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Ladd piloted his dust-streaked pickup along the border wall, discussing each stage of fencing as though it was a tree ring marking a distinct political era.

They will use dendrochronology, also known as tree ring dating, where the rings in the wood are counted to establish the age of a tree.

A laboratory measurement method can then be used to determine how much 14C is still contained in a particular tree ring and thus calculate the tree's approximate time of death over the known half-life.

The researchers compared early instrumental data with a large-scale tree ring dataset and found the 19th century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought.

While El Niño events were first noted by fisherman in the 17th century, they can be observed in the tree ring data much further back in time.