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A "Forests as biotic pump” hypothesis discredited due to errors in basic atmospheric physics

Última modificación 08/10/2009 14:48

The role of vegetation cover in the earth hydrological cycle remains controversial and difficult to model. Land cover change affects regional climate through impacts on the surface albedo and radiative forcing, partitioning of available energy between sensible and latent heat, boundary layer temperature, moisture profile and depth, and the partitioning of rainfall between evaporation and runoff (Pitman et al., 2009). Local people in many partially forested regions believe that forests “attract” rain, but climatology has no scientific explanation for that believe (Stigter, 2010). There is some evidence that the presence of extended tracts of forest (e.g. in the humid tropics) promotes inland transport of moisture. The most important mechanism in this regard seems to be the recycling of moisture by the forest (e.g. Salati and Vose, 1984; Meesters et al., 2009).

Kees Stigter1 and Antoon Meesters2

1Agromet Vision, Bruchem, the Netherlands, and Bondowoso, Indonesia

2 Faculty of Earth and Life Sciences, VU University, Amsterdam, the Netherlands

The role of vegetation cover in the earth hydrological cycle remains controversial and difficult to model. Land cover change affects regional climate through impacts on the surface albedo and radiative forcing, partitioning of available energy between sensible and latent heat, boundary layer temperature, moisture profile and depth, and the partitioning of rainfall between evaporation and runoff (Pitman et al., 2009). Local people in many partially forested regions believe that forests “attract” rain, but climatology has no scientific explanation for that believe (Stigter, 2010). There is some evidence that the presence of extended tracts of forest (e.g. in the humid tropics) promotes inland transport of moisture. The most important mechanism in this regard seems to be the recycling of moisture by the forest (e.g. Salati and Vose, 1984; Meesters et al., 2009).

 

On the other hand decreased atmospheric moisture availability due to reduced soil water uptake and reduced rainfall interception after forest removal is considered a key underlying cause of model predictions of reduced atmospheric moisture convergence as well as of tele-connection patterns between deforestation and changes in precipitation in remote areas (see Meesters et al. (2009) and the references given there). The latter were not found in a recent modelling exercise on climate responses to past land cover change but this may be due to limitations of the model inputs (Pitman et al., 2009). Deforestation has already reduced vapor flows from forests by about 5% per year with little sign of slowing. The need for understanding how vegetation cover influences climate has never been more urgent (Sheil and Murdiyarso, 2009).

 

Recently a new hypothesis, called the “biotic pump” suggested that the above mentioned local people were absolutely correct and that forest cover plays a much greater role in determining rainfall than previously recognized. Makarieva et al. (2006), Makarieva and Gorshkov (2007) developed an explanation how forests attract moist air and how continental regions such as the Amazon basin remain wet. The implications would be substantial (Stigter, 2010).

 

What follows is based on or taken from Sheil and Murdiyarso (2009), who have attempted to explain the basic ideas of Makarieva and Gorshkov (M&G) and their significance for a wider audience. See also Hance (2009) and Pearce (2009) about this paper. They state that deforestation has been implicated as contributing to declining rainfall in various regions (including the Sahel, West Africa, Cameroon, Central Amazonia, and India), as well as weakening monsoons, but the links remained uncertain. Observations suggest that extensive deforestation often reduces cloud formation and rainfall, and accentuates seasonality. Forest clearings can cause a distinct, convection driven “vegetation breeze” in which moist air is drawn out of the forest. Atmospheric turbulence resulting from canopy roughness and temperature driven convection are thought to explain the localized increase in rainfall sometimes associated with fragmented forest cover. Much of the latter statements were taken by Sheil and Murdiyarso (2009) from Bonan (2008). Researchers have previously puzzled over a missing mechanism to account for observed precipitation patterns and M&G’s hypothesis offers a solution.

 

Pressure gradients driven by temperature and convection are considered to be principal drivers of air flows in conventional meteorological science. M&G argue that the importance of evaporation and condensation have been overlooked. At the global average lapse rate water vapour rises and condenses. The reduction in atmospheric volume that takes place during this gas to liquid phase change causes a reduction in air pressure. This drop in pressure has routinely been overlooked, they say. So, atmospheric volume reduces at a higher rate over areas with more intensive evaporation. The resulting low pressure draws in additional moist air from areas with weaker evaporation. This leads to a net transfer of atmospheric moisture to the areas with the highest evaporation.

Sheil and Murdiyarso (2009) discuss various local consequences. Forest loss and diminished evaporation can for example reduce the penetration of monsoon rains and reduce the duration of the wet season. Clearing enough forest within a larger forest zone may switch net moisture transport “from ocean to land” into “from land to ocean”, leaving forest remnants to be desiccated. Clearing a band of forest near the coast may suffice to dry out a wet continental interior.

 

M&G’s hypothesis suggests that forest loss will be associated with a loss of stabilizing feedbacks and increased climatic instability. In Brazil’s Atlantic forests just such a correlation has been detected between reduced tree cover and increased local interannual variation in rainfall. Have forests evolved to generate rain? This idea touches on the much-debated possibilities of emergent self-stabilizing behavior (or “Gaia”). Trees and forests have evolved numerous times in the history of the earth, suggesting a repeated trend to generate rich self-watering terrestrial habitats. There is scope for self-stabilizing interactions to arise. We need to unravel the feedback processes and thresholds that operate spatially at different scales, and the influences that act upon them (Sheil and Murdiyarso, 2009)

 

This all sounds great, but what if the hypothesis would not be true? Now Meesters et al. (2009) showed some months ago that it is far from true. Makarieva and Gorshkov (2007) made errors in basic atmospheric physics that completely discredit their biotic pump hypothesis. The “evaporative force” on which the theory is built, is not supported by atmospheric physical principles. The following is from Meesters et al. (2009).

 

A very important notion is hydrostatic equilibrium (the pressure at each height equals the weight per horizontal area of the column above). But M&G apply this notion to air components (dry air and water vapor) separately. A key question is whether component-equilibrium is of importance to macroscopic transport in the atmosphere. M&G deny this correctly for water vapor. The reason is that vapor is continuously entering the atmosphere at the surface by evaporation, whereas it is removed at greater altitudes by condensation and precipitation. The biotic pump hypothesis is not required to explain this deviation from non-condensing gases. Dry air has a near constant composition in the atmosphere and can be treated as a single bulk component. Now M&G have made the assumption that for dry air component-equilibrium should always hold. However, this is in stark contradiction with the generally everywhere assumed bulk-equilibrium to hold in the atmosphere.

 

Component-equilibrium might be appropriate for certain situations in which macroscopic flows can be excluded (so that molecular diffusion is the only transport mechanism to be considered) such as in laboratory conditions, but it appears inappropriate for open-air conditions where these macroscopic flows are so dominant that component-equilibrium becomes of marginal importance. Separate behavior of the components becomes untenable. The critical issue neglected by M&G is the response of the dry-air component to the motions caused by a bulk-disequilibrium. M&G appear to imagine an atmosphere in which the dry-air component stays immobile in the presence of (even violent) vertical motion. This contradiction leads to an atmosphere in which bulk-equilibrium cannot be restored (Meesters et al., 2009).

 

A point which has been increasingly emphasized in M&G’s successive expositions of the biotic pump theory, is the “pressure drop” which occurs on condensation. Since condensation implies disappearance of water molecules from the vapor phase, there remain indeed less molecules which exert pressure. But on the other hand, condensation heats the air parcel and hence causes faster molecular motion and a rise in pressure, which is neglected in the calculations of M&G. Actually, condensation causes not a drop but a rise in local pressure (compared to parcels at the same height but without condensation). This is accompanied with expansion and thinning, contributing to the well-known buoyancy of convective clouds. This mechanism of heating and expansion is a fact which has been very well observed, and it strongly contrasts with the one proposed by M&G in which condensation is regarded as the cause of an ongoing implosion.

 

Given that the M&G “forests as biotic pump” hypothesis is now discredited due to basic errors in their atmospheric physics, what about the above issues that Sheil and Murdiyarso (2009) brought up. Should we forget about them? Not at all. As long as modelling of the climatic responses, including precipitation patterns, to land cover change gives such high uncertainties and as long as it is not yet feasible to impose a common land cover change across models (Pitman et al., 2009), we should improve these and other weaknesses.

 

Pitman et al. (2009) mention also the representation of crop phenology, the parameterisation of albedo, and the representation of evapotranspiration for different land cover types as such weaknesses of present models. Changes in land-atmosphere exchange of greenhouse gases, reactive trace gasses and aerosols as a function of land cover change may also have to be introduced into the models, they say. We may also observe that perhaps still other modelling approaches to responses of land cover change may be added to get into an ensemble approach, to make use of instead of being defeated by modelling uncertainties.

 

References

  1. Bonan GB (2008) Forests and climate change: Forcing feedbacks and the climate benefits of forests. Science 320:1444-1449.
  2. Hance J (2009) Revolutionary new theory overturns modern meteorology with claim that forests move rain.
  3. http://news.mongabay.com/2009/0401-hance_revolutionarytheory.html
  4. Makarieva AM,GorshkovVG (2007) Biotic pump of atmospheric moisture as driver of the hydrological cycle on land .Hydrol. Earth Syst. Sci. 11:1013-1033.
  5. Makarieva AM, Gorshkov VG, Li BL (2006) Conservation of water cycle on land via restoration of natural closed-canopy forests: Implications for regional landscape planning. Ecol Res 21:897-906
  6. Meesters AGCA, Dolman AJ, Bruijnzeel LA (2009) Comment on “Biotic pump of atmospheric moisture as driver of the hydrological cycle on land” by Makarieva AM, Gorshkov VG (2007). Hydrol. Earth Syst. Sci. 13:1299-1305.
  7. Pearce F (2009) Rainforests may pump wind worldwide.
  8. http://www.newscientist.com/article/mg20227024.400-rainforests-may-pump-winds-worldwide.html?full=true
  9. Pitman AJ, De Noblet-Ducoudre N, Cruz FT, Davin EL, Bonan GB, Brovkin V, Claussen M, Delire C, Ganzeveld L, Gayler V, Van den Hurk BJJM, Lawrence PJ, Van der Molen MK, Muller C, Reick CH, Seneviratne SH, Strengers BJ, Voldoire A (2009) Uncertainties in climate responses to past land cover change: First results from the LUCID intercomparison study. Geophys. Res. Lett. 36, L14814, 6pp.
  10. Salati E, Vose PB (1984) Amazon Basin – A system in equilibrium. Science 225: 129–138.
  11. Sheil D, Murdiyarso D (2009) How forests attract rain: an examination of a new hypothesis. Bioscience 59:341-347.
  12. Stigter K (2010, Ed) Applied agrometeorology. Springer, Heidelberg etc., in press.
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