Transcript The Changing Land Climate System －Chapter 7.2 from IPCC

The Changing Land Climate System
－Chapter 7.2 from IPCC AR4 WG1
Lei Huang
04/03/2008
Introduction to Land Climate
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Land surface relevant to climate consists of fabric of
soils, vegetation and other biological components.
Land climate consists of internal variables and
external drivers, including the various surface energy,
carbon and moisture stores, and their response to
precipitation, incoming radiation and near-surface
atmospheric variables. All of these can change over
various temporal and spatial scales.
Introduction to Land Climate
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These variables and drivers can be divided into
biophysical, biological, biogeochemical and human
processes.
The exchanges of energy and moisture between the
atmosphere and land surface are driven by radiation,
precipitation and the temperature, humidity and
winds of the overlying atmosphere.
Earth energy balance
Earth’s energy balance
diagram from Kiehl and
Trenberth (1997)
Earth’s energy balance
diagram from Piexoto
and Oort (1992)
Earth water cycle
Earth hydrologic cycle
91
100
Water moves from one reservoir to another by way of processes like evaporation,
condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration,
melting, and groundwater flow.
Earth carbon cycle
Diagram of the carbon cycle.
The black numbers indicate
how much carbon is stored in
various reservoirs, in billions of
tons. The purple numbers
indicate how much carbon
moves between reservoirs
each year. The sediments, as
defined in this diagram, do not
include the ~70 million GtC of
carbonate rock and kerogen.
Dependence of Land Processes and
Climate on Scale
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Temporal variability ranges from the daily and weather time
scales to annual, interannual, and decadal or longer scales. The
land climate system has controls on amplitudes of variables on all
these time scales, varying with season and geography.
Low clouds strongly control surface temperatures, especially in
cold regions where they make the surface warmer.
In warm regions without precipitation, the land surface can
become warmer because of lack of evaporation or lack of clouds.
Details of surface properties at scales as small as a few kilometres
can be important for larger scales.
Spatial Dependence
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Drivers of the land climate system have larger effects at
regional and local scales than on global climate, which is
controlled primarily by processes of global radiation balance.
Land comprises only about 30% of the Earth’s surface, but it
can have the largest effects on the reflection of global solar
radiation in conjunction with changes in ice and snow cover.
At a regional scale and at the surface, additional more localised
and shorter time-scale processes besides radiative forcing can
affect climate in other ways, and possibly be of comparable
importance to the effects of the greenhouse gases.
Urban Effects on Climate
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The consequences of urban development may be especially significant for local
climates. However, urban development may have different features in different
parts of an urban area and between geographical regions.
Buildings cover a relatively small area but in urban cores may strongly modify
local wind flow and surface energy balance.
Besides the near-surface effects, urban areas can provide high concentrations of
aerosols with local or downwind impacts on clouds and precipitation.
Change to dark dry surfaces such as roads will generally increase daytime
temperatures and lower humidity while irrigation will do the opposite.
Changes at night may depend on the retention of heat by buildings and can be
exacerbated by the thinness of the layer of atmosphere connected to the
surface by mixing of air.
Daily and Seasonal Variability
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Diurnal and seasonal variability result directly from the temporal
variation of the solar radiation driver.
Land is more sensitive to changes in radiative drivers under cold
stable conditions and weak winds than under warm unstable
conditions.
Winter or nighttime temperatures (hence diurnal temperature
range) are strongly correlated with downward longwave
radiation. Thus, modification of downward longwave radiation
by changes in clouds can affect land surface temperatures.
Daily and Seasonal Variability
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In moist warm regions, large changes are possible in the
fraction of energy going into water fluxes, by changes in
vegetation cover or precipitation, and hence in soil moisture.
Changes in reflected solar radiation due to changing vegetation,
hence feedbacks, are most pronounced in areas with vegetation
underlain by snow or light-colored soil.
Climate models simulate the diurnal precipitation cycle but
apparently not yet very well.
Coupling of Precipitation Intensities to Leaf Water
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Leaves initially intercept much of the precipitation over
vegetation, and a significant fraction of this leaf water reevaporates in an hour or less.
This loss reduces the amount of water stored in the soil for use
by plants. Its magnitude depends inversely on the intensity of
the precipitation, which can be larger at smaller temporal and
spatial scales.
Leaf water evaporation may have little effect on the
determination of monthly evapotranspiration but may still
produce important changes in temperature and precipitation.
Coupling of Precipitation Intensities to Leaf Water
Rainfall, runoff and evapotranspiration derived from climate
simulation results of Hahmann and Wang and Eltahir.
Hahmann’s results are for the Amazon centred on the equator,
and Wang and Eltahir’s for Africa at the equator. Both studies
examined the differences between ‘uniform’ precipitation over
a model grid square and ‘variable’ precipitation.
Vegetative Controls on Soil Water
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In the absence of leaves, forests appear as especially dry
surfaces with consequent large sensible fluxes that mix the
atmosphere to a great depth.
Trees in the Amazon can have the largest water fluxes in the dry
season by development of deep roots. Forests can also retard
fluxes through control by their leaves.
Such control by vegetation of water fluxes is most pronounced
for taller or sparser vegetation in cooler or drier climates, and
from leaves that are sparse or exert the strongest resistance to
water movement.
Land Feedback to Precipitation
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The interannual variation of precipitation over the Amazon is
largely controlled by the timing of the onset and end of the
rainy season. (Liebmann, 2001)
Removal of tropical forest reduces surface moisture fluxes, and
that such land use changes should contribute to a lengthening
of the Amazon dry season. (Fu, 2004)
More rainfall in the deforested area in the wet season and a
reduction of the dry season precipitation over deforested
regions. (Durieux, 2003)
Properties Affecting Radiation
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Albedo and emissivity are two important variables for the
radiative balance.
Surfaces that have more or taller vegetation are commonly
darker than those with sparse or shorter vegetation.
With sparse vegetation, the net surface albedo also depends on
the albedo of the underlying surfaces, especially if snow or a
light-colored soil.
Modelling the Coupling of Vegetation, Moisture
Availability, Precipitation and Surface Temperature
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The most important factors affected by vegetation are soil water
availability, leaf area and surface roughness.
Shorter vegetation with more leaves has the most latent heat
flux and the least sensible flux.
Replacement of forests with shorter vegetation together with
higher albedo could then cool the surface. However, if the
replacement vegetation has much less foliage or cannot access
soil water successfully, a warming may occur.
Deforestation can modify surface temperatures by up to several
degrees celsius in either direction depending on what type of
vegetation replaces the forest and the climate regime.
Evaluation of Models Through Intercomparison
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Intercomparison of vegetation models usually involves comparing
surface fluxes and their feedbacks.
Both the land and atmosphere models are major sources of
uncertainty for feedbacks. Coupled models agree more closely
due to offsetting differences in the atmospheric and land models.
Coupling strength between summer
rainfall and soil water in models
assessed by the GLACE study (Guo
et al., 2006), divided into how
strongly soil water causes
evaporation and how strongly this
evaporation causes rainfall.
Linking Biophysical to Biogeochemical and
Ecohydrological Components
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Changing soil temperatures and snow cover affect soil
microbiota and their processing of soil organic matter.
Biomass burning is a major mechanism for changing vegetation
cover and generation of atmospheric aerosols and is directly
coupled to the land climate variables of moisture and nearsurface winds.
Aerosols and clouds can reduce the availability of visible light
needed by plants for photosynthesis, thus to affect leaf carbon
assimilation and transpiration.
Summary
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Soil moisture and surface temperatures work together in
response to precipitation and radiative inputs.
Vegetation influences these terms through its controls on
energy and water fluxes, and through these fluxes, precipitation.
It also affects the radiative heating.
Clouds and precipitation are affected through modifications of
the temperature and water vapor content of near-surface air.
How the feedbacks of land to the atmosphere work remains
difficult to quantify from either observations or modelling.