On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added "At a glance" section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a "bump" for our ask. This week features "The albedo effect and global warming". More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.
What is albedo? It is an expression of how much sunshine is reflected by a surface. The word stems from the Latin for 'whiteness'. Albedo is expressed on a scale from 0 to 1, zero being a surface that absorbs everything and 1 being a surface that reflects everything. Most everyday surfaces lie somewhere in between.
An easy way to think about albedo is the difference between wearing a white or a black shirt on a cloudless summer's day. The white shirt makes you feel more comfortable, whereas in the black one you'll cook. That difference is because paler surfaces reflect more sunshine whereas darker ones absorb a lot of it, heating you up.
Solar energy reaching the top of our atmosphere hardly varies at all. How that energy interacts with the planet, though, does vary. This is because the reflectivity of surfaces can change.
Arctic sea-ice provides an example of albedo-change. A late spring snowstorm covers the ice with a sparkly carpet of new snow. That pristine snow can reflect up to 90% of inbound sunshine. But during the summer it warms up and the new snow melts away. The remaining sea-ice has a tired, mucky look to it and can only reflect some 50% of incoming sunshine. It absorbs the rest and that absorbed energy helps the sea-ice to melt even more. If it melts totally, you are left with the dark surface of the ocean. That can only reflect around 6% of the incoming sunshine.
That example shows that albedo-change is not a forcing. That's the first big mistake in this myth. Instead it is a very good example of a climate feedback process. It is occurring in response to an external climate forcing - the increased greenhouse effect caused by our carbon emissions. Due to that forcing, the Arctic is warming quickly and snow/ice coverage shows a long-term decrease. Less reflective surfaces become uncovered, leading to more absorption of sunshine and more energy goes into the system. It's a self-reinforcing process.
If you look at satellite images of the planet, you will notice the clouds in weather-systems appear bright. Cloud-tops have a high albedo but it varies depending on the type of cloud. Wispy high clouds do not reflect as much incoming sunshine as do dense low-level cloud-decks.
Since the early 2000s we have been able to measure the amount of energy reflected back to space through sophisticated instruments aboard satellites. Recently published data (2021) indicate planetary albedo, although highly variable, is showing an overall slow decrease. The main cause is thought to be warming of parts of the Pacific Ocean leading to less coverage of those reflective low-level cloud-decks, but it's early days yet.
Albedo is an important cog in the climate gearbox. It appears to be in a long-term slow decline but varies a lot over shorter periods. That 'noise' makes it unscientific to cite shorter observation-periods. Conclusive climatological trend-statements are generally based on at least 30 years of observations, not the last half-decade.
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Posted by John Mason on Tuesday, 12 March, 2024
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