A layered cake in the oven heats unevenly: the top layer is already browning while the middle is still raw. Earth is the same way, it doesn't warm all at once. Scientists took land temperature records from 1880 to 2017 and split them into layers: six continents, seven latitude bands, and eight seasons. Then they identified the main patterns of variability in the data and tested when warming becomes detectable.
The first pattern explains three-quarters of all changes: temperatures rose until the 1940s, then fell, then rose again after the 1970s. But other patterns turned out to be more important. One of them, with a period of 60–70 years, is linked to the slow sway of Atlantic Ocean temperatures. It was this pattern that caused the early-century warming. The test showed that in the Northern Hemisphere this warming was significant in all seasons, while in the Southern Hemisphere it wasn't significant in any. Oceania didn't notice it at all.
Why is that? The Southern Hemisphere has more ocean, which heats and cools more slowly. So some parts of the planet felt the heat earlier, others later. Now that we know which layers warm faster, we can check how accurately climate models reproduce the past and better predict the future.
The early 20th-century warming in the Northern Hemisphere was so pronounced that in the 1930s, the Arctic experienced a significant reduction in sea ice, comparable to modern levels.