CAPE GRIM, Tasmania — On the best days, the wind howling across this rugged promontory has not touched land for thousands of miles, and the arriving air seems as if it should be the cleanest in the world. But on a cliff above the sea, inside a low-slung government building, a bank of sophisticated machines sniffs that air day and night, revealing telltale indicators of the way human activity is altering the planet on a major scale.