When it comes to climate change, scientific evidence provides critical information for decision making. Because governments need to understand the consequences of choosing one strategy over another they also need to understand what will happen if targets are missed or cannot be agreed by all countries. Failures could have far-reaching consequences and so the Met Office has conducted a series of ‘what if?’ climate projections, to give a better understanding of what those consequences might be.

Latest climate projections from the Met Office Hadley Centre show the possible range of temperature rises, depending on what action is taken to reduce Greenhouse gas emissions. Even with large and early cuts in emissions, the indications are that temperatures are likely to rise to around 2 °C above pre-industrial levels by the end of the century. If action is delayed or not quick enough, there is a large risk of much bigger increases in temperature, with some severe impacts.

In a worst-case scenario, where no action is taken to check the rise in Greenhouse gas emissions, temperatures would most likely rise by more than 5 °C by the end of the century. This would lead to significant risks of severe and irreversible impacts.

In the most optimistic scenario, action to reduce emissions would need to start in 2010 and reach a rapid and sustained rate of decline of 3 per cent every year. Even then there would still only be a 50-50 chance of keeping temperature rises below around 2°C. This contrasts sharply with current trends, where the world’s overall emissions are currently increasing at 1 per cent every year

Whatever path is chosen it’s clear that some increase in temperature is inevitable in the next century. This is because carbon dioxide already in the atmosphere will be present for many years and the climate takes time to respond to these changes.

Only an early and rapid decline in emissions gets anywhere near to the 50 per cent reduction in emissions needed by 2050 to avoid large increases in temperature as recommended in the latest IPCC report.

Two further scenarios have been considered, which fall into the middle-ground of temperature projections. Delaying until 2030 together with a slow reduction of emissions results in a 4ºC rise by 2100 compared with almost 3ºC linked to an early but slow reduction in emissions. All of these temperature projections are the ‘most likely’ rises.

If the climate turns out to be particularly sensitive to increases in Greenhouse gases and the Earth’s biological systems cannot absorb very much carbon then temperature rises could be even higher.

Today, plants, soils and oceans absorb about half of the carbon dioxide emitted by man’s activities, limiting rises in atmospheric carbon dioxide, slowing global warming. But as temperatures increase the rate of absorption is very likely to decrease, a process called the ‘carbon cycle effect’.

At higher temperatures plant matter in the soil breaks down more quickly releasing carbon more quickly and amplifying any warming. In addition methane and carbon dioxide released from the thawing of permafrost will add to the warming.

Hence the risks of dangerous climate change will not increase slowly as Greenhouse gases increase. Instead, the risks will multiply if we do not reduce emissions fast enough.

The implications of climate change extend well beyond global temperature changes.

For example many Greenhouse gases and aerosols linked to climate change are also linked to air quality and human health. For example, ozone contributes to global warming and is also a powerful respiratory irritant in levels frequently observed in urban areas.

By the 2090s close to one-fifth of the world's population will be exposed to ozone levels well above the World Health Organization recommended safe-health level. Met Office Hadley Centre models project year-round increases in ozone will be large, representing more than a threefold increase in the percentage of population affected. In some regions, particularly industrialised developing countries such as India and China, increases will be considerably larger.

New scientific evidence shows much stronger interactions between the carbon cycle, low-level ozone and atmospheric aerosols than previously thought, strengthening evidence for linking action to curb different types of pollution.

This complex cycle means that increased carbon dioxide reduces the removal of ozone by plants during the growing season, resulting in higher atmospheric ozone concentrations. High levels of ozone poison plants and reduce the rate of photosynthesis which, in turn, reduces the absorption of carbon dioxide by plants, leading to increased global warming."

Actions to improve air quality and reduce climate change therefore need to be linked. In the UK for example, many people suffer from air quality problems as well as from heat stress. In many cases the two go hand in hand. It is an additional problem that people have not really taken into consideration that now needs to be taken into account.

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