Weather is today. Climate is 30-years of todays. Climate science looks at weather patterns over a suitable period, usually at least three decades. 30-years is okay, a century is better, a millennium better yet. Hundreds of thousands of years, the stuff of ice core samples, is possible for some purposes.
Still, we're stuck on this 30-year threshold. For some it's a handy way to forestall meaningful action.
Take the polar jet stream for example. Just a few years ago it was believed that the Arctic might be free of summer sea ice by the end of this century. Now the U.S. Navy figures the summer sea ice may be gone by 2016, 2019 at the outside. That's a game changer. As the Arctic sea ice retreats, the ocean absorbs more solar energy ( a fancy way of saying it warms), and an exposed, warming ocean gives off more heat to the atmosphere which, dutifully obeying the laws of physics, creates this polar jet stream that introduces 'long wave' or Rossby wave patterns that bring cold air way down into the south and hot southern air way up into the far north while slowing the west to east progression of weather patterns and, at times, establishing blocking patterns where heavy rainstorms just happen to sit over Calgary for days on end until the place is inundated with flooding.
That doesn't meet the 30-year test so it's just a weather event. The dreaded "polar vortex" is, for the same reason, just a weather event. As George Will would say, "it's winter, get over it."
Some climate scientists are catching Hell for ditching the 30-year stricture and opining that these weather events are very likely/probably/almost certainly the result of climate change. Can't say that - against the rules. To some, climate change must be proven to a point of near certainty before government should act on it. Why?
In Common Law the rule is that claims of fraud must be 'strictly proven.' Well if a fraudster is any damned good chances are that person can obscure enough evidence to make it very difficult to prove fraud to a point of near certainty. The Supreme Court of Canada tackled this dilemma back in 1915 in the case of Koop v. Smith. They explored the idea of 'badges of fraud' which basically meant conduct from which fraud could be inferred. Boiled down it meant if it looks like a duck and it walks like a duck and it quacks like a duck, you should figure it's a duck. The burden then shifts to the duck to prove that it's actually a swan.
The problem with the 30-year rule is that, as we've witnessed with the Arctic sea ice, climate change impacts can happen with unexpected rapidity. And some climate change impacts, such as heat waves, megadroughts, intense flooding on a sustained basis could be catastrophic to things like agriculture and infrastructure which our societies are utterly dependent upon.
Labour party leader Ed Miliband dealt with the problem of weather versus climate by noting that, if you roll a dice and it keeps coming up 6, you don't have to keep rolling that dice for 30-years to realize that it's loaded. Likewise if these 'weather events' become recurring and are consistent with climate projections, we need to accept them as climate change, not weather, and respond accordingly.
Time is not on our side. We need leaders who realize that.