![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Malay Archipelago Forest Loss to Cash Crops and Urban Expansion
Contributes to Weaken the Asian Summer Monsoon: An Atmospheric Modelling
Study
Forest is lost to large scale cash crops plantations (oil palm, rubber
and acacia, including fallow lands) and urban expansion in the Malay
Archipelago (Indonesia and Malaysia). Land surface properties and fluxes
are changed by deforestation, which thereby modify wind and rainfall.
The impact that results on the Asian summer monsoon has not been
studied, in spite of land-cover change over a climatologically sensitive
region of the tropics. This study investigated the atmospheric response
that is caused by the island surface change resulting from deforestation
into cash crop plantation and urban expansion. It is shown by this study
that deforestation warms the Malay Archipelago, which is caused by an
increase in soil warming that is due to a decrease in
evapotrasnspirative cooling. Island warming agrees well with
in situ and satellite
observations; it causes moisture to converge from the surrounding seas
into Sumatra and Malaya, and updrafts, rainfall, and cyclonic
circulations to spread northeastwards into southern India and the
Arabian Sea as well as a drying anticyclonic circulation over the
Indo-Gangetic plains, Indochina, and the South China Sea, which weakens
the Asian summer monsoon. The modelled monsoon weakening agrees well
with, and tends to enhance, the long-term trend that is observed which
suggests the potential for continued weakening with protracted cash-crop
plantation and urban expansion.
Biodiversity is threatened and irreversible anthropogenic changes to the
biosphere are caused by logging and conversion of tropical rainforest
(e.g., Indonesia) into cash crops such as oil palm, rubber and acacia,
as well as urban expansion (Bradshaw & Muller, 1998; Wilcove & Koh,
2010; Seto et al., 2012;
National Geographic, 2017). Land surface characteristics are altered by
cash-crop plantation and urban expansion (CPU), as well as affecting the
regional climate by regulating energy and moisture fluxes at the
land-atmosphere interface (Bradshaw & Muller, 1998; Rosenfeld, 2000;
Shepherd et al., 2002;
Arnfield, 2003; Dallmeyer & Claussen, 2011; Kusaka et
al., 2012; Mawalagedara &
Oglesby, 2012; Li et al.,
2013; Devaraju et al., 2015).
The highest rate of forest loss, globally, occurs in the Malay
Archipelago of the Maritime Continent (Economist, 2011; Hansen et
al., 2013; Crowther et
al., 2015); for accuracy
Wallace’s term “Malay Archipelago” is used in the paper (Wallace, 1863)
to include only Indonesia and Malaysia of the present. The loss of
forest from 2000 to 2014 is approximately 263,900 km2, an
area that is larger than the UK, mostly over Sumatra, Malaya
(i.e., the Malay Peninsula), and Borneo. At the same time areas
of CPU continue to expand (Grimm et
al., 2008; Ellis, 2009; Seto
et al., 2011; Mertes et
al., 2015; Drescher et
al., 2016; Ratnasari et
al., 2016; United Nations,
2018). CPU area has grown to about 50,000 to 100,000 km2
since the 1990s in the western Malay Archipelago (west of 120oE).
As the vegetation of the island (such as the leaf area, canopy heights
and root depths) and surface properties (such as Albedo, emissivity and
surface roughness) changes, the fluxes such as albedo, emissivity and
evapotranspiration also change (an introductory summary is provided in
the supplemental material). An idealised regional climate model
experiment of “Southeast Asia”
defined as (15oS-17oN and 92o-140oE,
which included Indochina and the Philippians as well as parts of
northern Australia was conducted (Tölle et
al., 2017), by changing all
grids in that region that had 80% or more vegetation into grassland, and
found that the surface would warm and local precipitation would
generally increase. The issues of deforestation that resulted from cash
crop plantations and urbanisation in the Malay Archipelago, in
particular its western portion, and how there may be changes in large
scale wind and precipitation were not addressed. There are 2 climate
models (with horizontal resolution of 1.9o x 2.5o
and 1.9o x 3.8o, respectively) that were used
(Devaraju et al., 2018) to
assess the effects of deforestation into grassland, finding that the
increase that resulted in aerodynamic resistance reduced turbulent
transfer of heat from the surface. The surface would warm over the
tropics, though the degree of warming would differ by 10 times between
the 2 models, from ~11 to 1.11 K. Cash-crops surface properties differ
markedly from those of grasslands and further analysis that targets
specifically the land type changes of the Malay Archipelago are
necessary to identify the effects. That the atmospheric circulation,
including the transport of water vapour and precipitation can be
particularly sensitive to changes in the surface flux near the equator;
have been shown by observations, theory and numerical experiment (Ramage,
1968; Gill, 1980; Neale & Slingo, 2003; Schiemann et
al., 2014). Yet there have
been no studies of the potential impact on the atmospheric circulation
and rainfall by CPU expansion in the Malay Archipelago.
The study was conducted using a suite of moderately large (36 member)
ensembles of atmospheric general circulation model experiments with
different land type changes in order to identify and understand changes
in wind and rainfall that were due to expansion of CPU. This study
demonstrates that the simulated wind and rainfall are sensitive to GPU
changes near the equator, especially in the western Malay Archipelago;
northern Sumatra, Malaya and Borneo. It is shown by the model that
expansion of CPU results in significant island warming, which agrees
with observations, which causes moisture to converge from the
surrounding seas; the impact on wind and rainfall reaches as far west
and north as the Indian subcontinent and southern China, contributing to
weakening of the South Asian summer monsoon has a long-term weakening
trend (Naidu et al., 2009;
Turner and Annamalai, 2012). The weakening has been attributed to
various factors such as:
·
increased cropland in India (Devaraju et
al., 2015; Krishnan et
al., 2016; Paul et
al., 2016),
·
increased aerosols (Krishnan et
al., 2016; Bollasina et al.,
2011; Ganguly et al., 2012;
Sanap et al.,2015),
·
Indian Ocean warming (Roxy et al.,
2015; Saha et al., 2014;
Liang et al., 2017) and/or
·
Tropospheric cooling over China (Menon et
al., 2002; Yu et
al., 2004; Wu et
al., 2005).
According to Huang & Oey a single common thread to these different
attributions is that the various factors combine to contribute to
weakening the summertime land-sea temperature contrast between
continental Asia in the Indian Ocean, either because the land has cooled
or the sea has warmed, or both the cooled land and warmed sea occur
simultaneously. CPU expansion in the Malay Archipelago is indicated by
the model of Huang & Oey to also contribute to the weakening of the
monsoon, though it does so from the equator side of the north Indian
Ocean via the faster-than-ocean warming of the Malay Archipelago
surfaces caused by reduced evapotranspiration of reduced forested trees.
While the island areas of the Malay Archipelago are much smaller than
the Indo-Asian continent, the strategic location of the islands on the
equator and plentiful availability of moisture from the surrounding seas
mean the warming can have a significant impact on the circulation of the
monsoon. The result of the warming is increased rainfall under the
northwestwards-tilted cyclone that emanates from the Malay Archipelago,
and decreased rainfall under the accompanying cyclone to the north. It
increases the sea-island contrast between the Indian Ocean and the Malay
Archipelago, which forces a northeasterly wind anomaly that weakens the
summer monsoon. This study emphasises the need for high resolution
observations and modelling that resolves the warming of the Malay
Archipelago. According to the findings of this study, e.g., the
reanalysis data of the NCEP, do not indicate the warming trend of the
islands that has been shown in this paper for high resolution APHRODITE
and CRU datasets. Of interest is that the increased precipitation and
updraft over the western Malay Archipelago may potentially contribute to
the well-recognised dry bias over the same region in climate models
(Neale and Slingo, 2003; Schiemann et
al., 2014), which often also
have a similarly coarse resolution as NCEP. Potentially, the weakened
monsoon wind may also decrease the frequency and/or weaken the strength,
of the north/northeastwards propagating Madden-Julian oscillation events
that originate over the central-eastern Indian Ocean directly west of
Sumatra during summer (the so-called monsoon intraseasonal oscillation
[MISO]; Goswami, 2012). Then, the reduced MISO events may lead to drier
conditions over northern Bay of Bengal, Indochina, and the South China
Sea. Further research is needed.
While the experiments using the atmosphere-land
surface model that were conducted in this study could isolate the
atmospheric response to expansion of CPU in the Malay Archipelago, the
lack of an air-sea coupling in the model may modify the result. In a
fully coupled model of the atmosphere, land, and ocean, a weakened
summer monsoon would reduce evaporative cooling over the Indian Ocean
and lead to a warmer SST (du et
al., 2009). The temperature between the Malay Archipelago and the
Indian Ocean, in the near surface, and therefore the convergence of wind
onto the islands may be reduced sufficiently to potentially moderate the
weakened effects of the monsoon on CPU expansion.
On the other hand, as marine layer moisture increases as a result of the
warming of the sea, the moisture flux convergence and associated CAPE
that fuel the updraft over the western Malay Archipelago may actually be
enhanced which would then boost the CPU-induced response. Again, further
research is needed.
It is expected that cash-crop will continue into the foreseeable future,
particularly in Indonesia. It is dictated by population growth and
economic growth that urban areas will also continue to expand (Seto et
al., 2012; Seto et
al., 2011; Gamba & Herold,
2009). It is projected by the results of this study that a protracted
weakening of the Asian summer monsoon. It is suggested by the research
in this study that there is urgency in implementing policies that can
soften or mitigate the resulting irreversible and possibly unanticipated
consequence of the altered climate.
Huang, S. and L. Oey (2019). "Malay Archipelago Forest Loss to Cash
Crops and Urban Expansion Contributes to Weaken the Asian Summer
Monsoon: An Atmospheric Modeling Study." Journal of Climate 32(11):
3189-3205.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |