Shame on us if 100 or 200 years from now our grandchildren and great-grandchildren are living on a planet that has been irreparably damaged by global warming, and they ask, “How could those who came before us, who saw this coming, have let this happen?”
Joe Lieberman
 
While human-induced global warming is not going to turn present-day Earth into present-day Mars, global warming is dire enough that our most distinguished scientists recently concluded that as many as 1 million species on the planet could be extinct by 2050 if affairs do not change. 
Jay Inslee
 
Climate change is the most severe problem that we are facing today, more serious even than the threat of terrorism.
David King

Global warming is too serious for the world any longer to ignore its danger or split into opposing factions on it.
Tony Blair
 
The world won't come to an end, but the incidence of disasters will have a very big impact, and in ways we can't predict.... Rises in sea levels will displace millions of people. It's estimated there will be 150 million refugees by 2050, homeless as a result of global warming. It's how we deal with these problems that is as much the challenge as tackling the causes of global warming. 
John Theodore Houghton