Due to the record-breaking wildfires that occurred in Canada in 2023, unprecedented quantities of air pollutants and greenhouse gases were released into the atmosphere. The wildfires had emitted more than 1.3 Pg CO2 and 0.14 Pg CO2 equivalent of other greenhouse gases (GHG) including CH4 and N2O as of 31 August. The wildfire-related GHG emissions constituted more than doubled Canada’s planned cumulative anthropogenic emissions reductions in 10 years, which represents a significant challenge to climate mitigation efforts. The model simulations showed that the Canadian wildfires impacted not only the local air quality but also that of most areas in the northern hemisphere due to long-range transport, causing severe PM2.5 pollution in the northeastern United States and increasing daily mean PM2.5 concentration in northwestern China by up to 2 µg m−3. The observed maximum daily mean PM2.5 concentration in New York City reached 148.3 µg m−3, which was their worst air quality in more than 50 years, nearly 10 times that of the air quality guideline (i.e., 15 µg m−3) issued by the World Health Organization (WHO). Aside from the direct emissions from forest fires, the peat fires beneath the surface might smolder for several months or even longer and release substantial amounts of CO2. The substantial amounts of greenhouse gases from forest and peat fires might contribute to the positive feedback to the climate, potentially accelerating global warming. To better understand the comprehensive environmental effects of wildfires and their interactions with the climate system, more detailed research based on advanced observations and Earth System Models is essential.