A warm river in the sea, and why Britain is milder than Canada.
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London and Goose Bay in Canada are almost the same distance from the North Pole. They have almost the same latitude. But in January, one is about 5 °C and the other is about −17 °C (blue means freezing land). Why?
The Gulf Stream is a warm current. It starts near Florida, fed by warm water from the Gulf of Mexico, and carries that heat north-east across the Atlantic. It moves more water than all the rivers on Earth put together. Canada’s coast gets a cold current from the Arctic instead.
Why does the Gulf Stream flow at all? The wind pushes it. Near the equator, the trade winds push the sea west. Further north, west winds push it east. Together they spin the whole North Atlantic into a giant, slow whirlpool called a gyre. The Gulf Stream is its fast western edge.
Here, the wind mostly blows from west to east. For Britain, that wind comes across the warm ocean first. For Goose Bay, it comes across thousands of kilometres of frozen land. Many scientists think this wind is the biggest part of the secret, but not all of them agree.
Up north, the warm water cools down. Cold, salty water is heavy, so it sinks to the bottom and creeps back south. This loop is part of a giant ocean conveyor belt that goes all round the planet. One trip takes about 1,000 years!
Melting ice pours fresh water into the sea up north. Fresh water is lighter, so it doesn’t sink as well, and that could slow the conveyor belt down. Scientists watch it closely. Try switching it off.
On this map, red means the sea has warmed up over the last 100 years, and blue means it has cooled. Almost the whole ocean has warmed. But one patch south of Greenland has cooled down. Scientists call it the “cold blob”. Some think it’s a sign the conveyor belt is already slowing, so less warm water gets there.
The Gulf Stream carries warm water across the Atlantic.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).