Even there, Nature (as in naturally occuring geophysical forces) can and has been far more destructive, far more quickly. There have been multiple volcanic eruptions that have drastically altered global climate patterns for years after the eruptions. Far more than anything humans have thus far been accused of doing.
That isn't to say we aren't capable of significant localised impacts on the environment.
The oil spill in the gulf is a great example of that. But as with past "disasters" the long term impact will quite likely be far less than all the hysteria I've been hearing the past year. Just as it was with Chernobyl, Hanford, Three Mile Island, etc.