My hometown, Vancouver, is in a rainforest, so we celebrate sunny days. People I talk to are enjoying the recent warm, dry weather , but they invariably add, “This isn’t normal” — especially with all the smoke from nearby forest fires.
With no mountain snowpack and almost no spring rain, rivers, creeks and reservoirs are at levels typically not seen until fall. Parks are brown. Blueberries, strawberries and other crops have arrived weeks earlier than usual. Wildfires are burning here and throughout Western Canada. Meanwhile, normally dry Kamloops has had record flooding , as has Toronto. Manitoba has been hit with several tornadoes and golf-ball-sized hail .
Unusual weather is everywhere. California is in its fourth year of severe drought. Temperatures in Spain, Portugal , India and Pakistan  have reached record levels, sparking wildfires and causing thousands of deaths and heat-related ailments. Heavy rains, flooding and an unusually high number of tornadoes have caused extensive damage and loss of life in Texas, Oklahoma and Mexico.
The likely causes are complex : a stuck jet stream, the Pacific El Niño, natural variation and climate change. Even though it’s difficult to link all events directly to global warming, climate scientists have warned for years that we can expect these kinds of extremes to continue and worsen as the world warms. Some hypothesize that the strange behaviours of this year’s jet stream and El Niño are related to climate change, with shrinking Arctic sea ice affecting the former.
Several recent studies indicate a clear connection  between increasing extreme weather and climate change. One, by climatologists at the National Center for Atmospheric Research in Colorado, looked at rising global atmospheric and sea-surface temperatures, which have increased water vapour in the atmosphere by about five per cent since the 1950s. According to the paper, published in Nature Climate Change, “This has fuelled larger storms, and in the case of hurricanes and typhoons, ones that ride atop oceans that are 19 cm higher than they were in the early 1900s. That sea-level rise increases the height of waves and tidal surges as storms make landfall.”
A Stanford University study  found, “accumulation of heat in the atmosphere can account for much of the increase in extreme high temperatures, as well as an average decrease in cold extremes, across parts of North America, Europe and Asia,” but also concluded the influence of human activity on atmospheric circulation, another factor in climate change, is not well understood.