Though brief, humanity's existence on earth has damaged it dramatically. We have polluted the atmosphere, cut down forests, over-irrigated for agriculture, released CFCs into the ozone layer, and drastically increased CO2 levels. All of these things contribute to rising global temperatures. The length of our existence on earth has nothing to do with the damage we have succeeded in causing during our stay.
While it is true that countless other catastrophes have rocked the earth before, mankind has created unique issues. Though many try to rationalize humanity's existence as just another "natural event" altering the earth, the damage we have caused to the climate is anything but natural. It is misguided to assume that we can do nothing to change the earth's climate when the vast majority of scientific literature claims otherwise.