So the IPCC will say. But what I am curious about this morning is the greatly-increased frequency of "La Nina" ENSO years. I know that a system with more energy in it will occupy more of its available states--is that what is going on, and does that make it more likely that we will also have more and bigger "El Nino" years in the future? Or is something more complicated going on?
On the average since 1950, one year in every five has been the hottest year on record. The last hottest year on record was only three years ago. The last ten years have seen two of the hottest years on record. The last twenty years have seen five.
Back in 1990, climatologists had a reasonably good grasp of how humans were warming the planet: "Emissions resulting from human activities are substantially increasing the atmospheric concentrations of greenhouse gases," they wrote in the very first IPCC report. "These increases will enhance the greenhouse effect, resulting on average in an increase in global warming."
Over the years, a vast pile of climate research has simply reaffirmed that conclusion. In 2001, the third IPCC report said it was "likely" that most of the rise in surface temperatures over the past half-century was due to human activity. The 2007 report bumped this up to "very likely." And the 2013 report is expected to say that it's "extremely likely" that humans are responsible for more than half that warming--a more than 95 percent chance…. Scientists are now as certain that humans are responsible for rising temperatures as they are that cigarette smoke causes lung cancer. But they're still basically saying the same things they said in 1990….
It's also notable that the big uncertainties that existed in 1990 still exist today. One crucial question is how much temperatures will rise each time we double the amount of carbon-dioxide in the atmosphere. In 1990, scientists predicted the Earth would warm somewhere between 1.5ºC to 4ºC. This year? According to early leaked drafts, the new report will peg climate sensitivity at... between 1.5ºC and 4ºC for every doubling of carbon-dioxide.