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Twentieth-Century Warming of the Northern Hemisphere
Volume 7, Number 41: 13 October 2004

In a monumental undertaking, Briffa et al. (2002) worked with a network of high-latitude and high-elevation tree-ring chronologies obtained from 387 conifer sites that circle the extra-tropical Northern Hemisphere (comprised of almost 10,000 individual tree cores with chronologies ranging in length from 85 to over 600 years) to reconstruct what they describe as a "near-hemispheric scale" April to September temperature history of the land area of the northern half of the planet that stretches back in time a full six centuries.  Based on the trees' maximum latewood density, this temperature reconstruction exhibited a strong, spatially coherent response to decadal-scale temperature variability; but because of the particular standardization procedure they employed in their analysis, Briffa et al. say that their reconstruction "is likely to remove the century to multicentury timescale power."  Consequently, we here restrict our discussion to a consideration of what their work implies about the warming of the Northern Hemisphere in the late 20th-century.

The year of coldest growing-season temperature in Briffa et al.'s reconstruction was 1912, after which temperatures rose in zigzag fashion to about 1930, whereupon they flattened out until approximately 1945, after which they declined in zigzag fashion to somewhere in the late 1970s, where they began to fluctuate about a well-defined mean that persisted to the end of the century.  At the dawning of the new millennium, therefore, Briffa et al.'s proxy temperatures were considerably below the peak warmth of the 1930s and early 40s, in striking contrast to the instrumental temperatures, which soared to new heights in the 1980s and 90s to achieve the century's highest values, which were several tenths of a degree Centigrade greater than the temperatures derived from the 10,000 trees' maximum latewood density data.  And, of course, it was the instrumental temperatures that Mann et al. (1998, 1999) used in lieu of the proxy temperatures over the latter part of the century to arrive at what they have called the "unprecedented" warming of its last two decades.

The dilemma we thus face is this: is it appropriate to "switch horses" part-way through the century and compare "apples and oranges," i.e., early-century proxy temperatures with late-century instrumental temperatures, to reach the conclusion that the 20th century experienced unprecedented warming over its final two decades, as Mann et al. do?  Or is it more appropriate to finish the dance with the parameter with which you began, which course leads to the conclusion that the end of the century was likely no warmer than it was in the 1930s and early 40s?

Briffa et al. approach this dilemma by first acknowledging the existence of the problem, which they describe as "a deviation between reconstructed and observed temperature during the most recent three or four decades," noting that the deviation increases with time, "particularly since about 1970 (although perhaps starting as early as 1935)."  They then admit that they cannot find a "substantiated explanation for it," so, instead, they "make the assumption" that the deviation "is likely to be a response to some kind of recent anthropogenic forcing," with which conclusion we agree.  Last of all, however, they make the most crucial assumption of all, with which we do not agree, i.e., that the anthropogenic forcing is perturbing the proxy temperature record.

A much less convoluted course of action is to assume that the anthropogenic forcing is perturbing the instrumental temperature record.  Why?  Because there does exist a well known and "substantiated explanation" for this point of view, i.e., the hypothesis that the ever-increasing population of the planet is ever increasing the strength of the urban heat island effect nearly everywhere on the face of the earth, but that sufficiently accurate adjustments to the instrumental temperature record for this phenomenon have not yet been made everywhere.

Since 1970, for example, the world's population has grown by approximately 64%, while since 1935 (when the first signs of the deviation between the proxy and instrumental temperature records began to occur) the number of people on the planet has grown by over 200%.  Consequently, since urban heat islands routinely raise city air temperatures several degrees Centigrade above the temperatures of their surrounding rural environs (see Urban Heat Island in our Subject Index), and since towns with as few as a thousand inhabitants have been demonstrated to possess heat islands of two degrees Centigrade or more (Oke, 1973; Torok et al., 2001), it is clear that the world's historical population growth could easily have produced a gradually-increasing elevation of the instrumental temperature record (which is typically developed from temperatures measured in cities and towns) relative to the basically rural temperature record typical of the locations where the tree-ring chronologies studied by Briffa et al. were obtained.  Therefore, it is equally clear that this phenomenon could easily account for the several tenths of a degree Centigrade by which the proxy and instrumental temperature records differed at the end of the 20th century.

There are also a number of other substantial reasons for accepting our analysis of the diverging-temperature-records dilemma, in lieu of yielding to Briffa et al.'s default assumption.  For one thing, and contrary to what we have just done, they could identify no credible and widely-occurring mechanism for producing the presumed problem they imagined to exist with the late 20th-century proxy temperatures.  Lacking such, in fact, they actually gave up on this point, saying that "to search for an explanation is beyond the scope of this paper."  And why was it beyond the scope of their paper?  Because they could not produce an explanation that would apply to the entire Northern Hemisphere and be readily comprehended by most reasonable people as having at least the potential to be correct.

Secondly, what kind of "recent anthropogenic forcing" could possibly have had such a huge negative effect on tree growth and wood density over the latter part of the 20th century?  Atmospheric pollution is one thing that comes to mind in this regard, especially rising ozone (O3) concentrations.  However, in numerous studies of the net effect of elevated O3 and CO2 acting together on trees, it has been found that the positive effects of elevated CO2 often totally compensate for the negative effects of elevated O3 [see Ozone (Effects on Plants - Tree Species: Aspen, Birch and Miscellaneous) in our Subject Index].  Likewise, it has been found that the positive effects of elevated CO2 also tend to totally compensate for the negative effects of elevated sulfur dioxide concentrations (Deepak and Agrawal, 2001; Izrael et al. (2002); Agrawal and Deepak, 2003).

Third, in addition to countering the negative effects of increasing concentrations of some of the world's worst air pollutants, rising atmospheric CO2 concentrations are known to significantly stimulate the growth rates of essentially all woody plants (see Trees and its many subheadings in our Subject Index).  Fourth and finally, atmospheric CO2 enrichment typically increases the density of the wood produced by trees (see Wood Density in our Subject Index).

In light of these several observations, it is highly unlikely that humanity is doing anything that would be actually reducing the growth and wood density of earth's trees -- especially those growing in the relatively remote locations scattered throughout the entire Northern Hemisphere that were used to develop the data analyzed by Briffa et al. -- that is not being effectively countered by the ongoing rise in the air's CO2 content.  Clearly, therefore, the explanation we have provided for the divergence of the instrumental and proxy temperature records analyzed by Briffa et al. is far more likely to be correct than the unidentified mechanism they have yet to discover, but in which they ask us to blindly believe.

The final upshot of all this, of course, is that the hockeystick temperature record of Mann et al. (1998, 1999) would appear to be patently erroneous over the last few decades of the 20th century, substituting, as it does, the erroneous and increasingly over-inflated instrumental temperature record for the more correct and stable proxy temperature record.  It will be difficult for most climate alarmists and many politicians to accept this conclusion, however, for with it must come the admission that (1) the temperature history of the 20th century has been almost totally determined by natural factors that are independent of the air's CO2 content, and that (2) the mean growing-season temperature of the 20th century was no warmer at its end than it was in the 1930s and early 40s, as we have long claimed.  Nevertheless, the day will come when the truth will be so obvious that they will be forced to admit it.  If they were wise, therefore, they would do it now; for history will not be kind to those who could see the truth of this matter in our day but adamantly refused to acknowledge it.

Sherwood, Keith and Craig Idso

Agrawal, M. and Deepak, S.S.  2003.  Physiological and biochemical responses of two cultivars of wheat to elevated levels of CO2 and SO2, singly and in combination.  Environmental Pollution 121: 189-197.

Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jones, P.D., Shiyatov, S.G. and Vaganov, E.A.  2002.  Tree-ring width and density data around the Northern Hemisphere: Part 1, local and regional climate signals.  The Holocene 12: 737-757.

Deepak, S.S. and Agrawal, M.  2001.  Influence of elevated CO2 on the sensitivity of two soybean cultivars to sulphur dioxide.  Environmental and Experimental Botany 46: 81-91.

Izrael, Yu.A., Gytarsky, M.L., Karaban, R.T., Lelyakin, A.L. and Nazarov, I.M.  2002.  Consequences of climate change for forestry and carbon dioxide sink in Russian forests.  Isvestiya, Atmospheric and Oceanic Physics 38: S84-S98.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Oke, T.R.  1973.  City size and the urban heat island.  Atmospheric Environment 7: 769-779.

Torok, S.J., Morris, C.J.G., Skinner, C. and Plummer, N.  2001.  Urban heat island features of southeast Australian towns.  Australian Meteorological Magazine 50: 1-13.