Modern Climate Change in the Boreal Zone
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Changes in global climate over the last one hundred fifty years have been brought about by anthropogenic emissions of greenhouse gases and by changes in land use and associated land cover. Although global climate typically varies on both long and relatively short time scales, the magnitude and rate of warming in the latter half of the twentieth century is without precedent in the within the record of paleoclimate. On average, the Earth warmed approximately six tenths of a degree Celsius from 1900 to 2000.
During the same time period, however, temperatures in the western Canadian Arctic warmed from three to five degrees Celsius (5,9), which is significantly greater than the global mean increase of six tenths of a degree. This rapid jump in temperature in the boreal Arctic has had several noticeable impacts on the region, including melting ice and snow, thawing permafrost, and a lengthened growing season (3,5). The physical impacts of climate change on the Arctic have affected ecosystem processes and ecosystem health on both the scale of individual trees and communities of microbes, and of ecosystems and landscapes. Higher temperatures have a strong impact the cycling of organic carbon in the soil--decomposition occurs more rapidly at higher temperatures, resulting in an increase in the flux of carbon (in the form of carbon dioxide) from the soil to the atmosphere (3). Enhanced soil respiration due to temperature increase may already be occurring in the northern hemisphere boreal zone (3). Future interactions of climate change with fire
in boreal forests will have a profound impact on both the individuals
and ecosystems of the boreal
zone. The susceptibility of a forest to fire is dependent
on multiple factors, including the availability of fuel, temperature,
precipitation, wind, and an ignition source (3,7). Changes in the climate
of the Canadian boreal zone will impact each of these factors in some
way. Warmer temperatures and increased concentrations of carbon dioxide
may cause an increase in the growth of some vegetation, resulting in
greater accumulation of organic matter and debris to serve as fuel
for fire. Additionally, warmer temperatures will cause stronger drying
of
these organic fuels, again increasing the fire potential. The effects
of climate change on wind and precipitation regimes are not well known;
however, it is likely that changes in precipitation will not
not be widespread enough to cancel the warming and drying effects increased temperature has on forest fire fuels. Wind speed and direction determine the rate
of spread of a fire once it has been ignited. Although the behavior
of wind in the future has not been quantified, the magnitude of its
influence
on boreal forest fires will probably remain constant. Currently, the
dominant non-anthropogenic ignition source for fires is lightning.
In order for a forest fire to build and spread, lightning must strike
a
fuel source that is readily combustible. With warmer temperatures over
boreal regions in the future, storms and the production of lightening
will become more frequent; forest fires ignited by lightning strikes
are predicted to increase
(4). Taken together, these climate-driven changes may increase the
potential for fire (the seasonal severity rating, or SSR) by 10-50% in the Arctic boreal forests.
Climate change will also impact the relationship between disturbances, including fire, and ecosystems. Disturbances open up patches within the forest, freeing space and resources for future recruitment. Future climate variability may increase the frequency of disturbances, thus altering the life cycles of species and the age structure of the forest. Additionally, warmer temperatures will allow new species to migrate into the boreal zone. These species may colonize areas of the forest recently cleared by disturbances, resulting in species assemblages that have not previously existed in the boreal forests of the Northern Hemisphere (1).
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