Posted on 10/21/2003 6:32:19 AM PDT by blam
Source: University Of Arizona
Date: 2003-10-20
Medieval Climate Not So Hot
The idea that there's no need to worry about human-induced global warming because the world's climate in medieval time was at least as warm as today's is flawed, according to a recent analysis.
There's not enough evidence to conclude that the Medieval Warm Period was global, or that regional warm spells between 500 and 1500 A.D. occurred simultaneously, leading paleoclimatologists report in the Oct. 17 issue of Science.
"The balance of evidence does not point to a High Medieval period (1100 to 1200 A.D.) that was as warm or warmer than the late 20th century," the team wrote. They pointed out that temperatures during High Medieval time in the Northern Hemisphere were almost the same as they were from 1901 to 1970, a time period which was cooler than the last three decades of the 20th century by about one-third of a degree Celsius (six-tenths of a degree Fahrenheit).
The authors of the article, "Climate in Medieval Time," are Raymond S. Bradley of the University of Massachusetts - Amherst, Malcolm K. Hughes of the University of Arizona Laboratory of Tree-Ring Research, and Henry F. Diaz of the National Oceanic and Atmospheric Administration Office of Atmospheric Research.
The scientists concluded that average temperatures in the Northern Hemisphere "were not exceptional" because some regions cooled whereas other regions warmed. The Northern Hemisphere is the region of the world for which scientists have the best natural records of past climate.
To reconstruct climate, scientists use natural archives, including tree rings, ice cores and laminated sediments. These natural records of temperature, precipitation and other environmental history have been carefully calibrated with instrumental observations, giving scientists quantitative information on past conditions at particular locations.
Scientists have well-calibrated, detailed data from such proxy records covering the past 1,500 years of temperature for only a few locations around the globe, the researchers noted. Most of these records are from temperate zones in the Northern Hemisphere. Few come from the tropics, and only a handful come from the Southern Hemisphere.
A common argument for the existence of a global Medieval Warm Period is that the solar radiation hitting Earth's atmosphere, or solar irradiance, was as high in medieval time as in the 20th century. Therefore, the argument goes, 20th century global warming has been largely driven by the sun, not by higher concentrations of heat-trapping greenhouse gases in the atmosphere.
However, recent modeling studies show that increased solar irradiance does not warm Earth's surface at all locations, the research team wrote. Rather, ozone absorbs ultraviolet radiation, warming the stratosphere and altering atmospheric circulation patterns. If such changes happened in the 12th century, they could well have altered large-scale atmospheric circulation patterns linked to the Arctic Oscillation, thereby warming some regions but not others.
Frequent volcanic eruptions from 1100 to 1260 A.D. may have added to regional warming. Volcanic eruptions may have resulted in mild winters in northern and western Europe during High Medieval time, just as erupting volcanoes have led to very warm winters in northern Europe and northwestern Russia in the 20th century, they added.
As first articulated in 1965, the concept of a "Medieval Warm Period" pertained only to western Europe during the High Medieval time, the authors noted.
But scientists have since stretched the term to include any climatic anomaly that occurred after the fall of Rome in 476 A.D. to the dawn of the Renaissance, around 1500 A.D., even if the climate aberration had nothing to do with temperature.
"The ill-defined evidence for a range of climate anomalies occurring over a wide time interval has created the notion that the MWE (Medieval Warm Epoch) was a definitive global phenomenon," the team wrote.
The proxy records reveal other important components of medieval climate that need to be better understood. Bradley, Hughes and Diaz said there is evidence for unusually intense and prolonged droughts in several parts of the world, including the western United States, from 900 to 1300 A.D.
Understanding the underlying mechanisms of persistent droughts and wet periods in medieval time has to be a priority, they added: "With more than 10 times as many people on Earth as in High Medieval time, (a recurrence) could be catastrophic."
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This story has been adapted from a news release issued by University Of Arizona.
How about the archives that include the facts that people lived and grew crops in Iceland? Or is that too subjective?
Well-calibrated??? How do you calibrate tree ring data which tells you more about the length of the growing season than the tempreture?
However, recent modeling studies show that increased solar irradiance does not warm Earth's surface at all locations, the research team wrote.
The is just great scientific work. Use your computer models to prove your theory. Of course your models assume your theory is correct. Computers just tell you what you told them to tell you, which isn't proof of anything.
Uh, the calibration is inherent in the ring patterns themselves. Individual rings may be wider or narrower due to variations in climate, but there is always "one set of rings per year". Measurement of the stable isotope ratios of oxygen (I believe oxygen 16 vs. oxygen 18 are the ones used) within any given ring tell you the average temperature over that ring/year.
Do a web search on "dendrochronology" to learn more.
Gee, what does it tell us when a single ring or series of rings is wider on one side of the tree than on the other?
Because it's a decimal: 0.6.
Exactly. Tree rings are very dependent on rainfall. Also most climatologist believe volcanic eruptions lead to global cooling.
The problem is that politics (and maybe grant money(?)) has been such a major motivator in this argument that it is impossible to distinguish fact from propaganda.
Basically that the tree was lopsided. But that doesn't change anything about the validity of the isotope ratio temperature data. In your postulated case, one would take measurements in the wide and narrow areas and compare them.
It tells you the tree was leaning. The ratios of width ring-to-ring are still the same on each side. Wider rings are the same percentage wider than narrower rings.
Which is why you need the isotope ratio, as that is independent of rainfall. The tree ring pattern gives you the dates, the isotope ratios in the rings give you the temperatures for the year of a specific date.
I suspect they also cross check these against the same isotope ratios in seashells deposited in silt layers (again, the silt layer gives you the year, and the isotope ratio again gives temperature).
You have to remember that these measurements always involve many, many data points--they are not trying to get one temperature from a single tree ring.
Let me know if you wish to be added or removed from this list.
I don't get offended if you want to be removed.
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