Introduction. The role of water vapor and cloud cover in assessing the long-term warming of the earth is complex, both with respect to observation (data gathering) and modeling. A schematic identifying the processes by which water vapor and clouds can affect the energy balance at the earth’s surface is shown below.
Processes involved in the global rate of absorbing or radiating energy due to clouds and water vapor. Units are given in watts per square meter (W m-2), where 1 watt is a unit of power, i.e. a unit describing the rate of energy gain or loss per second. The numbers given in the graphic represent the result of measurements and modeled calculation by the authors for March 2000 to May 2004.
Darker yellow downward arrows, left, show incoming power per meter squared for visible solar light. Paler yellow arrows, right, show outgoing power per meter squared due to heat (infrared) radiation, as well as heat radiation from the atmosphere back to the earth’s surface due to the greenhouse effect from CO2, water vapor and clouds. Evapotranspiration (center) combines bulk evaporation and transport of water from the ground to the air by the transpiration of green plants. Latent heat (cloud in center) is explained in this post http://warmgloblog.blogspot.com/2011/03/ice-water-and-water-vapor.html.
Source: Trenberth and coworkers, BAMS March 2009, pp. 311-323; http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/TFK_bams09.pdfFor sunlight reaching the earth, the sun's energy (visible light) is re-emitted as heat (infrared) energy. Water vapor is transparent to visible sunlight (just as is CO2), but water vapor and clouds act as greenhouse elements with respect to heat energy (also just as does CO2). As shown in the diagram, a) clouds directly reflect a portion of the visible light from the sun back into space, and b) clouds as well as atmospheric water vapor exert a greenhouse effect on heat (infrared) radiation originating at the earth’s surface. This greenhouse effect absorbs a large portion of the heat and re-emits it in all directions, shown in the diagram as continuing on out into space and returning to the earth’s surface as heat.
At the very bottom of the diagram, the net total result of the all the positive and negative contributions to the energy balance is shown as 0.9 W m-2, a small net warming effect. It is important to see from this diagram that since this final result is a very small number arrived at by adding and subtracting very large numbers, any small error in the inputs will have a disproportionately large effect on the final result, and could easily turn a positive energy balance into a negative balance. For example, even the reported outcome for the global average is the result of a cooling of 15.6 W m-2for land (about 30% of the earth's surface) and a warming of 6.9 W m-2 for the oceans (about 70% of the surface).
The New York Times recently published a report discussing scientists’ current understanding of the role of clouds in the long-term increase in the global average temperature. This involves new enhanced measurement methods as well as refined inputs into global climate models (GCMs; also general circulation models). As noted above, clouds can contribute both to more cooling (reflection of incoming sunlight), and to warming (because clouds and water vapor contribute a greenhouse effect based on the heat (infrared) radiation leaving the earth’s surface). According to the report, the broad conclusion of the great majority of scientists is that, in balance, a neutral or positive contribution to the overall global temperature dominates. (Clouds are only one of many factors accounted for in GCMs.)
Background. Climate scientists have reached a broad consensus that our planet is warming. By measuring the long-term average temperature at stations all around the globe, as well as by satellite in recent decades, they find that the global temperature is increasing, starting with the industrial revolution. Scientists attribute this warming to carbon dioxide, a greenhouse gas, that results from burning the fossil fuels that power global industrialization, as well as to other greenhouse gases produced industrially. These gases act to trap part of the heat radiation released by sunlight striking the surface of the earth that would otherwise escape into space. CO2 has been a component of the earth's atmosphere for millions of years. Yet its concentration has increased abruptly since the industrial revolution began due to mankind's burning of fossil fuels to provide energy. The greenhouse effect that it exerts on the planet's climate has been enhanced as a result.
Of the CO2that enters the atmosphere, a portion is absorbed by green plants as they grow (but is released as they die and decay), and a portion is absorbed into the waters of the oceans. The majority stays in the atmosphere for at least 100 years, or longer, as there is no additional mechanism that removes it. Before the industrial revolution the CO2 cycle was in equilibrium; the gas produced by animals and decaying vegetation was absorbed by the oceans and growing plants. But the carbon contained in fossil fuels is not recycled back to the geological reservoirs that the fuels came from. This carbon follows a one-way route from underground reservoirs to new, additional atmospheric CO2once burned to supply energy.
Water is a greenhouse substance. Water also exerts a greenhouse effect, whether as water vapor (i.e., a gas) or a liquid (including droplets in clouds and fog). In this regard, atmospheric water differs in many ways from CO2. Its vapor concentration in air is much higher than that of CO2; at "room temperature" the capacity of water in air is about 25 parts per thousand (25,000 parts per million) whereas currently the content of CO2 is about 390 parts per million. For this reason, the greenhouse effect from atmospheric water is much stronger than that of atmospheric CO2. Without the greenhouse effect of water, ambient temperatures on the earth would be far below freezing. Second, locally the actual water vapor content can be anywhere from 0 to 100% of the upper limit (the relative humidity). Globally the long-term cycle of water between water vapor, clouds and fog, rain and snow, glaciers and groundwater, and the oceans remains at equilibrium, in the absence of global warming. But thirdly, the capacity of air to hold water vapor (as the gas) increases by about 7% per degree C (3.9% per degree F). Thus as the long-term global average temperature rises because of the CO2greenhouse effect, the overall intensity of the global water cycle will grow.
The water cycle, including all the components mentioned above, is included in global climate models. The role played by clouds in various GCMs is modeled with different parameters. As shown in the graphic, some of the sunlight directly striking clouds, especially low clouds (cumulus) and middle, layered clouds (stratus), from space is reflected back into space as unaltered visible light. This reflected light never reaches the earth and does not contribute to the greenhouse effect. The highest (cirrus) clouds, however, are high enough to be formed of ice microcrystals rather than droplets of liquid water. It is believed that cirrus clouds permit most sunlight to pass through to the earth, in contrast to the behavior of lower clouds, while still retaining the ability to act as greenhouse elements, retaining a portion of the heat energy of re-emitted light.
Skeptics: Clouds will help cool the planet. The New York Times article devoted considerable emphasis to the views of certain scientist skeptics, especially the meteorologist Richard S. Lindzen of the Massachusetts Institute of Technology, affirming that clouds will contribute a cooling effect as the global temperature rises. Dr. Lindzen has studied climate for more than five decades. According to the Times, he believes that cirrus clouds, especially over the tropics, will serve as an “iris” (i.e. the portion of the mammalian eye, or of a camera, that regulates how much light reaches the retina, or the film) as the earth warms. Warmer atmospheric temperatures, in his view, will lead to a thinning of cirrus clouds that will permit more heat (infrared) radiation to escape into space. This negative effect on retention of heat will reduce the overall warming of the planet.
The Times reports that these views have been warmly received by politicians and others, such as the Heartland Institute, who are skeptical of the role of CO2 and other greenhouse gases in the long-term warming of the planet. According to the Times “most mainstream researchers consider Dr. Lindzen’s theory discredited”. As an example, an article in 2009 by Trenberth and Fassulo, states “Many papers refute the negative feedback and iris hypothesis of Lindzen et al. [2001]”, citing as examples Hartmann and Michelsen, 2002, “No evidence for iris”, Bull. Am. Meteorol. Soc., 83, 249–254; Randall et al., 2007, “Climate models and their evaluation”, in Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon et al., pp. 590– 662, Cambridge Univ. Press, New York; and evidence for a slight positive feedback by Lin et al., 2002, “The iris hypothesis: A negative or positive cloud feedback?”, J. Clim., 15, 3–7.
The Times further reports that a paper published by Dr. Lindzen in 2009 included errors in data analysis that were identified by other scientists and subsequently affirmed by him. In addition, a more recent manuscript was criticized by peer reviewers for a “prestigious American journal” and was rejected for publication. This is significant (see this earlier post on this blog), since review by anonymous peers ensures that analysis and conclusions expressed are supported by the data (usually peer review does notassess the accuracy or validity of the data themselves). Conversely, contemporary authors of journal articles thank their peer reviewers for offering constructive suggestions that improvethe final form of the paper (for examples see Science 27 April 2012: Vol. 336 no. 6080 pp. 455-458, DOI: 10.1126/science.1212222 (see Acknowledgements); and Science 27 April 2012: Vol. 336 no. 6080 pp. 462-466; DOI: 10.1126/science.1218389 (see Acknowledgements)).
Other articles also assess cloud feedbacks. NASA discussed (accessed May 5, 2012 ) long-term warming of the earth. In addition to forecasting significant warming by the end of this century, this article states climate feedbacks could more than double predicted warming, including feedbacks “due to snow and ice, water vapor, clouds, and the carbon cycle.” As the air warms, the ability of air to hold water vapor increases, as noted earlier. As described above, clouds have both positive (greenhouse effect) and negative (reflection of sunlight) feedback effects on warming. On balance, according to NASA, “most climate models predict a slight overall positive feedback or amplification of warming due to a reduction in low cloud cover.”
Discussing the role of cirrus clouds in this same post, NASA points out that they emit only small amounts of radiation because of their cold temperature. Thus, being composed of (solid) water, cirrus clouds strongly absorb heat (infrared) radiation reaching them from below, and retain a significant fraction of that heat, leading to higher atmospheric temperature than would be the case if they were absent. NASA states that in a world with higher average global temperatures, the air would have more water content that leads to formation of more cirrus clouds. In this view CO2-induced greenhouse warming would be amplified by the presence of more heat-retaining cirrus clouds in the upper atmosphere.
In a different post dated Dec. 13, 2010 , NASA summarized work (accessed May 5, 2012 ) by Andrew Dessler of Texas A&M University
Clement and coworkers (see also a commentary by a nonparticipating scientist) analyzed the correlation of cloud cover (low- and mid-level clouds, excluding cirrus clouds) and sea surface temperature over a large portion of the northeast Pacific ocean at subtropical latitudes, using existing records, for the period 1952-2007. In the region monitored there is a reduction in cloud cover when the sea surface temperature is warmer and vice versa. This indicates that clouds interact with sea surface temperature in a way that amplifies warming. The scientists then assessed whether existing GCMs in the archive of the worldwide consortium of climate scientists could reproduce their findings. Only one of 18 models assessed succeeded in reproducing their findings in response to the warming induced by the known increase in greenhouse gases that occurred over this period.
Trenberth and Fassulo, in the article mentioned earlier, published in 2009, modeled the effects of the complete cloud cover from 1950 to 2100 using all models in the worldwide archive. Although they found considerable variation among models, some yielded projections for positive feedback effects from clouds, i.e., that increased surface temperatures would lead to effects on the cloud cover that amplified the increase.
Conclusions
The New York Times published an article analyzing the effects of clouds on the warming of the planet. It devoted considerable attention to skeptics who doubt that mankind’s activities and the greenhouse effect have led to long-term warming, and who have subscribed to the renegade view of Dr. Lindzen that cirrus clouds will act as an iris, releasing more heat energy to space as the earth warms.
This post has presented background information showing that the contributions of clouds to the global climate are many, varied, and may be subject to considerable variability both in data analysis and in modeling their effects in GCMs. It is important to understand that final effects are small numbers arrived at as the difference between large positive and negative contributions from individual processes. Small changes in evaluating these processes can therefore lead to large changes in the final contribution, including changing from a net warming effect to a net cooling effect.
© 2012 Henry Auer
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