I'm trying to find a justification to the sea level on Earth decreasing by a large amount (in the range of 1000 / 2000 m).
The change would begin during the Modern Era or in the future, and last long enough to allow for new political entities to be established on the newly exposed lands. The interested area doesn't need to be the entire surface of the planet, but it should be extensive enough to allow for said political entities to form.
I imagine for example that something could cause, over a long span of time, the amount of water evaporating to be constantly bigger than the amount of water returned to the ground as rain, snow, sleet. But this requires an explanation of its own.
What kind of event, natural or caused by human actions either deliberate or accidental, could lead to this phenomenon (feel free to use my prompt in the paragraph above, or give a completely different reason)? What sort of timeline would it require?
If this is not possible without large amounts of handwaving with Earth, please use an Earth-like planet as similar as possible to ours.