Changes In Albedo Are Called An Enhanced “Greenhouse Effect”
Written by Carl Brehmer
A study done by John Christy (IRRIGATION-INDUCED WARMING IN CENTRAL CALIFORNIA?) has been touted by some as being “proof” that humidity causes an enhanced “greenhouse effect”.
The conclusions drawn in the paper are based on the following two graphs (the red trend lines were added for clarity):
At first glance the increase in the daily minimum temperature (just before sunrise) exceeds the decrease in daily maximum temperature (~2:30PM) giving the impression that there was an over-all increase in the daily mean temperature from 1930-2000, but look at the scale. Each line in the TMin graph is 2 °C while each line in the TMax scale is 4 °C.
So, as you can see the daily minimum temperature increased about 2 °C while the daily maximum temperature decreased by about 2 °C meaning that the overall affect of irrigation on the daily mean temperature was nil. Rather, the affect of irrigation in this study shows that ground water decreases the diurnal temperature swing. This is not surprising since water has a higher specific heat than does dry soil. As a result the specific heat of wet soil is nearly double that of dry soil.
Specific heat water = 4.179 j/g/°C
Ergo, wet soil both warms and cools more slowly than does dry soil given the same thermal input/output. Thus any assertion that this study demonstrates that irrigation in the San Joaquin Valley during the 19th century caused a net increase in average daily temperature is false unless you both milk the data and ignore the warm temperatures present in the 1930’s. For example, if you were to start the nighttime minimum trend line in the 1940’s instead of the 1930’s then the nighttime warming trend would appear to have increased about 3 °C compared to the actual nighttime warming trend of about 2 °C, which is cancelled out by an equivalent amount of daytime cooling.
It is interesting to note that the original paper does not quantify the total increase in the daily minimum nighttime temperatures over the time period studied, but only says that it was “positive”. It was a commentary on his paper that quantified the increase to have been ~3 °C, which, if true, would suggest that the study showed a ~1 °C net increase in daily mean temperatures from 1930-2000. In other words, whether the data shows a nil effect on the daily mean temperature or a slight increase depends upon where one arbitrarily places the trend line on the graphs.
Let’s keep something else in mind. Water vapor at a global average of 70% R/H is said to increase the global mean temperature by some 22 °C all by itself (2/3rds of the total 33 °C of “greenhouse effect” warming that is said to exist.)
This is roughly 3 °C for every 10% increase in humidity. If therefore the irrigation of this otherwise desert landscape caused even a doubling of the R/H from about 35% to 70% (70% is the current yearly mean humidity in the San Joaquin Valley) then a water vapor enhanced “greenhouse effect” should have been around 10 °C! Instead the data has to be milked and the daytime cooling ignored in order to suggest that the 35% increase in the San Joaquin Valley’s humidity has caused a significant increase in the daily mean temperature via an enhanced “greenhouse effect”.
What is odd about this paper is that it purports to assess the affect of humidity on the nighttime temperature increase within the San Joaquin Valley yet fails to report what the humidity actually was in that valley prior it being irrigated to grow crops or even when the irrigation reached sufficient levels to affect the regional climate.
All that it says is, “With very low humidity, such an environment saw diurnal temperature ranges of over 15°C in the dry season. Additionally, the hard, dry natural surface had little heat capacity and relatively high albedo.” Curiously in this statement Christy accurately attributes the change that the regional climate has experienced to 1) a change in the ground’s heat capacity [as mentioned above] and 2) to a change in the ground’s albedo.
Yet within his summary statement he drops mention of the change in heat capacity and adds the “greenhouse effect” to his list of hypothetical causes of the increased nighttime temperatures measured within the San Joaquin Valley during the 20th century.
So, let’s jump to the summary of the paper that again ignores the daytime cooling trend and bases its conclusions exclusively on the increase in nighttime minimum temperatures. “Our hypothesis at this point is that irrigation has altered the surface energy balance of the valley floor, causing nighttime temperatures to remain warm.” The paper then advances three possible reasons why irrigation might be the cause an increase in the nighttime temperatures seen in the San Joaquin Valley.
1) “The additional water vapor supplied through evaporation, not present formerly, enhances the downward flux of thermal radiation.” In other words increases the “greenhouse effect”.
2) “Second, the additional vapor allows aerosols to reach the swelling point at which they become very active in the thermal spectrum.”
3) “Last, the moist ground and vegetation absorb solar energy during the ubiquitous cloudless days, and release the energy in the evening.”
Since Christy only hypothesizes about the cause of the increase in nighttime temperatures and ignores in his summary the concurrent decrease in daytime temperatures, he is only looking at one half of a dampened diurnal temperature swing. Do the laws of physics change when the sun goes down? Why a doubling of the humidity wouldn’t also cause an enhanced “greenhouse effect” during the day is not explored.
To be fair to Christy, this question is never explored because doing so would not support the meme being advanced. It is an observable phenomenon that both up going long-wave radiation and the absolute humidity are the highest during the daytime hours yet the daytime temperatures in humid climates are seen to be significantly less than the daytime temperatures in arid climates.
This would suggest that the hypothetical “greenhouse effect” is only a mirage and that something else is causing the increased nighttime temperatures in humid climates. The fact is Christy himself identifies that “something else” in this very paper.
He concludes, “Preliminary calculations indicate the enhanced water-vapor greenhouse effect would be relatively small . . . Thus, the presence of liquid water in the ground and vegetation (with lower albedo) increases the thermal capacity of the surface, thus keeping the nighttime temperatures warmer than would be otherwise through sensible heat flux.”
So, in the end Christy affirms the reality that increasing the humidity enhances a “greenhouse effect” is only a hypothesis based on a “calculation” and that the most likely causes of the increase in the nighttime temperatures measured within the San Joaquin Valley since the advent of irrigation are most likely due to 1) an increase in the ground’s heat capacity and 2) a decrease in the regional albedo.
Let’s take a closer look at this albedo question. The USDA has put out a paper that states, “Dark, wet soils have an albedo roughly of 6 to 15 percent, and dry soils have an albedo of 22 to 34 percent.” This means that when you irrigate a field you decrease its albedo by 17% on average. This causes a 17% increase in the amount of thermal energy that the ground absorbs every day! All things being equal, this should cause a > 40 °C (288 K x 17%) temperature increase in daytime soil temperatures but it doesn’t.
As we see in Fig #2 above, the daytime ground level temperatures in the San Joaquin Valley have actually decreased in spite of its 17% increase in albedo. If you average that expected <40 °C of additional daytime warming over the whole day, the increase in albedo should have increased the daily mean temperature by some 20 °C.
Therefore, even if you accept those interpretations of Christy’s data that asserts that there has been a ~1 °C increase in the daily mean temperature since the advent of irrigation in the San Joaquin Valley, the cooling affect of the presence of water in the San Joaquin Valley, due to latent heat transfer and the enhanced emissivity of the air (increasing the emissivity of the air, due to well established laws of radiation thermodynamics, increases the atmosphere’s net radiation loss rate to space), brings a would be 20 °C increase in the daily mean temperature down to only 1 °C!
To put a fine point on it, since the extra water vapor in the air can be seen to be assisting in the disgorgement of an extra 17% of absorbed heat, how can it possibly be enhancing a hypothetical “greenhouse effect” at the same time?
In non-irrigated climates where the humidity is high no such net warming is seen because the increased cloud cover that comes with naturally occurring humidity significantly increases rather than decreases the albedo.
This graph, for example, is composed of daily mean temperature, insolation and absolute humidity readings taken from June 17th through July 17th 2012 in the Arizona high desert as it transitioned from the dry season into the wet season and as you can plainly see the humidity is inversely proportional to the insolation readings since the high humidity was brought into this area by clouds. The most extreme example of the ability of clouds to decrease the amount of insolation reaching the ground was circa July 14th when the insolation was reduced by 60%, with a resulting decrease in temperature of nearly 20 °C from the temperature highs of late June! Remember that the global average albedo is 30%. This requires that the local albedo under cloud cover (where the humidity is highest) be significantly higher.
What can we conclude from this? Christy’s paper actually demonstrates that changes in albedo were responsible for the possible slight increase in the daily mean temperatures seen within the San Joaquin Valley between 1930-2000 rather than a hypothetical enhanced “greenhouse effect”.