Global Warming

Global warming is the increase of average world temperatures as a result of what is known as the greenhouse effect. 


thermometerGlobal warming is the increase of average world temperatures as a result of what is known as the greenhouse effect. Certain gases in the atmosphere act like glass in a greenhouse, allowing sunlight through to heat the earth's surface but trapping the heat as it radiates back into space. As the greenhouse gases build up in the atmosphere the Earth gets hotter.  This process is leading to a rapid change in climate, also known as climate change.


One of the main greenhouse gases is carbon dioxide (CO2).

As trees grow they take in CO2 from the air. When the wood dies the CO2 is returned to the air. Forest clearance and wood burning (such as happens in tropical rain forests) is increasing the latter half of the process, adding to the CO2 in the atmosphere. Deforestation is now out of control. For example in 1987 an area of the Amazon rain forest the size of Britain was burned, adding 500 million tonnes of CO2 to the atmosphere. According to 2018 satellite data compiled by a deforestation monitoring program called Prodes, deforestation has hit its highest rate in a decade. About 7,900 km2 (3,050 sq miles) of the rainforest was destroyed between August 2017 and July 2018. The loss of the forests also means that there are fewer trees to absorb CO2.

However, as large a contribution as deforestation makes, studies in 2018 - 2019 suggest deforestation only causes around 8 to 10% of global CO2 emissions, the rest comes from the burning of coal, oil and other fossil fuels. These fossil fuels are burned in cars, power stations and factories of the wealthier nations such as the USA, Western Europe and the USSR.  The concentration of CO2 has increased 40% since the industrial revolution, half of this rise has been in the last 30 years. It is expected to double within decades. 

Televisions, lights and computers use electricity that is created mainly from burning oil and coal. This is why saving energy by doing simple things like turning off the lights helps to reduce pollution. Cars are also major sources of CO2.

It is also accepted that those of us living in the Western world, or more developed countries contribute far more to this problem than those in developing countries.  The average European is responsible for nearly 2.5 times as much atmospheric carbon as a Latin American.

For more information see Pollution factsheets and to find out more about fossil fuels take a look at Energy.  Related factsheets include Acid Rain, Meat free Mondays, Our Battered Biosphere.

Other Greenhouse Gases

CO2 contributes about 50% to the greenhouse effect. The other greenhouse gases are methane, chlorofluorocarbons (CFCs) and nitrous oxide (N2O).

Photograph by ericabreetoeMethane - is released during coal-mining activities, oil exploration and when vegetation is burnt during land clearance. The main source of methane though is agricultural activity. It is released from wetlands such as rice paddies and from animals, particularly cud-chewing species like cows. The problem with methane is that as the world population increases, agricultural activity must increase and so emissions of methane will also increase. Since the 1960s the amount of methane in the air has increased by 1% per year - twice as fast as the build up of CO2 .

Nitrous oxide - comes from both natural and man-made processes. Human influenced sources, which represent about 45% of output to the atmosphere, are mainly: fossil fuel combustion, as in power stations; use of nitrogenous fertilisers; burning rainforests and human and animal waste. N2O contributes about 6% to the greenhouse effect at the moment.

CFCs - once found in fridges, air conditioners, aerosols etc. are extremely effective greenhouse gases. Although there are lower concentrations of CFCs in the atmosphere than CO2 they trap more heat. A CFC molecule is 10,000 times more effective in trapping heat than a CO2 molecule, methane is about 30 times more effective. Methane molecules survive for 10 years in the atmosphere and CFCs for 110 years.

Due to their effect on the ozone, and their role in the ozone layer hole CFCs were widely banned and their use discontinued.  In 2013 a report that was issued in the journal Nature Geoscience suggested that this halt in CFC production and emissions actually contributed to the pause in rising global temperatures.

For more information see Pollution and to find out more about fossil fuels take a look at Energy.  Related factsheets include Acid Rain, Meat free Mondays, Our Battered Biosphere.

Feedback Process

As the world warms it causes feedback processes. Increases in temperature cause the liberation of CO2 and methane which then cause further warming. Another feedback mechanism arises through higher air temperatures evaporating more water and so providing more cloud which both traps heat from below and reflects back sunlight from above. As the world warms, the effect of clouds could become more and more significant.

Another feedback process is that of the melting ice - with higher temperatures more ice around the globe is melting. Ice contributes to the cooling of the planet by reflecting heat back into space.  With less ice around more heat is absorbed and trapped in the earth's atmosphere.

CO2 - about half the CO2 released by burning fossil fuels is absorbed by the oceans. It is taken up by tiny sea creatures or dragged to the ocean depths by the circulation of water. Research suggests that as the earth heats up, the oceans will be less efficient in absorbing CO2 , leaving more in the atmosphere and so adding further to global warming.

Methane - as global temperatures become greater, so large quantities of methane stored in the frozen tundra of the north may be released. Also methane trapped in the sea bed may be freed by temperature rises.


If no action is taken the greenhouse effect could lead to a rise in average global temperatures of between 0.3-0.7 degrees Celcius as early as the year 2035. These rises will be greater towards the poles and less at the tropics. There will also be more warming in winter than summer. In another 100 years such continued increases will make the world hotter than it has been for more than 100,000 years. The rise will also be faster than ever before; a rise of 3 degrees Celcius after the last ice age took thousands of years. The effects are already showing - the ten hottest years globally have all occurred since 1998.

Storms - Storms, tornadoes and hurricanes will become more frequent and stronger as oceans heat up causing more water to evaporate. Evidence is building up at an alarming rate. Tornadoes have been seen on all continents on earth except Antarctica but the United States has the most tornadoes of any country due to its size, location and geography.  In 2011, in just one week a record-breaking 362 tornadoes devastated southern states of the USA killing up to 350 people.  

Droughts - As temperatures rise, some areas will become dryer and water sources will evaporate or be used up sooner than they are replenished.  With such little rainfall rivers, streams and reservoirs runn dangerously low, yet continues to be used up in our homes and for farming, building and industry.

flickr_christian_revival_networkFloods - Long-term measurements of tide gauges and recent satellite data show that global sea level is rising, with the best estimate of the rate of global-average rise over the last decade being 3.6 mm per year. Continued rises in sea level will cause increased flooding in coastal areas and river estuaries such as Bangladesh and the Nile Delta. London and many other British coastal cities will be threatened also. It is now a priority to strengthen Britain's sea defences.


What can be Done?

It is important to slow the warming as much as possible. This means using less fossil fuel, eliminating CFCs altogether, and slowing down deforestation.

This can be achieved best through energy conservation, including better use of public transport and cleaner, more efficient cars; and energy efficiency by greater use of gas which produces less CO2 than coal and oil, and through renewable energy such as solar power. We need to stop destroying rain forests (deforestation) and start replanting trees (afforestation) to soak up carbon dioxide.


Image: Global Warming by NASA Goddard Space Flight Center

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