Page 321 - Earth's Climate Past and Future

CHAPTER 16 • Climate Changes During the Last 1000 Years 297

BOX 16–1 TOOLS OF CLIMATE SCIENCE

CONTINUED

and the instrumental record of climate change, scientists gradual warming in cold regions or a trend toward wetter
determine which aspects of climate control tree growth. climates in dry regions.
The final step is to use correlations defined from the In arid regions, some evidence suggests that rising
calibration period of tree ring and climate data to model CO levels and faster fertilization of leaf pores have
2
the tree ring/climate relationship and to estimate past cli- reduced the exposure of vegetation to the dry air that
mate using the tree ring data as predictors. Projections normally constrains growth in these regions. If this evi-
into the past assume that the climatic controls on tree ring dence is valid, then CO fertilization may have been a fac-
2
properties found over the calibration interval were also in tor in faster tree growth in dry regions, regardless of other
operation during the earlier interval. climate changes.
In cold regions, temperature early in the growing sea-
son is often the strongest influence on tree growth, but a Measured tree ring record
response to temperatures in other seasons is also present.
In relatively arid regions, precipitation is more important.
In some cases, precipitation in one year can affect growth
in the next by providing moisture to the soil, which
2 Use calibration 1 Calibrate
favors growth the following spring. On the other hand, to convert and verify
deep winter snows can slow growth in the spring in colder earlier tree ring record tree ring
to estimates of record to
areas.
past temperature observed
Many records from the circum-Arctic margins show an temperature
unexpected decrease in tree ring growth in the last several
decades, despite the fact that temperatures have been
warming and CO (fertilization) levels rising. This seem-
2 Observed
ingly anomalous trend is not well understood. Estimated temperature
temperature
The period of instrumental observations has also been
a time of steadily rising atmospheric CO levels. Con- 1600 1700 1800 1900 2000
2 Year
trolled experiments with vegetation in greenhouses show
that tree growth is enhanced by higher levels of CO and Calibrating tree ring signals Tree ring analysis is based on
2
photosynthesis. Some scientists speculate that correla- correlating the width (or density) of individual tree rings with
tions between tree ring properties and climate (tempera- monthly changes in temperature and precipitation recorded in
the last half of the instrument record of climate (the last few
ture and precipitation) during the calibration interval
decades). After these relationships are tested against the first
(the interval for which tree ring data can be compared
half of the instrumental record, they are used to convert older
against climatic measurements) have ignored the “fertil- tree-ring characteristics into estimates of past temperature
ization” effect of rising CO on plants. CO fertilization and precipitation using the entire instrumental record for
2 2
could masquerade as a climate signal, falsely indicating calibration.

similar to that of the circum-Arctic region. Intervals of of that region is ocean. Records from pines on the island
warmth occurred during the Little Ice Age, both in the of Tasmania, south of Australia, extend back more than
mid-eighteenth century and earlier, and colder temper- 2000 years, and the warmth of the last several decades
atures occurred in the late sixteenth, late seventeenth, matches any levels reached during the 2000-year inter-
and mid- to late nineteenth centuries. The warming of val (Figure 16–8). The interval of the Little Ice Age after
the middle and late twentieth century appears unprece- 1500 is cooler than the late twentieth century but does
dented within the 450 years of record. These signals not stand out distinctively against earlier centuries.
come from larch and pine trees growing on the high Other tree ring signals from the southern hemi-
flanks of mountains in Central Asia (Figure 16–7D). sphere vary considerably in character. Records from New
Far fewer data are available from trees at high lati- Zealand, Chile, and Argentina extending back a few hun-
tudes of the southern hemisphere, in part because most dred years show unique warmth in the twentieth century.