If, as the wildfires rage and the floodwaters rise, the world allows global warming to continue, it will ultimately result in the deaths of billions of people and the end of civilisation as we know it.
That, of course, is not going to happen. At some point, we are going to wake up to the danger and take effective action against it.
The question is whether we will start that process in earnest now or continue to put it off for another day, another year, or more.
The United Nations’ COP26 summit, which starts in Glasgow tomorrow, has been accurately described by US climate envoy John Kerry as our “last, best chance”.
Over the next two weeks, the countries of the world must take the opportunity to set out how they will halve greenhouse emissions by 2030 and then reduce them to net-zero by the middle of the century.
This will require huge changes to be made at a considerable pace, but while the path to net-zero is steep, it is still manageable.
If our leaders fail to do this, it is virtually certain that global warming will go above 1.5 degrees Celsius, the point at which scientists believe our climate will start to become particularly dangerous to humanity.
As the consequences of this become more apparent – the wildfires get yet bigger, the floods yet more devastating and the deathtoll climbs – people will eventually start to panic.
But by then the path to net-zero is likely to be too steep, the changes will need to be made at breakneck speed and we will risk falling on the way. The physical damage caused by climate change on a global scale and the mass movements of people as large areas of the world become uninhabitable will be accompanied by economic chaos of similar proportions.
So what is holding us back? One problem may be the lack of pressure from the people of the world on their leaders because just over 1C of warming may not sound like a lot.
However, a better way of thinking about this is in terms of the extra energy that the Earth has been accumulating. According to a paper published last year by an international team of scientists – including Professor James Hansen, who in 1988 as a senior Nasa official issued a famous warning to the world about climate change – the heat gained between 1971 and 2018 was about 358,000,000,000 terajoules.
To provide an idea of how much that represents, the nuclear bomb that destroyed Hiroshima in the Second World War produced about 63 terajoules. The comparison may not be as simple as this suggests, but it is as if we have been detonating several Hiroshima-sized bombs every single second of every single day for years on end.
That it takes such a colossal amount of energy to raise the temperature of the entire planet by a single degree should not be a surprise, particularly not to those who pay the household heating bills.
Continuing to add jaw-droppingly vast amounts of energy to the Earth is only going to result in increasingly vast effects. And, on our current course, global warming is expected to hit 2C by the middle of the century and 2.7C by the end.
Writing in the Scotsman last week, award-winning science writer Mark Lynas, author of Our Final Warning: Six Degrees of Climate Emergency, gave a taste of what 3C of warming would be like.
“At 3C vast areas of South Asia would become unliveably hot, with crop yields drastically reduced in all the world's major crop-producing regions. Much of the world's wildlife would be wiped out, with the Amazon rainforest tipping into full-scale collapse, and 12 million square kilometres of Arctic permafrost entering the melt zone and adding further methane and CO2 to the planet's atmosphere,” he said.
But there is another even more terrifying consequence of allowing global warming to cross the 1.5C threshold: the risk of triggering natural processes which result in further, irreversible warming.
Examples include the loss of ice, which reflects sunlight, exposing darker ground or sea which absorbs more of it; the melting of permafrost, releasing methane and carbon into the atmosphere, causing more warming and more melting; and wildfires that burn forests, releasing carbon into the atmosphere, leading to warming that causes more wildfires so severe that the forests do not regrow.
Leading climatologist Professor Tim Lenton, again writing in The Scotsman, explained that such “tipping points” could interact with each other and “in the worst case, this could lead to ‘domino dynamics’, where tipping one thing tips the next, and runaway climate change”. Once tipping points were viewed as a far-off threat, but Professor Lenton added: “Now we see we are in the danger zone at just over 1C of global warming.”
Of particular concern should be the effect of the melting of Greenland’s ice on the Atlantic currents that bring warm air from the Caribbean, making the climate in the UK and other parts of Europe relatively mild.
These currents have been weakening significantly and are “very likely” to continue to do so for the rest of the century, according to a recent report by the Intergovernmental Panel on Climate Change. It expressed only “medium confidence” that there would not be an “abrupt collapse” in these currents before 2100.
When our leaders begin negotiations in Glasgow tomorrow, humanity will have reached a fork in the road. We can choose to turn from our current course to take the steep path towards net-zero carbon emissions. As explained by economist Professor Lord Nicholas Stern in The Scotsman, a brighter, more prosperous future awaits if we do.
Or, ignoring warnings from the scientific community, we can press ahead down another steep slope which will become increasingly precipitous and, at some point in the near future, dangerously slippery.
Its final destination? Hell on Earth.