Goulden et al. (1998) report measurements that show the boreal forest soil to be a source of atmospheric carbon dioxide under global warming conditions. These high-latitude regions have warmed substantially in recent years and are projected to warm even further in the next several decades. Boreal forests contain a very large fraction on biospheric carbon on earth. The soil in this region remains frozen most of the year, which minimizes carbon loss through decomposition. However, higher temperatures increases the likelihood of thawing, and thawing increases decomposition 10-fold. The measurement site lost 0.3+/- 0.5 metric ton of carbon per hectare per year from 1994-1997. The addition of carbon dioxide to the atmosphere through decomposition in warmer soils is an example of a positive feedback in the climate system because more carbon dioxide leads to more warming which leads to more carbon dioxide release.
Reference
Goulden, M. L., S. C. Wofsy, J. W. Harden, S. E. Trumbore, P. M. Crill, S. T. Gower, T. Fries, B. C. Daube, S-M. Fan, D. J. Sutton, A. Bazzaz, and J. W. Munger, 1998: Sensitivity of boreal forest carbon balance to soil thaw. Science, 279, 214-216.