Climate change can transform ecosystems and ecosystems affect climate; 
hence ecosystem-mediated feedbacks may influence future climate change.  
We present evidence from past climate change for strong positive feedback 
effects that are not incorporated in our current climate models, and infer 
that global warming is likely to become more intense than is currently 
predicted.  To learn more about how ecosystems will respond to climate 
warming and how those changes feed back to climate, we have been 
experimentally warming a subalpine ecosystem in the Colorado Rockies for 
the past 17 years.  Results from this experiment will be presented, and 
its implications for climate change discussed.  We show that a number of 
simplifying assumptions currently made in climate-ecosystem investigations 
are inadequate and conclude that the task of quantifying ecosystem 
responses to global warming poses a daunting challenge.