Carbon moves between land, sea and air. This cycle was in a finely tuned natural equilibrium, but it’s being disturbed by humankind’s use of fossil fuels, land use change and deforestation. This diagram shows where and how much carbon is stored and the yearly changes in billions of tons of carbon (GtC).
CHILDREN OF THE REVOLUTION:
With their economies run on fossil fuels, the countries of the developed world have been responsible for 77 per cent of carbon dioxide emissions since the start of the Industrial Revolution.
carbon and carbon stores
Often described as the building block of life, carbon is present in all living things. It is the fourth most common element in the universe and is found in millions of different compounds. Commonly found in mineral form as coal, it is also in oil and methane (the main constituent of natural gas). When burnt, carbon combines with oxygen to form carbon monoxide (CO) and, more often, carbon dioxide (CO2). The world’s oceans, forests and soils absorb huge quantities of carbon, in the form of carbon dioxide, from the atmosphere.
Plants take carbon dioxide out of the atmosphere through photosynthesis. When a tree is cut down and burnt, some of the stored carbon in the tree converts back into carbon dioxide and escapes into the atmosphere.
Oceans absorb around half of the carbon dioxide in the atmosphere mostly as dissolved bicarbonate. Like plants, plankton in the sea take up carbon dioxide through photosynthesis. But as oceans warm they absorb less. The increasing concentration of carbon dioxide in the atmosphere is also increasing the acidity of the oceans and damaging marine life.
Organic material in soil takes carbon dioxide out of the atmosphere. More carbon is held in the world’s soils than in the atmosphere. But the capacity of the soil is limited and over time the amount of carbon dioxide escaping into the atmosphere will increase.
methane
Methane (CH4) is a naturally occurring greenhouse gas but levels in the atmosphere have more than doubled since the pre-industrial era. Farming contributes a huge amount: one dairy cow produces an estimated 500 litres of methane daily, mainly when burping – and there are some 10 million cattle in the UK. But methane also comes from landfill sites, burning fossil fuels, wetlands and drying peat bogs (swamp gas). It has a relatively short life span, of 11-12 years. Even so, methane is a powerful greenhouse gas, absorbing 20-25 times more infrared energy than carbon dioxide, and it is responsible for 24 per cent of global warming.
AN ILL WIND:
Domestic animals produce about a quarter of the world’s methane emissions – around 100 million tonnes annually. That’s more in carbon dioxide equivalent than emissions from transportation.
nitrous oxide
Like other greenhouse gases levels of nitrous oxide (N2O) have increased as a result of human activity. Chemical fertilisers and the burning of wood and fossil fuels are contributors. Nitrous oxide stays in the atmosphere for around 150 years and is responsible for about a tenth of global warming.
halocarbons
CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) – a group of greenhouse gases collectively known as halocarbons – have been building up in the atmosphere as a result of industrial processes. For many years CFCs were used in aerosol sprays and fridges. But scientists realised that these gases were damaging the Earth’s ozone layer – which blocks out harmful ultraviolet radiation from the sun. Public opinion and environmental campaigners helped persuade politicians to phase out CFCs. Although some of the gases used to replace them (HFCs and HCFCs) are also greenhouse gases, they are responsible for less than 3 per cent of global warming.
Carbon dioxide is the main greenhouse gas
Each greenhouse gas has a different capacity to cause global warming. This, combined with the actual amounts in the atmosphere, gives the ‘global warming potential’ – shown in this case for the next 100 years. Carbon dioxide is the main culprit, followed by methane. Source: Met Office Hadley Centre
the montreal protocol
The Montreal Protocol on Substances that Deplete the Ozone Layer has been described as the most effective international agreement designed to tackle environmental pollution, with developing countries being offered money to help phase out CFCs. Twenty years after it was signed CFC levels began to fall and experts are now predicting that the ozone layer will recover in the next 100 years.
ozone
Ozone in the upper atmosphere acts as a greenhouse gas while in the lower atmosphere it is part of summer smog. It may have an indirect effect on climate change. High concentrations of low-atmosphere ozone prevent plants taking up carbon dioxide.
water vapour
Although water vapour is a greenhouse gas – and the most abundant of them – human activity has only a small direct effect on the amount of it in the atmosphere. The precise role of water vapour and clouds in global warming is unclear, but scientists know that as the atmosphere warms it can hold more water vapour, causing more clouds to form. Clouds in turn can have an insulating effect, trapping warm air. But clouds may also cool things down by reflecting sunlight away from the Earth.
BLUE SKY SINKING:
The oceans absorb carbon dioxide from the atmosphere in a cycle that takes around 1,000 years to complete. The extra pollution thrown into the atmosphere by humans is distorting this process.
rising levels of greenhouse gases
Levels of greenhouse gases in the atmosphere are now more than a third higher than they were in pre-industrial times.
Scientists have compared recent rises in temperature and levels of greenhouse gases with historic data based on ice samples and tree rings. On a graph this data produces a ‘hockey stick’-shaped curve depicting a rapid increase in temperature from 1900 onwards. Levels of carbon dioxide are now rising faster than at any time in the past 20,000 years.
In 2007 the Intergovernmental Panel on Climate Change (IPCC) concluded that human activities are very likely to have been responsible for most of the global warming in the past 50 years. And because some greenhouse gases stay in the atmosphere for many years, current emissions will go on warming the Earth for centuries.
Rising levels of greenhouse gases – ‘the hockey stick’ curves
keeling curve
In 1958 US scientist