U.S. Heatwaves: "Weather" versus "Climate"

It's summer, and that means it's time for another round of record-breaking heatwaves in the United States.

The heatwave of the past week has triggered forest fires across the western states. Washington, DC staggered under 104-degree (F) temperatures - the air conditioner load helped prolong a five-day blackout across the eastern states. Back in the 1990s aid agencies used photos of the earth at night to flag underdeveloped countries where people had to live without electricity.  I never thought I'd see those types of images for the suburbs of Baltimore and Washington DC:

Washington-Baltimore on June 28, 2012

A less brightly-lit Washington-Baltimore on June 30, 2012

This is not the first time large parts of the U.S. have faced a massive heat wave.  In fact, they are becoming so common that it might be safe to consider record-breaking temperatures the "new normal".  Which brings us once more to the topic of climate change.

We might define "weather" as the meteorological conditions when you look out the window.  Is it raining? Is it hot?  Weather varies day by day, and it's difficult to predict more than a week in advance.  "Climate" refers to the typical conditions we might expect at a given time of year.  San Francisco is normally foggy on summer afternoons, Montana is typically frigid in winter.  A freak storm or unexpected heat wave is bad weather.  Searing temperatures every summer, year in and year out - that sounds more like climate. If that's not the climate we used to have, then it would be fair to say that the climate is changing.

Climate scientists are generally careful not to attribute any particular weather event to climate change.  Their models of overall change are predictions of longer-term trends.  But the weather we're seeing is beginning to match those predictions.  How long before "longer term" becomes "now"?