It is not new for scientists to forecast that the current warming of Earth's climate due to carbon dioxide, methane and other "greenhouse" gases already has led to increased weather extremes and will continue to do so.
The new study shows the jet stream pattern that brings North American wintertime weather extremes is millennia old – "a longstanding and persistent pattern of climate variability," Bowen says. Yet it also suggests global warming may enhance the pattern so there will be more frequent or more severe winter weather extremes or both.
"This is one more reason why we may have more winter extremes in North America, as well as something of a model for what those extremes may look like," Bowen says. Human-caused climate change is reducing equator-to-pole temperature differences; the atmosphere is warming more at the poles than at the equator. Based on what happened in past millennia, that could make a curvy jet stream even more frequent and-or intense than it is now, he says.
Bowen and his co-authors analyzed previously published data on oxygen isotope ratios in lake sediment cores and cave deposits from sites in the eastern and western United States and Canada. Those isotopes were deposited in ancient rainfall and incorporated into calcium carbonate. They reveal jet stream directions during the past 8,000 years, a geological time known as middle and late stages of the Holocene Epoch.
Next, the researchers did computer modeling or simulations of jet stream patterns – both curvy and more direct west to east – to show how changes in those patterns can explain changes in the isotope ratios left by rainfall in the old lake and cave deposits.
They found that the jet stream pattern – known technically as the Pacific North American teleconnection – shifted to a generally more "positive phase" – meaning a curvy jet stream – over a 500-year period starting about 4,000 years ago. In addition to this millennial-scale change in jet stream patterns, they also noted a cycle in which increases in the sun's intensity every 200 years make the jet stream flatter.
So, essentially, the climate grant-seekers admit that all this stuff we are seeing is more or less normal. That is what the actual evidence shows. They speculate that carbon dioxide and methane and "other" gases -- which would be water vapor, trap more heat and exacerbate patterns. But there is no evidence for that. They plug some data into computer models which seem to corroborate what the grant-seekers what them to corroborate.
In fact the primary factor in "climate change" is the intensity of solar radiation. More carbon dioxide and water vapor are a result of sun-driven warming rather than a cause. The earth will regulate its own temperature. Let's just hope we are not headed into a new ice age. That would be a lot more catastrophic than a slight rise in sea levels.
Weather patterns change in large and dramatic ways over time, and they always have. Temperatures are always going to vary from the mean. That's why they call it "average". It has predictive value only in a range defined by the standard deviation. Sometimes it's hotter; sometime colder. We used to call it "weather".