The earth warms and cools on it's own. I worry that humans are impacting the earth's ability to react naturally to it's warming/cooling. The ocean's response to the Gulf oil spill was pretty remarkable. The earth reminds me of the human body in that there are certain repairs that take place naturally. My "leftist" fear is that we will inhibit the earth's ability to fix itself.
I tend to worry more about oceanic ecosystems and things like that seem to get lost in the shuffle over temps, humidity, etc.