Nobody's certain yet, but some scientists are speculating that increases in atmospheric CO2 is causing a hauntingly beautiful phenomenon called "night shining" clouds.

Mysterious, glowing clouds previously seen almost exclusively in Earth’s polar regions have appeared in the skies over the United States and Europe over the past several days.

Photographers and other sky watchers in Omaha, Paris, Seattle, and other locations have run outside to capture images of what scientists call noctilucent (”night shining”) clouds. Formed by ice literally at the boundary where the earth’s atmosphere meets space 50 miles up, they shine because they are so high that they remain lit by the sun even after our star is below the horizon.

Like anything else, our atmosphere expands when heated up. One current theory is that climate change is causing noctilucent clouds to become more frequent because the upper regions of the atmosphere are being pushed further into cold regions of space - leading to more high altitude ice and more shining clouds.

If so, perhaps "disturbingly beautiful" is a better description for these haunting forms.