From CO² Science

There Has Been Little Net Global Warming Over the Past 70 Years
Volume 3, Number 13: 1 July 2000

In our editorial of 15 June 2000 - The Global Surface Air Temperature Record May Be Significantly in Error - we reviewed a large body of evidence that suggests the highly-hyped "unprecedented global warming" of the past two decades was not unprecedented.  This evidence includes (1) the satellite microwave-sounding-unit temperature record, which in the absence of the massive 1998 El Niño heat pulse shows very little warming from 1979 to the present, (2) the weather-balloon temperature record, which for the same circumstances also shows very little warming, (3) the surface- and satellite-derived temperature records of earth's polar regions, which also show very little warming, and (4) the high-quality U.S. Historical Climatology Network data base, which, not surprisingly, also shows very little warming over this period.  We now augment this substantial body of empirical evidence with observations gleaned from tree-ring reconstructions of surface air temperature.

First, there is the growing-season temperature history of the entire northern boreal forest region, which has been published most recently in the review of Briffa (2000) and is referred to by him as "the best overall indicator to date of long-term temperature changes over the higher northern land areas."  Derived from a large number of tree-ring density chronologies obtained from some 400 sites in the western United States, Canada, Europe, Fennoscandia and northern Siberia, this temperature record shows a dramatic departure from the instrumental temperature record over the last 70 years, with the instrumental record depicting dramatic 20th century warming, but with the tree-ring record showing nothing of the sort.  And the reason for the discrepancy? In the words of Briffa, "the reason is not known," leaving open the possibility that the instrumental temperature record may be wrong.

Second, there is the somewhat contradictory story told by a number of temperature reconstructions derived from tree-ring width chronologies.  As Briffa (2000) recounts it, "tree-growth, as represented in various standardized tree-ring chronologies in various parts of the world, often seems anomalous in the 20th century as compared to earlier centuries."  This widespread anomaly is extremely important, for he notes that "the recent high growth rates . . . provide major pieces of evidence being used to assemble a case for anomalous global warming, interpreted by many as evidence of anthropogenic activity," specifically mentioning Mann et al. (1998, 1999) in this regard.  But as Briffa further notes, the empirically derived regression equations upon which the temperature reconstructions are based may be compromised if the growth rates of earth's trees have been substantially enhanced over the past century or so by some other global environmental influence that has increasingly manifested itself over the same time period.

What might this influence be?  Briffa cites a number of possibilities, including the historical rise in the air's CO2 content and a number of plant physiological processes that become increasingly more efficient in response to this phenomenon; and he explains how this influence could significantly differ from that implied by the much less responsive tree-ring density signal described in the preceding paragraph.  Indeed, LaMarche et al. (1984) and Graybill and Idso (1993) demonstrated several years ago that the historical rise in the air's CO2 content could readily explain the anomalous 20th-century tree-ring width expansion; and Briffa states that "widespread evidence is accumulating of 'enhanced' productivity (ring-width, basal area and wood mass) in the 19th and 20th centuries, similar to positive growth trends observed in earlier studies," that is, in the studies of LaMarche et al. and Graybill and Idso.

It is very likely, therefore, that enhanced tree growth induced by the historical rise in the air's CO2 content - possibly augmented by enhanced nitrogen deposition (Idso, 1995) - has been increasing the growth rates of trees all around the world for over a century or more (see, for example, our editorials of 15 April 1999 and 1 April 2000).  Furthermore, this growth enhancement has been accelerating over time (Phillips and Gentry, 1994); and it is this ever-intensifying biological phenomenon that some are using to bolster their claim that the earth is warming at an ever-increasing rate, when such may not really be the case.

In view of the likelihood that there has thus been little net warming of the planet over the past 70 years, during which time the vast majority of all anthropogenically-produced CO2 has been emitted to the atmosphere, we conclude that since there should have been a sizeable CO2-induced increase in atmospheric radiative forcing over this period, there must have been a suite of compensatory negative feedbacks that largely overwhelmed the standard "greenhouse" impetus for warming.  Hence, there would appear to be little reason for supposing that any further man-induced increases in the air's CO2 content would substantially warm the planet either.