Hansen's Law refers to the principle that the effects of climate change, particularly the warming of the Earth, will continue to manifest for many years, even after greenhouse gas emissions are reduced. This is due to the long-lasting nature of carbon dioxide in the atmosphere, which can remain for centuries, leading to ongoing temperature increases and changes in weather patterns. The law is named after James Hansen, a prominent climate scientist who has been a key figure in raising awareness about global warming. He emphasizes that immediate action is necessary to mitigate these long-term effects, as the climate system has a significant lag time in responding to changes in emissions.