CLOUDS COOL AND WARM

CLOUD FEEDBACK

After that excursion to MA versus EPA, I will return to science. I want to finish my walk through reflection and absorption because there has been so much discussion about the role of clouds and aerosols in the comments to the last two blogs. First, an answer to a comment--the sulfur and nitrogen radicals in the troposphere (SOx and NOx in the comments) have a lifetime of a few days to a few weeks. They do not have long lifetimes like carbon dioxide. Nitrous oxide (N2O) does have a long lifetime, and it is an important greenhouse gas. It is increasing, and the increase is related to agricultural fertilizers. N2O is not the same as NOx.

Getting back to the physical climate system--what I laid out was that if you think about the Earth as a system in balance, then the energy that comes in from the sun is matched by the energy emitted back to space. There are only two things that happen to the energy from the sun--it is absorbed or it is reflected. So things that reflect will send solar energy back to space; they will help cool the earth. One way that we are changing the Earth is we are changing the absorption and reflection at the surface. The other important way we are changing the Earth is by changing the greenhouse gases in the atmosphere. This impacts the absorption and especially the absorption of energy emitted by the Earth. Energy is held close to the Earth's surface and the surface warms. If there is more energy held near the Earth's surface, then the Earth has to respond to this. As a first instinct you would expect the weather to respond. The role of weather in climate is to transport energy from the equator to the poles, and there will be more energy to transport. Clouds are associated with how the atmosphere moves; they form when moist air moves upward.

In case you want to look back at them, here are links to my last few figures: Sun-Earth System, Absorption, Reflection. The figure below shows an expansion of "cloud world."




Figure 1: The role of clouds in the energy balance of the Earth.


The cloud reflects solar energy back to space, which contributes to cooling. But the cloud also absorbs the Earth's radiation and re-emits it back to the surface. It therefore has a greenhouse effect. The clouds also emit infrared radiation upwards. If the clouds are low in the atmosphere, then they contribute to surface warming. Since, you are a weather community, this is why it does not get as cold at night even if there are very thin clouds. If the clouds are very high in the atmosphere, like those associated with thunderstorms, they are very cold at the top, the atmosphere above them is thin, hence, they emit efficiently to space. So whether it is day or night, and whether clouds are high or low in the atmosphere impacts how effectively they add to cooling or warming.

In a warmer environment the atmosphere will hold more water. (Remember, water is a greenhouse gas; hence, will add to warming! A positive feedback.) If there is more water, then it is reasonable to expect more clouds. This will contribute to cooling because of reflection of solar energy, but there is also a warming effect, especially at night. Without calculations it is not easy to argue if clouds cool or if they warm. However, clouds do have the possibility of helping to regulate, cool, the Earth in a warming environment; hence, they can be a negative feedback. As you think and rethink all that we have seen, the ice age cycles, the role of greenhouse gases, the increase of greenhouse gases, you develop an intuition. That intuition is that a stable climate on Earth is strongly reliant on water being able to exist in all three phases. As a gas it is supreme absorber. As ice it is supreme reflector. As clouds it's a balance. And as a liquid, water is life and energy. It's ultimately about water, water, water.

Next aerosols, then how aerosols impact clouds. From the corner of Colusa and Solano,


ricky