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The environmental time series

Environmental data (NAO index, LOD variations and temperatures) were

collected from scientific literature, online databases or communicated by climatologists

working in research institutes (Table 2). Regular and reliable climate observations,

allowing the calculation of climate indices, started about 150 years ago. Such field

records were primarily used when available. To follow environmental changes before

the instrumental period, we used reconstructed series.

The North Atlantic Oscillation (NAO) is one of major multiannual climate

fluctuations of the Northern Hemisphere (Hurrell et al., 2001; Rogers, 1984). It governs

the pattern and strength of wind, temperature and precipitation over the North Atlantic,

Northeast American and western European coasts (Hurrell, 1995). The NAO results

from the oscillation between the subtropical high surface pressure, centred on the

Azores, and the subpolar low surface pressures, centred on Iceland. To gain a

satisfactory representation of what might be the fluctuations of the NAO over a long

period, 3 time series were retained. The first one is the classical NAO index calculated

by the difference in normalised sea level pressures between Ponta Delgadas (Azores)

and Akureyri (Iceland) over the winter season (December to February, when the NAO is

most pronounced, Hurrell, 1995; Rogers, 1984, Figure 3). The other indices are two

NAO proxies: (i) a reconstruction based on tree-ring chronologies of North America and

Europe which spreads from 1701 to 1980 (Cook et al., 1998, Figure 3) and (ii) a proxy

reconstructed over the past 350 years from Greenland Ice Cores (Appenzeller et al.,

1998, Figure 3). These two proxies explain 58% and 30% of the variance of the

instrumental NAO index over the 1865-1980 period, respectively.

The excess Length Of the Day (LOD) is a global geophysical index, which

characterises variations in the earth rotation velocity. The rotational speed of the Earth

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