The bars in red on the graph below are the 20 hottest years on human record. There were hotter years, like when the Earth was still mostly molten rock. But since mankind has had the capability to actually measure temperatures around the world, the 20 bars below indicate the hottest years we've seen.

(Those temperatures are in Celsius and are relative to the average land and sea temperature over the course of the 20th century.)

You may notice a pattern. Of those 20 years, 15 have been since 2001, with 2015 setting a new record high. A goal of those seeking to slow global warming is to keep the Earth's temperatures from getting more than 2 degrees Celsius above the 20th century average. The year 2015 was at 0.9 degrees above the average.

The record prior to 2015 was set back in in the good old days, back when "How I Met Your Mother" was concluding its series and the song "Happy" ruled the airwaves. Yes, way back in 2014. There's a lot of volatility, year-over-year, but the increase between the record in 2014 and that in 2015 is still stunning. It was up by over a fifth. It's as though Joey Chestnut won the Nathan's hot-dog-eating contest by eating 73 hot dogs to Takeru Kobayashi's 60. As though the Knicks beat the Cavaliers 121-100. It was not close.

Over the past 15 years (during which we've seen 14 of the 15 hottest years on record), attitudes toward global warming as a threat have evolved in interesting ways. Gallup has repeatedly asked Americans if they think that the seriousness of global warming is exaggerated, underestimated or generally correct.

Attitudes were pretty muddled until 2006, which happens to be the year of Al Gore's Oscar-winning climate change documentary, "An Inconvenient Truth." In polls the years after 2006, the number of people saying the threat was exaggerated shot up, jumping from 30 percent in 2006 to 48 percent in 2010. We've noted before that the issue of climate change has become deeply politicized; it's almost certainly not a coincidence that the peak of skepticism immediately follows Barack Obama's first year in office and the failure of a Congressional cap-and-trade bill that would have imposed limits on carbon emissions.

Over the past few years, that attitude has stayed flat, as the number of people saying that the threat is underestimated has risen -- alongside temperatures. You can think of "generally correct" as sort of the neutral position here -- that the status quo is proper. The debate has polarized more as more Americans are concerned that the threat is being underestimated.

Last year, we noted that the place least-affected by the record heat of 2014 was centered on Washington, suggesting unsubtly that this might play a role in the skepticism displayed by some members of Congress. This year, the coldest place was just off the coast of Greenland, as you can see here.

Why's that spot there? Maybe, as our Chris Mooney reported in September, because of ice melting in Greenland.