If the temperature outside changes by half a degree, you won't even feel it.
Such a small change is almost imperceptible to human senses.
But half a degree of global temperature change could be little short of the end of the world we recognise, or at least the world humanity has inhabited for the last few millennia.
In 2015, governments agreed a deal in Paris to stop global warming rising past 2 degrees and to try to keep it below 1.5 degrees.
So what's in half a degree?
At 1.5 degrees, there would still be some precious rainforests, polar bears and coral reefs on planet Earth, albeit seriously diminished.
Sea levels would rise, but relatively slowly, so major cities like London, Jakarta and Mumbai have a better chance of continuing life as normal.
Drought and flooding would be at a level where food production can keep up with population growth.
Millions of extra climate refugees could stay at home and others could avoid starvation.
Getting to this world isn’t impossible, it means cutting greenhouse gas emissions by half within a decade and then being carbon neutral by mid-century.
So we'd be closing coal power stations instead of building new ones and every car would be electric.
We'd be restoring forests instead of cutting them down, and even aircraft would be carbon neutral.
It's not impossible, but it is unlikely. The fossil fuels lobby is powerful.
Some heads of state even deny the reality of climate change.
Transforming the global energy system will take time, and trillions of dollars of investment.
But it's also an opportunity. So if we're too slow and instead of 1.5, we get the extra half degree - what then?
According to the latest IPCC report, in a world two degrees warmer, you'd go diving in the tropical seas but the flourishing coral reefs are gone.
Instead, you would likely see a graveyard of rubble and algae.
Back on land, heatwaves are becoming increasingly deadly, with approximately 65 million extra people exposed to exceptional heatwaves each year.
Imagine yourself at the North Pole, but all you see is open water.
The polar bears are gone, along with the sea ice that used to be their home.
You take a boat to the edge of Greenland, but that frozen wasteland is thawing.
Huge meltwater rivers thunder into the ocean, where they add to the rising sea levels that are beginning to flood the world's coastlines.
Take a trip to Miami, Mumbai or Melbourne and you'll probably need a boat.
Faced with the rising seas, 10 million people could be forced to pack up and move.
Global harvests would also suffer, and in developing countries especially, the number of people experiencing water scarcity doubles.
This would be a world of increasing poverty and human misery.
All for what? For half a degree. And for a few more years of coal smoke stacks and oil industry profits.
The choice is ours, but not for long. Every bit of warming, every year, every choice matters.
But like a desert mirage, the 1.5 degree world is receding into the distance with every year we delay.
If carbon emissions keep growing for decades to come, then even two degrees looks hopeful.
That's a world - a darker world - of flood, fire and conflict that we can barely even imagine.
We can still choose that half a degree window between unsettling dream and full-blown nightmare.
But it's closing fast.