How do we know whether today's weather event is caused by climate change? While we can't yet link specific events to climate change, scientists can look at patterns of extreme weather and ask whether the likelihood of these events was heightened by human-driven climate change.
Think of a pair of dice that you throw every day. Rolling a pair of sixes happens now and then. But rolling five in a row looks suspicious. Human-induced climate change has loaded the dice toward certain extreme events.
In their paper, Public Perception of Climate Change and the New Climate Dice, James Hansen, Makiko Sato, Reto Ruedy find that
Hot extremes, which covered much less than 1% of Earth's surface in the period of climatology, now typically cover about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming, because their likelihood in the absence of global warming was exceedingly small.
Proceedings of the National Academy of Sciences, April 2012
The loaded dice analogy was used by Dim Coumou, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany in this Washington Post article on 29 March 2012.
Climate Bites has a good post on dice. They note that climate change has put an extra six on the dice (e.g. the five got an extra dot). When you roll the dice now you are twice as likely to throw a six, and you never know whether this particular six is the ordinary, by chance, six, or whether it is the new climate change six.
Joe Romm used the loaded dice analogy in this article about the unprecedented March (2012) heat wave that was “unmatched in recorded history” for the U.S. (and Canada). New heat records swamped cold records by the stunning ratio of 35.3 to 1.