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Idaho Forest Fire-Climate Connection: A Testament to the Magnitude of the Medieval Warm Period?
Volume 7, Number 46: 17 November 2004

In a provocative and politically-charged study, Pierce et al. (2004) dated fire-related sediment deposits in alluvial fans in central Idaho, USA, in a research program designed to reconstruct Holocene fire history in xeric ponderosa pine forests and to look for links to past climate change.  Their work centered on tributary alluvial fans of the South Fork Payette (SFP) River area, where fans receive sediment from small but steep basins, in weathered batholith granitic rocks, that are conducive to post-fire erosion.  Altogether, they obtained 133 AMS 14C-derived dates from 33 stratigraphic sites in 32 different alluvial fans.  In addition, they compared their findings with those of Meyer et al. (1995), who had earlier reconstructed a similar fire history for nearby Yellowstone National Park in Wyoming, USA.

Pierce et al.'s work revealed, in their words, that "intervals of stand-replacing fires and large debris-flow events are largely coincident in SFP ponderosa pine forests and Yellowstone, most notably during the "Medieval Climatic Anomaly' (MCA), ~1,050-650 cal. yr BP."  What is more, they note that "in the western USA, the MCA included widespread, severe miltidecadal droughts (Stine, 1998; Woodhouse and Overpeck, 1998), with increased fire activity across diverse northwestern conifer forests (Meyer et al., 1995; Rollins et al., 2002)."

Following the Medieval Warm Period and its frequent large-event fires was the Little Ice Age, when, as Pierce et al. describe it, "colder conditions maintained high canopy moisture, inhibiting stand-replacing fires in both Yellowstone lodgepole pine forests and SFP ponderosa pine forests (Meyer et al., 1995; Rollins et al., 2002; Whitlock et al., 2003)."  Subsequently, however, they report that "over the twentieth century, fire size and severity have increased in most ponderosa pine forests," which they suggest may be largely due to "the rapidity and magnitude of twentieth-century global climate change," i.e., warming, which they say "is probably greater than has occurred for millennia (Crowley, 2000; Bradley et al., 2003)."

With respect to their central thesis, which appears to be well supported by both the SFP and Yellowstone data, we agree with Pierce et al. that the size and severity of large-event stand-replacing fires tend to increase with temperature ? at least in the part of the world and for the specific forests they studied [for examples of contrary fire behavior, however, see Fire in our Subject Index].  We also note, in this regard, that the Yellowstone data additionally depict a sharp drop in large-event fire frequency and severity that coincided with the earlier Dark Ages Cold Period, as well as the preceding peak in such fires that was concomitant with the still earlier Roman Warm Period.  However, we disagree with their seemingly cavalier acceptance of the contention of Crowley and Bradley et al. that 20th-century global warming has been as unprecedented as they say it has, i.e., "probably greater than has occurred for millennia," particularly when their own data suggest otherwise.

By way of explanation, if Pierce et al. expect us to believe that the recent increase in large-event forest fires is chiefly a consequence of 20th-century global warming, they need to acknowledge the fact that their data clearly indicate it must have been much warmer during the Medieval Warm Period than it was over the past century.  Their summed probability distributions for both the SFP and Yellowstone data sets, for example, make the 20th century literally pale in comparison to the "Medieval Climatic Anomaly" in this respect.

In the case of the SFP percentage of total dated thickness, however, such a comparison cannot be made; for Pierce et al. say that "because of calibration problems for very young radiocarbon samples, deposits < 100 cal. yr BP are not shown."  This reason, however, sounds more like a subterfuge, for they plot their data in terms of 100-year totals, which convention merely requires that it be known that a particular fire occurred some time (= any time) in the 20th century; and if they can plot the result for the 19th century (which they do), they clearly know what the 20th century number is, as it is whatever remains at the young end of the data distribution, i.e., above and beyond the data points that make up the 19th-century result.

For Pierce et al.'s paper to be lacking this data point is particularly disturbing to us.  Did it ever appear in the original manuscript?  Was it omitted because it would cause embarrassment to the scientific-political establishment that supports the Kyoto Protocol?  Or was it even removed at the request of someone?  A reviewer, perhaps?  Or possibly an editor?  Or is there an altogether innocuous reason we are somehow missing?  We have never clamed to be omniscient, and we have sometimes missed the obvious, so we could well be way off base in our speculations.  Nevertheless, and whatever the answer is, it needs to be known.  The absence of that final data point, which is truly the most crucial point of all, simply leaves too many questions unanswered.

Sherwood, Keith and Craig Idso

Bradley, R.S., Hughes, M.K. and Diaz, H.F.  2003.  Climate in medieval time.  Science 302: 404-405.

Crowley, T.J.  2000.  Causes of climate change over the past 1000 years.  Science 289: 270-277.

Meyer, G.A., Wells, S.G. and Jull, A.J.T.  1995.  Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes.  Geological Society of America Bulletin 107: 1211-1230.

Pierce, J.L., Meyer, G.A. and Jull, A.J.T.  2004.  Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests.  Nature 432: 87-90.

Rollins, M.G., Morgan, P. and Swetnam, T.  2002.  Landscape-scale controls over 20th century fire occurrence in two large Rocky Mountain (USA) wilderness areas.  Landscape Ecology 17: 539-557.

Stine, S.  1998.  In: Issar, A.S. and Brown, N. (Eds.), Water, Environment and Society in Times of Climatic Change.  Kluwer, Dordrecth, The Netherlands, pp. 43-67.

Whitlock, C., Shafer, S.L. and Marlon, J.  2003.  The role of climate and vegetation change in shaping past and future fire regimes in the northwestern US and the implications for ecosystem management.  Forest Ecology and Management 178: 163-181.

Woodhouse, C.A. and Overpeck, J.T.  1998.  2000 years of drought variability in the central United States.  Bulletin of the American Meteorological Society 79: 2693-2714.