If you flip a coin how good are you at predicting which side comes up? At best your a bit better than 50% accurate.

Now flip that same coin a million times. I can't tell you with any degree of accuracy what any one flip will come out. But I can give you extremely good odds that over a million flips very close to 50% will end up heads. And the more flips you make the closer to the ideal the numbers will get.

In statistics it is known as the 'rule of large numbers'. Which is why even the best meteorologists often can't predict what the weather will be tomorrow but any competent climatologist with a statistical background can, with very high confidence, predict global warming based on the preponderance of the evidence available recording sources back thousands of years. When you get a large number of data points that record things over a very long time the trends become startlingly clear.