Like a lot of people, I worry a good deal about what we humans are doing to the planet, by which I really mean I worry a good deal about what we’re doing to life and the biosphere. Between anthropogenic global warming, ozone depletion, and the threat of nuclear war, as a species we could well end up responsible for a mass extinction event (though we’re be by no means the first organisms to fundamentally alter the planet’s biosphere – all the anaerobic bacteria spewing out oxygen during the first few billion years of life’s history on the planet did far more than we’ve done, or probably can do, to alter the biosphere – which is not at all to diminish the significance of the mass extinction of animals and plants we may be in the process of producing).
I’m not particularly worried that humans will ever wipe out life on the planet, or even our own species, though I do think it’s possible. Probably the best strategy to attempt to wipe out life on the planet, or simply the human species, would be to incite global thermonuclear war. The trouble with such a strategy is that in any conceivable actual situation, including at the most dangerous moments in the history of the Cold War, while the vast majority of individual human beings might be wiped out through the utter obliteration of the populations of the primary targeted regions (obviously a tragedy far beyond anything human beings have managed to do to one another thus far, even over the course of the bloody 20th century), far too many areas would go untargeted to wipe out the species. It’s hard for me to imagine ICBMs being targeted to wipe out all human life in the many rugged valleys of the highlands of New Guinea, or in all areas of the Amazon Basin, or on all the many small islands of the Caribbean or the Pacific, etc. Fall out, Nuclear Winter, and the like might do many of them in, but it’s hard for me to imagine a “naturally occurring” nuclear war wiping out the human species, much less life in general. To wipe out all humans, and much of the rest of life on the planet, I think you’d really need to engineer a conspiracy to end all conspiracies (in the metaphorical and literal senses) cutting across all the nuclear states to give you access to the world’s total nuclear arsenal so that you could target even the smallest Pacific island and every last valley in New Guinea. With access to the world’s total nuclear arsenal, this might be technically possible, though clearly utterly implausible to implement.
Alternately, one could attempt to wipe out the human species via a bioengineered epidemic. While killing billions in such a manner is potentially feasible if you have the ability to engineer and deliver the disease, wiping out the species would likely run into the same sort of mopping up problem as above.
I hadn’t before given much thought to destroying the planet, figuring that was simply not a possibility. Apparently it is possible, even if highly, highly implausible, as I found out when I read this highly entertaining essay on top ways to destroy the Earth. (I’d note that destroying the Earth would certainly destroy all life on the planet, too, though it wouldn’t necessarily wipe out the human species, as many of the methods for destroying the planet require technologies implying that at least some humans would not be confined to the planet.)