Late Holocene cooling and the LIA to present

As just discussed, the HTM in some regions occurred quite early, while for others it ended as late as 3 ka. The HTM was nevertheless followed by a period of cooling. For example, reconstructions by Koc etal. (1993) indicate that by about 5 ka, warm Atlantic waters had retreated to the central GIN seas, attended by strengthening of the cold East Greenland Current. By 3 ka, the sea ice cover had expanded along eastern Greenland.

A variety of sources points to a subsequent period of warming between the ninth and fourteenth centuries. During this Medieval Warm Epoch, temperatures may have been higher than at the start of the twentieth century in some areas, such as Scandinavia and Greenland. Historical evidence includes records of crop planting in more northern latitudes than the present. However, the picture is still unclear. Hughes and Diaz (1994) give a useful review.

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Figure 10.10 Reconstructed Arctic temperatures over the past 400 years (each panel) along with estimated time series of (a) methane, (b) atmospheric carbon dioxide concentration, (c) total irradiance, (d) sulfate. Asterisks indicate volcanic events known to be over-represented in the GISP-2 ice core record (from Overpeck et al., 1997, by permission of AAAS).

Figure 10.10 Reconstructed Arctic temperatures over the past 400 years (each panel) along with estimated time series of (a) methane, (b) atmospheric carbon dioxide concentration, (c) total irradiance, (d) sulfate. Asterisks indicate volcanic events known to be over-represented in the GISP-2 ice core record (from Overpeck et al., 1997, by permission of AAAS).

The LIA represents the most recent major cooling (Grove, 2004). The LIA is dated anywhere from 1250-1920 to 1550-1850 (Christiansen, 1998) and is clearly recorded in Greenland ice cores (Fischer et al., 1998). Records at GISP2 indicate that temperatures reached their lowest point in the past millennium between 1579 and 1730 (Kreutz et al., 1997). Temperatures in the northern to mid latitudes dropped an average of 0.5 to 1.2 °C. In Norway, advance and retreat histories of glaciers during the LIA seem to have varied according to distance of the glaciers from moisture sources (Benn and Evans, 1998). Evidence in the form of lichen-free zones on the plateau of north Baffin Island suggests lowering of the snowline (Locke and Locke, 1977), an analog of the processes envisioned in the theory of "instantaneous glacierization", although this interpretation is open to question. Causes of the LIA are highly debated and rather uncertain. Volcanism played some part according to Crowley (2000); volcanic output increased over 40% from 1400 to 1850 and this would lower atmospheric temperatures. The Maunder Minimum from 1680 to 1730 was a period of near absence of sunspots. Low numbers of sunspots are known to reduce solar output. Lean et al. (1995) attribute most of the temperature variability in the Northern Hemisphere from 1610 to 1800 to irradiance changes.

The LIA has in turn been followed by warming. Overpeck et al. (1997) examined Arctic temperature changes over the past 400 years, using a time series based on tree rings and other proxy sources. The time series is most representative of summer conditions. They attempted to explain variability in this reconstructed record in terms of the relative roles of changes in atmospheric trace gases, solar variability and aerosol loading from volcanic eruptions. Their results are summarized in Figure 10.10. Each panel shows the same reconstructed temperature time series, but with overlaid time series of the different climate forcings - methane concentration, carbon dioxide concentration, total solar radiation (irradiance) and sulfate concentration. Their reconstruction indicates that the coldest conditions in the Arctic of the past 400 years occurred around 1840. From their statistical analysis, based on correlating different parts of the record against the time series of estimated forcings, they conclude that the general warming trend from about 1820 to 1920 was primarily linked to reduced forcing by volcanic aerosols and increasing irradiance. Both high irradiance and low aerosol forcing likely continued to influence Arctic climate after 1920, but exponentially increasing concentrations of atmospheric greenhouse gases (carbon dioxide and methane) probably played an increasingly dominant role. The reconstructed forcings, especially irradi-ance, are open to question. It is nevertheless notable that the late twentieth century appears to have been the warmest period in the Arctic of the past 400 years. While a human influence on recent Arctic warming is suggested from these results, this is a lively area of debate. Which leads us to our next and final chapter.

Recent climate variability, trends and the future

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