Water vapor feedback: evaporation

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The press release further states:

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Evaporative cooling is the process by which a local area is cooled by the energy used in the evaporation process, energy that would have otherwise heated the area’s surface. It is well known that the paving over of urban areas and the clearing of forests can contribute to local warming by decreasing local evaporative cooling, but it was not understood whether this decreased evaporation would also contribute to global warming

The Earth has been getting warmer over at least the past several decades, primarily as a result of the emissions of carbon dioxide from the burning of coal, oil, and gas, as well as the clearing of forests. But because water vapor plays so many roles in the climate system, the global climate effects of changes in evaporation were not well understood.

The researchers even thought it was possible that evaporation could have a warming effect on global climate, because water vapor acts as a greenhouse gas in the atmosphere. Also, the energy taken up in evaporating water is released back into the environment when the water vapor condenses and returns to earth, mostly as rain. Globally, this cycle of evaporation and condensation moves energy around, but cannot create or destroy energy. So, evaporation cannot directly affect the global balance of energy on our planet.

Increased evaporation tends to cause clouds to form low in the atmosphere, which act to reflect the sun’s warming rays back out into space. This has a cooling influence.

Video of Julia Pongratz speaking about the cooling influence of evaporation

Video of Ken Caldeira asking: increased evaporation lead to global cooling?

The press release doesn’t mention a specific publication, but I spotted this one, which appears to be the relevant publication:

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George A. Ban-Weiss, Govindasamy Bala, Long Cao, Julia Pongratz, Ken Caldeira

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Abstract. Land use and land cover changes affect the partitioning of latent and sensible heat, which impacts the broader climate system. Increased latent heat flux to the atmosphere has a local cooling influence known as ‘evaporative cooling’, but this energy will be released back to the atmosphere wherever the water condenses. However, the extent to which local evaporative cooling provides a global cooling influence has not been well characterized. Here, we perform a highly idealized set of climate model simulations aimed at understanding the effects that changes in the balance between surface sensible and latent heating have on the global climate system. We find that globally adding a uniform 1 W m−2 source of latent heat flux along with a uniform 1 W m−2 sink of sensible heat leads to a decrease in global mean surface air temperature of 0.54 ± 0.04 K. This occurs largely as a consequence of planetary albedo increases associated with an increase in low elevation cloudiness caused by increased evaporation. Thus, our model results indicate that, on average, when latent heating replaces sensible heating, global, and not merely local, surface temperatures decrease.

Environ. Res. Lett. 6 (2011) 034032 (8pp)   Link to full paper [here].

I find this to be quite an interesting paper.  I tend to be more of an ocean person than a land person, so I find the idea of land use changes resulting in latent heat replacing sensible heating to be interesting (but not at all surprising once you think about it).

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The evaporative flux from the ocean surface  depends on surface wind speed (u) and the gradient of specific humidity (q) between the ocean surface and the lowest few meters of the atmosphere.     Surface temperature enters into the evaporative flux through its control on the humidity right at the ocean’s surface, which is determined by the Clausius-Clapeyron equation that relates saturated vapor pressure to temperature (note: to calculate the surface specific humidity, you also need to account for the solution effect of salts in the ocean water).  All other things being equal (which they rarely are), warming the ocean surface would increase the evaporative flux.

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In terms of the climatology of ocean evaporative flux, see this global estimate from satellite observations [here].  While generally higher values are seen in the tropics and lower values in the high latitudes, the largest values are seen in the boundary currents such as the Gulf Stream, and some of the lowest values are seen on the equator.  These variations reflect the impacts of atmosphere and ocean dynamics and land/sea temperature and humidity contrasts in influencing the variation of ocean evaporative flux.

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I have been thinking of some issues related to the ocean evaporative feedback, which are discussed in Chapter 13 on feedbacks in my book Thermodynamics of Atmospheres and Oceans, see especially section 13.6, pages in a discussion on the tropical ocean warm pool.  Here is a brief summary.

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 Using experimental data obtained from research ships (in the tropical Pacific), we determined the derivative of the surface wind speed and humidity gradient with respect to surface temperature (To).

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It was shown that dq/dT0 > 0, while du/dT0 < 0. Wind speed dependence of the surface latent heat flux dominates for T0 > 301 K, while the humidity dependence dominates for T0 < 301 K. The decrease in surface latent heat flux at high surface temperatures is hypothesized to arise from the following mechanism: high surface temperature —> increased instability and convection —> increased large-scale low-level convergence —> weaker surface wind — > lower latent heat flux. In interpreting the magnitudes and signs of these derivatives, it should be kept in mind that an apparent empirical relationship between wind speed and humidity with T0 is no guarantee that the primary factor giving rise to changes in wind speed or humidity is T0. Wind speed and T0 may appear to be related because both fields are related to a third and much more dominant factor, such as the large- scale coupled atmosphere-ocean circulation, which is controlled largely by the horizontal gradient in T0 rather than the value of T0 itself.

While googling around, I spotted this paper on Convection-Evaporative Feedback in the Equatorial Pacific, which is also relevant to this discussion.

Surface evaporative feedback as a component of the global water vapor feedback

A quick google search finds discussion of the evaporative feedback on local and short time scales, e.g. in the context of the Madden Julian Oscillation.  I am not finding much discussion of evaporation in the context of global water vapor feedback.

The basic physics giving rise to evaporative feedback is included in climate models, and causes surface cooling.  But there is an evaporative feedback, that is rarely discussed or included in the diagnosis of global thermodynamic feedbacks.  I am having a vague recollection that Will Kinnimonth has been making some of these arguments, but I recall the analysis that I saw had some significant errors, but the broader issues he was raising were interesting.  I can’t find anything right now, I will dig it up.   I know that skeptics have been talking about evaporation, I think I recall Tallbloke discussing how the shallow IR penetration depth couldn’t possibly warm the ocean, he argued that only the surface layer warms, which then increased evaporation.  This is incorrect since turbulence does mix heat in the upper ocean, and the physics of the cool skin layer right at the surface does not preclude heat exchange between the skin layer and the ocean mixed layer.

The broader issue raised in the Carnegie study that is of interest is to me is that the thermodynamic feedbacks – water vapor, cloud, surface albedo, lapse rate, evaporation – are not really separable and additive, they are linked.  Of these thermodynamic feedbacks, evaporation is much less frequently discussed and should arguably be integrated with the water vapor feedback (which is difficult to really separate from the lapse rate feedback and the cloud feedback).

So hopefully the Carnegie study will reinvigorate the discussion of evaporative feedback processes (over both land on ocean) in the context of global climate.

Moderation note:  this is a technical thread, comments will be moderated for relevance.

JC note:  a very busy week for me, I hoped to research this topic more before posting, but this is all I had time for.  Hopefully this will generate some good discussion and others can provide links and arguments.