If you spend a lot of time talking to scientists about climate change, there's one word you'll hear time and time again, and yet it's hardly ever mentioned in the public discussion of climate change. The word is "non-linear".
Most people think of global warming as an incremental thing. It may be inexorable, but it's also predictable. Alas, most people are wrong. The climate is a very complex system, and complex systems can change in non-linear ways.
In other words, you cannot count on the average global temperature rising steadily but slowly as we pump more and more greenhouse gases into the atmosphere.
It may do that - but there may also be a sudden jump in the average global temperature that lands you in a world of hurt. That may be happening now.
"We are moving into uncharted territory with frightening speed," said Michel Jarraud, secretary-general of the World Meteorological Organisation, last November. He was referring to the unprecedented way the warming is now accelerating. The hottest year ever was 2014 - until 2015 beat it by a wide margin; 2016 may beat it by an even wider margin.
It was the hottest January ever - and then the average global temperature in February was a full fifth of a degree Celsius higher than January. That was a huge jump, since the "average global temperature" is an average of all the temperatures over the seas and the land in both the summer hemisphere and the winter hemisphere. It is normally a very stable figure, changing no more than a few hundredths of a degree from year to year.
But March was not only hotter than February, it was hotter by an even wider margin than February was over January.
Indeed, each of the past 11 months has beaten the highest previously recorded average temperature for that month.
It could be some random short-term fluctuation ... [but] it feels a lot more like part of a trend.
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Some people try to explain this all away by blaming it on El Nino, a periodical rise in the ocean surface temperature in the eastern Pacific that moves the rainfall patterns around worldwide, causing droughts here and floods there. But El Nino is a LOCAL rise in temperature, it does not normally affect the average global temperature much.
The frightening acceleration in the warming in the past three months has no precedent in any El Nino year, or indeed in any previous year. It could be some random short-term fluctuation in average global temperature, but coming on top of the record warming of 2014 and 2015 it feels a lot more like part of a trend.
Could this be non-linear change, an abrupt and irreversible change? Yes. And if it is, how far will it go before it stabilises again at some higher average global temperature? Nobody knows.
Last year the average global temperature hit 1C higher than the pre-industrial average. That is halfway to the plus-two degree level which all the world's Governments have agreed we must never exceed, but at least we got to plus-one slowly, over a period of two centuries.
The plus-two threshold matters because that's when the warming we have already caused triggers effects we cannot control: the loss of the Arctic sea-ice, the melting of the permafrost, and immense releases of CO2 from the warming oceans. After plus-two, we will no longer be able to stop the warming by ending our own greenhouse gas emissions.
Even at the global climate summit in Paris in December, there was still hope we might avoid triggering the effects, because the historic rate of warming would still give us about 25 years to work on cutting our emissions before we reach plus-two. But if the current non-linear surge in warming persists, we could have covered half the remaining distance and reached plus 1.5C by the end of this year.
Obviously most scientists will not go this far in public, but they are very worried.
As Professor Stefan Rahmstorf of the Potsdam Institute of Climate Impact Research told the Guardian recently: "We are in a kind of climate emergency now."
Ten years ago I spent almost a year interviewing the world's leading climate scientists for a book I was writing. I learned all our calculations for dealing with climate change could be swept aside by a non-linear event. This could be it.