What 2 degrees means

It's a favourite line of some climate change skeptics that, if the world has already warmed up .8 degree over the 20th century, and the effects haven't been so bad, would we even notice an average warming of 2 degrees in future.

The argument is, I would have thought, obviously flawed for many reasons, not the least of which being that what climate scientists are actually saying is "hey, you'd better start working freaking hard even to have half a hope of keeping it to 2 degrees." The stupidest version of the skeptic argument says "well, so what if a previous hot day of 35 degrees becomes one of 36.5 degree?" You can point people to this well know bell curve:



but it doesn't seem to register that what is means hotter seasons, not just individual days.

So, they should read about research like this, indicating that what we currently consider an extreme summer will, in large parts of the world, become extremely common:

Researchers from Stanford University recently set out to learn at what point exceptionally hot summers will to become more commonplace around the world. Climate scientist Noah Diffenbaugh has studied how the warming to date has influenced the weather patterns that lead to unusually hot seasons. Projecting forward over the next few decades, he says the combination of warmer temperatures and changing weather patterns mean that the extremes will be changing quickly.

"According to our projections, large areas of the globe are likely to warm up so quickly that, by the middle of this century, even the coolest summers will be hotter than the hottest summers of the past 50 years," Diffenbaugh said when his study was published earlier this summer in the journal Climatic Change Letters.

Scientists say the trend towards more hot extremes has already begun. In the U.S., for example, record breaking hot days have already become more common than they once were. According to climate scientist Jerry Meehl, recording breaking hot days used to be as common as cold ones. But in 2000, there were twice as many warm temperature records as cold records in the U.S., and he says that in 2011, so far there have been three times as many.

Go on, skeptics, keep reading. I know it's difficult to get you to think outside your ideological comfort zone, but do try. Here's the paper's abstract:

Given current international efforts to reduce greenhouse gas emissions and limit human-induced global-mean near-surface temperature increases to 2°C, relative to the pre-industrial era, we seek to determine the impact such a temperature increase might have upon the frequency of seasonal-mean temperature extremes; further we seek to determine what global-mean temperature increase would prevent extreme temperature values from becoming the norm. Results indicate that given a 2°C global mean temperature increase it is expected that for 70–80% of the land surface maximum seasonal-mean temperatures will exceed historical extremes (as determined from the 95th percentile threshold value over the second half of the 20th Century) in at least half of all years, i.e. the current historical extreme values will effectively become the norm. Many regions of the globe—including much of Africa, the southeastern and central portions of Asia, Indonesia, and the Amazon—will reach this point given the “committed” future global-mean temperature increase of 0.6°C (1.4°C relative to the pre-industrial era) and 50% of the land surface will reach it given a future global-mean temperature increase of between 0.8 and 0.95°C (1.6–1.75°C relative to the pre-industrial era). These results suggest substantial fractions of the globe could experience seasonal-mean temperature extremes with high regularity, even if the global-mean temperature increase remains below the 2°C target.

Given what happens as a result of extremely hot summers in Australia (bushfires, water shortages) it's also obvious that it's not just extreme temperatures that are the issue.