evidence: The United States is witnessing a massive, dangerous heat wave, as a huge system of high pressure covers the central part of the country. It’s a big enough deal that yesterday President Obama even tweeted about it, including a map showing the maximum heat index in some parts of the Midwest and Southeast reaching 110 or 115 degrees on Saturday. [...] This will, inevitably, lead to much talk of climate change in the coming days. So it’s important to separate the scientific wheat from the chaff and figure out what science can, and can’t, reliably say about the link between an event like this and a warming planet — especially in a year that, on a global scale, has shattered past temperature records for six out of the last six months. [...] And we’ve definitely already had changes in not only "long-term mean conditions," but in heat waves themselves. The U.S. National Climate Assessment found that U.S. heat waves have already "become more frequent and intense," that the U.S. is shattering high temperature records far more frequently than it is shattering low temperature records (just as you’d expect), and that it is seeing correspondingly fewer cold spells.