Americans sweltering in the recent record-breaking heatwave may not believe it – but it seems that our ancestors suffered through much hotter summers in times gone by, several of them within the last 2,000 years. A new study measuring temperatures over the past two millennia has concluded that in fact the temperatures seen in the last decade are far from being the hottest in history.
A large team of scientists making a comprehensive study of data from tree rings say that in fact global temperatures have been on a falling trend for the past 2,000 years and they have often been noticeably higher than they are today – despite the absence of any significant amounts of human-released carbon dioxide in the atmosphere back then.
The reconstruction provides a high-resolution representation of temperature patterns in the Roman and Medieval warm periods, but also shows the cold phases that occurred during the Migration Period and the later Little Ice Age. (Credit: Illustration/Copyright: Institute of Geography, JGU)
“We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low,” says Professor-Doktor Jan Esper of the Johannes Gutenberg-Universität Mainz, one of the scientists leading the study. “Such findings are also significant with regard to climate policy.”
They certainly are, as it is a central plank of climate policy worldwide that the current temperatures are the highest ever seen for many millennia, and that this results from rising levels of atmospheric CO2 emitted by human activities such as industry, transport etc.
If it is the case that actually the climate has often been warmer without any significant CO2 emissions having taken place – suggesting that CO2 emissions simply aren’t that important – the case for huge efforts to cut those emissions largely disappears.
Needless to say, prominent alarmist scientists and the UN’s Intergovernmental Panel on Climate Change (IPCC) have not taken this view, arguing instead that the well-documented Roman and medieval warm periods may have taken place but either weren’t very warm or only happened in limited regions (though this latter idea has lately been seriously undermined by research in Antarctica).
In the IPCC view, the planet was cooler during Roman times and the medieval warm spell. Overall the temperature is headed up – perhaps wildly up, according to the famous/infamous “hockey stick” graph.
The new study indicates that that’s quite wrong, with the current warming less serious than the Romans and others since have seen – and the overall trend actually down by a noticeable 0.3°C per millennium, which the scientists believe is probably down to gradual long-term shifts in the position of the Sun and the Earth’s path around it.
Orbital forcing of tree-ring data
Nature Climate Change (2012) doi:10.1038/nclimate1589
Received 27 March 2012 Accepted 15 May 2012 Published online 08 July 2012
Jan Esper, David C. Frank, Mauri Timonen, Eduardo Zorita, Rob J. S. Wilson, Jürg Luterbacher, Steffen Holzkämper, Nils Fischer, Sebastian Wagner, Daniel Nievergelt, Anne Verstege & Ulf Büntgen
Solar insolation changes, resulting from long-term oscillations of orbital configurations (1), are an important driver of Holocene climate (2, 3). The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m−2 net anthropogenic forcing since 1750 (ref. 4), but the trend varies considerably over time, space and with season (5). Using numerous high-latitude proxy records, slow orbital changes have recently been shown (6) to gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models (7, 8) indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions (9, 10, 11, 12, 13) relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.
see also: Tree rings suggest Roman world was warmer than thought