Posted on 03/22/2002 7:36:11 AM PST by jimkress
Edited on 05/25/2004 3:02:59 PM PDT by Jim Robinson. [history]
WASHINGTON -- An unusually warm period a millennium ago may have been part of a natural planetary cycle, researchers say in a study of tree rings that scrutinizes the link between human activity and climate change.
The study, appearing today in the journal Science, analyzed ancient tree rings from 14 sites on three continents in the northern hemisphere and concluded that temperatures in an era known as the Medieval Warm Period some 800 to 1,000 years ago closely matched the warming trend of the 20th century.
(Excerpt) Read more at detroitnews.com ...
Abstract:
"Preserving multicentennial climate variability in long tree-ring records is critically important for reconstructing the full range of temperature variability over the past 1000 years. This allows the putative "Medieval Warm Period" (MWP) to be described and to be compared with 20th-century warming in modeling and attribution studies. We demonstrate that carefully selected tree-ring chronologies from 14 sites in the Northern Hemisphere (NH) extratropics can preserve such coherent large-scale, multicentennial temperature trends if proper methods of analysis are used. In addition, we show that the average of these chronologies supports the large-scale occurrence of the MWP over the NH extratropics."
Briffa and Osborn commentary (text only provided below)
Hiding behind a rather dry title, Esper et al., on page 2250 of this issue, provide a new and important vision of the detailed course of changing temperatures throughout the last millennium (1). Their analysis is based exclusively on tree-ring records from 14 locations spread over much of the northern extra-tropics. Though virtually all previous Northern Hemisphere temperature reconstructions use at least some tree-ring data, the authors use many new data and a processing technique that provides a largely independent history of widespread tree-growth variations, which they scale against modern temperature observations to estimate the relative magnitude of past temperature changes.
The new record differs in several respects from that highlighted in the Synthesis of the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (2), which focused on the 1000-year reconstruction of Mann et al. (see the purple line in the figure) (3). This record has a smaller amplitude of century-to-century variability and is consistently at, or near, the upper limit of the range of alternate records produced by other researchers (4-8).
The curve from Esper et al. (pink line) shows a pronounced cold phase in the 17th century, in qualitative agreement with the other records and especially with a record of borehole temperature data (see the figure) (9), more so when the latter are first gridded to reduce bias due to regional concentrations of these records. The borehole data (and data from Mann et al.) are interpreted as indications of true annual temperatures, incorporating both warm season and cold season signals. All records in the figure have been calibrated assuming that they portray annual warmth. It is possible, however, as Esper et al. state, that their tree-growth data are more influenced by summer than winter conditions. This affects not only their own record but also a number of the tree-ring series used in other reconstructions shown in the figure.
To place their record on an absolute scale and allow direct comparison of past temperature changes with 20th century observations, Esper et al. scale their series by matching the magnitude of its multidecadal trends to those in Northern Hemisphere mean (land and marine) annual temperatures from 1900 to 1977. After smoothing to remove year-to-year fluctuations, the records can be matched closely with either the annual or summer mean temperatures, because their trends over this period are very similar.
For the early 17th century, annual temperature estimates from Esper et al. differ by about 0.7ºC from those of Mann et al. [see figure 3 of (1)]. However, when we regressed the record of Esper et al. against nonsmoothed data (see the figure), this difference was reduced to about 0.4ºC. Recalibrating both curves against year-by-year warm season temperatures (10) reduces this difference further to about 0.35ºC.
The results of calibrating any proxy data depend on whether raw or smoothed records are used and on the chosen seasonal temperature predictand. Reconstructions of annual temperature records with predictors that are strongly influenced by summer conditions, assume stationary relationships between proxy and summer climate and between annual and summer climates (and hence between summer and winter). The relationship was stationary over the Esper et al. calibration period, but over other periods it may vary: summer warming of extra-tropical land has progressed at a slower rate than winter warming in the Northern Hemisphere (by about 0.6ºC since 1860, see dotted line in the figure) and this is predicted, on the basis of climate model experiments, to continue over the next century at least.
Whatever the true degree of cold in the 17th century, a surprising aspect of the results of Esper et al. is the indication of equally cold conditions throughout the 12th, 13th, and 14th centuries, where their reconstructed temperatures are consistently well below those indicated by all other records. On the evidence of this new series, the last millennium was much cooler than previously interpreted. The warming of the 20th century is seen more clearly as a continuation of a trend that began at the start of the 19th century, not the early 20th, and an early period of warmth in the late 10th and early 11th centuries is more pronounced than in previous large-scale reconstructions. This warmth also peaks slightly earlier than could be captured in the shorter Mann et al. record and is warmer than in any previous reconstruction.
Even accepting the knotty issue of reconstruction uncertainty, the curve of Esper et al. provides evidence for greater climate swings in the last 1000 years than has yet been generally accepted. We need more independent reconstructions like this, based on improved proxy records, and we need to know why it was once so warm and then so cool, before we can say whether 21st-century warming is likely to be nearer to the top or the bottom of the latest IPCC range of 1.4º to 5.8ºC (2).
References and Notes
1.J. Esper, E. R. Cook, F. H. Schweingruber, Science 295, 2250 (2002).
2.R. T. Watson et al., Eds., Climate Change 2001: Synthesis Report (Cambridge Univ. Press, Cambridge and New York, 2001) [publisher's information].
3.M. E. Mann, R. S. Bradley, M. K. Hughes, Geophys. Res. Lett. 26, 759 (1999) [ADS].
4.J. Overpeck et al., Science 278, 1251 (1997).
5.P. D. Jones et al., Holocene 8, 455 (1998) [GEOREF].
6.T. J. Crowley, T. S. Lowery, Ambio 29, 51 (2000).
7.K. R. Briffa, Quat. Sci. Rev. 19, 87 (2000) [GEOREF].
8.K. R. Briffa et al., J. Geophys. Res. 106, 2929 (2001) [ADS].
9.S. P. Huang, H. N. Pollack, P. Y. Shen, Nature 403, 756 (2000) [GEOREF].
10.K. Briffa, T. Osborn, data not shown.
11.T.O. was supported by the UK Met Office (PB/B3539).
Since the greenhouse models rely on an unidentified amplification effect which is unknown, to create the proposed warming, a simple explanitation like variations in the Sun's output seem far more plausible.
D'OH!
won't slow the fruits and nuts down a bit...
There's lots to debate about the amplification effects incorporated into the various climate models, but my layman's understanding of them is that all are quite specifically identified. The biggest assumed positive feedback mechanism is warming-induced evaporation, which produces water vapor. A 1995-vintage paper on water vapor at www.agu.org reports that the most common value assigned to this is a multiplier of around 0.6 (use this to multiply a model's assumed climate sensitivity value). Another assumed positive feedback mechanism in at least one of the models is warming-induced soil respiration, which the British model used by Cox et al. calculates will add a full degree C to their forecast for 2100. As I say, there's much to debate about these feedback mechanisms, but the modelers do identify them.
THis is the result when left wing politics informs scientific research.
None of these model results should be given ANY credibility until a full sensitivity analysis is performed, published, and independently verified!
I think you're referring to this:
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Mars' polar ice caps are slowly melting.
7 December 2001
PHILIP BALL
The martian ice caps are shrinking. As they are made mostly of frozen carbon dioxide, this evaporation could trigger an increase in Mars' own greenhouse effect.
Images from the Mars Global Surveyor spacecraft show that ice ridges and escarpments have retreated over the past two years or so. The orbiting probe has also captured the ice thickening and thinning with the passing seasons.
The reason for the change is not yet clear. But it means that Mars' climate may be changing. "These observations," say Michael Malin and co-workers at Malin Space Science Systems in San Diego, California, "suggest that the present martian environment is neither stable nor typical of the past."
Malin and his colleagues studied photos of the two ice caps taken between October 1999 and August 20011. The pictures show ridges and pits of ice, some just a few metres wide. In some places, the edges of these features seem to have retreated by up to three metres over the observation period.
In other words, the ice caps have shrunk, irrespective of seasonal changes. The researchers estimate that if all the losses are due to evaporation of carbon dioxide, the amount of this gas in the atmosphere must be increasing by about 1% every martian decade.
Mars' atmosphere is very thin - its pressure is less than 1% of that on Earth - and consists mostly of carbon dioxide. But enough carbon dioxide evaporating from the poles would make a big difference. Because atmospheric carbon dioxide prevents solar heat radiating back into space, it warms the planet."
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As the article says, the reasons for the shrinkage are not clear. Solar variability could be one possibility, but I don't think we know enough about the climatology of Mars to designate that as the only possibility (or even the likeliest). We need more data, obviously.
I expect that within the year we'll see papers with a couple of competing theories as to why the ice caps are shrinking.
;-)
An unusually warm period a millennium ago may have been part of a natural planetary cycle...This is capitalist running dog lies! The capitalist in his SUV did this, and must pay!
Celestial Mechanics, Carbon & Climate - Solving the Puzzle of The Ice Ages
It is the best treatment of this subject I have found, especially the explanation of the 120,000yr glacial 'Milankovitch Cycle'.
Also, a relatively unknown but well researched book on related science, myth and culture is...
I do believe that the climate models are of dubious predictive value for purposes of policy-making. But they do have important heuristic uses in understanding how the climate works and what can prompt change.
You mention that model predictions of greenhouse warming have been so off as to disprove the theory. I don't agree. The low end of some of the old predictions of global mean temperature are not so out of synch with the temperature record (which itself is subject to much interesting debate). Even climatologists well-known as "skeptics" in the climate change debate, such as MIT's Richard Lindzen and UVa's Pat Michaels think it's likely that rising atmospheric CO2 concentrations will result in higher temperatures.
A very thoughtful discussion of the limitations of climate models was published in the November 2001 issue of the peer reviewed scientific journal Climate Research. I don't think it's freely available on the web, but the lead author, Dr. Willie Soon, was very gracious about sending .pdf copy upon request. His email is wsoon@cfa.harvard.edu The citation is Soon, W., Baliunas, S., Idso, S.B., Kondratyev, K.Ya. and Posmentier, E.S. 2001. Modeling climatic effects of anthropogenic carbon dioxide emissions: unknowns and uncertainties. Climate Research 18: 259-275.
It seems intuitively obvious that the earth is not static in terms of weather patterns, volcanos, earthquakes, or rain cycles. So why do the believers of "Mother Earth" and "Gaia" think the earth should present a constant temperature?
Baffling.
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