J.C. Moore Online
Current Events from a Science Perspective

Posts Tagged ‘medieval-warm-period’

The Skeptic's Guide to the Medieval Warm Period

Sun ,01/04/2012

“How can science claim man is the cause of global warming when the temperature of the Earth was much warmer during the Medieval Warm Period (MWP)?”

The science Skeptics are quite adept at casting doubt on climate science by using clever arguments. One of their favorites is “How can science claim man is the cause of global warming when the temperature of the Earth was much warmer in the last millennium during the Medieval Warm Period (MWP)?”  You don’t really have to be a scientist to figure out that there were no accurate temperature records during the Medieval times and that much of the world was unknown.

The Skeptics usually point to historical records such as those by H.H. Lamb, which describe record heat waves in the known world during the MWP. Skeptics rather ignore the fact that H.H. Lamb was so concerned about the effect of global warming that he founded the Climate Research Unit (CRU) to study the Earth’s temperature records. The scientific questions comes down to  (1) whether the MWP was worldwide, (2) how warm the Earth actually was during the MWP, and (3) what caused the MWP?

Since there were no thermometers and no worldwide network of weather stations during the MWP, scientists have used a variety of proxy data from ice cores, isotope ratios, sediments, geological records, and even tree rings to try to reconstruct the temperatures for the last thousand years. Though there are large uncertainties in proxy records and they require careful calibration, they do show a similar pattern as you can see in the figure below, which is made up of 10 different reconstructions. The black line is the instrumental temperature record.* (See end of article for references.)


Though there are wide uncertainties in the proxy temperature records, taken together they form an overall pattern which answers the scientific questions. The proxy records show that that (1) there was a warm period from  AD 1000 to 1200, followed by a cooler period from  AD 1550 to1850  NASA identifies as the Little Ice Age, though it was not a true Ice Age. The record also show that (2) temperatures during the  MWP were quantitatively lower than the temperatures during the latter 20th century. To discover the cause of the MWP, it is necessary to look at another reconstruction.

While studying the cause of the past Ice Ages, scientists have identified the three  main factors which affect the Earth’s temperature, solar irradiance, greenhouse gases (primarily H2O and CO2), and particulates from volcanic activity. Below is a reconstruction of the three main factors controlling the Earth’s temperature. There are many interesting things in the records, but they show (3) the Earth was likely warmer worldwide during the MWP because of the higher solar radiation. It also shows that the solar radiation has been relatively constant during the last century while the other factors, primarily greenhouse gases, have increased.


 While it is possible to dispute or argue about the meaning of any of the individual records, it is rather disingenuous to claim that scientists “ have no data” or to dispute the obvious causes of the current global warming. To emphasize that, below is data from the last century, which is based on scientific records. While H20 and clouds accounts for about 75% of the greenhouse warming, their effect has increased only slightly( about 4%) while the amount of carbon dioxide has increased by about 39% in the last century.

CO2: Man is now putting about 30 billion tons of carbon dioxide into the air each year. About half of it dissolves in the ocean and they have become 20% more acidic in the last century. Much of the rest stays in the air, and  CO2 is building up in air:

Temperature: CO2 is one of the greenhouse gases that warm the Earth, and NASA’s graph shows how its increase is changing the Earth’s temperature. Note:  The effect of particulates from the explosion of Pinaturbo can be seen in the temperature decline from 1991 to 1995.




The Sun: The current global warming is often wrongly attributed to an increase in intensity of the sun. The sunspot activity does not show up above the noise in the temperature record above. The solar irradiance increased slightly until 1960 and has declined slightly since then.


There is the scientific story. Disputes in science are settled by the data. Though Skeptics may dispute the evidence showing the current global warming is caused by man, the question is ” Where is the evidence?”

* References for temperature reconstructions:  The original version of this figure was prepared by Robert A. Rohde from publicly available data from NOAA and the references therein. The article stated: “For the purposes of this comparison, the author is agnostic as to which, if any, of the reconstructions of global mean temperature is an accurate reflection of temperature fluctuations during the last 2000 years. “ The reconstructions used, in order from oldest to most recent publication are:

  • (dark blue 1000-1991): P.D. Jones, K.R. Briffa, T.P. Barnett, and S.F.B. Tett (1998). “High-resolution Palaeoclimatic Records for the last Millennium: Interpretation, Integration and Comparison with General Circulation Model Control-run Temperatures”. The Holocene 8: 455-471. doi:10.1191/095968398667194956
  • (blue 1000-1980): M.E. Mann, R.S. Bradley, and M.K. Hughes (1999). “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations”. Geophysical Research Letters 26 (6): 759-762.
  • (light blue 1000-1965): Crowley and Lowery (2000). “Northern Hemisphere Temperature Reconstruction”. Ambio 29: 51-54. Modified as published in Crowley (2000). “Causes of Climate Change Over the Past 1000 Years”. Science 289: 270-277. doi:10.1126/science.289.5477.270
  • (lightest blue 1402-1960): K.R. Briffa, T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov, S.G. and E.A. Vaganov (2001). “Low-frequency temperature variations from a northern tree-ring density network”. J. Geophys. Res. 106: 2929-2941.
  • (light turquoise 831-1992): J. Esper, E.R. Cook, and F.H. Schweingruber (2002). “Low-Frequency Signals in Long Tree-Ring Chronologies for Reconstructing Past Temperature Variability”. Science 295 (5563): 2250-2253. doi:10.1126/science.1066208.
  • (green 200-1980): M.E. Mann and P.D. Jones (2003). “Global Surface Temperatures over the Past Two Millennia”. Geophysical Research Letters 30 (15): 1820. doi:10.1029/2003GL017814.
  • (yellow 200-1995): P.D. Jones and M.E. Mann (2004). “Climate Over Past Millennia”. Reviews of Geophysics 42: RG2002. doi:10.1029/2003RG000143
  • (orange 1500-1980): S. Huang (2004). “Merging Information from Different Resources for New Insights into Climate Change in the Past and Future”. Geophys. Res Lett. 31: L13205. doi:10.1029/2004GL019781
  • (red 1-1979): A. Moberg, D.M. Sonechkin, K. Holmgren, N.M. Datsenko and W. Karlén (2005). “Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data”. nature 443: 613-617. doi:10.1038/nature03265
  • (dark red 1600-1990): J.H. Oerlemans (2005). “Extracting a Climate Signal from 169 Glacier Records”. Science 308: 675-677. doi:10.1126/science.1107046

(c) 2012 J.C. Moore

Note added on 07/09/2013 : In a new study by the PAGES 2k project, that was  published in Nature Geoscience , 78 researchers from 24 countries worked for seven years on the most extensive climate reconstruction to date of the past 2000 years.  It is based on 511 climate archives from around the world, from sediments, ice cores, tree rings, corals, stalagmites, historical documents and measurements.  Their  graph below confirms the basic “hockey stick” shape of the graph:Pages 2K



George Will: Blinded by Science of Climate Change

Tue ,23/02/2010

When we were kids, we used to spin ’round and ’round until, when we stopped, the horizon would appear to keep spinning. That is how I felt after I read George Will’s article “Blinded by science on climate change” (1). He thinks scientists are bad guys, selling cars is more important than clean air, polluters are good guys, and global warming is a religion. The Washington Post once had an ombudsman to handle complaints from scientists about George Will’s cherry-picking, misconstruance of data, spinning of facts, and ignorance of science. The opinion of most scientists is that George Will is entitled to his own opinions, but not his own facts.

Mr. Will does not get quotes right, either. He claims that Phil Jones, in his BBC interview, admits there has been no statistically significant global warming in the last 15 years. However, what Jones said was “I also calculated the trend for the period 1995 to 2009. This trend (0.12C per decade) is positive, but not significant at the 95% significance level.” Jones also presented data to show that from 1975-2009 ( the last 15 years) the Earth warmed by a statistically significant +0.16 0C per decade and said “most of the warming since the 1950s is due to human activity. “(2) Mr. Will apparently misquoted Jones in an attempt to back up one of his mistaken and much criticized claims. He keeps saying that there has been no global warming since 1998 even though a number of scientists, the author included, have written him to tell him he is misconstruing the data. (3)

Mr. Will started his good guys are bad guys diatribe by criticizing Dr. Rajendra Pachauri, chairman of the International Panel on Climate Change, for denouncing those who deny the effects of climate change on the planet. Dr. Pachauri earns very substantial consulting fees by advising governments on environmental issues but he takes only a $49,000 salary and donates the rest to those less fortunate. Does he sound like a bad guy? For his work, Dr. Pachauri shared the 2007 Nobel Peace Prize, a great honor- except to George Will who, by comparison, has been nominated for a Newsvine Environmental Hall of Shame award. Mr. Will also criticizes California’s Arnold Schwarzenegger for participating in the Western Climate Initiative and “trying to fix the planet on his own”. The WCI is made up of seven Western states and four Canadian provinces and their goal is to curb greenhouse gas emissions by 2012.

Not to be inconsistent, Mr. Will then praises Arizona’s Governor Jan Brewer for suspending participation in the WCI agreement because “she is afraid that strict emission rules may raise the cost of new cars”. It apparently is more important to sell cars than it is to have clean air and water. He also thinks it’s great that the Utah State Legislature, who wishes to exempt Utah from Federal emission standards, is considering a similar action. Mr. Will also praises BP America, Conoco/Phillips, and Caterpillar for withdrawing from the U.S. Climate Action Partnership, a coalition of companies and environmental groups. Mr. Will claims they withdrew because the cap-and-trade legislation is stalled and they are no longer worried about restrictions. Oops, wrong again. Those companies helped draw up the blueprint for Congresses’ cap-and-trade legislation and they still support the original cap-and-trade legislation. They withdrew because they felt the amendments added to the bill would unfairly penalize the petroleum industry. (4) Perhaps they object to the huge financial incentives added to the bill for the coal industry to develop the oxymoronic “clean coal”.

Mr. Will also ignores qualifiers. In his BBC interview, Phil Jones said that the Medieval Warm Period (circa 800 to 1300 A.D.) “may” have been warmer than today. Mr. Will apparently ignored the word “may” in Jones’ response as he claims Jones admitted to the MWP and went on that the MWP “complicates the task of indicting contemporary civilization for today’s supposedly unprecedented temperatures”. Jones said “may” as there is anecdotal evidence of a MWP in Europe but there is little evidence in the proxy data that it was worldwide. It is recorded that the Nile froze over in 1010 A.D., right in the middle of the MWP.(5) Mr. Will wants to save the MWP as he thinks he has a potent argument, but even if the MWP existed, his argument is mostly irrelevant to the documented environmental changes that have occurred in the last century.

Finally, Mr. Will claims (half seriously, he says) that Tom Stern, America’s Special Envoy on Climate Change, has violated the First Amendment which forbids the government from undertaking the establishment of religion. Mr. Will can say that because he claims, “a religion is what the faith in catastrophic man-made global warming has become.” Perhaps Mr. Will is unaware that there is a Christian environmental movement, sponsored by the National Council of Churches and made up of practicing Protestants and Catholics, that promotes good stewardship and respect for the Earth.

(1) http://www.realclearpolitics.com/articles/2010/02/21/blinded_by_science_104494.html
(2) http://news.bbc.co.uk/2/hi/8511670.stm
(3) http://que2646.newsvine.com/_news/2009/10/11/3372180-george-wills-climate-deception
(4) http://www.mnn.com/earth-matters/politics/stories/oil-firms-drop-out-of-group-lobbying-for-us-climate-bill

(5) Lamb, H. H. (1977) Climate: Present, Past and Future: Climatic History and the Future. Vol 2, Methuen and Co. Ltd