Transcript Document

Ecological aspects of the biotic pump
The role of forests in the water cycle
Victor Gorshkov, Anastassia Makarieva
National University of Colombia, Medellín
5 November 2014
Key features
• Land is
elevated over the
ocean. Because
of gravity, land
continuously
loses liquid
water
• There is little water on land: global runoff could deplete it in just a few years
• Water on land is replenished by ocean-to-land winds in the lower troposphere
• Winds bring moisture evaporated from the ocean (as water vapor)
• As moist air ascends over land, it cools. Water vapor condenses to form rainfall
Biotic pump of atmospheric moisture
Natural forests control the ocean-to-land atmospheric moisture inflow
Moist air
penetrates far
from the ocean
deep into the
continental
interior. Rainfall
is spatially and
temporarily
uniform
Unforested land: (a) water extremes (floods, droughts) closer to the ocean
(b) scarce continental precipitation
How does precipitation vary in space and time in world’s
forested versus unforested regions?
Red numbered arrows: transects from Fig. 2 of Makarieva, Gorshkov, Li (2009) Ecol. Complexity 6: 302
White arrows: transects from Fig. 6 of Makarieva, Gorshkov, Li (2013) Theor. Appl. Climatol. 111: 79
Continental precipitation dependence on distance
from the ocean: annually averaged patterns
Filled symbols: forested
Empty symbols: unforested
regions
Fig. 2 of Makarieva et al.
(2009) Ecol. Compl. 6: 302
In unforested regions
rainfall declines
exponentially with
distance from the ocean,
decreasing 3-fold per
each 600 km on average.
Forested regions enjoy
spatially uniform
precipitation.
• In summer, when forest is active, precipitation on land is higher than over the
ocean: biotic pump works at its most
• Summer rainfall is spatially uniform over several thousand kilometers
• Oceanic precipitation is small in summer and rises sharply in winter despite
lower temperature. The biotic pump is less intense in winter, so moisture
remains over the ocean
Source: Makarieva, Gorshkov, Li (2013) Theor. Appl. Clim. 111: 79 (white arrows from the map)
Moist air cannot penetrate into the
interior of an unforested continent
It is not the absolute availability of moisture that
distinguishes forested regions from unforested
ones.
It is the presence/absence of efficient ocean-toland moisture transport. Unforested land
regions are locked to oceanic moisture year
round, including seasons when moisture is
abundant over the adjacent ocean.
Biotic pump in action:
Example: Deforestation in Western Australia
Andrich and Imberger (2013) International Journal of Sustainable Development &
World Ecology, doi: 10.1080/13504509.2013.850752
Replacement of native vegetation by wheat fields (green line: % native
vegetation cover) resulted in a decline of the inland/coast precipitation ratio
(red line). Poorly vegetated land is locked for moisture.
Biotic pump in action example:
CHOCO low-level jet in Colombia
Notice the recurvature of the
CHOCO low-level jet toward the east
after crossing the Equator, and the
recurvarture of the Caribbean lowlevel jet (CLLJ) toward the southeast
after crossing the Panama isthmus,
both converging on the world-record
rainfall region of western Colombia.
Poveda, G., L. Jaramillo, and L.
F. Vallejo (2014), Seasonal
precipitation patterns along
pathways of South American
low-level jets and aerial rivers,
Water Resour. Res., 50, doi:
10.1002/2013WR014087.
Sahara desert: low pressure but no
moisture convergence
Wind power and air
convergence are
proportional to
condensation rate.
Where condensation is
absent, wind power
generation is low.
There can only exist a
non-dissipating
cyclostrophic circulation.
Chauvin et al. 2009 J. Climate 23: 2544
Non-vegetated versus vegetated regions:
temperature/pressure relationships
Vegetated versus unvegetated regions:
Sahara and East China (20N – 30N)
Vertical proles of air pressure and temperature dierences between
the zonal mean (20N - 30N) in July and (a) Sahara (20N - 30N, 0E
- 20 E) and (b) East China (20N - 30N, 100E - 120 E). P is
precipitation in July in these regions.
Great Horn of Africa and Papua New
Guinea (20N – 30N)
Vertical proles of air pressure and temperature dierences between
the zonal mean in February and (a) Papua New Guinea (-10°S 0°S, 140°E - 150 °E) and (b) Horn of Africa (0°N - 10°N, 40°E 50°E). P is precipitation in February in these regions.
How it works
Condensation-induced atmospheric dynamics
1.
2.
3.
4.
5.
When the atmosphere contains a lot of
water vapor, it is unstable to
condensation.
Vertical displacement of moist air leads
to its cooling.
Water vapor condenses forming clouds.
Local concentration of gas molecules is
reduced.
Local pressure drops. Air starts flowing
towards the condensation area.
The incoming air ascends. If it contains
enough water vapor, condensation
continues as well as the associated air
circulation.
Stages of cloud formation.
Condensation starts where
water vapor reaches
saturation.
Image credit: University of Albany
How it works
! Three important things to remember about condensation !
• Moist atmosphere is unstable to
condensation: the more water vapor it
contains, the higher the probability of
condensation.
• Condensation may occur at an
arbitrarily high rate determined by
vertical velocity of the ascending air.
• Evaporation that replenishes the
atmospheric water vapor store is a
much slower gradual process
determined by solar energy flux.
Probability of rain as a function
of atmospheric water vapor
content.
Sources: Holloway and Neelin (2010)
J. Atm. Sci. 67: 1091; Makarieva et al.
(2013) J. Hydrometeorol. doi:
10.1175/JHM-D-12-0190.1
We thus can compare atmospheric condensation to an
avalanche – a severe outburst of potential energy that suddenly
occurs after a long period of gradual accumulation. Partial
pressure of water vapor is a store of potential energy in the
atmosphere.
How do forests play in?
How it works: biotic pump ecology
Forest: winning the tug-of-war with the ocean
The air flows towards the low pressure
area where condensation occurs.
For there to be a stable ocean-to-land
inflow of moist air, condensation should
predominantly occur over the forest.
In this case the forest is the acceptor
area (receiving moist air) and the ocean
is the donor area (supplying moist air).
If condensation predominantly occurs
over the ocean, then the ocean steals
moisture from the continent.
High evaporation from the forest cover
allows the forest to win the moisture
“tug-of-war” with the ocean.
Forest properties important
for biotic pump functioning
• High leaf area index (many evaporating surfaces per unit
ground surface area) enables forest to enrich the
atmosphere with water vapor more efficiently than does
the ocean
• Large height of trees prevents formation of violent winds
(that can be otherwise driven by condensation over flat
surfaces, e.g. hurricanes)
• Controlled emission of biogenic condensation nuclei
enables forests to suppress or initiate condensation to
stabilize moisture flow largely independent of external
conditions.
How it works: biotic pump ecology

Evaporationcondensation
cycle
Immediately after precipitation
the atmosphere is dry (water vapor
has condensed and precipitated).
Winds are negligible. The
atmosphere starts to slowly regain
its water vapor via evaporation.


The pressure difference forces
the air from the ocean to flow
towards the forest. In the result,
moisture evaporated from the
ocean precipitates on land.
Liquid runoff is compensated by
aerial inflow.
 
Owing to its high leaf area index, the
forest enriches the atmosphere with
water vapor more rapidly than does the
ocean. Total air pressure in the area
slowly grows.
Even a small difference in
evaporation rates matters
given the sharp rise of rainfall
probability with increasing
water vapor content.
Once a critical amount of
water vapor has accumulated
over the forest, condensation
starts. Local air pressure drops.
Pressure-rainfall dependencies in unforested (a,b) and
forested regions (c) in Brasil
Makarieva, Gorshkov, Sheil,
Nobre, Bunyard, Li (2013)
Journal of
Hydrometeorology doi:
10.1175/JHM-D-12-0190.1
Prevalence of high pressure during rainy days as dependent on the length m
(days) of the smoothing interval in (a) region A, (b) region B, and (c) region C.
The value of s+ indicates the statistical significance of the effect, with s+ > 3 and
s+ <-3 indicating, respectively, significant prevalence of higher than average and
lower than average pressure during rainy days. The closer the region is to the
forest, the more prevalent higher than average pressure during rainy days.
Changes in circulation following
deforestation: Maritime Continent Region
Conversion of pristine forest
to palm plantation on Borneo
Deforestation and rainfall decline in
Borneo: forest cover loss from 80% to
~50% since 1950s. Kumagai et al.
(2013) Hydrological Processes doi:
10.1002/hyp.10060
Changes in circulation following
deforestation: Maritime Continent Region
In the last 59 years rainfall over the maritime continent decreased, while sea level pressure
increased in agreement with the biotic pump concept. Walker circulation has slowed
down. Sea surface temperatures increased. Tokinaga et al. 2012 J. Climate 25: 1689.
Biotic pump and modern meteorology
Theory: quantitative relationship between independently
measurable variables derived from fundamental physical laws.
Y=A*X
If we know A and X, we can predict Y.
Model: quantitative relationship between measurable variables
derived from empirical fitting some analytical functions to
observations.
For example, observations show that Y varies linearly with X (we
do not know why). Proportionality coefficient A is derived
from approximating the data by a straight curve. Then Y can be
“predicted” using the model Y = A*X and values of X outside
the original fitting interval.
Biotic pump results belong to the realm of theory.
What is wrong with the numerical
circulation models?
• Models are based on the physical premise that air circulation is
driven by temperature differences (warm air rises, cold air
descends).
• Large computer power allows one to incorporate much
observational evidence to obtain a reasonable fit of the model
with observations. This appears possible because in some
important cases the areas with high condensation rate are also
the warmest (like e.g. the equator or the hurricane core).
• Missing a correct theoretical basis for condensation-driven
winds, numerical models can be fitted to reproduce mean wind
velocities, but fail to correctly describe the water cycle.
• This deficiency of modern models in accounting for the various
terms in the water budget has been officially recognized by the
meteorological community to be one of the “real holes of
climate science” (Schiermeier 2010 Nature 463: 2010).
Example 1:
Runoff  Atmospheric moisture import
Water cycle on land:
Precipitation = Evaporation +
Atmospheric Moisture
Import
Atmospheric Moisture
Import = Runoff
Runoff measurements represent an independent check of the
model performance with respect to the atmospheric moisture
transport. For the Amazon, the largest biotic pump on Earth,
model derived moisture convergence appears twice less than the
observed runoff. This twofold discrepancy is equivalent to
complete neglect of the biotic pump and must involve a
significant overestimate of local evaporation (Makarieva et al.
2013 J. Hydrometeorol. doi: 0.1175/JHM-D-12-0190.1).
Example 2: Problems predicting hurricane intensity
Correct physics: hurricanes are driven by
atmospheric condensation (avalanche-like
release of potential energy of water vapor).
Hurricanes form when the atmosphere
contains a lot of water vapor.
Incorrect physics (models):
hurricanes are driven by heat
extraction from the ocean.
They develop when the ocean
is warm.
Usually the two conditions are fulfilled simultaneously: when the ocean is warm,
it means that the atmosphere is warm as well and contains a lot of water vapor.
The rare circumstances with a warm ocean and little water vapor (the latter exported
from the region of hurricane formation) makes for a test of the two explanations.
In August 2013, National
Atmospheric and Oceanic
Administration predicts a
hyperactive hurricane season
based on observed high ocean
temperatures.
Example 2: Problems predicting hurricane intensity
ACE
NOAA predicted 13-19 Named Storms (actually 12), 6-9 Hurricanes
(actually 2), 3-5 Major Hurricanes (actually 0), Accumulated Cyclone Energy
(ACE) range of 120%-190% of the median (actually about 35%).
“…the NOAA forecast can only be characterized as a complete bust. This
spectacular failure challenges our knowledge on the factors that 'control'
seasonal TC activity.” (Anonymous NOAA source)
Deforestation consequences for
the water cycle
• Deforestation leads to disruption of the ocean-to-land
atmospheric moisture flow.
• Moisture that would be otherwise uniformly distributed over
the continent precipitates instead in the coastal zone.
• This causes (a) severe floods in coastal regions and (b)
precipitation shortages in the continental interior.
• Floods in the coastal zone are followed by droughts since flood
water cannot be properly accumulated in soil (unlike the
rainwater gradually supplied by mild rains under normal
conditions).
• Large-scale conversion of primary forests (evergreens) to
successional forests (deciduous) in European Russia disturbs
the seasonal cycle of precipitation and temperature.
Some recent examples in Eurasia
“… (a) severe floods in coastal regions and (b) precipitation
shortages in the continental interior.”
Flood in Europe June 2013
Flood in Khabarovsk August 2013
Key feature: a high or low pressure system gets spatially “blocked” due
to the lack of normal atmospheric flows.
Complex dynamics! Temperate and boreal
forests had been re-growing in the 20th century
Global transition to the use of fossil fuels in the end of the 19th century
eased the anthropogenic pressure on forests. Forests had been re-growing
naturally on extensive previously deforested areas. More recently, global
economic growth has interfered with this recovery.
“East-European forests” 2004 Vol. 1, Ch. 5. Nauka, Moscow.
Recently this trend has changed.
In contrast, CO2 concentration has
been gradually increasing on a global scale.
Fitzjarrald et al. 2001 J. Clim. 14: 598
Data provided by Global Surface Intelligence Ltd. (GSi).
Above-ground biomass density is estimated from satellite data
(MODIS Normalised Difference Vegetation Index (NDVI), with >700
observations for each pixel for each year) calibrated using 10 million
field plots from US Forestry Service's Forest Inventory and Analysis.
Spatial resolution: 1 km.
Forest cover change in Russia 2001-2012
< -100
-100 to -30
Little change (-30 to +30)
>+30 Mg ha-1 (11 yr)-1
Yellow regions indicate areas with significant forest loss due to either cutting or
fire. Green areas indicate areas with significant forest re-growth. Rapid re-growth
characterizes early stages of forest recovery from disturbances (fire, cutting).
Above-ground biomass change in European Russia 2001-2012 (GSi data)
Yarie, Van Cleve 1983 Can. J.
For. Res. 13: 767
Chertov et al. 1999 Forest
Ecol. Manage. 119: 189.
< -100
-100 to -30
Little change (-30 to +30)
>+30 Mg ha-1 (11 yr)-1
Large areas in the North-West have been recently disturbed and are re-growing.
In the South forest loss dominates over re-growth.
Forest is a working mechanism,
not just a store of carbon
Not only the total forest area, biomass or any other
cumulative parameter, but also the stage of forest
development and the species composition that are
crucial for biotic pump functioning
Forest recovery
after disturbances
involves drastic
changes of important
parameters like leaf
area index, tree
height, foliage
seasonality
(evergreens,
decidious).
The stage of forest development is crucial for biotic pump functioning
Climax stage: evergreen trees (spruce)
Evergreen trees keep their foliage ready to work
year round. Immediately upon the onset of first
sunny days in spring they start pumping
moisture inland. They keep doing so until the
first frosts late in the autumn. In the result, the
moisture supply is homogeneous in time during
the entire vegetation season. Seasonal
temperature changes are gradual.
Leafy trees and herbs must develop foliage first before
they can pump moisture. Until then, there is little moisture
transport from the Atlantic ocean.
Thus, spring features drastic temperature changes: (1) first
it is very cold, (2) then land warms rapidly in the absence
of moisture transport from the Atlantics, then green leaves
flush and there is cool and very wet weather that can stay
until there is active photosynthesis (causing floods in
June). Then leaves become inactive in September, and the
moisture transport ceases.
Early successional stage:
deciduous trees (birch)
Conflict between hydrological competence
and commercial value of forests
The relative rate of biomass increment is highest at the earliest succession
stage; then it gradually slows down to become zero in the climax
forest: climax forest, as a healthy adult body, does not either gain or
lose "weight" (biomass). Biomass grows steadily from early succession
to climax. Because of these peculiarities, forestry industries are used to
value most the successional stages at about 50-70 years after
disturbance: the biomass is already there to harvest, while biomass
increment rate is not yet zero. Thus, from the commercial viewpoint, it
is prudent to keep all forests within 50-70 years of the last cutting. But
the water regulation potential of early successional stages and evenaged stands is low.
A conflict exists between the modern commercial value of a forest and the
forest's ability to regulate the regional water cycle and to be selfsustainable: these parameters cannot be maximized simultaneously.
Conclusions
• Condensation drives winds.
• Natural forests, based on the genetic
information of species in the ecological
community, perform an ultra-complex
control of condensation processes. This
control aims to stabilize a high-throughput
water cycle on land.
• The biotic pump should be urgently taken
into serious consideration by all concerned
about continental water security.