News broke last fall that 2015 was going to be the planet's hottest on record, though by saying "on record," scientists actually meant since 1880. A new study of European summer temperatures puts the region's killer heat waves in a somewhat broader context: The last few years have been substantially hotter than at any time in the last 2,100 years.
Although researchers know Europe (along with the rest of the world) is getting hotter, a crucial question is how unusual the continent's temperatures are. If temps routinely bounced up and down by a few degrees, then a few summers that are a couple degrees hotter than average isn't such a big deal. If, on the other hand, we're getting outside the range of normal, natural fluctuations—well, then we've got a serious problem. But how do we know what normal, natural fluctuations are?
To answer that question, climatologist Jürg Luterbacher and his colleagues turned to trees—in particular, tree rings, the width and density of which can be used to infer climate variables, including temperature. Using nine tree-ring records from different parts of Europe, each covering a period of several centuries and together covering the period between 138 B.C.E. to 2003 C.E., in conjunction with historical documents, the team re-constructed a temperature history dating back to the Roman Republic.
Whatever's going on with the climate in Europe, it's not just natural ups and downs.
Temperatures in Europe were relatively warm from the second century B.C.E. through the third century C.E. The fourth through seventh centuries were slightly cooler, though only by a little—around one degree Celsius on average—and with similar year-to-year fluctuations. Over the next millennium or so, Europe went through a few more warm and cool periods, but, starting in the 1200s, temperatures dropped—again, by only about one degree—and more or less stayed down until the late 19th century.
That's when things started warming up, right around the time of the Industrial Revolution. Where temperatures had been going up and down for a long time, they stayed more or less within about a degree of each other. During the 20th century, temperatures climbed back up to levels reached just a few times in the previous two millennia. In the last 30 years, they've climbed to unprecedented heights.
"Our primary findings indicate that the 1st and 10th centuries C.E. could have experienced European mean summer temperatures slightly but not [discernibly] warmer than those of the 20th century," the researchers write. "However, summer temperatures during the last 30 [years] (1986–2015) have been anomalously high and we find no evidence of any period in the last 2,000 years being as warm."
In other words, whatever's going on with the climate in Europe, it's not just natural ups and downs. Intriguingly, the re-constructions differ from the results of several climate simulations in the magnitude of the ups and downs—in fact, they're bigger in the tree-ring data than in the simulations. There are several possible explanations, the researchers write, and their results may help build better, more accurate climate models in the future.
"Catastrophic Consequences of Climate Change" is Pacific Standard's aggressive, year-long investigation into the devastating effects of climate change—and how scholars, legislators, and citizen-activists can help stave off its most dire consequences.