There is much talk about the temperature record going back a great many years. But of course we can’t stick a thermometer out the window into 49 BC so scientists reconstruct past temperatures using a variety of clever and indirect indicators. And the general picture that has emerged is of a Roman Warm Period (350 BC - 450 AD) followed by cold Dark Ages up to about 900 AD, then warm conditions in the medieval/Viking era until about 1300, followed by a long cooling until the early 1800s then the modern warming interval. The warming since the 1800s is not a source of controversy; whether it’s unusual compared to past cycles is the big question. Last year some scientists used a deep sediment core from the bottom of a Swedish fjord to say that, in that region at least, the answer is no.
The most famous attack on previous warming cycles was Michael Mann’s infamous hockey stick that essentially got rid of the Medieval Warm Period based on data from a few trees in the US Midwest. Al Gore sneered at the MWP in An Inconvenient Truth and one still encounters claims that it was a purely localized European phenomenon of no relevance to world history. Newer data shows the opposite, and it seems fairly clear that in Europe at least the MWP was warmer than today, with both crops and trees growing further north and at higher altitudes than at present, which is why the Vikings were famously able to get to and settle the optimistically named Greenland in that period.
So what of this Swedish research on that and earlier warmings? The scientists retrieved an 8 m column consisting of layers of sediment from the bottom of the Gullmarn Fjord northeast of Gothenburg. By examining the chemical properties of the layers they were able to infer what nearby winter temperatures likely were. And there in the resulting record were the familiar historical patterns.
The expansion of the Roman Empire occurred during a warm interval. The collapse of Rome and the onset of the Dark Ages coincided with a long descent into persistent cold conditions that reversed during the interval called the Medieval Warm Period (or Climate Anomaly), a period of relative abundance and prosperity across the Northern Hemisphere, suggesting that colder conditions are generally worse for civilization. And from 1300 to the mid-1800s the world cooled in what is now called the Little Ice Age. Fortunately for us it gave way to a new warming interval, the one we currently enjoy.
So where does it fit compared to earlier warm periods? Is late 20th-century warming unprecedented, the scary red bit shooting up at the right edge of those otherwise blue charts, proof that we’ve upset the balance and that current conditions cannot be linked to any cyclical pattern going back to the Holocene Climatic Optimum?
Well, no. The authors conclude that 20th century warming "does not stand out in the 2500-year perspective and is of the same magnitude as the Roman Warm Period and the Medieval Climate Anomaly." They also discuss other temperature reconstructions from Northern European locations, some of which suggest modern warming exceeds the medieval period and some the opposite. Not surprising given how imprecise the tools are and how complex the climate system is.
Yes, complex. One of the popular slogans in the climate debate is that 97% of scientists agree that climate change is a dangerous man-made catastrophe. Soon we'll be releasing our fact check video on this slogan, which emphasizes that the high level of agreement only covers the uncontroversial ideas, especially the claim that the world has warmed since the 1800s. The evidence supports it and it fits with the general picture of a climate system that's always changing, in centuries-long cycles. Where the disagreement lies is whether the warming today is unusual or dangerous. The evidence is not so clear and the scientists do not all agree, nor should they be expected to. Stay tuned.