The world is warming dangerously and humans are largely to blame. In simple terms, the sun's energy warms the earth's surface and the atmosphere. Because the earth's surface is warmer than outer space, it emits infrared radiation, which cools the planet.

However, water vapour, carbon dioxide and other 'greenhouse gases' like nitrous oxide and methane absorb part of this radiation and re-emit some of it downwards. This 'greenhouse effect' warms the earth's surface and cools part of the atmosphere.

Between the mid 20th century and 2006, the global average surface temperature increased by about 0.55°C. This may seem a small amount, but it was a much faster increase than in the previous half century. Eleven of the 12 years from 1995 to 2006 rank among the 12 warmest years since 1850.1IPCC, 'Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Summary for Policymakers', 2007, pp. 5-6.

This warming partly reflects a 'natural' shift in global temperatures, perhaps comparable to the unusually warm years around 1000 and – in the opposite direction – the 'Little Ice Age' of the 1700s.

The more important factor, most climate experts agree, has been the unprecedented increase in human fossil fuel consumption as the world's population has grown and incomes risen. Changes in land use, such as a decrease in the areas under forest, have made a smaller but significant contribution.

In early 2007 the Intergovernmental Panel on Climate Change (IPCC), which seeks to reflect the consensus among climate scientists, reported that 'there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.'2IPCC, 'A Report of Working Group 1 of the Intergovernmental Panel on Climate Change: Summary for Policymakers', 2007, p. 10.

Despite this broad agreement, considerable uncertainties remain. The diagram below illustrates the cascading effects of these uncertainties. Each uncertainty builds to the next. When you get to impacts at a regional level, the uncertainty is substantial.

The scientific evidence has hardened. The number of climate scientists doubting that humans are largely to blame for climate change has steadily fallen. Few would now dispute the consensus view.

The science academies of eleven of the world's most powerful countries issued an unprecedented declaration in 2005: the scientific understanding of climate change was sufficiently clear to justify nations taking prompt action.3‘Joint science academies' statement: Global response to climate change', http://nationalacademies.org.

Supporting the science has come the need for energy security, which has helped push climate change up the international agenda. (See » Will we have enough energy?)

The world's reliance on the Middle East for oil creates high-level security dilemmas. They have come sharply into focus at the start of the new century:

To become more energy secure, consuming countries would have to increase energy efficiency, develop renewable alternatives to oil and gas, and find ways of cleaning coal so that they could tap the world's abundant supplies, often within their own borders, without causing unacceptable pollution.

These measures would also top any expert's 'action list' on how to reduce global warming. Securing energy and mitigating climate change go hand in hand.

A sea-change in world opinion is occurring, as science and energy security have come together.

In the United States, for instance, the East and West coasts have tended to be more concerned about global warming than states in the middle. Yet some leaders within the evangelical right, heavily represented in middle America, have begun to support measures to mitigate climate change.10The New York Times, 8 February 2006.

Business has been signing up to the climate change agenda. 'Clean tech' is now the fifth largest home for venture capital investment in the US, running behind biotech, software, medical and telecoms.

By September 2006, 360 of the world's top 500 companies had joined the Carbon Disclosure Project, under which they disclose their greenhouse gas emissions.11The Times, 28 October 2006. The 2007 Davos World Economic Forum announced the formation of the Climate Disclosure Standards Board, which will encourage these disclosures to take a standardised form in annual reports.12The Times, 27 January 2007.

In February 2007 an informal meeting took place of legislators from the G8 rich countries, plus Brazil, China, India, Mexico and South Africa. They agreed that developing countries would have to set targets for cutting greenhouse gas emissions, as well as rich ones.

Previously, developing countries have said that this would be unfair, since rich nations are responsible for by far the most emissions. The United States has replied that it could never sign up to targets unless developing countries did so, too – its companies would be at a disadvantage.

Though not binding on governments, might the changes of view reflected by these legislators herald a more serious willingness to limit greenhouse gas emissions?

In March 2007, the EU reached a 'ground-breaking' agreement to cut carbon emissions by 20% by 2020, and by a further 10% if the United States and China (among others) agreed to cut their emissions too. By demonstrating that it is possible to 'go green' and grow its economy, Europe hopes to spur other countries to act more urgently on climate change.

At the summit of world leaders later in the year, President Bush reversed his previous position and showed a willingness to join the Kyoto process for reaching a global agreement on climate change. He allowed the possibility of a global target (cutting emissions 50% below 1990 levels by 2050) to be included in the communique, even though targets had once been an anathema to him.

The debate has moved on from a question of evidence (are humans responsible for climate change) to a question of management (what are we going to do about it?).

Adapting to climate change is rising up the agenda. While public debate concentrates on mitigating climate change, the world is starting to adapt to the reality of global warming.

In the UK, a 2005 survey found that 60% of farmers thought that the growing season was lengthening. Farmers were grazing their livestock on natural feeds for more months, cutting winter feed requirements.13UK agriculture and climate change, London: National Farmers Union, 2005, p. 2. New crops have begun to appear, such as olives in the South West.

In Australia, ravaged by a 6-year drought, the Federal government has declared the viability of the Murray-Darling basin to be at risk. The basin accounts for more than 70% of the country's irrigated croplands and pastures.

In January 2007 the Prime Minister, John Howard, announced a $A10.5 billion (£4 billion) programme to restore the nation's dwindling rivers. Howard, a climate change sceptic, acknowledged that the southerly weather systems which bring rains to Australia were failing. Australia's climate was getting hotter.14The Times, 27 January 2007.

Although the world has begun to adapt to climate change, with some exceptions adaptation remains a low priority for policy makers: will this alter in the years ahead?