1. The extinction event at the K-T boundary may have taken place in a matter of days if the Alvarez hypothesis is correct, i.e. the Chicxulub impact is the primary cause of the extinction. There is a lot of evidence to support Alvarez, but there are competing and complementary hypotheses.
2. Of course what's happening to the biosphere due to human activity is not good, at least in terms of the continuation of the world that we know. The Permian extinction, however, was even greater than that which ended the Cretaceous.
3. Life has existed on Earth more or less since the planet became hospitable to it. Nobody knows whether biogenesis occurred here, nor exactly how, or whether single-celled organisms might have been brought to Earth on comets or through some similar means (the Panspermia Hypothesis). At the moment Panspermia seems at least possible, especially since extremophile microorganisms were discovered in recent years. It turns out that life is very tenacious and hardy, much more so than was previously thought.
4. Human beings, on the other hand, are not so hardy and durable. It is now clear that many species of hominids have gone extinct over the last seven million years or so, as well as one "human" species (homo neanderthalensis). Our time will come eventually, as it has for every multicellular species over the last 550 megayears or so.
5. Finally, I certainly agree that the destruction of phytoplankton is very alarming. I can't say that I'm indifferent to an imminent end to our species; nevertheless, I'm not concerned that life on Earth will continue to thrive for quite some time. We certainly should be doing more to care for the environment, but whether we do or not will not be the deciding factor in the fate of the Earth's biosphere.