Stories tagged greenhouse effect

Sep
20
2011

There’s been some buzz about the relationship between clouds and climate recently, prompting Andrew Revkin of the New York Times’ Dot Earth blog to get his panties in a twist about the “…over-interpretation of a couple of [scientific] papers…”

What gives? I wanted to know too, so I’ve done a bit – ok, a lot – of research and this is what I can tell you: The heart of the discussion is not whether there is a cloud-climate connection (that’s clear), but rather over what that relationship behaves like. There are at least three possible theories, but before we get to those, let’s review some important background concepts.

Gimme the Basics First

Cloud Formation

First, scientists think of air as units of volume called air masses. Each air mass is identified by its temperature and moisture content. Clouds are basically wet air masses that form when rising air masses expand and cool, causing the moisture in the air to condense. You can see the process in action yourself just by exhaling outside on a cool morning. The Center for Multiscale Modeling of Atmospheric Processes has a webpage to answer your other questions about clouds.

Earth’s Energy Budget
Earth's energy budget: Incoming solar energy is either absorbed (orange) or reflected (yellow).  Outgoing energy is radiated (red).  The arrows show the direction and magnitude of movement where thick arrows signify bigger movements.
Earth's energy budget: Incoming solar energy is either absorbed (orange) or reflected (yellow). Outgoing energy is radiated (red). The arrows show the direction and magnitude of movement where thick arrows signify bigger movements.Courtesy NASA

Energy from the Sun is essential for life on Earth. Let’s pretend the Earth has an “energy budget” where solar energy is like money, absorption is like a deposit, reflection is like a transfer, and radiation is like a withdrawal. It’s not a perfect analogy, but it’ll work for starters: Most of the incoming solar energy (money) is absorbed by (deposited into) the ocean and earth surface, but some is absorbed or reflected (transferred) by the atmosphere and clouds. Most of the outgoing energy is radiated (withdrawn) to space from the atmosphere and clouds. The figure to the right illustrates this process.

The Greenhouse Effect

Thanks to the greenhouse effect, our planet is warm enough to live on. The greenhouse effect occurs within the earth’s energy budget when some of the heat radiating (withdrawing… remember our budget analogy from above?) from the ocean and earth surface is reflected (transferred) back to Earth by greenhouse gases in the atmosphere. Greenhouse gases include carbon dioxide, methane, and water vapor. This National Geographic interactive website entertains the concept.

Climate Change

Climate change is occurring largely because humans are adding more greenhouse gases to the atmosphere. More greenhouse gases in the atmosphere means more heat reflected back to earth and warmer temperatures. Warmer temperatures might sound pretty good to your right now (especially if you live in Minnesota and could see your breath this morning as you walked to school or work), but it’s not. Why? Check out NASA’s really great website on the effects of climate change.

Alright, already. What’s the climate-cloud relationship?

From what I can tell, there are three possible theories about the climate-cloud relationship:

  • Clouds actively drive climate change. This is a linear process where clouds reflect too much heat back to Earth, which increases the average global temperature and causes climate change.
  • Clouds passively blunt climate change. This is a cyclical process where more climate change includes increasing average global temperature, which increases average global evaporation, which creates more clouds. More clouds absorb more heat, keeping the average global temperature from rising even faster and lessening climate change. This slows down (note: it does not stop) the rate of climate change.
  • Clouds passively amplify climate change. This is a cyclical process where more climate change includes increasing the average global temperature, which increases average global evaporation, which creates more clouds. More clouds reflect more heat back to Earth, which raises the average global temperatures and worsens climate change. This speeds up the rate of climate change.
  • So which is it? Probably NOT Theory #1. Maybe Theory #2… or maybe it’s Theory #3? Scientists aren’t quite sure yet, so neither am I, but the evidence is stacking against Theory #1 leaving two possible options. The next big question seems to be surrounding the size of the effects of Theory #2 and Theory #3.

    Using what you just read about cloud formation, the earth’s energy budget, greenhouse gases, and climate change (Woah. You just learned a lot!), what do you think? What’s the climate-cloud relationship?

    If you want, you can read more about what scientists are saying about the climate-cloud relationship here:

Jul
07
2010

Fascinating article in the June 23 issue of Science. A major puzzle of paleoclimatology is why after tens of thousands of years of glacial conditions, recent ice ages have ended with relatively sudden warm ups. Six authors have devised a comprehensive hypothesis as to why. Here is my attempt to summarize the process:

  1. First, you need very large ice sheets around the Northern Hemisphere. These ice sheets are so large that they depress the continents, pushing them down into the Earth’s mantle.
  2. Next, the Milankovitch Cycle plays a role by increasing the amount of solar energy reaching the ice sheets during summer, resulting in large amount of melt water entering the North Atlantic. Sea levels rise worldwide and encourage extensive calving of icebergs from the continental glaciers lining the North Atlantic.
  3. The freshwater and icebergs result in the formation of vast areas of winter sea ice in the North Atlantic, abruptly returning the Northern Hemisphere to glacial conditions with severe winters.
  4. The return of glacial conditions to the Northern Hemisphere affects the atmospheric circulation of the entire planet, in particular causing a southward shift of westerly winds in the Southern Hemisphere that warm Antarctica and encourage the upwelling of carbon dioxide-rich deep water in the Southern Ocean.
  5. The glacial conditions in the Northern Hemisphere last long enough to encourage prolonged off gassing of carbon dioxide from upwelling Southern Ocean deep water, resulting in the crossing of a threshold where there is enough carbon dioxide in the atmosphere to tip the whole planet into a new interglacial warm period.

If this research holds up to scientific scrutiny, it will bear on the current global warming debates. Some have interpreted the rise of carbon dioxide in the atmosphere at the end of the last ice age not as a cause of deglaciation but rather as an effect of deglaciation. These six authors see carbon dioxide as playing a key role in finally bringing to an end the last ice age because carbon dioxide is a greenhouse gas.