Recent changes to the fire regime and permafrost

In addition to the warming of the climate over the past half-century, interior Alaska has also seen changes to the fire regime and a warming of permafrost. Figure 13.6 presents the annual burned area in the entire state of Alaska for 1950 to 2006, where over 95 per cent of the total occurs in the interior region. For the past 22 years (1985—2006), the average burned area increased by 70 per cent compared to the previous 35 years (1950-1984), reaching 520,000 ha y-1. The increase in fire activity is the result of a significant rise in the frequency of larger fire years (i.e., years with large, very large or ultra-large areas burned) (see Figure 13.6, Plate 20). Prior to 1985, 1.7 larger fire years occurred per decade. Since 1985, 3.2 larger fire years have occurred each decade. During the record fire years of2004 and 2005, nearly 4.5 million hectares of land was impacted by fire, which represents >10 per cent of the land surface of the interior ecozones of Alaska. Recent changes in the frequency of larger fire years have also been observed in western Canada as well (Kasischke and Turetsky, 2006).

Over the past two decades, the occurrence of fire in interior Alaska has been strongly influenced by the seasonal patterns of precipitation. During small fire years, the non-human fire season begins in early June, when enough moisture enters the atmosphere to result in late-afternoon, convective lightning storms. Precipitation during April and May averages less than 4 cm throughout most of the interior, which results in the low fuel moistures and relative humidities that create conditions that support the ignition and spread of wildland fire. During

Figure 13.6 Historical patterns of annual burned area in Alaska (see also Plate 20 for color version)

Note: The blue bars represent small fire years, when <1 per cent of the land surface in interior Alaska burned. The yellow bars represent large fire years, when 1—2 per cent of the land surface burned. The orange bars represent very large fire years, when 2—3 per cent of the land surface burned. The red bars represent ultra-large fire years, when 3 per cent of the land surface burned.

Source: Based on fire records from the Alaska Fire Service, Bureau of Land Management.

low fire years, precipitation increases beginning in mid-July and August, raising both fuel moisture and relative humidity to the point where the ignition and spread of fires cannot be sustained. During larger fire seasons, however, stationary high-pressure centers interrupt seasonal precipitation patterns and create drought conditions that allow fires to burn longer, as well as leading to more fires being ignited. During larger fire years in the period 1985 to 2006, there was a substantial increase in the amount of late season burning (see Figure 13.7, Plate 21). These seasonal differences in burned area between smaller and larger fire years were not present during 1950-1984. As a result, there has been a doubling in the rate of burning in late-season fires, from 82,600 ha y-1 during 1950-1984 to 175,300 ha y-1 during 1985-2006.

Over the longer term, average air temperature directly controls average permafrost temperature. Because of this, long-term measurements show the temperature of deep permafrost (1-10 m) has been rising in interior Alaska at nearly the same rate as air temperatures since the beginning of the 1970s (Osterkamp, 2005).

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