A baker puts a layered cake in the oven: the dough is the same, the heat is the same, but each layer bakes differently. The thin crust on top is already browning, while the middle is still underdone. Earth's climate is similar: the planet is warming, but different parts of it are warming at different rates and at different times. To see this unevenness, scientists have broken 130 years of observations into layers: six continents, seven latitude bands, and eight seasons. And they found that the early 20th-century warming was not global but local.
Land temperature records from 1880 to 2017, compiled by NOAA and NASA, resemble a long tape with readings from thousands of thermometers. From this tape, the main patterns of variability were extracted, that is, the ways temperature fluctuates from year to year. The first pattern turned out to be the strongest: it explains three-quarters of all fluctuations and shows the overall course, with a rise until the 1940s, a decline until the late 1950s, a plateau until the mid-1970s, and a sharp increase afterward. But beneath this main layer, other, subtler patterns are hidden.
The third pattern turned out to be the key to the early-century puzzle. It has a period of 60–70 years and correlates noticeably with the Atlantic Multidecadal Oscillation, a slow swaying of ocean temperature. This layer is responsible for the warming of 1900–1940s, which long defied explanation. The reverse arrangement test, which checks when a temperature rise becomes statistically significant, showed that in North America and Europe, modern warming became noticeable only around 1990, while in South America it has been continuous since the 1930s.
The Southern Hemisphere, with more ocean and less land, responded to the same forcings differently. Oceania showed no significant warming at the beginning of the century. This changes our understanding of how climate responds to external forcings: not as a single blanket, but as a cake where each layer has its own thermal conductivity.
The baker takes the cake out of the oven: the crust crunches, the middle is soft, but the whole cake is hot. So is Earth: some regions warmed earlier, others later, but the overall trend is relentless. Understanding which layers warm faster helps test how accurately climate models reproduce the past, and thus how much they can be trusted for future projections.
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.