If a city drowns beneath a once-in-a-hundred-years flood, that's weather. Such things have happened in the past.
But when hundred-year floods start happening every few decades, that's no longer just weather. The dice have become loaded for different outcomes. Climate — that is, the average of weather — is changing.
So let's get down to the question everyone's asking. Were this summer's floods the result of climate change?
Justifiably, scientists are cagey about assigning single causes to complex events. But there's a question we can legitimately ask: do the floods fit into a broad picture of weather trends that would confirm global warming? Here the answer is clearly yes.
We don't, of course, need floods to prove the average global temperature is rising. That's well demonstrated, and we know the amount: some 0.8°C since the mid-19th century.
As you'd expect, global records for heat in recent decades have far outnumbered those for cold. The staggering thing, in various cases, is by how much the old records have been swept aside. Adelaide in March 2008 sweltered beneath 15 consecutive days above 35°C. The old record was eight.
Rain events — drought or deluge — have also shown a trend toward extremes, especially in the tropics and subtropics.
As its temperature rises, air can hold sharply rising amounts of water vapour. Climate scientist Kevin Trenberth, head of the Climate Analysis Section at the US National Center for Atmospheric Research, has explained that the global atmosphere now contains about 4% more water vapour than just 30 years ago.
To this, add the effect of warmer oceans in speeding evaporation. The oceans are where as much as 90% of the extra heat held in by the earth's growing blanket of greenhouse gases finishes up. Average sea surface temperatures, the European Environment Agency reports, have risen by an average of about 1°C since pre-industrial times.
The British Met Office's Vicky Pope says every 1°C rise in sea surface temperature brings an increase of 6% to 8% in atmospheric moisture over the oceans.
To understand the floods in eastern Australia, we also need to look at another phenomenon — ENSO, the El Nino Southern Oscillation.
This is best understood as the appearance, several times a decade, of broad pools of relatively warm and cool water in the tropical Pacific. In the "El Nino" phase the cool anomaly is in the west, to the north of Australia. Local air pressure is high, and the easterly trade winds are relatively light.
In the opposite "La Nina" phase, the cool water is in the central and eastern Pacific, and the trade winds blow more strongly than usual.
We are now in one of the strongest "La Nina" events yet measured. Ocean surface temperatures off northern Australia have been at record levels — according to CSIRO buoys, as much as 5°C above average at some points.
With strong trade winds picking up moisture, it becomes clear how the famously dry town of Birdsville in western Queensland could have received well over three times its average rainfall in 2010. For eastern Australia as a whole, last spring was the wettest recorded.
Nor is it any mystery how a line of thunderstorms could have sprung up off southern Queensland, crossed the coast near Noosa as a "superstorm", and dumped as much as 200 mm of rain in less than an hour on Toowoomba and the Lockyer Valley.
So are events like the recent floods linked to global warming? Trenberth sees such associations as elementary:
"It's not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there's always an element of both."
Paradoxically, all but far northern Australia may face a future of aridity rather than floods. There is evidence (the really curious can google "Walker Circulation slowing") that "La Nina" years will become fewer as warming proceeds, and dry "El Nino" events more normal.
Whatever the case, this is only the beginning. The extreme weather we are now seeing is linked to a mere 0.8°C of atmospheric warming.
If emissions cuts are limited to the meagre pledges made at the Cancun climate conference, predictions run, the eventual rise will be around 4°C, perhaps as soon as the 2060s.
Referring to extreme rainfall events, Trenberth told blogger Joseph Romm in a recent interview: "The prospects are that these kinds of things will only get bigger and worse in the future."
Economist Professor Warwick McKibbin has suggested that the cost of the Queensland floods could come to as much as $13 billion. Is there anyone still arguing that failure to act resolutely against climate change is a cheap option?