Canada's awful summer gives way to a burning fall

Threat level: The fires have released hundreds of megatonnes of carbon dioxide that will contribute to global warming, which made the blazes more likely and intense to begin with.

• Wildfire smoke, as pictured above, has repeatedly degraded air quality in cities across North America, even at times crossing the Atlantic Ocean to affect European skies.

By the numbers: Over 900 fires were burning across Canada on Wednesday, including more than 500 that were out of control.

• So far, over 6,300 fires have burned an estimated 43.4 million acres — an area almost the size of Oklahoma.

State of play: Fire seasons typically taper off and are mostly over by the end of October, but Canadian officials say this year is unprecedented.

• Dozens of blazes have formed massive smoke-infused storm clouds, called pyrocumulonimbus, that blast large amounts of smoke particles into Earth's atmosphere.

The big picture: The U.S. has had a below-average season in terms of acreage burned, but the number of fires it has had is mostly in line with 10-year averages.