The basic science behind climate change is actually quite simple.
The story began nearly 150 years ago when Irish physicist John Tyndall discovered ‘carbonic acid’ – today known as carbon dioxide – was one of a number of “perfectly colourless and invisible gases and vapours” to absorb radiant heat.
He also realised just how important this was to life on Earth as without such gases it would be “held fast in the iron grip of frost”.
About 40 years later, the Swedish scientist Svante Arrhenius first suggested that increasing the amount of carbon dioxide in the atmosphere would cause the global temperature to go up, particularly in the Arctic. But the idea remained mostly an academic question until, as he predicted, the mercury started to rise.
This is one of the certainties in climate science – the world has got warmer.
How do we know that the world has really warmed up?
Last year, the hottest on record for the third year in a row, was about 1.1 degrees Celsius above the pre-industrial level, according to Nasa and the Met Office, boosted slightly by the natural El Nino effect. Svalbard in the Arctic has seen average winter temperatures rise by up to a staggering 11C compared to the average between 1961 and 1990.
In addition to thermometer records going back to the 1880s, there is a large amount of natural evidence in the form of glaciers retreating and sea ice melting to record lows, sea levels rising by 20cm, islands disappearing, animals and plants shifting their ranges and spring coming several weeks earlier.
Ironically, the famously impassable Northwest Passage – which claimed the lives of numerous explorers trying to find a new trade route to China – is now so free of ice that people can take a cruise through it on a ship powered by the fossil fuels that help cause global warming.
It is also certain that greenhouse gases in the atmosphere have increased and that human activity is responsible for this. Helpfully, atmospheric carbon dioxide resulting from fossil fuels has a different isotope signature to the gas produced naturally.
As Nasa puts it: “There is no question that increased levels of greenhouse gases must cause the Earth to warm in response.”
How do we know that warming has anything to do with humans?
All this does not scientifically prove beyond all doubt that our greenhouse gases, which also include water vapour, nitrous oxide, ozone and methane, are causing global warming.
But the correlation between the two and the evidence of causation first supplied by Tyndall make a compelling argument. It is as near to certain as it is possible to get without conducting a planet-scale experiment.
Outside of conspiracy theorists, cranks and some of the ruling US politicians, no one disputes the above is true. Even the UK’s leading ‘sceptic’ think tank, Nigel Lawson’s Global Warming Policy Foundation (GWPF), agrees.
What will happen in the future?
Some commentators, including the GWPF, adopt a so-called ‘lukewarmist’ position, arguing that climate change will not be too bad so there is no need to stop using fossil fuels. They point to beneficial effects such as 'global greening' in which plant growth is boosted by the extra carbon in the air.
Others have claimed humanity is plunging headlong towards catastrophe and possibly even a future in which a tiny band of survivors cluster around the last remaining habitable territory near the poles.
Both groups are at odds with the science.
As Professor Tim Palmer, an Oxford University physicist who has worked on reports for the Intergovernmental Panel on Climate Change (IPCC), explained in a recent talk at the Royal Society, the ‘scientific consensus’ on climate change is that it could be anything between those two extremes.
According to computer models, developed using mathematics, the laws of physics and the best available knowledge, the world could be on course for between about 1.5C and more than 5C of warming as a result of the doubling of atmospheric carbon dioxide. So far it has risen from about 280 parts per million – a level that had remained fairly constant from the end of the last Ice Age to the 1800s – to more than 400ppm today.
“When we talk about scientific consensus, I would say it’s basically about this [temperature] distribution, it’s not individual predictions,” Professor Palmer told the Royal Society.
But, according to the models, there is a much higher probability of going beyond 2C of warming, than staying below this point, often regarded as when climate change becomes particularly ‘dangerous’.
And, as Professor Palmer also pointed out, there is also “a non-negligible probability of 5C or more, which would be utterly catastrophic”.
Models are often the source of derision from climate ‘sceptics’ or ‘deniers’, but they have actually proved remarkably successful when measured against actual observations. Even Arrhenius’ prediction that the Arctic would warm more quickly has proved to be true.
What should humans do about it?
Most of the world – Donald Trump and some of his supporters aside – has come to the conclusion that fossil fuel use should be phased out rapidly to try to minimise the chances that devastating storms, deadly heatwaves, floods, droughts and other extreme weather events will wreak havoc to an unprecedented extent.
Some of the hottest and most humid parts of the world are already getting close to the point where humans are no longer be able to lose enough heat to stay alive outdoors.
A 5C average temperature rise might not sound like much to some, but that was about the difference between now and the last global Ice Age when Britain was covered with glaciers.
It is thought prudent to restrict warming to a much lower level.
Under the terms of the Paris Agreement on climate change, the world agreed to try to keep the temperature rise to "well below 2C ... and to pursue efforts to limit the temperature increase even further to 1.5C".
The tougher target was set amid evidence that at some point between 1.5C to 2C of warming changes could occur that lock in major sea level rise for hundreds or even thousands of years.
Professor Richard Betts, a leading Met Office scientist who has also worked on the IPCC's reports, said: “It comes down to how much risk you want to take.
“The lukewarmist position that [global warming] is going to be at the low end is not, sadly, based on the full range of the science.
“It is within the range of possibilities, but I don’t know how you can be confident in that position.
“You are arguing from a position in the hope that it is true.”
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