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Scientists predict that human-caused global warming should result in certain specific patterns of warming. Because these patterns are consistent with what we expect to happen as a result of the increased greenhouse effect, they’re considered “fingerprints” of the human influence on the Earth’s climate. As far back as 1865, physicist John Tyndall predicted that warming caused by the increased greenhouse effect should cause nights to warm faster than days, and winters to warm faster than summers. He was able to make this prediction by knowing that at night and during the winter, the Earth’s surface cools by radiating heat out to space. Greenhouse gases trap some of this heat, slowing that nighttime and winter cooling. The sun doesn’t shine all the time, the greenhouse effect is at work 24/7. Additionally the moon gives us a good counter-example because it doesn’t have an atmosphere. During the day, there’s nothing between the Sun and the Moon’s surface to block incoming sunlight.

At night, there are no greenhouse gases to trap the outgoing heat from the Moon. As a result, the difference between day and night temperatures is extreme. Daytime temperatures on the moon reach 120˚ Celsius, or 250˚ Fahrenheit. Nighttime temperatures fall below minus 200˚ Celsius, or minus 330˚ Fahrenheit. At the other extreme, Venus has a runaway greenhouse effect, much bigger than the greenhouse effect on Earth. Its temperature is an intense 460˚ Celsius, or 730˚ Fahrenheit. It’s like this day and night, all year long. Venus doesn’t even have seasons because its greenhouse effect is so strong. As these two examples illustrate, the bigger the greenhouse effect, the smaller the difference between daytime and nighttime temperatures. We know humans are increasing the greenhouse effect on Earth by burning more and more fossil fuels. If the greenhouse effect is increasing, then the difference between nighttime and daytime temperatures, and between winter and summer temperatures, should be shrinking. There’s a common myth that global warming is caused by the Sun rather than humans.

That myth fails to account for the available evidence. If the Sun were responsible, we would see an entirely different pattern of global warming. In that scenario, we would expect to see the Earth warming most when sunlight is bombarding the surface the most – during the daytime, and during the summer season. That means that if the Sun were responsible, we would see days warming faster than nights, and summers warming faster than winters. These expected patterns of global warming give scientists a clear test to determine whether the evidence matches the fingerprints of human or solar-caused warming. It took over 130 years before John Tyndall’s prediction was confirmed, but over the last few decades, surface measurements have found nights warming faster than days, and winters warming faster than summers. The difference between nighttime and daytime temperatures, and winter and summer temperatures, is shrinking, just as Tyndall anticipated would happen due to the increased greenhouse effect.

Fingerprints in the Earth’s climate change, like these changes in global warming patterns, clearly point to humans, and not the Sun, as the culprit responsible for global warming over the past century..