B.D. Amiro, M.D. Flannigan, B.J. Stocks, J.B. Todd and B.M. Wotton 1
The area burned in Canada has doubled during the past 20 years compared to most of the twentieth century, with wide interannual fluctuations. Global and regional climate models suggest that the area burned and severity of boreal fires will increase as a result of climate change. Landscape changes, and alterations of the cycles of other disturbances are also likely in the future. These pose increased impacts on social and economic values, and it is unlikely that there will be sufficient resources to respond to increasing fire. Some of the effects of these fires can be mitigated, but society will need to adapt to many of the impacts, some of which have global implications on climate feedbacks.
Changes to climate caused by increasing atmospheric greenhouse gas concentrations are expected to cause general warming across the boreal zone in the order of a few degrees over the next century (IPCC 2001). This warming, coupled with drier conditions in some parts of the boreal region, is expected to create weather conditions conducive to more fire (Flannigan et al. 1998), as well as increases in other disturbances such as insects and diseases (Dale et al. 2001). About one-quarter of the world's boreal forest is in Canada, thus many of the expected climate changes affecting the global boreal forest will greatly affect this country. In this paper, we discuss Canada as a case study of the impacts of climate change on forest fires. We consider the extent of the impacts associated with a changing climate, as well as the potential to mitigate or adapt to some of these impacts.
Wildland fires typically burn one to three million hectares (ha) annually in Canada (Figure 1). These fires exclude agricultural and rangeland areas but do include regions that may only be sparsely forested or wetlands contiguous to forested areas. A 5-year running average shows an increase since about 1980 and greater variability in the last two decades compared to earlier periods. We have less confidence in measurements of area burned before about 1970 and it is likely that it has been underestimated during this period when detection methods were not as reliable as today. However, the mean and variability through the 1960s and 1970s is quite similar to previous decades, which may indicate that the 1980s and 1990s are anomalous. The area burned has doubled over the last four decades, from an annual average of 1.4 million ha in the 1960s and 1970s, to 2.7 million ha burned annually through the 1980s and 1990s. This large burned area occurs in spite of substantial resources being deployed for fire management. Data on fires greater than 200 ha in area are well documented in the Canadian large-fire database (Stocks et al. 2002), with these fires accounting for most (~97%) of the area burned. This database allows a retrospective look at trends in fire size, location and cause and forms a national monitoring system to detect changes that could be caused by a changing climate.
Projections of future fire weather using GCMs (General Circulation Models) and a RCM (Regional Climate Model) suggest that weather will be more conducive to increased fire activity through much of Canada, especially in the western part of the country (Flannigan et al. 1998, Flannigan et al. 2001, Stocks et al. 1998). This is caused by warmer temperatures and less precipitation, which couple to increase the fire severity rating. Figure 2 illustrates a simplified overview of the future fire weather outlook for the country, based on knowledge from two GCMs and the RCM output. Although GCM results vary among models, Figure 2 considers some of the uncertainty to capture the most important features. These models indicate that the seasonal severity rating (an index relating to the difficulty of controlling a fire based on weather) will increase through much of Canada. In addition, the fire season may lengthen (Wotton and Flannigan 1993), and drier and warmer conditions will sustain increased ignitions. Our expectation is for more fire to occur at a national scale.
A changing climate will affect vegetation over many parts of Canada. Simulations and logical arguments have predicted a variety of possible outcomes (Aber et al. 2001, Hogg and Hurdle 1995). Availability of water will be the greatest limitation, especially in the southwestern part of the Canadian boreal forest. Some estimates suggest that the aspen parkland area at the southern limit of the western boreal forest will grow at the expense of the more coniferous component (Hogg and Hurdle 1995). However, the persistence of the existing vegetation is unclear. Many researchers believe that disturbance by fire, insects, diseases, or harvesting will be a major catalyst for species replacement in a changing climate (Dale et al. 2001, Volney and Fleming 2000). The logical conclusion is that disturbance will initiate change, and that the species composition will adjust in response to the new climate, disturbance regime, and rates of migration. Interactions are expected among many of these disturbances, some of which will be competing while other interactions could be additive or even synergistic.
What will this mean for the future fire regime? For much of the southern boreal forest, the deciduous tree component may increase. Observations by fire suppression personnel, experimental data (Forestry Canada 1992), and landscape analyses (Cumming 2001) clearly show that greater deciduous components in forests reduce fire rates of spread. In many areas, recent burns also cause fuel reductions where fire cannot propagate as easily. Of course, there are exceptions, notably in recently harvested areas where dried grasses provide an environment for a rapidly moving fire front. Recently disturbed forests can still support fire and it is unknown whether more fires will change the landscape and be self-limiting. The net impact depends on the time scale and the amount of disturbance that has occurred.
The urbanization of Canada is fairly recent compared to many other countries. The majority of the population lives in areas not frequented by forest fires, either because of lack of forests, or because the fuel type does not normally sustain severe fires. However, there are sizable populations living in the wildland-urban interface, and the trend is for increased construction of buildings in the interface that are at greater risk to fire. Infrastructure includes roads and power lines that are impacted by fire. Smoke can make major transport conduits impassable for many hours, and power lines can be destroyed by fire. Forests in much of the northern and western parts of Canada are being harvested for the first time, with fire now competing with timber harvesting. Although burned wood is salvaged where road and mill infrastructures exist, the resource is generally of lower value.
Over the last few decades, ecological arguments have been made to maintain fire regimes that are approximately consistent with historical ones (Parks Canada 2000). It has always been difficult to select a particular fire regime, since these have changed over time at any particular location. For some parts of Canada, the fire cycle was longer throughout most of the 20th century compared to the previous few centuries (Bergeron 1991, Tande 1979). Perhaps the greater incidence of fire during the past two decades is more similar to previous fire cycles. But how much fire can the boreal forest experience and maintain a diverse mix of plant and animal communities? It is possible that too much fire in the future could affect biodiversity, just as too little fire favours mature forest types. Hence, ecological values could be included as additional values at risk if climate change substantially increases fire activity.
Approximately half of the area burned in Canada during the past 40 years has resulted from fires that, while monitored, did not receive substantial suppression efforts (Stocks et al. 2002). It is difficult to establish whether increased suppression would have had a big impact on these other fires. Canadian forest fire control expenditures have increased by almost a factor of 10 since 1970 (CCFM 1997) with current annual estimates being about Cdn$500 million. At least part of this cost is associated with greater fire incidence and it is reasonable to believe that more future fire will increase costs dramatically. Analyses suggest that we are approaching a limit of diminishing returns, with less incremental improvement with dollar spent (McAlpine and Hirsch 1999). This is caused by severe fire weather creating a situation where it is almost impossible to contain all large fires, and huge resources must be deployed to achieve marginal gains. Severe fire weather also creates multiple ignitions, making it difficult to address all fires. Recognizing this limitation, agencies have been investigating schemes for landscape management, where it is hoped that fuel manipulations will provide a fire break or at least reduce fire intensity to help suppression efforts, often termed "fire-smart forests" (Hirsch et al. 2001). This may be possible to protect specific values, such as communities, infrastructure, and some timber. Effective fuel management should mean that no structures are consumed by wildfire, which would reduce many of the pressures faced by fire suppression agencies. However, smoke will remain an issue and create the need for evacuations of towns in close proximity to fires. This impacts many of Canada's northern communities, many of which have limited transportation routes.
Funds for fire suppression compete with other socioeconomic needs. In Canada, climate change could impact major sectors, such as natural resources and health. These sectors will also require funding to cope, and it is not clear that fire suppression will be a top priority. Much of the need to adapt to a more dangerous fire regime will likely rely on efforts of individuals and companies to protect their lives, property, infrastructure, and timber. Insurance companies will likely increase their role to cover these losses, but premiums will need to increase. At least one Canadian fire management agency is managing the financial risk of high fire years through an insurance policy covered by private underwriters. Given the wide cyclic nature of fire shown in Figure 1, it is likely that many agencies will consider this option to smooth out annual costs. Some fire impacts have a much greater potential for mitigation than others; for example, we should be able to minimize loss of human life, property and infrastructure. However, we will need to adapt to effects that cannot be mitigated, since we believe that increased fire is inevitable for much of Canada.
The current increase in atmospheric greenhouse gases is mostly caused by fossil fuel emissions (IPCC 2001). However, landscape change and biomass burning also add to the effective emissions. For example, forest fires have contributed direct carbon emissions averaging 18% of the fossil fuel emissions in Canada over the past four decades with additional contributions caused by post-fire effects (Amiro et al. 2001). These fire carbon emissions have increased in recent years following the trend in increased area burned. If the modelling scenarios hold true and global warming will increase fire in Canada, then we see a positive feedback scenario where a warming climate also increases fire carbon dioxide emissions, further contributing to warming. This will also be true for other trace gases that are released by fires such as carbon monoxide and methane (Cofer et al. 1998). However, this "runaway" positive feedback scenario is mostly controlled by fossil fuel emissions, and biomass burning emissions alone would not likely have a great climate change effect. Canadian fires have been implicated in elevated carbon monoxide concentrations in the U.S. (Wotawa and Trainer 2000), and black carbon particles may influence global climates (Lavoue et al. 2000, Hansen et al. 2000, Jacobson 2001). In addition, there is evidence that smoke promotes positive lightning strikes (positive strikes have more ignition potential than negative strikes) (Lyons et al. 1998), while reducing precipitation (Rosenfeld 1999). Fire affects the local and regional climate by changing the surface characteristics, such as increasing winter surface albedo (Betts 2000) and summer daytime surface temperature (Amiro et al. 1999). Very large fires can have regional effects, with perhaps global implications.
Climate change will likely increase the incidence of fire, resulting in more area burned in Canada, and across much of the global boreal zone. These fires have global implications because they also release greenhouse gases and change the energy balance of the Earth's surface. In parallel, landscapes will also be changing in response to climate, human developments, and disturbances. It is likely that disturbances such as fire, insects, and diseases will be the catalyst for change in many forested areas, but the interactions among these agents are still unclear. Socioeconomic values will increase in the future, with many more of these at risk because of fire. Only some of these impacts can be mitigated, while we will need to have adaptation strategies for others.
We thank a large number of individuals who have contributed to data and analyses of forest fire incidence in Canada. In particular, we thank E. Bosch and K. Logan for recent work on the fire and climate change projects. Partial funding was provided by the Canadian government Program on Energy Research and Development (PERD), the Climate Change Action Fund, and Action Plan 2000.
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Fig. 1. Annual area burned in Canada during the 20th century.
Fig. 2. Changes to the seasonal severity rating (SSR) for fire in Canada as a ratio of 3x CO2 (about 2100) scenarios to the 1xCO2 (present) state (value of 1 indicates no change). The SSR reflects difficulty of controlling fire, based on weather conditions, with higher values indicating more difficulty. Estimates for the extreme west of the country are based on a regional climate model (Flannigan et al. 2001), whereas the rest of the country is based on averaging results from the Canadian and Hadley general circulation models (Logan et al. 2002). Regions where the models disagree are uncertain and are marked as "no change".
1 Canadian Forest Service, Northern Forestry Centre, 5320 - 122 Street, Edmonton, Alberta, Canada, T6H 3S5. [email protected]