One of the most popular myths promoted by those skeptical about man-made global warming is that warming has stopped and, in fact, the trend has even reversed in recent years. This illusion largely stems from the fact that 1998 was an exceptionally hot year and 2008 relatively cool due to the respective el Niño and la Niña events of those years.
Despite the fact that the current decade is actually the hottest on record, that eight of the last ten years are among the twelve hottest ever and that ten years is in any case too short a period to represent a meaningful trend, this myth has been remarkably successful – it is not just the usual suspects who are promoting it, even the BBC seem to have been taken in.
Climate scientists and other interested parties have already attempted to correct this misapprehension based on science, unfortunately without much success, but now Associated Press has made an excellent contribution to the debate which demonstrates that the claims made by the deniers are nonsensical from a statistical point of view.
In a blind test, the AP gave temperature data to four independent statisticians and asked them to look for trends, without telling them what the numbers represented. The experts found no true temperature declines over time.
The AP sent expert statisticians NOAA’s year-to-year ground temperature changes over 130 years and the 30 years of satellite-measured temperatures preferred by skeptics and gathered by scientists at the University of Alabama in Huntsville.
Statisticians who analyzed the data found a distinct decades-long upward trend in the numbers, but could not find a significant drop in the past 10 years in either data set. The ups and downs during the last decade repeat random variability in data as far back as 1880.
Saying there’s a downward trend since 1998 is not scientifically legitimate, said David Peterson, a retired Duke University statistics professor and one of those analyzing the numbers.
Identifying a downward trend is a case of “people coming at the data with preconceived notions,” said Peterson, author of the book “Why Did They Do That? An Introduction to Forensic Decision Analysis.”
They also nailed the way the deniers have cherry picked the data to suit their agenda.
Grego produced three charts to show how choosing a starting date can alter perceptions. Using the skeptics’ satellite data beginning in 1998, there is a “mild downward trend,” he said. But doing that is “deceptive.”
The trend disappears if the analysis starts in 1997. And it trends upward if you begin in 1999, he said.
I doubt it will silence the deniers but we now know that on this particular question their arguments are both scientifically and statistically illiterate.
Hat tip: Climate Progress