The boreal forest biome occupies an area of 18.5 million km2, which is approximately 14% of the vegetated cover of the earth’s surface (McGuire et al. 1995b). North of 50°N, terrestrial interactions with the climate system are dominated by the boreal forest biome because of its large aerial extent (Bonan et al. 1992, Chapin et al. 2000b; Fig. 19.1). There are three major pathways through which the function and structure of boreal forests may influence the climate system: (1) water/energy exchange with the atmosphere, (2) the exchange of radiatively active gases with the atmosphere, and (3) delivery of fresh water to the Arctic Ocean. The exchange of water and energy has implications for regional climate that may influence global climate, while the exchange of radiatively active gases and the delivery of fresh water to the Arctic Ocean are processes that could directly influence climate at the global scale. In this chapter, we first discuss the current understanding of the role that boreal forests play in each of these pathways and identify key issues that remain to be explored. We then discuss the implications for the earth’s climate system of likely responses of boreal forests to various dimensions of ongoing global change. Most of the energy that heats the earth’s atmosphere is first absorbed by the land surface and then transferred to the atmosphere. The energy exchange properties of the land surface therefore have a strong direct influence on climate. Boreal forest differs from more southerly biomes in having a long period of snow cover, when white surfaces might be expected to reflect incoming radiation (high albedo) and therefore absorb less energy for transfer to the atmosphere. Observed winter albedo in the boreal forest varies between 0.11 (conifer stands) and 0.21 (deciduous stands; Betts and Ball 1997). This is much closer to the summer albedo (0.08–0.15) than to the winter albedo of tundra (0.6–0.8), which weather models had previously assumed to be appropriate for boreal forests. The incorporation of true boreal albedo into climate models led to substantial improvements in medium-range weather forecasting (Viterbo and Betts 1999).