The recent record-shattering heat wave in the Pacific Northwest and devastating floods in western Europe have both been ascribed to global warming by many climate scientists. But an alternative explanation, voiced by some climatologists yet ignored by the mainstream media, is that the disasters were caused by the phenomenon of jet-stream blocking – which may or may not be a result of global warming, and could instead arise from a weakening of the sun’s output.

Blocking refers to the locking in place for several days or weeks of the jet stream, a narrow, high-altitude air current that flows rapidly from west to east in each hemisphere and governs much of our weather. One of the more common blocking patterns is known as an “omega block,” a buckling of the jet stream named for its resemblance to the upper-case Greek letter omega, that produces alternating, stationary highs and lows in pressure as shown in the figure below. Under normal weather conditions, highs and lows move on quickly.

According to the blocking explanation, the torrential rains that hovered over parts of eastern Germany, Belgium and the Netherlands came from a low-pressure system trapped between two blocking highs to the west and east – the opposite situation to that shown in the figure.

Precipitation tends to increase in a warmer world because of enhanced evap­oration from tropical oceans, resulting in more water vapor in the atmosphere. So with a blocking low stuck over the Rhine valley and the ground already saturated from previous rainfall, it’s not surprising that swollen rivers overflowed and engulfed whole villages.

A similar argument can be invoked to explain the intense “heat dome” that parked itself over British Columbia, Washington and Oregon for five blisteringly hot days last month. In this case, it was a region of high pressure that was pinned in place by lows on either side, with the sweltering heat intensified by the effects of La Niña on North America. Several Pacific Northwest cities experienced temperatures a full 5 degrees Celsius (9 degrees Fahrenheit) above previous records.

There’s little doubt that both of these calamitous events resulted from jet-stream omega blocks. Blocking can also induce cold extremes, such as the deep freeze endured by Texas earlier this year. But how can blocking be caused by the sun?

Over the 11-year solar cycle, the sun’s heat and visible light fluctuate, as does its production of invisible UV, which varies much more than the tenth of a percent change in total solar output. It’s thought that changes in solar UV irradiance cause wind shifts in the stratosphere (the layer of the atmosphere above the troposphere), which in turn induce blocking in the tropospheric jet stream via a feedback effect. Blocking can also stem from other mechanisms. In the North Atlantic at least, a 2008 research paper found that during periods of low solar activity, blocking events in more eastward locations are longer and more intense than during higher solar activity.

Right now we’re entering a stretch of diminished solar output, signified by a falloff in the average monthly number of sunspots as depicted in the next figure. The decline in the maximum number of sunspots over the last few cycles likely heralds the onset of a grand solar minimum, which could usher in a period of global cooling.

However, climate scientists are divided on the question of whether global warming exacerbates jet-stream blocking. Computer climate models in fact predict that blocking will diminish in a warming climate, though the frequency of past blocking events has been simulated poorly by the models. Many climate scientists admit that it’s not yet clear how large a role is played by natural variability – which includes solar fluctuations.

Adherents to the global warming explanation got a boost from a just-completed attribution study, claiming that the deadly US-Canada heat wave was “virtually impossible” without climate change. The study authors used 21 climate models to estimate how much global warming contributed to excessive heat in the areas around the cities of Vancouver, Seattle and Portland. Their conclusion was that the heat wave was a one-in-1,000-year event, and would have been at least 150 times rarer in the past.

But such a highly exaggerated claim is based on dubious computer models, not on actual scientific evidence. June’s scorching temperatures in the Pacific Northwest were no match for the persistently searing heat waves that afflicted North America in the 1930s.

And despite the hoopla over the European floods, it’s not the first time some of the flooded areas have suffered catastrophic flooding. For example, the Ayr valley in Germany, struck again this month, experienced major floods in the same locations on June 12, 1910, when at least 52 people were killed.

Next: Has the Sun’s Role in Climate Change Been Trivialized?