EPOW - Ecology Picture of the Week

Climate Signals from the Moon

Crescent Moon and Earthshine

Explanation:  What does the moon have to do with climate change on Earth?  The answer sits in two concepts:  earthshine and albedo.  I'll explain.

As shown in my photo sequence above, the dark part of an early crescent moon often shows earthshine, which is sunlight that is reflected off the Earth onto the moon and then reflected back to Earth.  This makes the unlit portion of a crescent moon glow softly.  

The brightness of that glow, in turn, is determined by how much of the sunlight hitting Earth is reflected.  And that is called Earth's albedo, which is the proportion of sunlight reflected off the Earth.  

So what does this have to do with Earth's climate, and especially climate change?

Well, Earth's albedo is influenced by Earth's surface and atmospheric conditions that can absorb or reflect sunlight radiation.  Such conditions include average and extent of cloud cover, wildfire smoke, ice cover (think of glaciers, snow-covered mountains, and polar regions), vegetation, and other factors.  And, aha, those are the conditions that are very much being affected by climate change.  

So now astronomers can track the intensity of the moon's earthshine and use that as a signal of climate conditions and changes on Earth!  

One study (Goode et al. 2021) found significant reductions in Earth's albedo, using earthshine signals, over 1998 to 2017, potentially caused by climate change reducing Earth's reflectance capacity. 

Indeed, researchers have concluded that the Earth is dimming because of climate change.

And beyond that -- note to any extraterrestrial explorers -- at least one study (Palle and Montañes-Rodríguez 2012) suggests that studying specific frequencies of reflected sunlight might serve as a fingerprint of biomarkers of life.  

Information:
    Glenar, D. A., T. J. Stubbs, E. W. Schwieterman, T. D. Robinson, and T. A. Livengood. 2019. Earthshine as an illumination source at the Moon. Icarus 321:841-856.
    Goode, P. R., E. Pallé, A. Shoumko, S. Shoumko, P. Montañes-Rodriguez, and S. E. Koonin. 2021. Earth's albedo 1998–2017 as measured from earthshine. Geophysical Research Letters 48(17):e2021GL094888.
    Goode, P. R., and W. A. Dziembowski. 2003. Sunshine, earthshine and climate change I. Origin of, and limits of solar variability. Journal of the Korean Astronomical Society 36(1):75-81.
    Goode, P. R., E. Palle, J. Hickey, M.-C. Chu, C. T. Brown, and S. E. Koonin. 2003. Sunshine, earthshine and climate change II. Solar origins of variations in the Earth's albedo. Journal of the Korean Astronomical Society 36(1):83-91.
    Palle, E., and P. Montañes-Rodríguez. 2012. The earthshine observations: from climate change to astrobiology.  Pp. 25-34 in: J. C. Guirado, L. M. Lara, V. Quilis, and J. Gorgas (Ed.). Spanish Astrophysics VII, Proceedings of the X Scientific Meeting of the Spanish Astronomical Society. Valencia, Spain. 
    Wielicki, B. A., T. Wong, N. Loeb, P. Minnis, K. Preistley, and R. Kandel. 2005. Changes in Earth's albedo measured by satellite. Science 308(5723):825 DOI:10.1126/science.110648.