Many people ask how sure we are about the science of climate change. The most definitive examination of the scientific evidence is to be found in the work of the Intergovernmental Panel on Climate Change (IPCC) and its last major report published in 2007. I had the privilege of being chairman or co-chairman of the panel’s scientific assessments from 1988 to 2002.

Many hundreds of scientists from different countries were involved as contributors and reviewers for these reports, which are probably the most comprehensive and thorough international assessments on any scientific subject ever carried out. In June 1995, just before the G8 summit in Scotland, the academies of science of the world’s 11 largest economies (the G8 plus India, China, and Brazil) issued a statement endorsing the IPCC’s conclusions and urging world governments to take urgent action to address climate change. The world’s top scientists could not have spoken more strongly.

Unfortunately, strong vested interests have spent millions of dollars on spreading misinformation about climate change. First, they tried to deny the existence of any scientific evidence for global warming. More recently, they have largely accepted the fact of anthropogenic (man-made) climate change but argue that its impacts will not be great, that we can “wait and see,” and that in any case we can always fix the problem if it turns out to be substantial.

The scientific evidence does not support such arguments. Urgent action is needed both to adapt to the climate change that is inevitable and to reduce emissions of greenhouse gases, especially CO₂, to prevent further damage as far as possible.

At the Earth summit in Rio de Janeiro in 1992, the world’s nations signed up to the Framework Convention on Climate Change (FCCC), the objective of which is “to stabilise the concentration of greenhouse gases in the atmosphere at a level that does not cause dangerous interference with the climate system … that allows ecosystems to adapt naturally to climate change, that ensures food production is not threatened, and that enables economic development to proceed in a sustainable manner.” Such stabilisation would also eventually stop further climate change.

It is now recognised that widespread damage due, for instance, to sea level rise and more frequent and intense heat waves, floods and droughts, will occur even for small increases of global average temperature. Therefore it is necessary that very strong efforts be made to hold the average global temperature rise below 2C relative to its preindustrial level.

If we are to have a good chance of achieving that target, the concentration of CO₂ must not be allowed to exceed 450 parts per million (it is now nearly 390 ppm). This implies that before 2050 global emissions of CO₂ must be reduced to below 50% of the 1990 level (they are currently 15% above that level), and that average emissions in developed countries must be reduced by at least 80% of the 1990 level. The UK has already committed itself to a binding target to reduce emissions by that amount, and President Barack Obama has expressed intention that the United States should also set that target.

One clear requirement is that tropical deforestation, which is responsible for 20% of greenhouse gas emissions, be halted within the next decade or two. Regarding emissions from the burning of fossil fuels, the International Energy Agency (IEA) in its Energy Technology Perspectives has set out in detail the technologies and actions that are needed in different countries and sectors to meet these targets.

For the short term, the IEA points out that very strong and determined action will be necessary to ensure that global CO₂ emissions stop rising (the current increase is more than 3% per year), reach a peak by about 2015, and then decline steadily toward the 2050 target. The IEA also points out that the targets can be achieved without unacceptable economic damage. In fact, the IEA lists many benefits that will be realised if its recommendations are followed.

What is required now is recognition that anthropogenic climate change will severely affect our children, grandchildren, the world’s ecosystems, and the world’s poorer communities, and that the severity of the impact can be substantially alleviated by taking action now.

John Theodore Houghton, a former professor of atmospheric physics at Oxford University, and founder of the Hadley Centre for Climate Prediction and Research, was the co-chair of the IPCC’s scientific assessment working group and lead editor of its first three reports