How Do Clouds Affect Earth's Climate?
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Clouds have both cooling and warming effects on Earth's surface. Figuring how they balance is tricky. Determining global cloud cover over time is a major aspect of understanding climate change.
Some clouds help cool the Earth, but other clouds help keep Earth warm – in part depending on how high up they are in our atmosphere. That's according to Steven Platnick, a satellite researcher with NASA. He studies clouds and how they connect with Earth's climate. He said that low, fluffy clouds keep us cooler.
Steven Platnick: You can appreciate that if you go out on a hot and sunny day, and a cloud passes by overhead, it's a great relief from the heat. And the reason of course is because the cloud is reflecting sunlight.
But it's a different story for clouds that are high up in the atmosphere, said Platnick. Those high, wispy clouds actually keep Earth warm, like a blanket, by preventing heat from escaping into space. Platnick uses NASA's Aqua satellite to determine the height of clouds, and other properties of clouds as well -- like what's inside them, how much water they have, and whether a cloud is primarily liquid or ice. The satellite also tracks the amount of cloud cover all over the world.
SP: The most important thing to trying to understand their effect on global climate is we need to know how they're globally distributed and how they vary over the course of a year. And you need satellites to really obtain the statistics over those scales. It's really impossible for most of those quantities to get the same statistics from ground-based or aircraft observations.
Platnick explained to EarthSky what scientists mean when they talk about climate.
SP: When scientists talk about climate, what we're talking about is the statistics of weather. And normally we're talking about that on multi-decadal time scales, and longer, even centuries and millennium. An example might be the average surface temperatures, something that most people can relate to. Or also, the minimum and maximum surface temperatures. And there's other examples -– precipitation, the amount of rainfall during the month, or snowfall, or snow pack. But when we talk about recent climate change, and the causes of recent climate change, and that's over the last several decades including the last several centuries since the start of the industrial revolution, then we start to study the effect of human activities, and how humans can directly modify climate.
Platnick said that there are several ways that humans can directly impact climate. The best understood of these, he said, and what's likely the most important in the long run, is the release of greenhouse gases into the atmosphere.
SP: And that includes not just carbon dioxide, but methane, ozone, there's other gases as well. And these greenhouse gases perturb the radiation balance of climate. And what they do is move the climate system to a warmer state. And it does that because of the way that gases reduce the emissions of infrared radiation to space by the Earth and its atmosphere. And this so-called greenhouse gas forcing on the climate system is well-understood.
He said that what's less well-understood is how the climate responds to this forcing.
SP: And that is the focus of much climate modeling research today. It's certainly the motivation behind many of NASA's satellite observations and the modeling studies that we do, and that's in fact what a large part of my own work involves.
Platnick told EarthSky what he thought was the most important thing people should know about Earth's clouds and climate.
SP: Earth is a dynamic system of components. On the largest scales you have the atmosphere, the land, the ocean, snow and ice on the ground. And these systems connect with each other in different ways, through the exchange of water, and energy, and chemistry, such as the exchange of carbon. We need to be able to better understand the consequences of these connections. Clouds are the prime example of this interconnection, because they play a key role in these systems, especially in the water and energy connections.
NASA satellites and instruments, along with our international partners are helping us to get better observations we need to understand these cloud physical processes and to enable cloud model improvements as well.
You can listen to an audio version of this interview here.