Two-thirds of the way through the year, this year seems destined to be remembered as the year that extreme weather events left the Northern Hemisphere reeling. The summer months brought a litany of extreme weather events: Wildfires across Canada. Flooding in Vermont. An unusually early and aggressive start to the Atlantic hurricane season. A devastating wildfire on the Hawaiian island of Maui, fuelled by strong heat and cyclonic winds that destroyed a historic town etc. According to the Copernicus Climate Change Service, the three-month period from June to August was by far the warmest northern hemisphere summer ever recorded. And a new analysis by the non-profit organization Climate Central has drawn a direct line between those high temperatures and climate change, arguing that about half of the global population  almost four billion people  experienced thirty or more days between June and August with extreme temperatures made at least three times more likely by climate change. Virtually no one on Earth escaped the significant influence of global warming during the past three months. In every country we could analyse, including the southern hemisphere where this is the coolest time of year, we experienced temperatures that would be difficult, and in some cases nearly impossible, without human-caused climate change. Carbon pollution is clearly responsible for record-setting heat of this season. Connecting the dots between climate change and extreme weather this year is complicated, but some scientists now are feeling increasingly confident in their ability to do so. We can say, with a great deal of confidence, that we would not be seeing the unprecedented weather extremes we saw play out this past summer in the absence of human-caused warming from fossil fuel-generated carbon pollution. However, analysis that attempt to attribute specific events to climate change can sometimes miss the bigger picture of how global warming actually affects broader, longer-term weather patterns, such as by distorting the jet stream, the band of westerly winds that circles the northern hemisphere. A great number of researchers are concerned that a warming Arctic is weakening the jet stream, allowing hot air from the tropics to flow north, prolonging heatwaves, fuelling wildfires, and melting the Arctic. For that reason, it is often preferred to talk about how climate change is impacting the basic processes relevant to all of these events, which includes the obvious fact that current heat waves are much more frequent and intense on a hotter planet. 
