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Figure 11.14. Mg/Ca analyses of the white variety of the planktic foraminifera delta, which
were obtained from the northern Gulf of Mexico as a measure of sea surface temperatures
(Richey et al., 2007).
Rorvik et al. (2009) examined sediment cores from a Norwegian fiord to infer
temperature changes over the past thousand years. They found that ''The periods
from c. ad 500 to 790 and c. ad 1500 to 1940, stand out as cold periods.''
Kobashi et al. (2010) derived Greenland temperatures over the past thousand
years using nitrogen and argon isotope data, rather than oxygen isotope data.
Their procedure is complex and appears to involve a number of assumptions.
Nevertheless, they addressed these issues in considerable detail. Their conclusion
was: ''The data show clear evidence of the Medieval Warm Period and Little Ice
Age in agreement with documentary evidence.''
The degree to which the current global warming trend is due to industrializa-
tion and land clearing remains to be determined but, clearly, the range of past
climate variability is large enough that it is possible—at least in principle—that we
are simply witnessing another natural fluctuation.
11.1.4.3 Retreat of mountain glaciers
Holzhauser et al. (2005) discussed the advance and retreat of mountain glaciers as
climate indicators. As they pointed out:
''Glaciers in mountain areas are highly sensitive to climate changes and thus
provide one of nature's clearest signals of warming or cooling and/or dry and wet
 
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