Cloudy With a Chance of Climate Change

April 25, 2016

As most homeowners, I have a thermometer that gives me a digital reading of the temperature outside my house. Since wireless types are easy to install and they're inexpensive, it's easy to have one of these, today, but I've had mine for thirty years. The only way to have such a thermometer in those days was to build your own, so I designed and built my unit using a handful of TTL and analog integrated circuits, and a semiconductor temperature sensor. The temperature sensor was the LM335[1] (see figure) that's now superseded by the easier to use LM35[2].

Temperature sensing with the LM335.[1] The voltage output is 10 millivolts per kelvin, and setting the calibration pot to give the correct temperature at any temperature (e.g., the ice point) calibrates the sensor to give the correct temperature over its entire range. (Drawn using Inkscape.)

One thing that my thermometer taught me is that I could still expect to find ice on my automobile windshield even when the outside temperature was several degrees above freezing. The reason for this was radiative cooling of the windshield exposed to the clear night sky. The background temperature of the night sky is about 3 kelvin (K); so, when my windshield was radiating energy appropriate to a 300 K black body, it was only collecting radiation from a 3 K black body. Such a non-equilibrium condition meant that the windshield was losing energy and cooling in the process.

Such radiative cooling will also cause black ice on road surfaces when temperatures are above freezing. This effect also allows an effective way of ice-making by exposing a shallow, water-filled, insulated tray to a clear sky on cold nights. As I wrote in a previous article (Energy-Harvesting the Earth's Heat, March 10, 2014), this effect can be used to generate electricity.

Cloud cover, however, will allay most cooling, since the clouds are at a much higher temperature than 3 K. Altostratus clouds, which appear several miles above ground, have temperatures as low as about -30°C, while the somewhat higher cirrostratus clouds might be as cold as -50°C. However, the temperature of low altitude cumulus clouds is above freezing.

Cumulus clouds. Cumulus clouds are low altitude clouds occurring about a kilometer above ground. They are composed of liquid water, so their temperature is above freezing. (Photo by Michael Jastremski, via Wikimedia Commons.)

This one example should convince you that clouds have a role in weather, in the short term; and, perhaps, climate in the long term. Clouds will trap heat at Earth's surface, but they can also reflect sunlight to cause cooling. Not only will clouds affect climate, but the climate affects the formation and persistence of clouds.[3]

Human activity has lofted a considerable quantity of aerosol particles in the atmosphere. These have a cooling effect, since they reflect sunlight, and they act also as nuclei for formation of cloud drops, making clouds brighter. Current estimates are that half of cloud droplets are formed when trace gases combine in the atmosphere.[4]

A recent paper in the Proceedings of the National Academy of Sciences (PNAS) underscores the importance of clouds in determining climate. It also presents the interesting observation that pre-industrial clouds are different from today's post-industrial clouds. In fact, pre-industrial clouds could not be properly modeled using data from our post-industrial world, sometimes even giving the reverse trend.[5-6]

The research was done by a multi-national team from the Pacific Northwest National Laboratory (Richland, Washington), Nanjing University (Nanjing, China), ETH Zurich (Zurich, Switzerland), the National Center for Atmospheric Research (Boulder, Colorado), the Norwegian Meteorological Institute (Oslo, Norway), the University of Oxford (Oxford, United Kingdom), Stockholm University (Stockholm, Sweden), and Kyushu University (Fukuoka, Japan).[5]

The PNAS study indicates that the specific effects of aerosols are hard to pin down, and they may be incorrectly incorporated into climate models.[6] The problem is that there are no cloud data from the preindustrial era before aerosol pollution, and this problem might be addressed by examining more pristine cloud cover in relatively non-polluted areas.[6] Says study co-author, Steve Ghan of the Pacific Northwest National Laboratory,

"We might have to find clouds far away from civilization... but, there are parts of the world that are pretty darn clean."[6]

The "clean" parts of the atmosphere appear in the southern hemisphere between the latitudes of 40 and 50 degrees.[6]

While the geological record contains a history of greenhouse gases as bubbles in ice cores, fossils, and tree rings, clouds are ephemeral things that leave no trace.[6] The influence of greenhouse gases on temperature rise is well established, but how much clouds have shielded the Earth by reflecting solar radiation into space is not known.[6]

An assortment of cloud types, illustrating how difficult assessing the climate influence of clouds must be. Clockwise from upper left, cirrus, cirrocumulus-altocumulus, cumulus, stratocumulus, all from Wikimedia Commons.

As the above photographs demonstrate, clouds come in a variety of types, each of which may have their own affect on climate. Internal layering of clouds, which would influence how sunlight is transfered in and out of clouds, might be a factor; or, clouds may behave differently depending on the extent of aerosol content.[6]

This research was funded by the US Department of Energy, the US National Science Foundation, the National Natural Science Foundation of China, the Austrian Science Fund, the Swiss National Supercomputing Centre, the UK Natural Environment Research Council, the UK European Research Council, the Japan Ministry of the Environment, and the Japan Society for the Promotion of Science.[6]

A cartoon from Randall Munroe's xkcd Comics, licensed under a Creative Commons Attribution-NonCommercial 2.5 License. (xkcd comic 287.)

References:

1. Data Sheet, LMx35, LMx35A Precision Temperature Sensors, Texas Instruments
2. Data Sheet, LM35 Precision Centigrade Temperature Sensors, Texas Instruments.
3. International Satellite Cloud Climatology Project, NASA Web Site.
4. Jasper Kirkby, et al., "Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation," Nature, vol. 476, no. 7361 (August 25, 2011), pp. 429-433.
5. Steven Ghan, Minghuai Wang, Shipeng Zhang, Sylvaine Ferrachat, Andrew Gettelman, Jan Griesfeller, Zak Kipling, Ulrike Lohmann, Hugh Morrison, David Neubauer, Daniel G. Partridge, Philip Stier, Toshihiko Takemura, Hailong Wang, and Kai Zhang, "Challenges in constraining anthropogenic aerosol effects on cloud radiative forcing using present-day spatiotemporal variability," Proc. Natl. Acad. Sci., Early Edition, February 26, 2016, doi: 10.1073/pnas.1514036113. This is an open access publication with a PDF file available here.
6. Mary Beckman, "Cloudy problems: Today's clouds might not be the same as pre-industrial ones," Pacific Northwest National Laboratory Press Release, March 3, 2016.