Carbon dioxide (CO2) warms the planet by absorbing infrared heat energy from the Earth’s surface, temporarily trapping the heat in the atmosphere. The resulting rise in global temperature leads to various responses in the climate system, called feedbacks, which either increase or reduce an initial temperature rise. The final response of the climate system following a doubling of CO2 over preindustrial levels – after all feedbacks have run their course – is known as the ‘climate sensitivity’. Because the climate is a complex system there are many uncertainties in scientists’ understanding of its behaviour. Taking these into account, scientists calculate that climate sensitivity is likely to lie somewhere in the range 2–4.5 °C, with a most likely value of about 3 °C.

In the mid to late 1700s humans began to extract fossil fuels – oil, coal and natural gas – and exploit them as an energy source on an ever-increasing scale. These fuels contain ancient stores of carbon. Burning fossil fuels to produce power releases this carbon into the atmosphere, where it combines with oxygen to form carbon dioxide (CO2), a greenhouse gas. From samples of air in ice-core bubbles, scientists have measured that in 1750 the concentration of CO2 in the atmosphere was about 280 parts per million (ppm) – known as the ‘preindustrial’ CO2 concentration. In 2010 measurements showed that CO2 levels were 385 ppm – an increase of more than 30%. If global emissions keep rising at the current rate, CO2 could be double preindustrial levels by the middle of the century and would continue to rise beyond that.

By studying the absorption characteristics of carbon dioxide (CO2) molecules in laboratory experiments, scientists can calculate the amount of additional heat that would be trapped near the Earth’s surface by different amounts of CO2 increase, such as a doubling over preindustrial levels. Scientists then use mathematical equations describing the physical behaviour of the climate system to predict the resulting rise in global average surface temperature – the ‘climate sensitivity’. These equations are based on the laws of physics and scientists consider many of the processes involved to be well understood. However, there are still uncertainties in the calculations, especially in estimating the strengths of the relevant feedback effects.

There are many feedback effects in the climate system, both positive feedbacks which amplify an initial temperature rise and negative feedbacks which reduce it. Atmospheric measurements, climate model studies and examples of past climate change indicate that the overall strength of positive feedbacks outweighs the strength of negative ones. Scientists calculate that the effect of a doubling of carbon dioxide (CO2) alone would be a global temperature rise of about 1 °C. However, when the various climate feedbacks are taken into account, the most likely temperature increase following a doubling of CO2 – known as the ‘climate sensitivity’ – is about 3 °C, with an uncertainty range of 2–4.5 °C.

After decades of laboratory studies, atmospheric measurements and accumulated knowledge, scientists consider many elements of the climate system to be well understood. But other aspects of the climate’s behaviour are less certain. Scientists therefore compare estimates of ‘climate sensitivity’ derived from basic physical principles with estimates derived from alternative methods, notably computer model simulations of the climate and calculations based on proxy measurements of past climate change. Despite the many uncertainties, there is broad agreement on the ‘best estimate’ value of climate sensitivity – about 3 °C. This agreement between different, independent methods gives scientists confidence in their predictions of how the climate will respond to increasing levels of carbon dioxide.

‘Climate sensitivity’ is the rise in global average surface temperature following a doubling of atmospheric carbon dioxide (CO2) and is estimated to lie in the range 2–4.5 °C. If human emissions keep increasing at the current rate, CO2 could be doubled by the middle of the 21st century and would continue to rise after that. This could lead to a warming of more than the 2–4.5 °C estimate. However, the full extent of this temperature rise wouldn’t occur straight away. It takes time for the many components of the climate system to respond to changes, and the oceans in particular have high thermal inertia. This results in a time-lag between the CO2 increase and the consequent warming. Scientists estimate that if emissions keep increasing at the current rate, global temperatures could rise between 2 and 6 °C by 2100.

• Met Office: Climate Projections