A growing number of present-day geologists are now arguing that the holocene epoch is effectively over, and that it's time to call our times by a new name: the anthropocene epoch, named for the dominant geological force of the period. Let's look at our credentials: we've relocated billions of tons of carboniferous-era rocks out of the ground and into the atmosphere; we've accelerated erosion in rivers worldwide, we're melting away ice caps that are hundreds of thousands of years old, and we've precipitated one of the largest mass extinctions in the history of life on Earth.
The Holocene Epoch began about 10,000 years ago. It is, by two orders of magnitude, the shortest geological epoch, and we may have just ended it. Whatever - it's an arbitrary division of time, a scientific invention. But what impresses me most about this is the fact that the geological record is almost four billion years old - and in a few thousand years, humans have already managed to leave a permanent mark.
Some geologists even say that the entire quaternary period, a scant 1.8 million years young, may be extinct. The quaternary period is defined geologically by periodic ice ages, which seem increasingly unlikely, but it's still entirely within the realm of possibility that something like The Day After Tomorrow might happen someday. Or, as the geologists put it, "Given the large uncertainties in the future trajectory of climate and biodiversity, and the large and currently unpredictable action of feedbacks in the earth system, we prefer to remain conservative. Thus, while there is strong evidence to suggest that we are no longer living in the Holocene (as regards the processes affecting the production and character of contemporary strata), it is too early to state whether or not the Quaternary has come to an end" (source: GSA Today, February 2008).
On the one hand, I'm encouraged by the fact that geologists are heeding the fact that humans are a part and a force of nature. Kind of puts the environmental movement in perspective: Ted Turner and the Nature Conservancy can buy as many Montanan ranches as they want, but they won't be doing squat in the grand scheme of geological time.
Raise cain about greenhouse gases and move our society towards leaving carbon in the ground, though, and that's an accomplishment that will be scribed in the rocks for billions of years to come.