At the heart of climate science lies the greenhouse effect — how the Earth’s atmosphere traps some of the heat that reaches us from the sun.
Life depends on this warming. Without it, if all solar radiation were reflected straight back into space, the average global temperature would be around -18C rather than the 14C we actually enjoy.
When human activities add more heat-absorbing gases to the atmosphere than would naturally be present, climate changes in a way that may disrupt life on the planet.
Although water vapour contributes more than other gases to the natural greenhouse effect, the most important human-made contributor is carbon dioxide, because of the vast volumes of CO2 generated by burning fossil fuels: coal, oil and natural gas. Trillions of tonnes of these carbon-rich compounds exist in geological strata across the world — the remains of countless plants, animals and microbes.
By consuming these fossil fuels over a period of decades, we have increased the amount of CO2 in the atmosphere by more than 40 per cent since pre-industrial times.
The next most important warming gas is methane (CH4), generated by agricultural, industrial and energy production. Each molecule of methane can absorb far more heat than CO2 but, fortunately, there is much less of it in the atmosphere and it does not hang around for quite so long — on average about 12 years.
Other greenhouse gases include nitrous oxide and fluorine compounds.
Of course, the climate has varied throughout Earth’s history as a result of the complex interaction between processes including volcanic activity, solar variability, and cyclical changes in ocean currents.
Some climate sceptics have tried to pin the warming of about 1C observed since industrial activity began on natural variability, rather than a result of human actions. However, that view has become scientifically untenable due to how much the temperature has risen and the absence of plausible natural explanations.
More stories from this report
That 1.1C temperature rise is already causing disruption to weather patterns. Rainfall distribution is changing. A warmer atmosphere holds more moisture and energy, which means storms and flooding are becoming more severe. Conversely, droughts and more intense heat in other areas are causing forest fires and crop failures.
The momentum of industrial activity and the delayed response of the atmosphere and oceans mean a further increase to at least 1.5C warming is inevitable — and many scientists expect 2C to be breached even if the world takes the strongest action that we can realistically expect.
There is still a lot of uncertainty about the future extent of climate change and also about regional differences. For example, temperatures are rising faster over the polar areas and particularly the Arctic than in the tropics. One pressing question is how much climate change will be exacerbated by positive feedbacks, whereby one warming effect drives another — such as how melting permafrost in the Siberian tundra releases methane and carbon dioxide.
One of the more predictable outcomes of global warming is raised sea levels. The average rise across the world’s oceans over the past 30 years has been about 10cm, propelled both by melting ice from Greenland and Antarctic and by the thermal expansion of water.
Larger sea level increases are expected during this century. A report in February by the US National Ocean Service projected a rise along the American coastline of 25cm to 30cm over the next 30 years, with damaging floods occurring 10 times more frequently. By 2100, the increase is likely to be at least 60cm — and could be more than a metre.
Fighting global warming depends, above all, on drastically cutting the world’s carbon emissions. Coal results in the most CO2 per unit of energy produced, followed by oil and then natural gas.
The world is therefore investing in alternative non-carbon sources, particularly wind and solar, together with batteries and other technology to store this energy for when the weather is calm and the sky dark. Electricity generated by renewable sources could produce fuel for transport, with very little greenhouse gas impact, by splitting water into its component elements: hydrogen and oxygen. Nuclear power is another carbon-free energy source.
Many companies have committed to “net zero” targets, resolving to reduce their carbon emissions and offset the impact of those they cannot cut, by investing in schemes to remove CO2 from the atmosphere.
The most straightforward way to do this is reforestation, because trees suck CO2 out of the atmosphere as they grow, but this reduces the land available for farming.
More tech-intensive methods include removing CO2 from the air or from the emissions of power stations and storing it underground.
But these “negative emissions” technologies cannot be scaled up in the short term. Long-term success will depend on individuals, companies and governments making a real political and economic commitment to decarbonise their activities.
How have sceptics tried to refute human responsibility for climate change?
Why has this been rejected by experts?
Why is there still uncertainty about the future trajectory of climate change?
Michael Dunn, theoryofknowledge.net
See more at ft.com/ibtokclass