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Arbitrary?
How do you think the atmosphere keeps in the heat from the sun?
Changes have always been happening sure -- but how FAST were the changes in the past? Going up or down a degree C in 100,000 years is easy, but having that change in ~100 years is another thing altogether.
We will probably lose all the ice in the Arctic in the summer within the next 10 years. This will accelerate the warming, because of the low albedo of open water vs the higher albedo of snow and ice. Additional warming will then melt the permafrost more quickly -- which will release lots of methane -- which is ~20X stronger greenhouse gas than is carbon dioxide. And so the very quick warming will become even quicker.
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