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Chapter 2
the science of climate change
We know climate change is happening, but what’s causing it? This chapter looks at the facts behind the freaky weather, and the implications for our planet if things carry on as they are.
why climate change is happening
Vital signs Three charts that tell a story – as global temperatures and sea levels have risen snow cover in the northern hemisphere has shrunk. The red lines give a bird’s eye view of key changes over the past 150 years based on yearly measurements (the grey dots), while the narrowing blue bands shows the room for doubt declining as measurement techniques have improved. Source: IPCC.
the greenhouse effect
Most life on Earth relies on energy from the sun to provide warmth and light. Some of the sun’s energy reflects off the Earth’s surface in the form of infrared rays, and gases in the atmosphere trap and/or reflect this energy - just as glass keeps heat in a greenhouse. The greenhouse effect helps keep the Earth warm enough to support life.
But some of these gases, particularly carbon dioxide, are building up because of pollution created by human activity. The result is more trapped heat and a sharp rise in the rate of warming.
SUN TRAP:
A thin blanket of gases around the Earth prevents some of the sun’s energy from escaping back into space.
five warming signs
Higher temperatures: Scientists have established that the global average temperature has increased by 0.76°C in the past century. Records going back over 150 years show that globally 19 of the 20 hottest years have occurred since 1980.
Melting ice: Arctic and Antarctic ice is thinning and sections of ice shelves are breaking off completely. Temperatures in the Arctic are rising twice as quickly as the global average.
Coral bleaching: Scientists have found a rise of 2°C can kill coral. Reefs are home to around a quarter of known marine species.
Rising seas: We saw an increase of 17 cm during the 20th century.
Drier: Droughts have become more intense over the past 30 years, and have lasted longer, particularly in the tropics and sub-tropics.
fossil fuels and the greenhouse effect
Plants, trees and ocean plankton containing carbon absorbed many millions of years ago fossilised underground to form oil, coal and gas. When these are burnt the carbon is released, and combines with oxygen to form carbon dioxide. The first person on record to recognise the power of fossil fuels to change the climate was Svante Arrhenius, a Swedish scientist who published his ‘greenhouse law’ in 1896. Arrhenius estimated that doubling carbon dioxide levels in the atmosphere would lead to a rise in temperature of 5 °C and thought this promised a warmer climate in colder parts of the world. His maths wasn’t far off but few people today would agree with his predictions that rising temperatures are a good thing.
greenhouse gases
The greenhouse gases include carbon dioxide, methane, nitrous oxide ozone, water vapour, sulphur hexafluoride and halocarbons. They make up quite a small proportion of our atmosphere; some have a more powerful warming effect than others.
carbon dioxide
Carbon dioxide (CO2) is naturally occurring. People and animals produce it when breathing out whereas plants absorb and metabolise it, releasing oxygen in return.
The level of carbon dioxide in the atmosphere is measured in parts per million (ppm); or simply by weight (kg, tonnes).
Scientists believe carbon dioxide levels have been much higher in the Earth’s distant past. Geological evidence suggests that in the Cretaceous period (65-144 million years ago) it was three to six times higher than today.
RING LEADERS:
Trees are in the frontline of defence against climate change – the vast sub-arctic forests of the north being among the most important. The United States, for instance, would be having an even greater impact on global warming if its forests weren’t absorbing around a tenth of its carbon dioxide emissions.
The world was also 10-15 °C warmer then and there was no ice at the Poles. For much of the time that people have been on Earth, carbon Carbon moves between dioxide levels have been stable at around 270-280 ppm. But land, sea and air. This cycle since the Industrial Revolution they have been rising quickly, was in a finely tuned reaching 380 ppm in 2005. This is largely a result of our use of natural equilibrium, but coal, oil and gas: producing, distributing and burning fossil fuels it’s being disturbed by accounts for three quarters of all the emissions of carbon humankind’s use of fossil dioxide caused by humans. The rest comes from changes in fuels, land use change and the way land is used, particularly cutting down and burning deforestation. This trees. Scientists have calculated that carbon dioxide is diagram shows where and responsible for 63 per cent of global warming. And it can stay how much carbon is stored in the atmosphere for up to 200 years. Its long lifespan means and the yearly changes in that carbon dioxide released today will still be affecting the billions of tons of carbon climate for hundreds of years. (GtC).
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 Dr Charles D Keeling at the Mauna Loa Observatory in Hawaii began measuring the concentration of carbon dioxide in the atmosphere. Plotted on a graph, his results form a rising curve now known as the Keeling Curve. This shows that carbon dioxide levels vary with the seasons – by as much as 3 per cent over 12 months – because many trees in the Northern Hemisphere do not take up carbon dioxide during winter. Over and above this variation, however, we can detect a steady increase in the past 50 years.
Dr Keeling was the first scientist to report that carbon dioxide in the atmosphere was increasing, alerting the world to the threat of climate change.
dimming – hot or cold?
Whereas greenhouse gases warm the atmosphere, aerosols and solid particles reflect heat and light from the sun and prevent sunlight reaching the ground. This has a cooling effect – sometimes referred to as global dimming – and is caused by pollution from industrial processes, transport, wood-burning stoves, forest fires and volcanoes. Conversely, particles such as dust and soot can also increase warming by absorbing sunlight.
No one knows the precise impact of such pollution, but it may be masking the full impact of the greenhouse gases. It may also play a role in cloud formation, causing further complex influences on the climate. Areas downwind of pollution sources may suffer regional impacts. A recent study observed that a brown cloud of pollution over the Indian Ocean was warming the sea and could be contributing to melting of glaciers in the Himalayas.
snapshot of our future climate:
less snow and ice
more severe storms and floods, particularly along coasts
more rainfall at higher latitudes
less rainfall over land in the tropics – more drought
less predictable winds, rain and temperature
more heatwaves
the threat of extreme change
Some scientists believe that warming presents a risk of more extreme irreversible climate change as the Earth’s natural systems swing out of balance.
Thawing permafrost: Peat bogs in Siberia and Alaska are thawing, releasing carbon dioxide and methane that they have stored for thousands of years.
The albedo effect: melting snow and ice exposes rocks, trees and tundra which are less reflective so they absorb more of the sun’s energy, adding to warming and melting more snow.
Ocean warming: Methane deposits, known as clathrates, are stored in sediments under the oceans. Warming might lead to the release of huge quantities of methane gas into the atmosphere.
Scientists say there is a greater than 90 per cent chance that the great ocean conveyor – the Meridional Overturning Circulation – will slow down this century, and it could switch off in the longer term. This would imply a huge change in natural cycles.
Recent studies by NASA director and scientist James Hansen suggest that ice sheets can melt much faster than the thousands of years envisaged by the IPCC. This is because melt water under the ice sheet speeds up the breaking up and melting of the ice. Hansen also found that other greenhouse gases – and soot particles – are speeding up melting in the Arctic. This is because the dark flecks absorb more heat. As a result, he warned, an average temperature rise of just 1°C could lead to a sea level rise of between 2 and 6 metres. If Greenland’s ice sheets melt quickly this will change the temperature in the Atlantic ocean, with dramatic effects on the world’s weather systems.
If a small amount of human-induced global warming triggers massive natural warming, the result is likely to be climate change that happens over decades rather than centuries.
The great ocean conveyor belt Ocean currents redistribute heat around the planet, (below), for example bringing warm water from the tropics to the UK via the Gulf Stream, and cold water to Newfoundland, with striking effects on both countries’ climates. Source: IPCC.
uncertainty and the sceptics
The complexity of the Earth’s climate means that there has been some scientific uncertainty about global warming. Although the theory of the greenhouse effect was first put forward more than 100 years ago, clear evidence that it was happening was not readily available until the 1970s.
Today there is little room for doubt that climate change is happening and that it is caused by human activity. The IPCC – a naturally cautious body – stated in 2007, ‘Most of the observed increase in global average temperatures since the mid-20th century is very likely (>90 per cent) due to the observed increase in anthropogenic greenhouse gas concentrations.’
But earlier scientific uncertainty has proved a fruitful area for media debate, amplifying the doubts of people who have questioned climate change. Some have exploited scientific doubt to spread complacency and confusion because they have seen addressing climate change as a threat to their interests. Certain companies, for example, have tried to divert attention from the environmental impact of fossil fuels.
“Some will always make a case for doubt in an issue such as this, partly because its implications are so frightening. But what is not in doubt is that the scientific evidence of global warming caused by greenhouse gas emissions is now overwhelming.
Tony Blair, UK Prime Minister (1997-2007)
Scientists have explored alternative theories on the rising temperatures – for example, that they are down to variation in the sun’s cycles. But this does not explain the fit between rising levels of greenhouse gases and rising temperatures. In fact, computer models looking at the effect of the sun’s output on our current climate show that if natural variations were the cause, the Earth should now be cooling, not getting hotter.
The sun’s activity has in fact been decreasing since 1985. Natural phenomena simply do not explain why temperatures have risen in the past 30 years.
An assessment of more than 900 scientific studies on climate change, published over a ten-year period, found that none of the research disputed the consensus view that human activity is responsible for global warming.
fuelling the scepticism
While some scientists have pursued legitimate lines of enquiry over the science of climate change, others have deliberately exploited doubts for commercial and political gain. In the United States PR strategists advised the oil industry on how to set up groups to stir up doubts over the science and influence public opinion – in much the same way as the tobacco industry had earlier tried to persuade the public that smoking did not damage their health.
Oil has been key to the global economy for a century, and action to tackle emissions from fossil fuels has been seen to threaten the industry. So it is perhaps not surprising that politicians and many others were at first persuaded not to take global warming seriously.
the scientific consensus
The Intergovernmental Panel on Climate Change (IPCC) is an international body set up by the United Nations and the World Meteorological Organisation (WMO) to assess scientific information on climate change. It brings together climate scientists and government experts to consider research from around the world. In 2007 it shared the Nobel Peace Prize with Al Gore, the maker of An Inconvenient Truth.
