the sea will rise by another 56 metres. (See country maps for worst cases.)
Populations also increasing with pressure on land, food and fuel.
In spite of disease and war the world population of 1.7 billion in year 1900 reached 7 billion in year 2011. At this rate the population would be 28 billion by year 2100 and although lower figures are predicted (9 billion by 2050), one must also consider the effect of the success of medicine, elimination of disease, prolongation of life, lower child deaths. With less productive land, more desert regions, greater infertile seas, worlds population rapidly increasing, land for food production, forests, and alternative energy production will be increasingly inadequate.
China is the only major country with a realistic policy for limiting excess population.
Other greenhouse gases
More potent than CO2 are Methane, Nitrous oxide and Sulphur hexafluoride (SF6) but they exist in smaller quantities. Methane is 33 times as potent as CO2 but decays faster and accounts for one fifth of the warming effect. It could be released in quantities as a result of warming (see feedback on page of past warming). SF6 is used in electrical transmission products (ref168).
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