Wildland fire is a frequent occurrence in Canada with 8000 fires burning over 2 million hectares on average every year during the past decade. Most of this area burned results from stand-renewing high-intensity crown fires. These high intensity fires can have significant impact on communities; for example, the fires in Slave Lake in 2011 and Kelowna in 2003 sustained damages measured in 100 s of millions of dollars (Filmon 2003). Most of the area burned is from a relatively few large fires that occur on a few severe fire weather days (Flannigan and Wotton 2001). Stocks et al. (2003) found that for Canada fires that are 200 ha or larger burn 97 % of the area burned but represent only 3 % of the total number of fires. This skewed distribution is in part due to a highly efficient initial-attack strategy used by fire management agencies in full suppression zones such that most fires are extinguished before they reach 1 ha in size; fires in northern modified or observation zones are often allowed to burn to large sizes when no values at risk are threatened.

Fire activity is strongly influenced by four factors: fuels, climate–weather, ignition agents and people (Flannigan et al. 2005). Fuel amount, type, continuity, structure, and moisture content are critical elements for fire occurrence and spread. In regards to fuel continuity some suggest that at least 30 % of the landscape needs to have fuel for a fire to spread (Hargrove et al. 2000; Finney et al. 2007). This is important in many drier parts of the world where precipitation is required preceding the fire season for the growth of vegetation and the subsequent generation of surface fuels available to carry fire on the landscape (Swetnam and Betancourt 1998; Meyn et al. 2007). This is generally not a concern in Canada where there are sufficient surface fuels for fires to start and spread except where recent fires have consumed the surface fuels. Fuel structure can also be important in fire dynamics; for example, understory trees and shrubs in a forest can act as ladder fuels that help a surface fire to reach the tree crowns and thereby generate a faster moving and much more intense fire. Although the amount of fuel, or fuel load, and fuel distribution (vertical and horizontal) affect fire activity, fuel moisture largely determines whether fuels can sustain ignition and spread (Blackmarr 1972; Wotton et al. 2010), and has been found to be an important factor in the amount of area burned (Flannigan et al. 2005).

There are two common mechanisms for wildfire ignition in Canada: people and lightning. During the past decade, people have started 65 % of the fires and these are responsible for 15 % of the area burned whereas lightning is responsible for the remainder. Lightning-caused fires are responsible for proportionally more area burned because lightning can occur in remote areas so detection and suppression, if any, are delayed as compared to human-caused fires that usually occur in southern full suppression zones. Additionally, lightning fires can occur in large numbers over a short period of time, which can overwhelm a fire management agency’s capacity to respond.

Weather and climate – including temperature, precipitation, wind, and atmospheric moisture – are critical aspects of fire activity. Weather is one of the four factors influencing fire activity but it also influences two other factors, fuel and ignitions. Fuel moisture, which may be the most important aspect of fuel flammability, is a function of the weather, and weather and climate also in part determine the type and amount of vegetation (fuel) at any given location. Additionally, lightning is determined by the meteorological conditions. Weather arguably is the best predictor of regional fire activity for time periods of a month or longer. For example, Cary et al. (2006) found that weather and climate best explained modelled area burned estimated from landscape fire models compared with variation in terrain and fuel pattern. Although wind speed may be the primary meteorological factor affecting fire growth of an individual fire, numerous studies suggest that temperature is the most important variable affecting overall annual wildland fire activity, with warmer temperatures leading to increased fire activity (Gillett et al. 2004; Flannigan et al. 2005; Parisien et al. 2011). The reason for the positive relationship between temperature and regional wildland fire is three-fold. First, warmer temperatures will increase evapotranspiration, as the ability for the atmosphere to hold moisture increases rapidly with higher temperatures (Williams et al. 2015), thereby lowering water table position and decreasing forest floor and dead fuel moisture content unless there are significant increases in precipitation. Second, warmer temperatures translate into more lightning activity that generally leads to increased ignitions (Price and Rind 1994; Romps et al. 2014). Lastly, warmer temperatures may lead to a lengthening of the fire season with a longer snow period (Wotton and Flannigan 1993; Westerling et al. 2006; Flannigan et al. 2013; Jolly et al. 2015). While testing the sensitivity of landscape fire models to climate change and other factors, Cary et al. (2006) found that area burned increased with higher temperatures. This increase was present even when precipitation increased, although the increase in area burned was greatest for the warmer and drier scenario. The bottom line is that we expect more fire in a warmer world. However, we need to assess the sensitivity of these expected increases in fire activity to changing temperature and precipitation associated with this warmer world.

The global climate is warming and this may have a profound and immediate impact on wildland fire activity. Some suggest that wildland fire activity has already increased due to climate change. For example, Gillett et al. (2004) suggest that the increase in area burned in Canada over the past four decades is due to human-caused increases in temperatures. Dennison et al. (2014) found regional increases in area burned over the western U.S. since 1984. These increases in area burned in Canada and western U.S. were occurring despite stable or increasing fire suppression effectiveness and increased coverage by fire suppression resources. Predicting the impacts of future climate change on fire activity will require an understanding of the impacts and interactions of temperature and precipitation on fuel moisture dynamics, which is a critical factor affecting fire ignition and spread.

The objective of this paper is to examine the sensitivity of fuel moisture as described by the Canadian Fire Weather Index System fuel moisture codes to changes in temperature and precipitation. We do this by determining how much precipitation has to increase for every degree of warming to maintain the same fuel moisture code value. Additionally, we examine the frequency of extremes or frequency of exceeding a threshold in the fuel moisture as fire activity is driven by extreme fire weather (Wang et al. 2015). For example, Podur and Wotton (2011) found that there was almost a 50 % probability of having a day of significant fire growth in the boreal forest region, given that a fire was spreading, occurred when the FFMC was above 92.6Lastly, we interpret the results in terms of future fuel moisture conditions based on temperature and precipitation changes from General Circulation Models (GCMs).