A favorite claim of the climate change deniers is to point out that in Medieval times, the Vikings built farms in Greenland and Europe was in a warm spell. That, they claim, is evidence that modern global warming is just natural climate variation.
In the latest issue of the journal Science, scientists put that claim to rest.
Using natural records going back about 1,000 years, the scientists were able to pinpoint causes of Europe's Medieval warm spell, and they can show that the mechanisms responsible for warmer temperatures in Europe then are not causing the warming seen today.
In the 1920s, a configuration of two pressure systems in the Atlantic Ocean called the North Atlantic Oscillation was discovered to be the main determinant of natural variability in European climate. Since its discovery, the difference between the two pressure systems – one high and one low – has oscillated from great differences (positive NAO) to small (negative NAO) over periods of a year or a few years.
In Medieval times, however, the NAO was in a predominantly positive pattern for about three centuries, which brought consistently warm weather into Europe, especially in winter, when NAO's impact is greatest. Since the NAO is no longer predominantly positive, it cannot be the cause of recent higher temperatures, says Valerie Trouet, a Belgian research scientist at the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf and lead author of the study.
"Those conditions are by far not enough to explain the global warmth we are experiencing," Trouet said.
The researchers were helped in their discovery by old Moroccan trees and a stalagmite in Scotland.
Morocco and Scotland happen to be near the two ends of the North Atlantic Oscillation, in which a high-pressure system exists semi-permanently over the Azores, islands in the Atlantic west of Portugal and Morocco, and a low-pressure one stays over Iceland. (The difference in the pressure systems oscillates between great and small from year to year, depending on how low the Icelandic Low is and how high the Azores High is.)
These two pressure systems determine what weather conditions blow into Europe on the westerly winds. The Azores High phenomenon causes dry conditions in Morocco, and the Icelandic Low causes wet conditions in Scotland and Scandinavia.
When the difference between the Azores High and the Icelandic Low is high (when the NAO is in a positive mode), "it drives winds from the Atlantic Ocean over Europe. So if the pressure difference is larger than normal, those winds are strong and bring more warm air over Europe," Trouet said.
In order to discover why Europe was so warm in Medieval times, Trouet and other researchers took a tree ring record from Morocco that contained trees 950 years old from the Atlas Mountains, a very dry region. There, the amount of rainfall drives tree growth, so dry years produce narrow rings and wet years, large ones.
"In that way, the tree ring record is actually a proxy record for drought conditions in Morocco. And rainfall or drought in Morocco is very much influenced by the North Atlantic Oscillation, because Morocco is very close to the Azores High," Trouet said.
A 3,000-year-old stalagmite in Scotland helped make the case that the difference in pressure between the northern and southern nodes was high throughout this period. The stalagmite grew in a cave under a peat bog as water dripped from the bog into the cave. Growth was influenced by how much rain had fallen in the peat bog above it, resulting in growth rings just like those of trees.
Putting the two records together showed that, for the last 1,000 years, whenever it was wet in Scotland, it was dry in Morocco and vice versa. But the tree and stalagmite rings showed the precipitation record and not the temperature history, so the scientists still did not have proof that the NAO was responsible for the warm Medieval period.
They matched these records with a temperature record from a winter-temperature-sensitive stalagmite in the Austrian Alps and found a strong correspondence.
"By using two drought-sensitive records from remote regions, we were able to reconstruct the mechanism that explains temperature for central Europe," Trouet said.
"NAO was in a persistent, centuries-long positive mode. Nowadays, NAO is nowhere near that persistent. Over the last 100 years, NAO has been up and down. We didn't know it was possible that NAO could be in the same state for such a long period of time."
"It was in a very different condition in the Middle Ages, so the warming now cannot be explained by the NAO alone."
Aside from teaching something new to those who reject the idea that human activity is causing climate change, the study has a lesson for climate modelers: "None of the models take into account that NAO can be in a persistent mode over such a long period of time," Trouet said. "This needs to be taken into account when modeling future climate conditions."
Another factor that climate modelers might want to take into account is the phenomenon that persisted in the Pacific in the Middle Ages.
While Medieval Europe experienced warmth, the Pacific experienced sustained colder than average temperatures due to the climate pattern La Niña. (In present times, El Niño fluctuates with La Niña.) "[NAO and La Niña] might have provided feedback to each other, and that might be why they stayed in such a persistent dominant mode over such a long time period," Trouet said.
The deniers claim that we're actually in a cooling period, not warming. However, a look at the temperature records from the past century (above) shows that temperature changes have varied year to year while the longterm trend is a temperature rise.
David Easterling of the National Climatic Data Center and Michael Wehner of Lawrence Berkeley National Laboratory used the scientific process to make it clear that there can be periods of a decade or two where the globally averaged surface air temperature does not show a warming trend while still being part of a longer-term warming trend.
The point of the study, Easterling told Revkin, was: "To show, in a peer-reviewed scientifically defensible way that there is no reason to expect the climate to warm in a monotonic type fashion, that there is natural variability along with anthropogenic forced warming and we shouldn't expect each year to be warmer than the next or even a run of 10 years always to show warming. That we can get a 10- or even 15-year period with no real change in globally averaged temperature even though in the end we have strong global warming."