The dominant contribution to the area burned by forest fires in Canada is from large, lightning-caused fires. Using anomalies calculated from Canadian General Circulation Model (CGCM) climate predictions, we generated future fire weather, fuel moisture and fire danger indices for baseline, 2 x CO2, and 3 x CO2 climate scenarios, and used these as the inputs to Prometheus: The Canadian Wildland Fire Growth Simulation Model for northwestern Ontario. The goal of this study was to provide a more quantitative methodology to assess the impact of climate change on area burned by large, lightning-caused forest fires in northwestern Ontario, by combining GCM predictions with the fire growth simulation model. Area burned was calculated for a total of 63 individual simulations (21 fires x 3 climate scenarios). Results indicated a 64.75% average increase in area burned by 2040, and a 174.45% average increase in area burned by 2090, from the reference baseline scenario. This represents almost a tripling of area burned by the end of the 21st century. These estimates do not explicitly take into account fire suppression, nor any changes in vegetation, length of the burning period or fire season that may influence area burned. Fire management agencies should consider planning and implementing mitigative and adaptive strategies to prepare for this scenario.