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Tree-ring data provide a summer-season climate record with the kind of yearly accuracy that could be useful for historical studies. Dendrochronologists in Europe have been diligently collecting data on oak and pine for decades, in an attempt to fill in the annual temperature and climate records.
When all the data are pooled together, some interesting trends emerge (Figure 1 B). From the beginning of the Roman Empire to its peak (c. 50 BC to AD 250), the climate was relatively stable, with warm and wet summers. During the two-and-half centuries or so that followed, though, Europe had “the most unfavourable conditions for agriculture that you can imagine”, says Fredrik Charpentier Ljungqvist, a history graduate student at Stockholm University. This interval corresponds to what is known as the Migration Period – a 250-year-period of turmoil and waves of migration.
Büntgen and his colleagues conclude that climate and variability in weather patterns ought to be considered for current-day decision-making, something that Overpeck finds entirely reasonable. The researchers “highlight two things climatically: mean trends and variability”, he says. “Too often people think [only] of slow increases in mean temperatures” when they think of climate change, Overpeck says, something that “could in itself cause problems. But it’s also the extremes superimposed on that long trend: hotter drier conditions such as in 2003 and 2010 in Europe and Russia.” In Russia last year in particular, three- to four-degree shifts in temperatures over the course of one week led to deaths, forest fires, crop losses and other problems. Of course, climate researchers are still trying to dissect whether those extremes are related to anthropogenic warming or are part of natural climate variability.