What are some threats to the Arctic tundra?

With global temperatures on the rise the tundra will undergo some major changes that end up acting as positive feedback on the global warming phenomenon. In the summer plants grow quickly, taking in carbon dioxide from the atmosphere. However, this season normally only lasts for a short period. Before any organic mater has a chance to decay, it freezes again. Thus, carbon from the atmosphere is sequestered and locked in the permafrost. However, with global rise in temperatures, the tundra is releasing carbon back into the atmosphere. It no longer acts as a sink.