2017-2018 North America wildfires and their impact on air quality and climate.

Wildfires have become a subject of increasing attention in recent years, especially as they continue to claim lives and wreak havoc on the economy. Wildfires are natural processes that are closely tied to warm temperature and drought conditions and are modulated by humans. Human activity has played an important role in altering the frequency, size, and duration of wildfire regimes by climate change and by managing land cover and land use. In turn, wildfires can impact climate by releasing carbon dioxide, by changing the surface albedo of the burned area, by altering vegetation, and by causing deforestation. In addition, atmospheric aerosols and trace gases emitted from wildfires can directly and indirectly impact physical and chemical processes in the atmosphere, mainly by changing Earth's radiative balance by scattering and absorbing incoming solar radiation and by interacting with clouds. Although it has been shown that there is a global decline in burned area as a result of fire suppression, there have also been changes in fire weather season length and in the number of high-fire-danger days as a result of the warmer climate in the Western United States. The years 2017-2018 were unusual in terms of the number, intensity and frequency of fire events, both on the global scale and in the Western United States in particular. We show the impacts of the wildfires on the air quality and the climate system using Community Earth System Model. The estimates of the direct and indirect aerosol radiative contributions of the wildfires are calculated using a simulation with a daily fire emission product Fire INventory from NCAR and a simulation without fires. We find that the radiative forcing from the wildfires of recent years is significant and is likely to have impacted the regional climate.