Geography Reference
In-Depth Information
on the whole, dryer than average on the scale of a few centuries. With no weather measure-
ments available, how, one might ask, is it possible to determine yearly weather patterns in
previous centuries? Here, a couple of tough little juniper trees that go by the Latin names
of Sabina przewalskii and S. tibetica come to the rescue. Although not present everywhere
throughout western China, these slow-growing and scrubby little trees find a niche where
precipitation is just high enough to allow for tree growth, and at elevations high enough
to avoid the parching desert below but not so high as to make for an insufficiently long
growing season. Such places are found in the Qilian Shan along the Qinghai-Gansu border,
in moister and warmer areas of southern Qinghai and northeastern Tibet, as well as in
isolated ranges surrounding the Chaidam Basin. Conveniently, individual trees can live
for hundreds of years, and stumps and snags of dead individuals stay on the landscape for
centuries longer. Thus, by measuring the width of rings representing annual growth (and
by careful calibration of those widths with weather conditions during years they could
be recorded), one can reach back hundreds of years to trace general weather patterns in
the area. The longest data set uses trees born as early as 159 C . E .
As with precipitation analyses, studies differ in their choice of geographic area, sample
size, and methods (although most use the same basic procedures), and no two studies
point to exactly the same trends. As well, most studies are more concerned with inferring
the periodicity and amplitude of wet and dry (as well as warm and cool) periods than
with depicting a long-term trend. However, from the various graphs and tables resulting
from this body of work, one can still search for evidence that the recent past has been, on
average, dryer than the distant past. Yet again, there is no evidence that the past fifty years,
or indeed the entire twentieth century, has been a particularly dry period. 22 Indeed, one
study, using 200 trees collected just east of the county town of Dulan, Qinghai, suggested
that the twentieth century was among the wettest of the past two millennia. 23
If precipitation itself has not declined, it could still be that western China has become
drier and generally less hospitable to vegetation growth if temperatures have risen suffi-
ciently to hasten evaporation, making less water available to plants. And unlike claims of
reduced precipitation, there is evidence that temperatures have gone up recently. The same
sources that claim a general drying in western China are quick to blame global warming for
a gradual rise in temperatures. Here, the bulk of the evidence seems to support their claim:
examination of the available data renders it incontrovertible that mean annual temperatures
have increased. 24 (In fact, they appear to have increased throughout most of China, but the
rise has been greater in the Tibetan highlands than the overall mean, with analyses vari-
ously placing the yearly warming trend as between 0.02°C-0.06°C per year.) But even
here, caution is warranted before jumping to the conclusion that such higher temperatures
have been drying out western China despite unchanged or slightly increased precipitation.
In assessing the possible effects of this warming on vegetation, one must consider the sea-
sonality of the trends (because vegetation is differentially affected by temperatures during
various growth stages, and is least affected when it is dormant in winter). Here, it turns out
that the overwhelming majority of mean annual warming is caused not by higher summer
temperatures but by milder winters, 25 a season when desiccation hardly seems a concern.
Finally, it is worth considering what effect the relatively small increase in summer
temperatures would have on vegetation growth in large portions of western China that
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