Here are two instances of the rapidity of climate change. It is this rapidity that is of serious concern and a primary indicator that fast-growing human generated emissions are a principal reason. Previous changes recorded in earth's history took millennia. We are now seeing dramatic effects within decades.
This first item is about National Parks concern about major changes to US western areas within twenty years and totally unliveable summer temperatures in Death Valley, where temperatures are expected to climb by 8 degrees (F.)
'Park Service director Jon Jarvis was not in a mood to mince words, calling climate change "the greatest threat to the integrity of the national park system that we've ever faced.'
http://blogs.kqed.org/climatewatch/2010/11/01/national-parks-wrestle-with-warming/
The second reference show clearly the changes demonstrated by the minimum ice cover of the Arctic regions over a period of 26 years. The legendary NorthWest passage has now become almost fully navigable at low ice in that short period.
http://climate.nasa.gov/blogs/index.cfm?FuseAction=ShowBlog&NewsID=281&CFID=1052136&CFTOKEN=11531440
edits: "summer temperatures", "a period of"