Late Holocene

The later part of the Holocene is by far the most well-documented period of paleoclimatic history, this is largely due to the presence of numerous, independent, and well-preserved archives which can be precisely dated and measured with annual resolution. Notable among such extremely precise paleoclimatic records are documentary archives, rings from living and fossil trees, and growth bands from speleothems (cave deposits) and tropical corals, and annual accumulation layers from high altitude ice cores. Inevitably, attention focuses on the so-called Little Ice Age and Medieval Warm Period. Well chronologically constrained evidence for climate changes broadly paralleling, but rarely perfectly synchronous with, the European Medieval Warm Period—Little Ice Age sequence has been identified in records right across the northern hemisphere (Alverson and Solomina 2004) and into low latitudes—for example, in ice core records from Quelccaya Ice Cap in Peru.

O Figure 12.7

Holocene lake level records from much of Africa show dramatic oscillations well beyond the range of variability captured by the instrumental record. (a) A record from lake Abhe Ethiopia showing periods of extreme drought lasting for centuries with associated lake level changes of order 100 m (Gasse 2000). (b) A higher resolution lake level record for the past millennium from lake Naivasha, Kenya showing periods of multidecadal drought and lake level variations of order 10 m (Verschuren 2000). Over this time period, lake level is well correlated with atmospheric 14C, a proxy for solar activity (figure from Oldfield and Alverson 2002)

Was the Medieval Warm Period as warm as the 1990s? Long tree ring chronologies are one of the most important sources of information available to address this question. > Figure 12.8 shows a recent, extratropical, treering based reconstruction (Esper et al. 2002) compared with an earlier hemispheric, multiproxy reconstruction (Mann et al. 1999) and their respective uncertainty

O Figure 12.8

Two proxy-based reconstructions of mean Northern Hemisphere Temperature changes over the past millennium including uncertainty levels (Esper et al. 2002) © 2002 American Association for the Advancement of Science

O Figure 12.8

Two proxy-based reconstructions of mean Northern Hemisphere Temperature changes over the past millennium including uncertainty levels (Esper et al. 2002) © 2002 American Association for the Advancement of Science

estimates. The differences between these curves have been the subject of an active debate in the community. Of particular interest has been the question of whether the Northern Hemisphere average warming measured in recent decades is indeed greater than that associated with the peak of the Medieval Warm Period approximately 1000 years ago. The amount of cooling that occurred several centuries later during the Little Ice Age and whether this cooling was geographically restricted to certain regions in the extratropics is another issue of considerable debate.

Notable among many differences in the construction of the curves in O Figure 12.8, the former (Mann et al. 1999) contains records from multiple proxies and from the tropics, whereas the later (Esper et al. 2002) makes use of a novel technique (Regional Curve Standardization) designed specifically to obviate the inherent loss of centennial scale variability in long chronologies constructed from a series ofshort, detrended records spliced together. Rather than highlighting the differences between these two curves, one might consider their similarities. They capture many of the same decadal scale events and lie at least 95% within each others' 95% confidence limits. They both show the remarkable power of long proxy-climate records to put modern changes in the perspective of the past and to use this perspective to better understand natural and anthropogenic drivers of global change. Together, they also indicate some of the most promising areas for future research: better calibration of the growth response of trees to climatic forcing, the need to quantify the range of frequencies that can be reasonably expected to be captured by given reconstructions of past climate variability, and the need to develop long, annually resolved, quantitative, paleoclimatic proxy records from the tropics.

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