Every cloud has a metaphorical silver lining, but it seems global warming has a literal one: increasing the frequency of high-altitude clouds that shine with exquisite beauty on summer nights. In a future world lacking the visual splendor of coral reefs or our most majestic cities, Pollyannas may take comfort in this compensating beauty. From a scientific point of view, however, these clouds help confirm the validity of atmospheric models.

Most clouds lie less than 18 kilometers (11 miles) above the Earth's surface. Yet even 80 kilometers (50 miles) up, there is sometimes enough water vapor to freeze around specks of meteor dust. When this happens, the clouds produced are too thin to be seen from the ground during daylight. Shortly after sunset or before dawn, however, the Sun lights them up, creating the phenomenon of noctilucent (night-shining) clouds.

The higher reaches of the atmosphere are usually too dry to allow this to occur. Large volcanic eruptions create an exception, pushing so much water vapor to such great heights that the clouds can appear. The first scientific records of the phenomenon come from 1885, after Krakatoa's explosion.

In recent years, these clouds have become more common, even without recent major eruptions. Measuring this has been difficult until recently, but satellites are addressing this problem. Speculation that human-induced climate change could be the cause dates to 1989, but it is only now that the evidence has been brought together to confirm this.

Professor Franz-Josef Lübken of the Leibniz Institute of Atmospheric Physics has studied the contribution of methane, the second-most important gas in human contributions to climate change, which reacts to produce water vapor at high altitudes. In Geophysical Research Letters, Lübken reports there has been a 40 percent increase in water vapor 80 kilometers high at mid-northern latitudes since 1871, more than doubling the volume of ice crystals. Since there is a threshold quantity of ice to make the clouds visible, there has been a much more dramatic increase in their frequency.

"We speculate that the clouds have always been there, but the chance to see one was very, very poor, in historical times," Lübken said in a statement.

An important question Lübken has yet to answer is whether these clouds, or other phenomenon at similar altitudes, are having an effect on the rate of global warming. Clouds at lower altitudes reflect incoming sunlight and cool the planet, while higher clouds prevent heat from escaping the Earth and increase warming. Climate modelers have put a lot of effort into calculating the net effect of both of these increasing as a result of human activities, but clouds as high as these haven't yet been taken into account.