Hubertus Fischer and his team reconstructed North American wildfire activity and soil ammonium emissions using continuous, high-resolution measurements of ammonium concentrations in Greenland ice cores between 110,000 to 10,000 years ago. Background ammonium concentrations found in these ice cores are controlled by North American soil emissions, while North American biomass burning events lead to pronounced ammonium peaks in the record. Their findings show that on orbital timescales soil emissions increased under warmer climate conditions when vegetation expanded northwards into previously ice-covered areas. For millennial-scale interstadial warm periods, the fire recurrence rate increased in parallel to the rapid warmings, whereas soil emissions rose more slowly, reflecting slow ice shrinkage and delayed ecosystem changes. The study concludes that sudden warming events had little direct impact on North American soil ammonium emissions and ammonium transport to Greenland, but did result in a substantial increase in the frequency of North American wildfires.
