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.
four climate hotspots
Scientists have identified a number of hotspots around the world that could trigger fundamental changes in the global climate, including:
Amazon rainforest: Changes in the Sahara could