Between 1998 and 2012 the global economy more than doubled in size—to some $71 trillion in GDP from $30 trillion. That's the good news. Over the same period the world pumped more than 100 billion tons of carbon dioxide into the atmosphere. That is supposedly the bad news. Yet global surface temperatures have remained essentially flat. That's the mystery: If emitting CO2 into the atmosphere causes global warming, why hasn't the globe been warming?

That's the question we would have liked to see answered by the U.N. Intergovernmental Panel on Climate Change (IPCC), which Friday published the summary of its fifth report on what co-chairman Thomas Stocker calls "the greatest challenge of our times." It would have also been nice to see some humility from the IPCC, which since its last report in 2007 has seen some of its leading scientists exposed as bullies, and some of its most eye-catching predictions debunked. (Remember the vanishing Himalayan glaciers?)

No such luck. "Warming of the climate system is unequivocal," insists the report in its first bold-face conclusion, followed by the claim that "each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850." What follows are warnings of shrinking ice sheets, rapidly rising sea levels and other scary events.

So what about the warming that hasn't been happening since 1998? Here's the key paragraph, buried on page 10: "The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean."

After noting that scientists have only low or medium confidence in various theories for this reduced warming trend, the report adds that "there may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing." (Our emphasis.)