Some deniers point to the failings of their local TV weatherperson and ask how any scientists can claim to predict Global Warming planetwide for the next thousand years or more. Reasonable question.
The answer is that it’s actually usually easier, scientifically, to analyze general worldwide trends, than which jet streams or cloud formations are going to appear over this or that particular point on the earth’s surface more than, say, 24 hours in the future. What GW scientists aren’t saying is that your block will get a thunderstorm more intense than usual next weekend, or for that matter, that your town’s daytime high (or worse, overnight low) next weekend will be 0.03 degrees Celsius warmer than usual, or whatever. All they’re saying is that on average, the long-term trends, verified by complex mathematical models that have successfully predicted what we’ve already gone through in recent generations as if it hadn’t happened yet, portend that the world’s overall temperature will keep rising as far ahead as we can see, caused by Greenhouse Gases that have had known effects and will continue to because we know how these gases behave in the atmosphere, how long they can be expected to last up there, etc etc etc.
For example, the global average temperature won’t even be affected by a relatively-more localized Upper North Atlantic Little Ice Age if it happens, though Western Europe and eastern North America will certainly feel different until the global average overcomes the local Little Ice Age effect.
Like dropping an ice cube into a boiling pot.