Water, Water, Water(2): Water Vapor Feedback
This follows from the previous water blog, links below.
Water is a greenhouse gas. In fact, in the Earth’s radiative balance, water is the most important greenhouse gas. Its influence on the radiative budget is larger than the influence of carbon dioxide. Water is, however, different from carbon dioxide in several important ways. First, water exists in all of its phases in the normal range of temperatures observed on Earth. When the phase changes, energy is consumed or released. As water vapor is carried around in the atmosphere and its phase changes, energy is transported. When water turns to ice, its normal path in clouds, and then into water, it falls out of the atmosphere. Hence, the second major difference compared with carbon dioxide, it cycles quickly. Any particular water molecule spends a much shorter time in the atmosphere than a molecule of carbon dioxide.
There is also the ocean. From the point of view of the atmosphere, the ocean is a nearly infinite supply of water vapor. The atmosphere is constantly replenished with water after in falls out. Since the air can hold more water if it gets warmer, one consequence of warming at the Earth’s surface is more water in the atmosphere. If water gets deposited as snow and ice on the surface, then it can stay there for a while. (This is the big difference when compared with Mars, on Mars a large portion of the atmosphere is deposited in the polar caps. Very different balance!)
This blog will focus on water in the atmosphere.
Because of this fast cycling of water with this large reservoir, we don’t think of water in the same way as we think of carbon dioxide. The atmosphere, more or less, holds the amount of water it can hold at any given the temperature. Yes, we emit water from industry and in cooling towers. But it immediately becomes integrated into the water cycle; it does not accumulate like carbon dioxide.
The cycling of water is closely related to vertical motion in the atmosphere. When warm wet surface air rises, the air cools. This will not change with climate change, warm air will rise and it will cool and water will condense and it will rain --- and snow. Again, in clouds water vapor is normally converted to ice and then turns into water or snow as it falls through the warmer air below.
When water vapor turns into ice or liquid, its role in the radiative balance of the Earth changes. As ice or liquid, water is a cloud and then becomes a reflector of solar radiation; hence, it has a cooling effect. Water in the atmosphere – it’s a greenhouse gas, it contributes to energy transport, it’s a reflective particle.
Imagine that the climate of the Earth is a balance of some type. Then when we add a long lived greenhouse gas like carbon dioxide to the atmosphere, then it is reasonable to ask the question how will the balance change? To first approximation the surface will warm from the additional carbon dioxide. Then, the atmosphere and ocean will respond. The responses in the atmosphere and ocean might amplify the warming by carbon dioxide; they might reduce the warming by carbon dioxide; they might do nothing. Because we are in this temperature range where water changes phases so readily, water is at the center of this response. The response to a change in a balance is called a feedback. A positive feedback amplifies the change, and a negative feedback reduces the change. The feedback called the water vapor feedback is one of the most important and most easily analyzed atmospheric responses. It is a positive feedback. Basically as the atmosphere warms it holds more water, which acts as a greenhouse gas, and warms the atmosphere some more.
Figure 1: Water Vapor Feedback in a Warming Atmosphere
If you wanted to think about this in the same way as carbon dioxide, as something we are adding to the atmosphere, then adding carbon dioxide leads to us adding water as well. This is due to temperature increases. In an earlier blog I mentioned a paper that I thought was especially important. It was a paper about what is changing in the radiative balance of the Arctic sea ice, and what was measured was an increase in surface warming due to an increase in water vapor due to an increase in air temperature. Here’s that reference again. (Sea Ice Arctic
There is a lot of chatter about all of the ice and snow we have seen this winter. In the U.S. you might also think of it as ice and snow followed by fog because of rapid melting. (Yes, there is fog at the Chicago airport today.) In an earlier blog, I talked about the fact that increased water vapor would likely build up the Greenland and East Antarctic ice sheets because they are at high elevation. It’s cold up there; it stills snows; there is more water to snow. This is true in the Sierra Nevada as well. It can still snow a lot, but it is likely to melt a lot as well.
Let’s think about cold a little more. In the winter, at the pole, it is dark. It still gets cold at the pole. With climate change, it will still get dark at the pole, and it will be cold. It might not be as cold, as long, but it is still cold. (Have you read To Build a Fire?
– “It was cold.”) And the atmosphere still has to get cold air away from the pole and warm air to the pole. That is the job of the atmosphere. When that cold air gets wrapped up with warm wet air, it still freezes and it still snows. That the globe gets warm on average does not mean it will not snow. It might not even mean in a cumulative way, it will snow less. What are the other attributes of snow, and snow cover, and melting, and fog, and water storage that might be a better measure of whether or not there is warming?
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