#DoEDeepDive: Declining planetary albedo

It sounds like one of those annoying internet ads: Have you noticed your albedo isn’t what it used to be? Studies show albedo levels decline in both men and women over 50. But we can help. Click here for our free information kit. Except what we’re talking about is the reflectivity of the planet, aka planetary albedo, and all the herbal supplements in the world won’t raise it. As the DOE Report says, “Arguably the most striking change in the Earth’s climate system during the 21st century is a significant reduction in planetary albedo since 2015, which has coincided with at least two years of record global warmth.” A lower albedo means more solar energy is absorbed by the Earth’s surface, which means a warmer atmosphere. The reduction in albedo since 2015 implies an increase of 1.7 watts per square meter in the amount of solar radiation being absorbed by the Earth’s surface, which is 73% as large an effect as all the greenhouse gas emissions since the 1700s. So it matters. Why has albedo declined? is it a natural change? Will it reverse in the years ahead? Good questions. Let’s see how the DOE Red Team answered.

Albedo varies due to many factors including ice extent, volcanoes, air pollution, and cloud cover. Arctic sea ice has barely changed since 2007 and Antarctic sea ice extend has been stable at least since 1979. Likewise North American fall and winter snow cover has held steady since the late 1960s. Global greening has increased forest cover, and forests have low albedo compared to open land or snow, but the overall effect is thought to be counteracted by increased cloud cover over forests. New air pollution rules for marine traffic came in after 2010, but the effects are believed by many experts to be too small to account for the declining albedo.

The most likely explanation is a global decrease in cloud cover. We’ve discussed this topic several times previously (e.g. here and here). What then explains the change in cloudiness? The DOE team explains that it is an open question. One possibility is natural variability, which would imply the recent temperature surges are natural in origin. A rival hypothesis is that it is a hitherto-undiscovered feedback of CO2-driven warming, which means the warming of the past decade is just a harbinger of even faster warming to come. They point out some problems with that explanation:

“It is not easy to justify a new positive low cloud feedback that began emerging in 2015 since there is no obvious feedback trigger starting at that time. However, there are numerous natural climate signals during this period that are associated with atmospheric circulation changes that can influence the distribution of clouds.”

These signals include the strong 2014-2016 El Nino, changes in ocean circulations in the North Atlantic and Pacific Ocean in 2015-16 and the Hunga-Tonga eruption in 2022. Overall, they highlight that the topic is extremely important but it will take time to find solid answers to the major questions they raise rather than settling:

“In summary, the decline in planetary albedo and the concurrent decline in cloudiness have emphasized the importance of clouds and their variations to global climate variability and change. A change of 1- 2 percent in global cloud cover has a greater radiative impact on the climate than the direct radiative effect of doubling CO2. While it is difficult to untangle causes of the recent trend, the competing explanations for the cause of the declining cloud cover have substantial implications for assessing the Equilibrium Climate Sensitivity and for the attribution of the recent warming. An additional 10 years of data should help clarify whether this is a strong positive cloud feedback associated with warming or a temporary fluctuation driven by natural variability.”

Coming up: climate impact drivers and extreme event attribution.