The Americas join
An isthmus closes the sea and starts the ice ages
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The Great American Interchange sent marsupials, sloths and opossums north, and deer, cats and camelids south. The outcome was asymmetric: North American groups fared far better, probably because they came from a larger and more competitive continent. Oceanographically, the isthmus closed the equatorial flow between Atlantic and Pacific and strengthened North Atlantic circulation. That made northern Europe warmer, but also delivered moisture to the Arctic — snow that does not melt is the precondition for ice sheets, and the ice ages begin shortly afterwards.
For children
North and South America had been separate for over a hundred million years, and their animals had evolved entirely apart. When the isthmus of Panama closed, animals crossed in both directions. At the same time ocean currents were forced north — and the Gulf Stream was born.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org