Grantville Gazette 37

In the Alps, we have the very visible evidence of the advance and retreat of the glaciers. Unfortunately, in the 17^th century, we do not have good maps of their positions, and hence we have to rely on diaries and legal documents. In 1600-19, there are repeated descriptions of the destruction of houses, the failure of crops and the decline in tithes as a result of glacial advance (Ladurie 143ff).

It appears that the glaciers were more quiescent in the 1620s and 1630s, but they remained dangerous. A third of the cultivable land at Chaimonix was destroyed in 1628-30, and the Mattmarksee (influenced by the Allalin glacier) flooded in 1620, 1626, 1629, and 1633 (Aug. 21). In 1636, the people in the valley of Randa thought that the whole Zermatt glacier was coming down on top of them; forty people were killed by ejecta. In the 1640s, there were new glacial advances. In May, 1642, the Les Bois glacier was reportedly moving by 'over a musket shot every day.' Such ominous developments led to the famous June 1644 procession, led by the local bishop, to seek divine intervention to hold the glacier at bay (Ladurie, 165-173).

Wine harvest dates for fifteen locations in the Swiss Plateau and northwestern Switzerland have been used to reconstruct April-August temperatures. Harvest on year-day 285 indicated temperatures identical to those of the base period 1961-90; earlier harvests implying warmer temperatures. In 1632-33, temperatures were a little below base, whereas in 1634-39 they were higher, peaking at 1.78^oC higher in 1638. The temperature anomalies in 1640-43 were negative (Meier).

Northern Italy. The second quarter of the seventeenth century was not marked, as was the first one, by any 'great' winters (enough for large bodies of water to have ice thick enough to support people) or even 'severe' winters (causing the death of animals and plants) (Alfani Graph 1.3). The flooding of tributaries (Tanaro and Bormida) of the Po was perhaps half as common as in the preceding quarter-century, but nonetheless more common than in the next one.

In 1629, a landslide, triggered by heavy rain, caused loss of life and property in the hamlet of Onera. In 1629 -30, a plague epidemic killed about 27% of the population of Northern Italy, but the extent to which climatic factors contributed to its occurrence remains in dispute (Alfani). In 1632, there were complaints about both heat and drought (Marusek 115). In the lower Po valley, cereal yields were 'seriously reduced in the period 1590-1630, especially.' That was, of course, attributable to the flooding (Grove 129).

The Italian price of wheat in the LIA reached a peak just after 1600, then descended to a broad low in the 18^th century, then climbed more moderately to twin peaks in the 19^th (Flohn 44).

Spain. The period 1575-1650 was 'generally wet,' at least in the southeast. 1617 and 1626 were 'deluge' years, and 'catastrophic floods were unusually frequent between 1571 and 1630, especially in Catalonia.' (Grove 129). There was major flood activity in 1630-1650, too (Llasat Fig. 5).

The Black Art of Reconstructing Past Climates

Crude thermometers appeared in the 17^th century, and our oldest continuously monthly temperature records date back to 1659 (for central England). For that region, the coldest winter was in 1684, the coldest summer in 1725, and the coldest year overall was 1704 (Manley). Other early records are those for Berlin from 1697, for Hoofddorp and Zwanenberg/De Bilt in the Netherlands from 1706 and 1735, respectively, for Uppsala (Sweden) from 1739, and for St. Petersburg from 1726 (Flohn).

Clearly, this direct data doesn't tell us anything about what the temperatures were in 1631-39. However, it does help in calibrating 'proxy' data.

A 'proxy' is any observable variable of the fossil record (this term used in a broad sense) that can be reliably correlated with direct temperatures for part of the range of the record, so that the historical temperatures can be reconstructed for the rest of that range.

For our purposes, it isn't sufficient that the proxy be highly correlated with the actual temperature, it also must be 'high resolution.' For example, if we couldn't determine the age of a proxy value more accurately than the nearest decade, or if the proxy value reflected the temperatures over the preceding decade, then the resolution it offers is just decadal. We want resolution down to the annual level.

Here are some of the sources of high-resolution proxy data:

Ice Cores-the upper portion of an ice core exhibits a layered structure with annual variation; the light bands are formed by freshly fallen, clean summer snow and the dark bands are formed by old, dust-contaminated winter snow. The thickness of the light band is indicative of how much snowfall there was. Air bubbles in the ice preserve 'fossil' air, in which the level of greenhouse gases can be measured. Also, oxygen isotope ratios are influenced by ocean temperatures. Obviously, ice cores are only available from a few parts of the world; notably Greenland, Antartica, and a few glaciers.

Tree Rings-the light colored layer grows in the spring and the dark colored one in late summer. Narrow rings are indicative of poor growth conditions, such as drought or severe winter. Tree ring data is available only where trees grow.

Corals-we can see annual variations in skeletal density and geochemical parameters. The light layers are from the summer and the dark layers from the winter. Oxygen isotope ratios are indicative of ocean temperatures. The most useful corals grow in shallow tropical waters.

Lake Sediments-these may exhibit seasonable variations (varving) in runoff sediment composition, which in turn are the result of summer temperature, rainfall, and winter snowfall.

Boreholes-the variation of temperature with depth has a detectable relationship to the history of temperature at the surface.

Speleotherms-these are stalactites, stalagmites and flowstones. Some provide annual resolution, as a result of visually detectable lamination, or a seasonal variation in trace elements. Layer thickness is related to surface rainfall and cave air temperature.

Historical accounts-these are most useful if they provide some kind of quantitative information.

There are technical problems with working with proxy data, but consideration of those problems is outside the scope of this article.

Climate Reconstructions: The North Atlantic Oscillation

In the mid-latitudes of the North Atlantic, the prevailing winds are from the west. These were convenient for mariners returning from the New World. However, those winds are also important because they bring moist air to Europe.

The direction and strength of the prevailing winds are controlled by the position and strength of a persistent low-pressure system over Iceland (the Icelandic Low), and a persistent high-pressure system over the Azores (the Azore High).

The atmosphere alternates between a state in which the pressure difference widens (positive phase, NAO+) and one in which it narrows (negative phase, NAO-). There are a number of ways the NAO may be quantified, but the simplest is as the normalized difference in pressure between a station in the Azores (or in Portugal) and one in Iceland. There is no significant periodicity in the switching between NAO+ and NAO-.

In NAO+, the westerlies are stronger, and more and stronger winter storms cross the north Atlantic, on a more northerly track. Temperatures are above average in the eastern United States and in northern Europe, and below average in northern Canada and Greenland and often in southern Europe, northern Africa and the Middle East. There is also above average precipitation in northern Europe, and below average in southern Europe. In NAO-, the effects are reversed.

The effects are strongest in winter. (NWS-CPC).

The North Atlantic Oscillation index has been reconstructed, on a seasonal basis, for 1500-1658 and monthly for 1659-2001 (LuterbacherNAO). Table 2-1shows its behavior for 1630-39. It can be seen that it was mostly in negative phase in that decade.

Climate Reconstructions: European Annual Average Temperatures

Looking first at reconstructed mean annual temperatures for Europe generally, Table 2-2A shows how the 1630s (with the years 1999-2000 for comparison) shape up.