Statisticians: "Global Cooling" a Myth
Have you heard that the world is now cooling instead of warming? You may have seen some news reports on the Internet or heard about it from a provocative new book.
Only one problem: It's not true, according to an analysis of the numbers done by several independent statisticians for The Associated Press.
The case that the Earth might be cooling partly stems from recent weather. Last year was cooler than previous years. It's been a while since the super-hot years of 1998 and 2005. So is this a longer climate trend or just weather's normal ups and downs?
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.
"If you look at the data and sort of cherry-pick a micro-trend within a bigger trend, that technique is particularly suspect," said John Grego, a professor of statistics at the University of South Carolina.
Yet the idea that things are cooling has been repeated in opinion columns, a BBC news story posted on the Drudge Report and in a new book by the authors of the best-seller "Freakonomics." Last week, a poll by the Pew Research Center found that only 57 percent of Americans now believe there is strong scientific evidence for global warming, down from 77 percent in 2006.
Global warming skeptics base their claims on an unusually hot year in 1998. Since then, they say, temperatures have dropped - thus, a cooling trend. But it's not that simple.
Since 1998, temperatures have dipped, soared, fallen again and are now rising once more. Records kept by the British meteorological office and satellite data used by climate skeptics still show 1998 as the hottest year. However, data from the National Oceanic and Atmospheric Administration and NASA show 2005 has topped 1998. Published peer-reviewed scientific research generally cites temperatures measured by ground sensors, which are from NOAA, NASA and the British, more than the satellite data.
The recent Internet chatter about cooling led NOAA's climate data center to re-examine its temperature data. It found no cooling trend.
"The last 10 years are the warmest 10-year period of the modern record," said NOAA climate monitoring chief Deke Arndt. "Even if you analyze the trend during that 10 years, the trend is actually positive, which means warming."
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."
One prominent skeptic said that to find the cooling trend, the 30 years of satellite temperatures must be used. The satellite data tends to be cooler than the ground data. And key is making sure 1998 is part of the trend, he added.
It's what happens within the past 10 years or so, not the overall average, that counts, contends Don Easterbrook, a Western Washington University geology professor and global warming skeptic.
"I don't argue with you that the 10-year average for the past 10 years is higher than the previous 10 years," said Easterbrook, who has self-published some of his research. "We started the cooling trend after 1998. You're going to get a different line depending on which year you choose.
"Should not the actual temperature be higher now than it was in 1998?" Easterbrook asked. "We can play the numbers games."
That's the problem, some of the statisticians said.
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.
Apart from the conflicting data analyses is the eyebrow-raising new book title from Steven D. Levitt and Stephen J. Dubner, "Super Freakonomics: Global Cooling, Patriotic Prostitutes and Why Suicide Bombers Should Buy Life Insurance."
A line in the book says: "Then there's this little-discussed fact about global warming: While the drumbeat of doom has grown louder over the past several years, the average global temperature during that time has in fact decreased."
That led to a sharp rebuke from the Union of Concerned Scientists, which said the book mischaracterizes climate science with "distorted statistics."
Levitt, a University of Chicago economist, said he does not believe there is a cooling trend. He said the line was just an attempt to note the irony of a cool couple of years at a time of intense discussion of global warming. Levitt said he did not do any statistical analysis of temperatures, but "eyeballed" the numbers and noticed 2005 was hotter than the last couple of years. Levitt said the "cooling" reference in the book title refers more to ideas about trying to cool the Earth artificially.
Statisticians say that in sizing up climate change, it's important to look at moving averages of about 10 years. They compare the average of 1999-2008 to the average of 2000-2009. In all data sets, 10-year moving averages have been higher in the last five years than in any previous years.
"To talk about global cooling at the end of the hottest decade the planet has experienced in many thousands of years is ridiculous," said Ken Caldeira, a climate scientist at the Carnegie Institution at Stanford.
Ben Santer, a climate scientist at the Department of Energy's Lawrence Livermore National Lab, called it "a concerted strategy to obfuscate and generate confusion in the minds of the public and policymakers" ahead of international climate talks in December in Copenhagen.
President Barack Obama weighed in on the topic Friday at MIT. He said some opponents "make cynical claims that contradict the overwhelming scientific evidence when it comes to climate change - claims whose only purpose is to defeat or delay the change that we know is necessary."
Earlier this year, climate scientists in two peer-reviewed publications statistically analyzed recent years' temperatures against claims of cooling and found them not valid.
Not all skeptical scientists make the flat-out cooling argument.
"It pretty much depends on when you start," wrote John Christy, the Alabama atmospheric scientist who collects the satellite data that skeptics use. He said in an e-mail that looking back 31 years, temperatures have gone up nearly three-quarters of a degree Fahrenheit (four-tenths of a degree Celsius). The last dozen years have been flat, and temperatures over the last eight years have declined a bit, he wrote.
Oceans, which take longer to heat up and longer to cool, greatly influence short-term weather, causing temperatures to rise and fall temporarily on top of the overall steady warming trend, scientists say. The biggest example of that is El Nino.
El Nino, a temporary warming of part of the Pacific Ocean, usually spikes global temperatures, scientists say. The two recent warm years, both 1998 and 2005, were El Nino years. The flip side of El Nino is La Nina, which lowers temperatures. A La Nina bloomed last year and temperatures slipped a bit, but 2008 was still the ninth hottest in 130 years of NOAA records.
Of the 10 hottest years recorded by NOAA, eight have occurred since 2000, and after this year it will be nine because this year is on track to be the sixth-warmest on record.
The current El Nino is forecast to get stronger, probably pushing global temperatures even higher next year, scientists say. NASA climate scientist Gavin Schmidt predicts 2010 may break a record, so a cooling trend "will be never talked about again."
AP Only one problem: It's not true, according to an analysis of the numbers done by several independent statisticians for The Associated Press.
The case that the Earth might be cooling partly stems from recent weather. Last year was cooler than previous years. It's been a while since the super-hot years of 1998 and 2005. So is this a longer climate trend or just weather's normal ups and downs?
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.
"If you look at the data and sort of cherry-pick a micro-trend within a bigger trend, that technique is particularly suspect," said John Grego, a professor of statistics at the University of South Carolina.
Yet the idea that things are cooling has been repeated in opinion columns, a BBC news story posted on the Drudge Report and in a new book by the authors of the best-seller "Freakonomics." Last week, a poll by the Pew Research Center found that only 57 percent of Americans now believe there is strong scientific evidence for global warming, down from 77 percent in 2006.
Global warming skeptics base their claims on an unusually hot year in 1998. Since then, they say, temperatures have dropped - thus, a cooling trend. But it's not that simple.
Since 1998, temperatures have dipped, soared, fallen again and are now rising once more. Records kept by the British meteorological office and satellite data used by climate skeptics still show 1998 as the hottest year. However, data from the National Oceanic and Atmospheric Administration and NASA show 2005 has topped 1998. Published peer-reviewed scientific research generally cites temperatures measured by ground sensors, which are from NOAA, NASA and the British, more than the satellite data.
The recent Internet chatter about cooling led NOAA's climate data center to re-examine its temperature data. It found no cooling trend.
"The last 10 years are the warmest 10-year period of the modern record," said NOAA climate monitoring chief Deke Arndt. "Even if you analyze the trend during that 10 years, the trend is actually positive, which means warming."
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."
One prominent skeptic said that to find the cooling trend, the 30 years of satellite temperatures must be used. The satellite data tends to be cooler than the ground data. And key is making sure 1998 is part of the trend, he added.
It's what happens within the past 10 years or so, not the overall average, that counts, contends Don Easterbrook, a Western Washington University geology professor and global warming skeptic.
"I don't argue with you that the 10-year average for the past 10 years is higher than the previous 10 years," said Easterbrook, who has self-published some of his research. "We started the cooling trend after 1998. You're going to get a different line depending on which year you choose.
"Should not the actual temperature be higher now than it was in 1998?" Easterbrook asked. "We can play the numbers games."
That's the problem, some of the statisticians said.
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.
Apart from the conflicting data analyses is the eyebrow-raising new book title from Steven D. Levitt and Stephen J. Dubner, "Super Freakonomics: Global Cooling, Patriotic Prostitutes and Why Suicide Bombers Should Buy Life Insurance."
A line in the book says: "Then there's this little-discussed fact about global warming: While the drumbeat of doom has grown louder over the past several years, the average global temperature during that time has in fact decreased."
That led to a sharp rebuke from the Union of Concerned Scientists, which said the book mischaracterizes climate science with "distorted statistics."
Levitt, a University of Chicago economist, said he does not believe there is a cooling trend. He said the line was just an attempt to note the irony of a cool couple of years at a time of intense discussion of global warming. Levitt said he did not do any statistical analysis of temperatures, but "eyeballed" the numbers and noticed 2005 was hotter than the last couple of years. Levitt said the "cooling" reference in the book title refers more to ideas about trying to cool the Earth artificially.
Statisticians say that in sizing up climate change, it's important to look at moving averages of about 10 years. They compare the average of 1999-2008 to the average of 2000-2009. In all data sets, 10-year moving averages have been higher in the last five years than in any previous years.
"To talk about global cooling at the end of the hottest decade the planet has experienced in many thousands of years is ridiculous," said Ken Caldeira, a climate scientist at the Carnegie Institution at Stanford.
Ben Santer, a climate scientist at the Department of Energy's Lawrence Livermore National Lab, called it "a concerted strategy to obfuscate and generate confusion in the minds of the public and policymakers" ahead of international climate talks in December in Copenhagen.
President Barack Obama weighed in on the topic Friday at MIT. He said some opponents "make cynical claims that contradict the overwhelming scientific evidence when it comes to climate change - claims whose only purpose is to defeat or delay the change that we know is necessary."
Earlier this year, climate scientists in two peer-reviewed publications statistically analyzed recent years' temperatures against claims of cooling and found them not valid.
Not all skeptical scientists make the flat-out cooling argument.
"It pretty much depends on when you start," wrote John Christy, the Alabama atmospheric scientist who collects the satellite data that skeptics use. He said in an e-mail that looking back 31 years, temperatures have gone up nearly three-quarters of a degree Fahrenheit (four-tenths of a degree Celsius). The last dozen years have been flat, and temperatures over the last eight years have declined a bit, he wrote.
Oceans, which take longer to heat up and longer to cool, greatly influence short-term weather, causing temperatures to rise and fall temporarily on top of the overall steady warming trend, scientists say. The biggest example of that is El Nino.
El Nino, a temporary warming of part of the Pacific Ocean, usually spikes global temperatures, scientists say. The two recent warm years, both 1998 and 2005, were El Nino years. The flip side of El Nino is La Nina, which lowers temperatures. A La Nina bloomed last year and temperatures slipped a bit, but 2008 was still the ninth hottest in 130 years of NOAA records.
Of the 10 hottest years recorded by NOAA, eight have occurred since 2000, and after this year it will be nine because this year is on track to be the sixth-warmest on record.
The current El Nino is forecast to get stronger, probably pushing global temperatures even higher next year, scientists say. NASA climate scientist Gavin Schmidt predicts 2010 may break a record, so a cooling trend "will be never talked about again."
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the Roman Warm Period, a 2000 year trend? That shows a terribley[sic]
cooling trend."
Bunk: See http://www.realclimate.org/images/m08.jpg
You have perhaps a gradual cooling up to about 1850, then several
times that amount of warming afterward.
You're pretty authoritative in your dismissal of the Roman Warm, and the Medieval Warm Periods. Is your recitation of climate history also based on the linked graph?
you're referring to.
The linked graph is just one of many I could have picked, but includes data from several
different sources and is relatively easy to read. It's drawback is that it's just the northern
hemisphere, but I was a bit rushed to find a global one for that time period.
It is much more relevant to reference heating/cooling to the long term heating that has been taking place since 10000 and 400 years ago, as this is the only relevant information to guide any political / economic decisions. It is obvious that humankind cannot control what nature does at that scale.
Thus, ignoring this factor makes the analysis suspicious, and continues feeding the skeptics. The real question should have been: is there statistical evidence that the earth has become cooler than the natural temperature rise?
Carlos W. Moreno
If you have data, please cite it.
If these trends continue we will see decades of cooling in the future. Try to get into astrophysics and astronomy and read about Jovian cycles, barycenters, strong correlations between deep solar minima as the Maunder, Wolff, Sp?erer, Dalton, etc, and deep coolings. The present solar minimum compares with the Dalton (1795-1823) and astronomers --a bad word in the AWG debate, I know :-), are predicting a 50-90-year deep cooling. And that really are bad news.
Only for those seeking "creative interpretations" (I'm being polite.)
There are standard statistical methods used to separate signal from
noise, the simplest being low-degree polynomial curve fitting using
something like a least squares fitting. Using Occam's razor, that's
where one should begin unless there are convincing reasons to do
otherwise. One should also look at periods that are significantly
longer than the typical periods of known noise factors. Since El Nino
and similar oscillation events last for about 1 to perhaps 5 years,
"trends" of less than about 10 years start to become meaningless.
"If you want a loo-o-o-ng trend then what about a trend beginning in
the Roman Warm Period, a 2000 year trend? That shows a terribley[sic]
cooling trend."
Bunk: See http://www.realclimate.org/images/m08.jpg
You have perhaps a gradual cooling up to about 1850, then several
times that amount of warming afterward.
When you attempt to fit simple curves to the global temperatures for
time periods that span 1850 you get a lot of variance (i.e. a poor
fit). But if you split the data into two curves, one from the distant
past (anywhere from 400 to 600,000 years ago) up to 1850, and another
from 1850 to the present, you get two distinct curves each with much
lower variance. And further splitting doesn't reduce the variance by
much. That indicates something very significant and unique happened
abound 1850 that initiated a new regime. The dramatic rise in
atmospheric CO2 that began then is both highly correlated (about
0.96!) and makes physical sense as an explanation, so it is the
obvious candidate explanation. Nothing else comes close.
"But during the 20th Century we had four cooling and four warming
trends of about 30 years each. It is all about solar cycles. The
irrefutable data analysis of the past 150 years clearly indicates the
existence of a 230-year-cycle and a 65-year-cycle, in addition to the
larger 1000-year-cycle. Since all three cycles had their maxima around
the year 2000 the number of relatively warm years was quite natural."
You provide no evidence, but I assume you're referring to
"Phenomenological solar signature in 400 years of reconstructed
Northern Hemisphere temperature record", by Scafetta & West, since it
seems to be a current favorite among the denialists.
This has been thoroughly debunked. The authors made the gross error
of basing their conclusions on an ill-conditioned formula. (See any
text on numerical analysis for the definition of ill-conditioned, but
the basic idea is that an algorithm is ill-conditioned when small
variances in the arguments lead to large variances in the answers.)
When their methodology is applied to data from other centuries it
predicts the 20th century would warm by one thousand degrees!
There was something of a cooling trend from 1940 to 1970, coincident
with vastly increased aerosol releases post WW2. But even that is
best described as more of a lull in the heating as opposed to actual
cooling if you look at global temperatures and not just the northern
hemisphere.
More signficantly, note that many glaciers disappearing now have
persisted for several hundred thousand years. This pretty much
completely discredits the notion that the cycles you allude to are
terribly significant, or such glaciers would not have survived through
several earlier coincident maxima. Something is happening now that
has not occurred in over a million years, and searching for magic
cycles to explain it smacks of desperation.
"astronomers --a bad word in the AWG debate, I know :-), are
predicting a 50-90-year deep cooling. "
And which astronomers would that be?
[I might not have a chance to post again for the next several days, so
my apologies in advance if I can't reply quickly to any reply you
might have.]
If 2008 was COOLER than 2007, and 2007 was COOLER than 2006, and 2006 was COOLER than 2005, then THERES Is A COOLING TREND.
"Trend" implies something more than 4 or 5 years.
"...it is dead set against modern/industry, technology, and capitalism. As the saying goes "Green is the new red."
--------------------------------------------
Heavens NO.....you have it backwards, since the DENIALISTS have been pushing antiquated technology of burning fossil fuels without any regard to the massive greenhouse gas emissions.
On the other hand, those that can see the harm being done to our environment, are pushing RENEWABLE ENERGY that produces no harmful greenhouse gases and is free from the sun and wind, and newer technology that captures CO2 from fossil fuel emissions such as coal-burning electricity plants, and grows OIL-producing algae that thrive on the CO2 and can be harvested for biofuels -- a win/win situation.
Modern industry must evolve with the times, and not be resigned to living in the past as the constipated conservitards would wish!