Researchers Say Climate Change Does Not Cause Extreme Winters

A new study out of Zurich says that extreme winters are not caused by climate change.

Researchers at ETH Zurich and the California Institute of Technology have shown that global warming tends to reduce temperature variability. Repeated cold snaps led to temperatures far below freezing across the eastern United States over the past two winters. In Canada, parts of Niagara Falls froze, the Great Lakes froze and ice floes could even be seen on Lake Michigan. Such low temperatures had become rare in recent years, but pictures of icy, snow-covered cities made their way around the world, this year. What is the cause of these extreme weather events?

It has been argued for years that the amplified warming of the Arctic, relative to lower latitudes in recent decades, has weakened the polar jet stream. This in turn has produced strong wind current several kilometres high in the atmosphere, driven by temperature differences between the warm tropics and cold Polar Regions.

One hypothesis is that a weaker jet stream may become more wavy, leading to greater fluctuations in temperature in mid-latitudes. Scientists at ETH Zurich and at the California Institute of Technology, led by Tapio Schneider, professor of climate dynamics at ETH Zurich, have come to a different conclusion.

“The study's point of departure was that higher latitudes are indeed warming faster than lower ones, which means that the temperature difference between the equator and the poles is decreasing,” said Schneider. “Imagine for a moment that this temperature difference no longer exists. This would mean that air masses would have the same temperature, regardless of whether they flow from the south or north. In theory there would no longer be any temperature variability. Such an extreme scenario will not occur, but it illustrates the scientists' theoretical approach.”

Using a highly simplified climate model, they examined various climate scenarios to verify their theory. It showed that the temperature variability in mid-latitudes indeed decreases as the temperature difference between the poles and the equator diminishes. Climate model simulations by the Intergovernmental Panel on Climate Change (IPCC) showed similar results: as the climate warms, temperature differences in mid-latitudes decrease, and so does temperature variability, especially in winter.

Temperature extremes will therefore become rarer as this variability is reduced, adds Schneider. And the jet stream? Schneider shrugs off the idea: "The waviness of the jet stream that makes our day-to-day weather does not change much." Changes in the north-south difference in temperatures play a greater role in modifying temperature variability.

Schneider wants to explore the implications these results have in further studies. In particular, he wants to pursue the question of whether heatwaves in Europe may become more common because the frequency of blocking highs may increase. And he wants to find why these high pressure systems become stationary and how they change with the climate.