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Michael Mann is a Professor at Penn State University and director of the Penn State Earth System Science Center. He is a co-founded of the award-winning climate science blog "" and author of  the recent book The Hockey Stick and the Climate Wars: Dispatches from the Front Lines about his experiences in the center of the debate over human-caused climate change. [You can follow Dr. Mann on twitter (@MichaelEMann) and Facebook].

James Hansen's latest findings linking extreme weather to climate change is science society cannot afford to ignore.

The first scientist to alert Americans to the prospect that human-caused climate change and global warming was already upon us was NASA climatologist James Hansen. In a sweltering Senate hall during the hot, dry summer of 1988, Hansen announced that "it is time to stop waffling.... The evidence is pretty strong that the [human-amplified] greenhouse effect is here."

At the time, many scientists felt his announcement to be premature. I was among them.

I was a young graduate student researching the importance of natural – rather than human-caused – variations in temperature, and I felt that the "signal" of human-caused climate change had not yet emerged from the "noise" of natural, long-term climate variation. As I discuss in my book, The Hockey Stick and the Climate Wars, scientists by their very nature tend to be conservative, even reticent, when it comes to discussing findings and observations that lie at the forefront of our understanding and that aren't yet part of the "accepted" body of scientific knowledge.

Dire warning

Hansen, it turns out, was right, and the critics were wrong. Rather than being reckless, as some of his critics charged, his announcement to the world proved to be prescient – and his critics were proven overly cautious.

Given the prescience of Hansen's science, we would be unwise to ignore his latest, more dire warning.

In a paper published today in the prestigious journal Proceedings of the National Academy of Sciences, Hansen and two colleagues argue convincingly that climate change is now not only upon us, but in fact we are fully immersed in it. Much of the extreme weather we have witnessed in recent years almost certainly contains a human-induced component.

Hansen, in his latest paper, shows that the increase in probability of hot summers due to global warming is such that what was once considered an unusually hot summer has now become typical, and what was once considered typical will soon become a thing of the past – a summer too improbably cool to anymore expect.

We need to view this summer's extreme weather in this wider context.

Not random

It is not simply a set of random events occurring in isolation, but part of a broader emerging pattern. We are seeing, in much of the extreme weather we are experiencing, the "loading of the weather dice." Over the past decade, records for daily maximum high temperatures in the U.S. have been broken at twice the rate we would expect from chance alone. Think of this as rolling double sixes twice as often as you'd expect – something you would readily notice in a high stakes game of dice. Thus far this year, that ratio is close to 10 to 1.  That's double sixes coming up ten times as often as you expect.

So the record-breaking heat this summer over so much of the United States, where records that have stood since the Dust Bowl years of the 1930s are now dropping like flies, isn't just a fluke of nature; it is the loading of the weather dice playing out in real time.

The record heat – and the dry soils associated with it – played a role in the unprecedented forest fires that wrought death and destruction in Colorado and New Mexico. It played a role in the hot and bone-dry conditions over the nation's breadbasket that has decimated U.S. agricultural yields. It played a role in the unprecedented 50 percent of the U.S. finding itself in extreme drought.

Other threats

Climate change is also threatening us in other ways of course, subjecting our coastal cities to increased erosion and inundation from rising sea level, and massive flooding events associated with an atmosphere that has warmed by nearly 2˚F, holding roughly 4 percent more water vapor than it used to – water vapor that is available to feed flooding rains when atmospheric conditions are right.

The state of Oklahoma became the hottest state ever with last summer's record heat. It is sadly ironic that the state's senior senator, Republican James Inhofe, has dismissed human-caused climate change as the "greatest hoax ever perpetrated on the American people." Just last week he insisted that concern over the impacts of climate change has "completely collapsed." This as Oklahoma City has just seen 18 days in a row over 100˚F (with more predicted to follow), Tulsa saw 112˚F Sunday, and 11 separate wildfires are burning in the state, with historic Route 66 and other state highways and interstates all closed.

The time for debate about the reality of human-caused climate change has now passed. We can have a good faith debate about how to deal with the problem – how to reduce future climate change and adapt to what is already upon us to reduce the risks that climate change poses to society. But we can no longer simply bury our heads in the sand.

Note: This is a re-post of a commentary previously published at The Daily Climate.

Originally posted to MichaelEMann on Mon Aug 20, 2012 at 04:00 PM PDT.

Also republished by DK GreenRoots, Climate Hawks, Climate Change SOS, and Science Matters.

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