21 Aug 2023
 | 21 Aug 2023

Geographically divergent trends in snowmelt timing and fire ignitions across boreal North America

Thomas D. Hessilt, Brendan M. Rogers, Rebecca C. Scholten, Stefano Potter, Thomas A. J. Janssen, and Sander Veraverbeke

Abstract. The snow cover extent across the Northern Hemisphere has diminished while fire extent and severity has increased over the last five decades with accelerated warming. However, the effects of earlier snowmelt on fire is largely unknown. Here, we assessed the influence of snowmelt timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snowmelt led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest-southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over Northwest (Southeast) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snowmelt induced earlier ignitions of between 0.22 and 1.43 days earlier per day of earlier snowmelt in all ecoregions between 2001 to 2019. Early-season ignitions (defined by the 20 % earliest fires per year) developed into significantly larger fires in 8 out of 16 ecoregions and 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snowmelt is a proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that led to earlier ignitions. This indicates that snowmelt timing is an important trigger of land-atmosphere dynamics. Future warming and consequent changes in snowmelt timing may contribute to further increases in western boreal fires while the number of fires in eastern boreal North America may increase too with climate change.

Thomas D. Hessilt et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1741', Anonymous Referee #1, 17 Sep 2023
  • RC2: 'Comment on egusphere-2023-1741', Quinn Barber, 29 Sep 2023

Thomas D. Hessilt et al.

Thomas D. Hessilt et al.


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Short summary
In boreal North America, snow and frozen ground prevails in winter while fires can occur in summer. Over the last 20 years, the northwestern parts experienced elevated temperatures, earlier snowmelt and more ignitions which was opposite to the southeastern parts. However, earlier ignitions led to larger fires and happened after earlier snowmelt timing across the domain. Snowmelt timing is a good proxy for ignition timing but may also influence atmospheric conditions controlling ignition timing.