Climate Change And Boreal Forest Fire Activity

Confirming a growing scientific consensus, the Intergovernmental Panel on Climate Change (IPCC) has recently concluded (IPCC 1995) that "the observed increase in global mean temperature over the last century (0.3-0.6°C) is unlikely to be entirely due to natural causes, and that a pattern of climate response to human activities is identifiable in the climatological record". There is also evidence of an emerging pattern of climate response to forcings by greenhouse gases and sulphate aerosols, as evidenced by geographical, seasonal and vertical temperature patterns. In North America and Russia this pattern of observed changes has taken the form of major winter and spring warming in west-central and northwestern Canada, Alaska, and virtually all of Siberia over the past three decades, resulting in temperature increases of 2-3°C over this period (Environment Canada 1995, Hansen et al. 1996).

Numerous General Circulation Models (GCMs) project a global mean temperature increase of0.8-3.5°C by 2100 AD, a change much more rapid than any experienced in the past 10,000 years. Most significant temperature changes are projected at higher latitudes and over land. In addition, greatest warming is expected to occur in winter and spring, similar to the trends measured recently, although warming is projected for all seasons. While GCM projections vary, in general winter temperatures are expected to rise 6-10°C and summer temperatures 4-6°C over much of Canada and Russia with a doubling of atmospheric carbon dioxide. Global precipitation forecasts under a climate are more variable among GCMs, but indications are that large increases in evaporation over land due to rising air temperatures will more than offset minor increases in precipitation amounts. In addition, changes in the regional and temporal patterns and intensity of precipitation are expected, increasing the tendency for extreme droughts and floods. Recent transient GCMs, which include ocean-atmosphere coupling and aerosols, and project climate continuously through the next century (e.g. Flato et al. 1999), support these earlier predictions.

Despite their coarse spatial and temporal resolution, GCMs provide the best means currently available to project future climate and forest fire danger on a broad scale. However, Regional Climate Models (RCMs) currently under development (e.g. Caya et al. 1995) and validation (Wotton et al. 1998), with much higher resolution, will permit more accurate regional-scale climate projections. In recent years GCM outputs have been used to estimate the magnitude of future fire problems. Flannigan and Van Wagner (1991) used results from three early GCMs to compare seasonal fire weather severity under a climate with historical climate records, and determined that fire danger would increase by nearly 50% across Canada with climate warming. Wotton and Flannigan (1993) used the Canadian GCM to predict that fire season length across Canada would increase by 30 days in a climate. An increase in lightning frequency across the northern hemisphere is also expected under a doubled C02 scenario (Fosberg et al 1990, 1996; Price and Rind 1994). In two recent studies, Fosberg et al.(1996) used the Canadian GCM, and Stocks et al.(1998) used four current GCMs, along with recent weather data, to evaluate the relative occurrence of extreme fire danger across Canada and Russia, and showed a significant increase in the geographical expanse of severe fire danger conditions in both countries under a warming climate. This increase does not appear to be universal across Canada though, as Flannigan et al. (1998) report results using the Canadian GCM that indicate increased precipitation over eastern Canada could result in a decrease in fire activity in that region. In addition, a dendrochronologi-cal analysis of fire scars from northern Quebec indicates a decrease in fire activity during the warming period since the end of the Little Ice Age (ca. 1850). However, most paleoecological studies of lake sediments in North America show fire frequency and intensity have increased in past warmer and drier climates (e.g. Clark 1988, 1990)

In addition to increased fire activity and severity, climate warming of the magnitude projected can be expected to have major impacts on boreal forest ecosystem structure and function in northern circumpolar countries (see Weber and Flannigan 1997). Based on GCM projections large-scale shifting of forest vegetation northward is expected (Solomon and Leemans 1989; Rizzo and Wilken 1992; Smith and Shugart 1993), at rates much faster than previously experienced during earlier climate fluctuations. Increased forest fire activity is expected to be an early and significant result of a trend toward warmer and drier conditions (Stocks 1993), resulting in shorter fire return intervals, a shift in age-class distribution towards younger forests, and a decrease in biospheric carbon storage (Kasischke et al. 1995; Stocks et al. 1996). This would likely result in a positive feedback loop between fires in boreal ecosystems and climate change, with more carbon being released from boreal ecosystems than is being stored (Kurz et al. 1995). Reinforcing this point, a retrospective analysis of carbon fluxes in the Canadian forest sector over the past 70 years (Kurz and Apps 1999) found that Canadian forests have been a net source of atmospheric carbon since 1980, primarily due to increasing disturbance regimes (fire and insects). It has been suggested that fire would be the likely agent for future vegetation shifting in response to climate change (Stocks 1993). Weber and Flannigan (1997) conclude that "Fire regime as an ecosystem process is highly sensitive to climate change because fire behaviour responds immediately to fuel moisture..." and that "interaction between climate change and fire regime has the potential to overshadow the direct effects of global warming on species distribution, migration, substitution, and extinction".

While fossil fuel burning contributes most significantly to increasing atmospheric greenhouse gas concentrations, emissions from biomass burning of the world's vegetation (forests, savannas, and agricultural lands) has recently been recognised as an additional major source of greenhouse gas emissions (Crutzen and Andreae 1990). Recent cooperative international experiments (e.g. Andreae et al. 1994, FIRESCAN Science Team 1996) have confirmed that biomass burning produces up to 40% of gross carbon dioxide and 38% of tropospheric ozone, along with a suite of less common, but equally important greenhouse gases (Levine et al. 1995). While most biomass burning emissions originate from savanna and forest conversion burning in the tropics, there is a growing realisation that boreal and tempe rate forest fire emissions are likely to play a much larger role under a warming climate. Cofer et al. (1996) recently outlined a number of reasons why the importance of atmospheric emissions from boreal fires may be underestimated: the tremendous fluctuations in annual area burned in the boreal zone, the fact that boreal fires are located at climatically sensitive northern latitudes, the potential for positive feedback between climate wanning and boreal fire activity, and the high energy level of boreal fires which traditionally produce smoke columns reaching into the upper troposphere.

The 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change calls for the "protection and enhancement of sinks and resevoirs of greenhouse gases", and will require all countries to monitor and understand the major factors influencing the exchange of carbon between the biosphere and the atmosphere. With a large amount (37%) of the total global terrestrial carbon stored in boreal forests, boreal countries will be required to be in the forefront of these efforts. As discussed here, fire is the major disturbance regime affecting carbon cycling in the boreal zone and, with the likelihood of significant increases in forest fire activity in this region, predicting future boreal fire regimes is an urgent international research goal. Policy development and adaptation strategies require this information as soon as possible.

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