The Changing Land Climate System－ Chapter 7.2 from IPCC AR4 WG1

Lei Huang04/03/2008

Introduction to Land Climate 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.

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.

Introduction to Land Climate

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

Water moves from one reservoir to another by way of processes like evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, melting, and groundwater flow.

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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 Temporal variability ranges from the daily and weather time sc

ales 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 Drivers of the land climate system have larger effects at regio

nal 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 The consequences of urban development may be especially significant for lo

cal 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 Diurnal and seasonal variability result directly from the temp

oral 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.

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.

Daily and Seasonal Variability

Coupling of Precipitation Intensities to Leaf Water

Leaves initially intercept much of the precipitation over vegetation, and a significant fraction of this leaf water re-evaporates 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 In the absence of leaves, forests appear as especially dry surf

aces 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 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 Albedo and emissivity are two important variables for the rad

iative 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

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

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

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 near-surface 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

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.