ditions. However, those records do not provide much spatial detail, nor do they sample the whole earth. For those, one must consider a global array of data sources of various types, as described in the following subsections.
The Younger Dryas was first discovered by studying the biological records found in terrestrial sediments. These records clearly reveal the global reach of the event. Owing to dating uncertainties, including those associated with the conversion of radiocarbon measurements to calendar years, the phasing of events between different locations is not known exactly. The ice cores show that much of the world must have changed nearly simultaneously to yield the observed changes in methane, Asian dust, and Greenland conditions, but we cannot say with confidence whether all events were simultaneous or some were sequential. A summary of much of the relevant terrestrial pollen information follows, organized by region.
As the Northern Hemisphere was recovering from the last ice age about 15,000 years ago, the climate warmed dramatically and trees started to colonize the landscape. Evidence of the warming was first found in Scandinavia by geologists who noticed tree fossils in organic sediment. They named the warming interval the Allerød for the locale where it was first observed. Overlying the Allerød layer were leaves and fruits of Dryas octopetala, an arctic-alpine herb, in sandy or silty (minerogenic) layers above the peaty tree remains; this suggested that the climate had reverted several times to very cold conditions. Two such reversals to frigid conditions were named the Older and Younger Dryas (Jansen, 1938). Considerable evidence of this sequence in hundreds of pollen diagrams throughout Europe (Iversen, l954; Watts, l980) brought attention to the strongest effects of the event, which occurred in coastal Europe. During the Younger Dryas, pollen of tundra plants, such as Artemisia (wormwood) and Chenopodiaceae, abruptly replaced birch and even conifer pollen (e.g., Lowe et al., 1995; Walker, 1995; Renssen and Isarin, 1998; Birks and Ammann, 2000). In Norway, mean July temperature was about 7-9°C lower than today and about 2-4°C lower than the preceding warm Allerød interval (Birks and Ammann, 2000). It is now apparent that regional climate changes were also large in southern Europe (Lowe and Watson, l993; Beaulieu et