Albedo reductions amplify climate change

Bad news, or good? Only time will tell.

I sometimes mention albedo in the context of our exposure to UV radiation. Albedo’s just another word for reflectivity. Over areas of high surface albedo - like snow-covered terrain - the amount of UV we receive is increased significantly by reflections from those surfaces. If you’ve ever been snow-skiing, you’ll be well aware of the issue. Protective glasses are a must. The extra UV skiers get isn’t just from the snow directly below their feet. Although that adds an important new upwelling component, reflections from snow-covered surfaces even tens of kilometers away also contribute to increases in the downwelling component because of subsequent scattering by molecules of air.

Albedo is important in the context of climate change too, but at an even broader scale. I’ve always been a bit concerned about the role of planetary reflectance (the albedo of planet Earth as a whole) in climate change. Naively, I’d have thought that even tiny changes in it could easily swamp the small energy imbalance that’s caused by increasing greenhouse gases.

A recent report by climate-guru James Hansen confirms that importance. It also adds another complication, which gives cause for extra concern. The plot below from his report shows that the amount of sunlight reflected from planet Earth has reduced by more than 0.5 percent since the turn of the century. That’s a LOT of energy, given that at noon the mean incoming energy is about 1kW/m2. Of course, there’s no absorption of incoming energy at nighttime, so the mean energy absorbed by planet Earth is around 300W/m2. But, in the last 25 years that’s increased by nearly 1 W/m2.

Hansen reckons that the only factor big enough to cause that is changes in cloud cover. That reduced reflectance from clouds leads to increased propagation to Earth’s surface and increased absorption. A warming effect.

Part of the reduction in cloud cover is caused by reduction of human-made atmospheric aerosols, which act as condensation nuclei for cloud formation. But, Hansen argues, most of the cloud change must be a cloud-feedback that occurs with global warming. The observed albedo change proves that clouds provide a large, amplifying, climate feedback. This large cloud feedback in turn confirms a high climate sensitivity, consistent with paleo-climate data and with the rate of global warming in the past century. His proposed climate sensitivity is thus greater than assumed by the IPCC, who estimate its value from model calculations. If Hansen is correct (and my money is with him), this means that that future warming due to increases in CO2 are likely to be much more severe than currently predicted by the IPCC.

My guess, for what it’s worth, is that the albedo will not continue to decrease as it has for the last 25 years. Higher temperatures mean that the atmosphere will be able to hold more water vapour. Surely that wetter air will eventually lead to more cloud cover. If so, the extra cloud - like the aerosol injections currently being contemplated - might just save our bacon by providing more shielding from sunlight. But it’s complicated. For example, if the extra cloud forms only at low altitudes it won’t help. I’m not sure if climate modellers share my optimism.

Time will tell.

In the meantime, I’ll keep an eye on future changes in the planetary albedo.