Introduction

Main publication
(abstract)

Overview of the results

Comments and responses
[you are here]
List [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Nobre A. (2006) Interactive comment on "Biotic pump of atmospheric moisture as driver of the hydrological cycle on land" by A. M. Makarieva and V. G. Gorshkov. Hydrology and Earth System Sciences Discussions, 3, S1235-S1240. www.cosis.net/copernicus/EGU/hessd/3/S1235/hessd-3-S1235.pdf
The comment discusses the main biotic pump publication.
Informal title of the comment:
Geography of climate

Living amidst a very active international community of meteorologists dedicated to the understanding of terrestrial biosphere-atmosphere interactions, and having spent 20 years conducting field observations and experiments in Amazonia, I was struck by the biotic pump proposition developed in this paper. Not because I think such a mechanism lacks physical substance, but rather because the source of what the authors call evaporative force appears to me, after reading the explanations in the paper and the debate in this discussion, as self evident, a realization stemming on well known and well established principles of gas physics. Therefore, how could mainstream meteorology, having developed highly sophisticated and powerful numerical representations of the atmosphere, plainly ignore such a key physical force? After some investigation, motivated by the reading of this paper, I found some clues. Although not written in papers, it is not uncommon to hear references to the fact that the representation of tropical convection is poor (understood as having poor physics) and somewhat controversial in virtually all atmospheric models. As a result, the representation of rainfall in the tropical areas tend to be poor. Some NCEP reanalysis data, for example, places more rainfall on Maranhao to the East of the Amazon, a transition to a semi-arid zone, than on some areas within Amazonia to the West. Problems of grid scale, sub-grid phenomena (like cloud representation), and other complexity and non-linearity issues are usually blamed for these mismatches. From our research we know that tropical rainforest trees are extremely efficient evaporators (Tomasella et al, subm, Cuartas et al, 2006). Because of this fact (already known for many decades), most of the available energy in such system is consumed by evaporation, being converted at the surface to latent heat. Resulting from this, surface temperature is drastically lowered if compared to a drier surface elsewhere at the same latitude. Even when the surrounding tropical Atlantic average SSTs are compared to average rainforest surface temperatures it becomes apparent that the Amazon *green ocean* (sensu Andrea et al, 2004) is in average consistently cooler than the tropical Atlantic. If I get it right, cool temperatures at the surface tend to be associated to higher atmospheric pressures, while hotter surface temperatures tend to be associated with lower atmospheric pressures. Following down pressure gradients, wind tend to blow from high to low pressures. But in the Amazon-Atlantic coupling the winds blow consistently from sea to land, precisely the contrary to what the surface temperature (and associated buoyancies) would entail. Therefore, conventional meteorological wisdom, as listed out by Dr. Dovgaluk, produces in Amazonia what looks like a paradox. When confronted with this indication, conventional meteorological wisdom is quick to point either to planetary circulation forcing or, mainly, to convection in Amazonia itself (?) as a source of lower pressure at the surface that then drives winds inland. But, isn't precisely convection that is poorly represented in the models? I am then left with the impending sensation that meteorological models have captured convection using a greater degree of parameterization and a smaller degree of representation for the physics, a numerical mimics of sorts. I can see that the logic of the evaporative force not only would resolve the paradox enunciated above, it also would give a good clue in the understanding of long term climate stability in Amazonia. Paleoclimatologists as well as paleontologists have suggested that South America enjoyed sufficiently stable and humid climate for at least tens of thousands of years (Baker et al, 2001), maybe even millions of years (Hooghiemstra et al, 2002). Given the fact that climate forcing over such long spans of time would hardly justify stable climates on land, due to profound changes in oceans, ice caps and inferred atmospheric circulation patterns, it ensues that South American forests must have enjoyed some special mechanisms to guarantee the availability of moisture on land, even when circulation was unfavorable if it was the case. Revisiting these inferences, now illuminated by the mechanism proposed in the biotic moisture pump, I can find a defensible hypothesis on South America paleoclimate stability, although it still remains a hypothesis in need of proof.

Makarieva A.M., Gorshkov V.G. (2006) Interactive comment on "Biotic pump of atmospheric moisture as driver of the hydrological cycle on land" by A. M. Makarieva and V. G. Gorshkov. Hydrology and Earth System Sciences Discussions, 3, S1258-S1263. www.cosis.net/copernicus/EGU/hessd/3/S1258/hessd-3-S1258.pdf

In response to the questions of A. Nobre:
The questions touch upon a few interesting problems, where the biotic pump approach could, in our view, be fruitfully applied to yield a solution. Within the limited time framework of the present discussion we now try to outline possible directions of further thought that stem from the biotic pump approach.

Questions 1) I reckon that during the time of the Pangea, circa 200 Myears ago, the single continent had vast interior regions completely arid, an aridity that has evolved after the continent earlier had vegetation cover. This aridity extended across the equatorial belt, something unthinkable today due to the inter tropical convergence zone that produces rainfall all around the globe. According with your formulations, and ignoring for now the complexities of atmospheric circulation changes, this ancient aridity in the single continent would be easily explained by the length-scale, distance from the mega-ocean and lack of extensive forests. With the continental splits and drifts, leading to the fragmentation of the huge land mass and also with the appearance of smaller and fragmented oceans, the relative distance of any given land point to the oceans was significantly reduced, leading to changes in rainfall distribution on land. Coastal lands in these fragmented continents, depending of prevailing winds, received geophysical fluxes of precipitable water, even with the interior areas still arid or desert. I previously thought that this availability of moisture in the coastal zones lead to the development of luxuriant forests there, which then started to condition the land hydrology, as you so well have formulated, which on its turn created conditions inland for further expansion of the coastal forests. But in your paper you suggest that deforestation inland could compromise coastal zone forests as well, giving a mechanism through which interior deserts could overcome narrow strips of coastal forests. How then to solve the paradox of a coastal forest evolving into the interior (as one supposes it happened at least once over the course of evolution) with your explanation that a narrow forest bands cannot survive to the effect of an interior desert?

1) We mention in the paper that deforestation in the inner parts of the continent can compromise the existence of coastal forests due to the degradation of the continental biotic pump mechanism. Dr. Nobre asks how then the ancient colonization of the continents by forests could have occured, because it should have had apparently started from a coastal forest band to proceed inland. In short, our response is that the modern state of inner deforestation with a coastal forest remnant is not equivalent to the ancient state with an arid inner part of the continent and first forests colonizing the continent from the coast.

Consider a narrow band of natural forest with high leaf area index, which borders with the ocean on one side and with the desert on the other. Initially, the evaporative force is highest above the forest canopy. The forest sucks in both the oceanic and, even more so, the desert air. The input of arid air from the desert makes the air above the forest canopy drier, so the evaporative force above the forest diminishes. In the result, such a desert-bordering forest will receive less precipitation from the ocean than if it were adjacent to an extensive continental forest cover. We conclude that in order to survive and advance further inland, the desert-bordering forest must be to a considerable degree resistant to the dryness conditions.

The difficult task of colonizing land could not be solved by forests alone. It is likely that the land was first covered by some xeric vegetation. Due to the low, but non-zero, transpiration of these plants and the associated evaporative force, it was possible to lure the oceanic air a bit more inland. The vegetation became more mesic. Then the inner continent got covered by some kind of dry forests, these already ensuring substantial transpiration. Finally, under these well-prepared conditions the rainforests started their march to the continent interior.

So, when modern forests are destroyed by humans in the inner parts of the continents and replaced by rapidly eroding agricultural fields or dryness areas, while there remain narrow rainforest belts near the coast, the situation is not the same as in those ancient times. After the hundred million years' evolution on the rainy continent, with no fires or large-scale droughts, the newly-evolved rainforest plants may have become much less resistant to moisture shortage than those ancient land invaders could have used to be.

This does not mean at the theorem level that a narrow band of forest near the coast is inevitably destined to perish. However, these arguments highlight such a possibility and prepare one to witness the dieback of forests at the coastline with no surprise, if extensive zones of aridity and deforestation are created in the interior of the continent. Note, for example, that in the Australian transect (region 1 in Fig. 1 of our paper) there are no forest remnants on the coast.
 

2) I found your explanations on the generation and maintenance of the Hadley circulation quite interesting. Then I started to imagine scenarios without forests and I felt like not able to solve some puzzles. For example Africa, where the Sahara lies precisely on the desert band (30^o N) associated with subsiding dry air from the Hadley circulation. It is well known that the Sahara has had forests sometime in the past. If that was the case, what happened then with the Hadley circulation over that area? Using your logic, with a forested Sahara there would be evaporative force rising in both middle latitudes and the equatorial zone. What kind of circulation would have existed then? In the same line, most of the present day deserts lie in the 30 degree latitude North or South. Someone might contend your association of human deforestation with desertification by arguing that humans settled everywhere, not only in these two bands, so why deserts did not develop elsewhere (thinking of the Gobi...)?

2) In the first question Dr. Nobre mentioned the areas of equatorial dryness on the primordial Pangea continent; he noted that this would be unthinkable today taking into account the Intertropical Convergence Zone. If we assume that the paleodata about that ancient dryness are reliable, this is another argument in favor of the proposed explanation of Hadley circulation based on the evaporative force. At present one explains Hadley circulation by higher equatorial temperatures making warm air to expand and rise etc., while we claim that the equatorial air rises due to the higher evaporative force on the equator as compared to the tropics. Temperature of the dry equatorial surface of Pangea should have also been higher than in the tropics, however, apparently, the ITCZ did not form there, although it should have formed, according to the traditional account of Hadley cells. In our approach, it is clear why ITCZ and Hadley cells could not form at that time - because it was dry there, the evaporative force was zero, the equatorial air did not rise and did not suck in rains from the tropical regions.

Regarding the particular shape of atmospheric circulation at the time when Sahara was covered by forests, we do not think that it can be responsibly reconstructed in great detail. However, it is easy, in our view, to imagine possible patterns. If one compares atmospheric circulation over the arid areas of North Africa with that over moist forests of Equatorial Africa (see, e.g., Fig. 1 of Nicholson (2000)), one can see that over both areas winds have a meridional velocity component towards the equator. But over the forested area winds blow inland (i.e. with an eastward velocity component), while over the deserts they blow from the continent to the ocean (i.e. with a westward velocity component). It is readily imaginable that when Sahara was covered by forests, winds used to blow inland in this region as well. Note also that equatorial African forests create an additional meridian-oriented convergence zone in the middle of the continent. This feature could have also been present when Sahara was green.
 

As Dr. Nobre mentioned, not all deserts are at 30 ^o North or South. For example, the deserts of Central Asia are at 40-50 ^o N; so are the arid zones of North America, Gobi in China. Additionally, and also importantly, not all areas at 30 ^o N or S are deserts, if we recall climates of China and India at these latitudes. Absence of deserts at high latitudes might have to do with a shorter history of human settlements or a different lifestyle (hunting) of ancient people living in temperate climates as compared to their tropical conspecifics.

On the other hand, some areas should apparently be more prone to desertification than the others, and here modern Hadley circulation can play a role. The evaporative force acting on a territory at 30 ^o N or S (e.g., Sahara) must counteract both the evaporative force of the oceanic surface at the same latitude, as well as the evaporative force of the oceanic surface or forests at lower latitudes. When forests on such an area are disturbed, the evaporative force there diminishes and moist air can be stolen from that area by the equatorial forests and adjacent ocean that feature high evaporative forces.

3) In South America there are no coastal savannas, but in the inner continent they are vast, and have a typical monsoon climate. In your paper you seem to suggest that, theoretically, a dense forest present no limitation in terms of extension of cover on big continents, giving the examples of Siberia and the Amazon. Why would you think then there are savannas far inland in South America? As mentioned in the introduction of this comment, there are other evidences that South America indeed have been covered by almost continuous forests, but why is not the case now? If humans are to blame, why their effect only appeared in central SA?

3) The origin of inland savannahs in South America is an interesting problem, which cannot be resolved without a serious study of all the relevant data, including the precise geography of savannas and the cultural history of humans on those areas. Our general idea is that aridity in the inner parts of the continent should owe itself to disturbances of coastal forests. Coastal forests can still persist as a narrow belt in modern South America, but these are not intact, pristine forests (with no fragmentation, no selective cutting, no burning, no human overexploitation). Weakened coastal forests cannot run the moisture pump the same efficiently as natural forests do. To sum up, recalling the first question of Dr. Nobre, we state that deforestation of the inner parts of the continent weakens the continental biotic pump and makes moisture conditions less favorable for the existence of coastal forests; vice versa and even more critically, disturbance of coastal forests can weaken the continental biotic pump to the degree totally prohibiting forest existence in the middle part of the continent.

Nevertheless, we mention in the paper that when the ocean is warm, it is more difficult (or even impossible) for the forest to pump moisture inland. In this case high transpiration does not serve to suck in moisture from the ocean. Instead, it serves to prevent the formation of strong land-to-ocean winds, which would originate be this transpiration negligible (as during the dry season in classical monsoon climates). These winds, should they originate, would blow forest moisture away to the ocean, thus adding to the gravitational losses of runoff water. Thus, high transpiration during the dry season is a protection against the worse - even if it is impossible to make the oceanic moisture come to the forest, the forest tries at least not to lose its own atmospheric moisture to the ocean. Remarkably, in this situation the more intensively the forest transpires moisture, the less moisture it loses.

Conclusion: I repute your paper as a potential groundbreaker, a rare and welcome contribution to the advancement of the understanding of how land vegetation can be of key importance to the hydrological cycle on land.

References:

Andreae MO., D. Rosenfeld, P. Artaxo, AA. Costa, GP. Frank, KM. Longo, MAF. Silva-Dias, Smoking Rain Clouds over the Amazon, Science, Vol 303, Issue 5662, 1337- 1342 , 2004

Baker PA., GO. Seltzer, SC. Fritz, RB. Dunbar, MJ. Grove, PM. Tapia, SL. Cross, HD. Rowe, JP. Broda, The History of South American Tropical Precipitation for the Past 25,000 Years, Science, Vol 291, Issue 5504, 640-643 , 2001

Hooghiemstra H, T. Van der Hammen, A. Cleef, Evolution of forests in the northern Andes and Amazonian lowlands during the Tertiary and Quaternary in Guariguata M & G Kattan, eds. Ecology of Neotropical Rainforests. Ediciones LUR, Cartago, Costa Rica, 2002

Tomasella J, Hodnet M, Cuartas A, Nobre AD, Water balance of an Amazonian micro-catchment, Hydrological Processes, subm 2006

Cuartas LA, Tomasella J, Nobre AD, Hodnet M, et al, Interception water-partitioning dynamics for a pristine rainforest in Central Amazonia: Marked differences between normal and dry years, Agricultural and Forest Meteorology, in press, 2006

References

Nicholson, S. E.: The nature of rainfall variability over Africa on time scales of decades to millenia, Global and Planetary Change, 26, 137-158, 2000.

Zhou, J. and Lau, K.-M.: Does a monsoon climate exist over South America?, J. Climate, 11, 1020-1040, 1998.