![]() Noctilucent clouds form in the mesosphere, high above where normal weather clouds form. This diagram shows the major layers of Earth’s atmosphere. Researchers have wondered what effect, if any, this has had on the middle atmosphere and the formation of noctilucent clouds. ![]() Humans have injected massive amounts of greenhouse gases into the atmosphere by burning fossil fuels since the start of the industrial period 150 years ago. And they’re only visible at dawn and dusk, when the Sun illuminates them from below the horizon. The clouds can only form at mid to high latitudes in the summertime, when mesospheric temperatures are cold enough for ice crystals to form. “We now detect a distinct response in clouds.”Ĭonditions must be just right for noctilucent clouds to be visible. “Our methane emissions are impacting the atmosphere beyond just temperature change and chemical composition,” said Ilissa Seroka, an atmospheric scientist at the Environmental Defense Fund in Washington, D.C. Whether thicker, more visible noctilucent clouds could influence Earth’s climate themselves is the subject of future research, Lübken said. The results suggest noctilucent clouds are a sign that human-caused climate change is affecting the middle atmosphere, according to the authors. “We speculate that the clouds have always been there, but the chance to see one was very, very poor, in historical times,” said Franz-Josef Lübken, an atmospheric scientist at the Leibniz Institute of Atmospheric Physics in Kühlungsborn, Germany and lead author of the new study in Geophysical Research Letters, a journal of the American Geophysical Union. They conclude human activities are the main reason why noctilucent clouds are significantly more visible now than they were 150 years ago. The study’s results suggest methane emissions have increased water vapor concentrations in the mesosphere by about 40 percent since the late 1800s, which has more than doubled the amount of ice that forms in the mesosphere. Extracting and burning fossil fuels delivers carbon dioxide, methane and water vapor into the atmosphere, all of which are greenhouse gases. In a new study, researchers used satellite observations and climate models to simulate how the effects of increased greenhouse gases from burning fossil fuels have contributed to noctilucent cloud formation over the past 150 years. Sightings of the clouds became more common during the 20 th century, and in the 1990s scientists began to wonder whether climate change was making them more visible. Humans first observed noctilucent clouds in 1885, after the eruption of Krakatoa volcano in Indonesia spewed massive amounts of water vapor in the air. New research suggests they are becoming more visible and forming more frequently due to climate change. Noctilucent clouds form only in the summertime and are only visible at dawn and dusk.
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