It doesn't; that's kind of a first-glance reading of the phrase without really thinking about it.

Something can said to change from a certain standard even if it wasn't perfectly constant to begin with. For example, if I always kept my house at 65-75 degrees for the past year, and now it's 85 degrees inside, I could certainly say that the temperature in my house recently changed and gotten warmer. That might lead me to check whether my AC's working, rather than say "well I guess the temperature has never really been constant, and 85 is within the range of possible non-constant temperatures, so everything's perfectly normal and nothing has changed."

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alt227 an hour ago | parent [-]

Your analogy doesnt work, becaue the earth has been warmer than it is now several times in the past. so the increased temperature is within the range of normal temperatures.

The problem is not that the earth is warming, it is that it is warming at an artificially increased rate.

	▲	larkost 13 minutes ago \| parent \| next [-]
		The rate of warming is a problem (i.e.: it determines what generations of humans are going to see this), but the major problem is the warming itself, or rather the change. We (humanity) have gotten comfortable with the way things are, and a change in that is going to mean that things are going to change for us, and we don't like change. Most of our biggest cities are all close to the coast and will be subject to massive flooding in the next 100 years (if not sooner). Much of those same large population centers are also fairly close to being too hot for general survival (without aggressive AC). Our agriculture is all setup for the temperatures we have now, and the rain patterns we are used to. So we are going to have to change both where we live, and how we grow our food (location and probably strains as well). Global warming is (almost) definitely not going to destroy all life on earth, but many of the forecasts are in extinction-level for most of the large animals. So life in general will continue, and probably humanity (since we are so good at making environment for ourselves), but the (eventual) changes are going to make the world very different, in ways that we are not going to like.
	▲	mithr an hour ago \| parent \| prev [-]
		It's not meant as a perfect analogy for global warming, but rather an illustration of how a constant state isn't necessary for something to be said to be "changing", which was OP's claim.