Wei et al. (2025) Hysteretic response of Western North American fire weather to CO₂ removal

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2025
• Date: 2025-12-16
• Authors: Ting Wei, Wenjie Dong, Yueli Chen
• DOI: 10.1007/s00704-025-05875-4

Research Groups

• State Key Laboratory of Disaster Weather Science and Technology, Chinese Academy of Meteorological Sciences, Beijing, China
• School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai, China
• Key Laboratory of Tropical Atmosphere-Ocean System Ministry of Education, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China

Short Summary

This study investigates the hysteretic response of Western North American fire weather to idealized CO₂ ramp-up and ramp-down scenarios, finding that fire weather peaks 7-11 years after maximum CO₂ and intensifies by approximately 5.5% during CO₂ ramp-down compared to ramp-up at identical CO₂ levels. This hysteresis is primarily driven by drier conditions, warmer temperatures, and reduced humidity, linked to shifts in atmospheric circulation and Pacific sea surface temperatures.

Objective

• To quantify the hysteretic response of fire weather in Western North America (WNA) to idealized symmetric CO₂ ramp-up and ramp-down scenarios.
• To identify dominant climatic drivers and explore underlying mechanisms contributing to fire weather hysteresis.

Study Configuration

• Spatial Scale: Western North America (WNA), including Baja California and the northern part of WNA, and the North Pacific region.
• Temporal Scale: Idealized symmetric CO₂ ramp-up (1% per year for 140 years, from 284.7 ppm to 1138.8 ppm) and ramp-down (1% per year for 140 years, returning to pre-industrial levels). Analysis focuses on two 30-year intervals at identical CO₂ levels (years 1–30 of ramp-up and years 251–280 of ramp-down) and the fire season (April–September).

Methodology and Data

• Models used: Coupled Model Intercomparison Project Phase 6 (CMIP6) outputs from three climate models: CanESM5, MIROC-ES2L, and NorESM2-LM. Experiments used are 1pctCO2 (ramp-up) and 1pctCO2-cdr (ramp-down).
• Data sources: Daily outputs of temperature, precipitation, surface wind speed, and relative humidity from the selected CMIP6 models. All model outputs were regridded to a 2° × 2.5° horizontal resolution. The Canadian Forest Fire Weather Index (FWI) system was used to calculate fire weather conditions, along with its component codes (Fine Fuel Moisture Code (FFMC), Duff Moisture Code (DMC), Drought Code (DC), Initial Spread Index (ISI), and Build-Up Index (BUI)). A factor separation method was employed to quantify the contribution of individual factors to FWI hysteresis.

Main Results

• Fire weather over WNA exhibits a delayed response, peaking 7–11 years after the maximum CO₂ concentration.
• An asymmetric response shows an additional intensification of approximately 5.5% in fire weather over WNA during the CO₂ ramp-down phase compared to the ramp-up phase at identical CO₂ levels.
• The largest asymmetry (increase in FWI) is observed over Baja California, while fire weather in the northern WNA shows a decrease when CO₂ returns to initial levels.
• The Initial Spread Index (ISI) is the largest contributor to FWI hysteresis (~66%), primarily driven by the Fine Fuel Moisture Code (FFMC). The Build-Up Index (BUI) contributes ~14%, mainly governed by the Duff Moisture Code (DMC) (~94%).
• Precipitation is the dominant meteorological factor contributing to FWI asymmetry (40%), followed by temperature (26%) and relative humidity (20%). Wind speed has a negligible contribution (–0.4%).
• The asymmetric response of precipitation is linked to a southeastward expansion of the North Pacific subtropical high, which is in turn associated with hysteretic Pacific sea surface temperature (SST) changes (relative warming at high latitudes and cooling over the subtropical eastern North Pacific during ramp-down).

Contributions

• This study is the first to quantify the hysteretic response of fire weather in Western North America to idealized CO₂ removal scenarios.
• It reveals that WNA fire weather exhibits a significant lag and intensification even after CO₂ concentrations return to pre-industrial levels, highlighting the persistence of fire danger.
• It identifies the dominant meteorological drivers (precipitation, temperature, relative humidity) and the underlying atmospheric and oceanic mechanisms (North Pacific subtropical high expansion, Pacific SST hysteresis) responsible for this asymmetry.
• The findings provide crucial insights into the protracted climate impacts on regional hazards under Carbon Dioxide Removal (CDR) pathways, informing future fire risk management and climate adaptation strategies.

Funding

Not explicitly stated in the provided text.

Citation

@article{Wei2025Hysteretic,
  author = {Wei, Ting and Dong, Wenjie and Chen, Yueli},
  title = {Hysteretic response of Western North American fire weather to CO₂ removal},
  journal = {Theoretical and Applied Climatology},
  year = {2025},
  doi = {10.1007/s00704-025-05875-4},
  url = {https://doi.org/10.1007/s00704-025-05875-4}
}