In not quite the words of the singer Bob Dylan, a hard rain is going to fall. Australian scientists predict that precipitation extremes with large variability will emerge in the Northern Hemisphere in the coming decades as part of a “wettening trend”.

In other words, an even harder rain will fall - and then you’ll really know that the climate has changed.

Andrew King, a climate system scientist at the Climate Change Research Centre at the University of New South Wales and at the University of Melbourne, is lead author of a report in the journal Environmental Research Letters.

He and colleagues say they have pinpointed the moment when global warming, as a result of human influences, emerged in the historic record in some parts of the world. They can now also predict when it will become even clearer in places so far untouched.

Story of warming

Although climate change can be predicted, it can only be identified after the event. That is because, while climate is the average of all the extremes of weather, the weather can get pretty extreme, even within a stable climate.

From the late 1980s onwards, global average temperatures began to tell a consistent story of warming.

But if those temperatures are now on the increase because of the continual rise in carbon dioxide concentrations in the atmosphere, as a consequence of the human exploitation of fossil fuels, then there must have been some moment in the past when the signs of change could have been detected, and when a new climate trend developed.

Such questions sound academic, but the better humans understand the past, the more likely they are to be prepared for the future.

So Dr King and his colleagues used state-of-the-art computer models to simulate not just climate change, but the way that extreme events have begun to change. “Both hot and cold extremes have already emerged across many areas,” they say.

The scientists examined average and extreme temperatures because these would, of course, be most sensitive to global warming - and therefore evidence should show in historic records.

“Remarkably, our research shows you could already see clear signs of global warming in the tropics by the 1960s, but in parts of Australia, south-east Asia and Africa it was visible as early as the 1940s,” Dr King says.

Changing pattern

First the scientists saw a rise in average temperatures; then the changing pattern of extremes of climate began to confirm the picture.

There are exceptions: the continental US, especially on the East Coast, has yet to show any obvious warming signals, but these could appear in the next decade.

And although greater warming means more evaporation and more precipitation, in many places this heavier rainfall is still so far within the range of “normal” extreme weather.

“We expect the first heavy precipitation events with a clear global warming signal will appear during winters in Russia, Canada and northern Europe over the next 10 to 30 years,” says Ed Hawkins, a meteorologist at the University of Reading in the UK, and one of the report’s co-authors.

“It is likely to bring pronounced precipitation events, on top of the already existing trend towards increasingly wet winters in these regions.”