At the high end of emissions, with business as usual for several decades to come, global mean warming is estimated to reach 4-7C by 2100, locking in climate change at a scale that would profoundly and adversely affect all of human civilization and all of the worlds major ecosystems. At the lower end of emissions, something that would require urgent, deep and long-lasting cuts in fossil fuel use, and active preservation of the worlds forests, global mean warming is projected to reach 2-3C by century's end. While clearly a better outcome than the high emissions route, global mean warming of even just 1.5-2.0C still carries a significant risk of adverse impacts on ecosystems and human society. For example, 2C global temperature rise could lead to sufficient warming over Greenland to eventually melt much of its ice sheet (Oppenheimer and Alley 2005), raising sea level by over six meters and displacing hundreds of millions of people worldwide.
-- The Copenhagen Diagnosis, 2009: Updating the World on the Latest Climate Science, http://www.copenhagendiagnosis.com