As global temperatures rise, the seasons shift in various ways depending on their latitude. For many areas such as Interior Alaska, the annual dry season has become longer and surface fuels have consequently become drier; this has led to an increase of wildfire frequency and severity. Wendler et al. (2010) conducted a 55 year analysis of the various conditions conducive for and associated with, predominantly lightning induced fires in Interior Alaska. With a special emphasis on the extreme year of 2004, the authors examined data for lightning and fire ignition, number of fires versus area burned, and air quality and composition. Special emphasis was placed on the year 2004 where specific fire and climate data, weather, particulate matter, and carbon monoxide concentration, were all examined within the broader context of the whole 55 year data set. They determined that 2004 was the worst fire season on record (in terms of area burned) and that there has been an overall trend in increased wildfire severity and occurrence. Although the unique weather patterns for the summer of 2004 were attributed to the severity of the year, they did show that climate change has been an underlying factor. Furthermore they demonstrated that there were four major fire seasons burning an area greater than 10,000 km² in the last 27 years, as opposed to only two in the previous 28 years. –Lindon Pronto
Wendler, G., Conner, J., Moore, B., Shulski, M., Stuefer, M., 2010. Climatology of Alaskan wildfires with special emphasis on the extreme year of 2004. Theoretical and Applied Climatology (2011), 104:459–472
The authors examined the Palmer Drought Severity Index and the Canadian Drought Code, against both the number of wildfires and the area burned in order to find any significant correlations. Lightning-caused fire data were collected by the Alaska Lightning Detection Network operated by the Bureau of Land Management and the Alaska Fire Service. These data were then used to show the spatial distribution of the lightning strikes, the mean monthly strike count, and the diurnal variation of the strikes. Additionally, temperatures and precipitation patterns were evaluated in terms of lightning activity levels; they found that as temperature increased, so did the lightning activity.
General historical data from 1955–2009 showing number of fires and area burned were retrieved from the Alaska Fire Service, while very detailed data from the 2004 fire season were compiled. On average, 3,775 km² burn annually in Alaska, of which about 90% occurs within the interior of the state as bounded by the Brooks Range in the north, and the Alaska Range in the south. During the 55 year study period, about 93% of fires were started by an average of 32,000 lightning strikes per year. Statistical analyses showed a correlation coefficient of r=0.67 in the ratio of lightning strikes to resulting wildfires, with an estimate one fire per every 600 strikes. Furthermore this relationship is much higher when positive and negative strikes are evaluated independently of overall fire starts. Positive down-strikes occurring during dry conditions are more infrequent than negative down-strikes which are usually accompanied by precipitation. Despite the lower frequency of positive down-strikes, they are four times more likely to result in a fire. The vast majority of lightning occurs during the months of June and July.
In the summer of 2004, a record 27,200 km² burned (well over 6,700,000 acres), or an area greater than any 6 of the smallest states in the US. It was in this year that many areas around the state set record high temperatures. Additionally, and what is believed responsible for the extremity of the fires in 2004, is the unique weather patterns that occurred. Anticyclonic conditions resulted in unusually clear skies and the third driest summer on record. For calculating the correlation of climate conditions and fire occurrence, the authors found that the Canadian Drought Code (CDC) was more effective than the more commonly used Palmer Drought Severity Index (PDSI). Beginning with below normal snowpack in the spring, a semi-permanent upper-ridge followed with above normal temperatures and below normal rainfall. During two distinct periods of 4 weeks and 3 weeks fires burned an area of 16, 200km² and 8,000km², respectively.
Air quality during this time period deteriorated severely, dramatically reducing visibility and causing significant health risks to the population. Wendler et al. chose to focus on visibility, particulate matter, carbon monoxide (CO) concentration, and radiative fluxes. They found that during the worst of one of the more severe smoke events, visibility was <1/4 mile, fine particle matter exceeded 1,000 μg/m³, while CO concentration levels reached a value of 10.3 ppm; maximum levels prior to smoke never exceeded 1.0 ppm. The transmissivity was calculated as the percent of outside solar radiation that reached the surface as direct solar radiation. During the smoke events, less than 10% of direct radiation occurred. When the CO concentration level and the particulate matter were correlated, there was an overall variance of 72% and 80% during the two major burn periods; they concluded that a relative abundance of CO in relation to particle matter is to be expected for the older smoke of the second period.
Due to the increase in temperature of 1°C in Alaska over the past half century, more frequent and severe wildfire events can be attributed to the latter increase, even when they start nearly exclusively from natural causes as only opposed to increasing human activities in wildland areas. For example, this study shows that during the first half of the study period, 6 years with mean summer temperatures above 16°C occur, while for the second half, this occurred nine times. Similarly, severe fire seasons (>10,000km² burned) increased from 2 to 4 events from the first 28 years to the second 27 years. Furthermore wildfire events where >5,000 km² burned, increased from three to eight events. This pattern can be contributed to an overall trend of quick or prolonged drying of the surface fuels and soils that increase the likelihood of ignition from a lightning strike. This study synthesizes data to try and recognize conditions for severe fire events and thereby perhaps aide in predicting efforts needed by fire suppression forces in near-future situations; however it does admit that predicting the severity and length of future fires seasons is largely speculative as it can only be based on previous climate patterns of the past 30 years.