The earth is dimming. What 'earthshine' means for our climate system

Look closely at a crescent moon and you may be able to see earthshine: light from the sun reflected by earth onto the moon. But, hurry, this light source is dimming over time, new research shows.

Our planet has no light source of its own but absorbs light and radiation from the sun, while reflecting a fraction of the sun’s radiation back into space. By methodically measuring the levels of earthshine Earth reflects onto the moon, scientists have found that its reflectance - or ‘albedo’ in technical terms - is diminishing over time.

Measuring earth’s albedo is a challenge. The level of reflectivity is subject to daily and seasonal changes and depends on the level of snow, ice and cloud cover. Clouds reflect about 50% of light from the sun, while snow and ice reflect more, compared to just 10% reflectivity from oceans.

Measurements could only be taken when there is no direct sunlight reaching the moon, further limiting data collection.

The study, led by Philip Goode of New Jersey Institute of Technology, observed the moon’s surface from California’s Big Bear Solar Observatory over the 20-year period between1998 and 2017. Comparing like-for-like measurements while controlling for a variety of factors, the team found a “gradual but climatologically significant” decline in earthshine levels, as the chart shows.

Study results suggest that warming ocean temperatures due to human-induced climate change are reducing the levels of low cloud cover over the eastern Pacific Ocean, which act like a mirror to reflect light and radiation from the sun back into space. The more sunlight the earth absorbs, the warmer it will be, the more of the sun’s light earth reflects, the cooler it will be.

If the drop in earthshine (Earth’s reflectivity) is a temporary effect of periodic changes in weather patterns, such as the decades-long fluctuations caused by phenomena like the Pacific Decadal Oscillation, there is no long-term problem. If, however, the earthshine continues to decrease, there is cause for concern as the process could add to the planet’s climate instability.

Global surface temperatures in the world’s oceans have been increasing since 1880 and continue to rise at an average rate of 0.14°F per decade, as the chart shows. Sea temperature changes vary between regions, while most ocean zones have seen temperatures increase, the North Atlantic has experienced cooling.

Warming or cooling oceans impact plant and animal species, alter marine ecosystems and have a profound effect on climate change, including reducing the levels of reflective cloud cover in some regions.

As climate change warms the planet, melting polar caps also mean less-reflective ice cover resulting in dimming earthshine which could exacerbate climate change.

The largest drop in the planet’s reflectivity occurred in the last three years that data was harvested for the research paper, adding to fears that climate change is at the heart of the problem.

In 2018, the World Economic Forum created the Ocean Action Agenda to help protect ocean environments and support a range of initiatives aimed at ensuring long-term sustainable use of our oceans. Initiatives like this could help fight climate change and help make our oceans more resilient.