What a good idea! In theory. If you live in a tropical climate it is a good idea to paint your roof white. In temperate and colder climates it's not a good idea. In the winter we want a dark surface to absorb heat.
Even if it were possible to paint every horizontal surface in the world white it would not equal the current surface whiteness of the North or South poles. If the poles reflected warming light, the poles would not be melting, would they?
I really think that global warming is (1) not caused by human endeavour and (2) it is cyclical with sun spots and has happened throughout history. Our puny activities cannot possibly cause the whole Earth to warm, given the atmospheric volume of Earth's surface and how miniscule an amount we even matter to such an enormous volume.
Also given that 4/5ths of this planet is water, we humans are giving ourselves swolen heads to think that we can make a difference.
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Some days it just doesn't seem worth trying to chew through the restraints.