ABSTRACT: Future permafrost response to climate change: lessons from ground temperature measurements in Takhini Valley, southern Yukon Territory.

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Ground temperatures have been measured at three sites in Takhini River valley to determine the thermal response of discontinuous permafrost terrain to climate change. The data have been collected by residents of Whitehorse. Takhini valley is in the sporadic discontinuous permafrost zone, with approximately 20% of the ground underlain by permafrost. The area was burned by forest fire in 1958, but, since it is a relatively dry environment, forest regeneration has been slow. The research sites are adjacent to the Alaska Highway, about 3 km east of the Takhini River bridge. One site is in a stand of spruce trees that were not burned. Permafrost at this site extends to a depth of 16 m beneath an active layer 1.4-m thick. The mean annual temperature in permafrost is –0.7 C. The snow depth at this site rarely exceeds 20 cm. A second site is in a grassy meadow where there is no permafrost, and the mean annual ground temperature is 1.5 C. The third site is in an area where the forest was burned, and permafrost has degraded since 1958. The top of permafrost is 4 m below the ground surface. At these two sites, the snow depth is commonly over 25 cm. At each site ground temperatures have been measured at seven depths to 5 m. The measurements are made on thermistor cables installed in 1” steel pipes. The measurement program was started in 1983, but continuous time series, collected at intervals of two weeks or one month, have been obtained since 1990. During this period, air temperatures at Whitehorse have fluctuated about a mean between –1 and 0C. Since the mid 1990s the air temperature has risen slightly. The ground temperature response to climate warming has varied between the sites. In permafrost, the maximum temperature has not changed over this time, but the minimum temperatures have increased steadily. In contrast, at the meadow site, where there is no permafrost, minimum temperatures have remained constant, near 0C, and the impact of climate warming has been observed in summer. The difference between the sites is associated with the exchange of latent heat. In permafrost, the frozen ground provides a sink for heat in summer, and therefore ground temperatures respond little to climate variation. In the meadow, where the seasonal frost may be thawed by the end of June, the ground warms in response to summer heat. If winter cooling of the ground is reduced, the ground thaws earlier and so the proportion of heat used to warm the ground is greater. At the third site, where permafrost is degrading, the ground thermal regime in the near surface is similar to conditions in the meadow, because the active layer is sufficiently deep at act as unfrozen ground. At this site is warming heat flows into the permafrost from above and below. Above permafrost in the forest, the dry active layer is commonly frozen by November, and surface temperatures then decline in winter. As a result, the mean ground temperature is low enough to sustain permafrost. At the meadow site, where there is seasonal frost penetration, freezing at depth supplies latent heat to maintain relatively warm surface temperatures in winter. Therefore, permafrost is to an extent self-sustaining. In future, we must expect sites with and without permafrost to respond differently to climate warming, and we must expect the degradation of permafrost widely predicted under scenarios of a warmer world to proceed slowly.

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