Climate models estimate that the extra water vapor will at least double the direct greenhouse effect. The third feedback, and the most uncertain, is clouds.
A lot of the water vapor in the air forms water droplets that coalesce into clouds. We generally think of clouds as keeping us cool, and more water vapor should make more clouds. That may sound helpful. But things are not so simple. While during the day, low clouds shade the planet, at night they act as an insulating blanket.
Meanwhile high cirrus clouds predominantly act as heat traps, warming the air below them. Generally, at a global level, models have suggested that the warming and cooling effects cancel each other out, and the presumption has been that that will continue as the world warms. But the new model analyses suggest otherwise. The data is now increasingly becoming public. Others soon followed. Last month, American and British researchers, led by Zelinka, reported that 10 of 27 models they had surveyed now reckoned warming from doubling CO2 could exceed 4.
The average warming projected by the suite of models was 3. French scientists at the National Center for Scientific Research concluded that the new models predicted that rapid economic growth driven by fossil fuels would deliver temperature rises averaging 6 to 7 degrees C They warned that keeping warming below 2 degrees C was all but impossible.
Zelinka said the new estimates of higher climate sensitivity were primarily due to changes made to how the models handled cloud dynamics. The models found that in a warmer world clouds would contain less water than previously thought. This tweaking of the models followed recent field research over the Southern Ocean, which is currently one of the cloudiest regions on Earth. Flying through those clouds, researchers found they contain much more water and less ice than previously assumed.
That sounds like good news. But it means that past models have overestimated how much ice in these clouds will turn to liquid water in a warmer world — and so overestimated both the thickness of future clouds and their ability to keep us cool.
Eliminating that bias, says Tan, could increase climate sensitivity by as much as 1. Modelers have also changed how they characterize the effect of anthropogenic aerosols from burning fuel, particularly in clouds. In general, the aerosols make clouds thicker and better able to shade the planet. The recent recalculation follows new estimates of aerosol emissions during the midth century, a time when booming emissions from rapid industrialization caused the planet to cool for several decades, masking the warming effect of accumulating CO2.
And why do different types of clouds form? The water or ice that make up clouds travels into the sky within air as water vapor, the gas form of water.
Water vapor gets into air mainly by evaporation — some of the liquid water from the ocean, lakes, and rivers turns into water vapor and travels in the air. When air rises in the atmosphere it gets cooler and is under less pressure. The vapor becomes small water droplets or ice crystals and a cloud is formed. These particles, such as dust and pollen, are called condensation nuclei.
Eventually, enough water vapor condenses upon pieces of dust, pollen or other condensation nuclei to form a cloud. Some clouds form as air warms up near the Earth's surface and rises. Heated by sunshine, the ground heats the air just above it. That warmed air starts to rise because, when warm, it is lighter and less dense than the air around it. Cumulus clouds are a cloud-spotting favorite: They are big, white, and cottony and—depending on your imagination—may look like a bear, an apple, or any other familiar object.
Cumulonimbus clouds are heavy and dense; they tend to build dramatically upward and are often harbingers of thunderstorms, hail, or tornadoes. Stratus clouds appear as a thin gray layer in the sky. Stratocumulus clouds are patchy, gray and white, and usually resemble a honeycomb.
Ever see one that looks like undulating waves? How about those pouch-like protrusions bulging from underneath larger clouds? Those are called mammatus clouds. Contrails are long bands of clouds produced by aircraft. Otherworldly looking lenticular clouds resemble stacks of giant disks rising in the sky. No matter what shape or size they are, clouds are essential to life on Earth. At night they act as a blanket to keep us from getting too cold.
They also provide precipitation and signal weather changes and patterns. Astrophysicists who study atmospheres on other planets know that Mars, for example, has clouds similar to ours.
Continuing to learn about weather on other worlds, they believe, will help us better understand—and predict—our own. All rights reserved. Colorful clouds and calm water create a striking sunset scene for canoeists on Winisk River in Ontario, Canada. A single cloud may hold billions of pounds of water—but not all clouds bring rain.
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