Ecological Climatology: Biosphere-Atmosphere Interactions & Climate Dynamics

Scholars of the eighteenth and nineteenth centuries lacked the scientific tools to properly ascertain forest influences on climate, but scientists in the latter part of the twentieth century had a new tool - global climate models - with which to study how plants and ecosystems affect climate. Scientific interest over the past few decades in the coupling between climate and life has paralleled the trend by atmospheric scientists to recognize the planet as a system of interacting spheres.

Today, scientists identify four main components of the Earth system: atmosphere, air; hydrosphere, water; biosphere, living things; and geosphere, solid portion of Earth. The geosphere can be subdivided into other spheres. The lithosphere is the solid outer layer of Earth including the crust and upper mantle. Its outermost layer is called the pedosphere, or soil. Some scientists separately identify the cryo- sphere, or frozen portion of Earth. The influence of humans is so prevalent, especially after the Industrial Revolution, that a new sphere, the anthroposphere, has been proposed to describe that part of Earth modified by people for human activities or habitats. Earth’s climate must be understood in terms of a system of interacting spheres (atmosphere, hydrosphere, biosphere, geosphere, and anthroposphere); the energy, water, and biogeochemical cycles that link these spheres; and the interactions with human systems that alter these cycles.

This parallels a progression in atmospheric sciences from (Figure 1.3): atmospheric general circulation models, which considered atmospheric physics and dynamics; to atmosphere-ocean general circulation models, which included the coupling of the atmosphere with models of ocean and sea-ice physics and dynamics; to global climate models, which additionally accounted for hydrometeorological coupling with land; and now to Earth system models, which also include atmospheric chemistry, terrestrial and marine ecology, and biogeochemistry.

Fig. 1.3. Components of the earth system, their processes, and interactions as represented in global models. new processes added to each model are highlighted in italics. the distinction among models is not precise, and the transition among models is in fact blurred. (a) atmospheric general circulation models circa 1970s. these models used prescribed inputs of atmospheric CO₂, other greenhouse gases, and aerosols. they calculated land and ocean physical fl ux exchanges using prescribed soil wetness and sea surface temperature. (b) global climate models circa 1990s. these models added the hydrologic cycle on land and plant canopies. they included ocean general circulation models to calculate sea surface temperature, sea ice, and ocean dynamics. (c) earth system models circa 2010s. these models added the carbon cycle and other biogeochemical processes, anthropogenic land use, wetlands, glaciers and cryospheric processes, atmospheric chemistry, and aerosols

At the intersection of these spheres is an interdisciplinary field of study called biogeoscience that bridges the earth and life sciences (Figure 1.4). Biogeoscience is the study of the interactions between life and Earth’s atmosphere, hydrosphere, and geosphere. It has long been synonymous with the study of biogeochemistry, but is broader and includes the interactions between living organisms and the physical environment and the manner in which organisms modify physical systems.

Fig. 1.4. Biogeoscience as the intersection of the atmosphere, hydrosphere, biosphere, and geosphere. human infl uences are shown in the text outside the spheres

This book examines one element of that science - the commonality between ecological and atmospheric sciences that affect weather, climate, and atmospheric composition. The study of plants and terrestrial ecosystems, and human appropriation of ecosystem functions, is as essential to the study of Earth’s climate as is the study of atmospheric physics and dynamics. This book merges the relevant areas of ecology and climatology, broadly defined to include weather, climate, and atmospheric composition, into an overlapping study of ecological climatology. Ecological climatology is an interdisciplinary framework to understand the functioning of plants and terrestrial ecosystems in the Earth system and the physical, chemical, and biological processes by which the biosphere affects atmospheric processes. A central theme of the book is that plants and terrestrial ecosystems, through their cycling of energy, water, chemical elements, and trace gases, are a critical determinate of climate. Changes in terrestrial ecosystems through natural vegetation dynamics, through human land uses, and through climate change itself significantly affect the trajectory of climate change.

Figure 1.5 illustrates five core areas: the biogeophysical and biogeochemical processes that regulate the exchanges of energy, water, momentum, and chemical materials with the atmosphere over periods of minutes to hours; watersheds and ecosystems and the hydrological and ecological processes that regulate these exchanges over periods of days to months; and landscape dynamics and the ecological and anthropogenic processes controlling the arrangement of plants into communities, the functioning of ecosystems, and temporal changes in response to disturbance over periods of years to centuries.

Fig. 1.5. A generalized scope of ecological climatology showing the biogeophysical and biogeochemical processes by which terrestrial ecosystems affect weather, climate, and atmospheric composition, the watershed and ecosystem processes that govern biosphere–atmosphere coupling, and the role of landscape dynamics in initiating change

Biogeophysics is the study of physical interactions of the biosphere and geosphere with the atmosphere. It considers the transfers of heat, moisture, and momentum between land and atmosphere and the meteorological, hydrological, and ecological processes regulating these exchanges. Momentum is transferred when plants and other rough elements of the land surface interfere with the flow of air. Heat and moisture are exchanged when net radiation at the surface (R_n) is returned to the atmosphere as sensible heat (H), latent heat (λE), or stored in the ground (G). Biogeophysical feedbacks are understood through the surface energy balance:

where S ↓ and L ↓ are downwelling solar radiation and longwave radiation onto the surface, respectively, and S ↑ and L ↑ are the upward radiative fluxes from the surface. Collectively, these four radiative fluxes comprise net radiation. The typical unit of measurement is the flux of energy per unit area (J s^-1 m^-2, or W m^-2).

The surface energy balance highlights several important land-atmosphere interactions. One relates to surface albedo (Figure 1.6a) . An increase in surface albedo, which can occur with loss of vegetation cover, increases reflected solar radiation, reduces the absorption of solar radiation at the surface, and cools the surface climate. Less energy returns to the atmosphere as sensible and latent heat, which promotes subsidence of air aloft and may reduce precipitation. Such albedo influence on rainfall is particularly important in semiarid climates. In cold, snowy climates, tall trees protrude above the snowpack and reduce surface albedo. Vegetation masking of the high albedo of snow creates a warmer climate than in the absence of trees.

Fig. 1.6. Key land–atmosphere interactions. (a) Surface albedo and net radiation. (b) Surface roughness and turbulent mixing. (c) Canopy ecophysiology, stomata, leaf area, and evapotranspiration. (d) Soil moisture, evapotranspiration, and precipitation. (e) the carbon cycle. (f) reactive nitrogen, atmospheric chemistry, and aerosols. (g) aerosols, radiation, and clouds. (h) Biogenic volatile organic compounds, atmospheric chemistry, and secondary organic aerosols

Another important aspect of land-atmosphere coupling is surface roughness (Figure 1.6b). Rough surfaces such as forests generate more turbulence and have higher sensible and latent heat fluxes than smoother surfaces such as grasslands, all other factors being equal. A decrease in roughness length, by decreasing turbulence and aerodynamic conductance, can lead to a warmer, drier atmospheric boundary layer.

Biogeochemistry is the study of element cycling among the biosphere, geosphere, and atmosphere. Carbon dioxide (CO₂), methane (CH₄) , and nitrous oxide (N₂O) are greenhouse gases regulated in part by terrestrial ecosystems. The net storage of carbon in the biosphere in the absence of fire and other losses, known as net ecosystem production (NEP), is the balance of carbon uptake during gross primary production (GPP), carbon loss during plant respiration (R_A), and carbon loss during decomposition (R_H):

The typical unit of measurement is the mass of carbon exchanged per unit area over some period of time (e.g., g C m^-2 yr^-1). The net carbon uptake by plants (GPP - R_A) is known as net primary production (NPP), and the total ecosystem respiration is R_E = R_A + R_H. The signature of terrestrial ecosystems is seen in the annual cycle of atmospheric CO₂ , which has low concentration during the growing season when plants absorb CO₂ and high concentration during the dormant season. It is also evident in the uptake of anthropogenic CO₂ emissions by terrestrial ecosystems. Only about one-half of current anthropogenic CO₂ emissions remain in the atmosphere. Oceans and terrestrial ecosystems absorb the other half. Two key terrestrial feedbacks that regulate this are the uptake of carbon during photosynthesis with elevated atmospheric CO₂ and the loss of carbon during respiration with a warmer climate. Terrestrial ecosystems differ in these feedbacks, their carbon storage, and their capacity to sequester anthropogenic carbon emissions (Figure 1.6e).

Terrestrial ecosystems similarly regulate the concentrations of CH₄ and N₂O in the atmosphere. Ecosystems are also sources of reactive nitrogen (Nr) that alters atmospheric chemistry and produces aerosols; sources of mineral dust and black carbon (soot) from wildfires, which are important aerosols; and sources of biogenic volatile organic compounds (BVOCs), which produce ozone (O₃), increase CH₄ in the atmosphere, and form aerosols (Figure 1.6f-h).

Biogeophysical and biogeochemical processes do not occur in isolation. For example, stomata open to absorb CO₂ during photosynthesis, but in doing so water diffuses out of the leaf during transpiration (Figure 1.6c). Consequently, water loss during transpiration is tied to carbon uptake during photosynthesis. This is seen in studies that relate leaf photosynthesis, transpiration, and stomatal conductance. The physiology of stomata represents a balance between the conflicting goals of maximizing CO₂ uptake while minimizing water loss.

The exchanges of energy, water, and other materials between biosphere and atmosphere depend on the hydrologic cycle. The fundamental system of study in hydrology is a watershed, or catchment. Over long periods of time, it is commonly assumed that water entering a watershed as precipitation (P) either returns to the atmosphere as evapotranspiration (E) or runs off into streams and rivers (R) so that the annual water balance is:

The typical unit of measurement is the mass of water flowing per unit area over some period of time (e.g., kg H₂O m^-2 yr^-1).

The hydrologic cycle influences climate in many ways. One prominent means is through latent heat flux, or evapotranspiration. A decrease in leaf area reduces the surface area for transpiration and for the interception of rainfall (Figure 1.6c). Evapotranspiration from the plant canopy decreases, but soil evaporation may increase. In general, a reduction in evapotranspiration, which occurs, for example, with deforestation, produces an increase in runoff. A decrease in vegetation cover that reduces latent heat flux warms surface climate and may reduce precipitation. This is particularly prominent in tropical deforestation. Wet soil can sustain a high latent heat flux and creates a cool, moist atmospheric boundary layer - conditions that may feed back to increase precipitation (Figure 1.6d). In contrast, dry soil decreases latent heat flux and amplifies droughts and heat waves.

Numerous topographic, edaphic, and ecological features control the hydrology of a watershed. Hydrologic processes such as evapo- transpiration, interception of precipitation by plants, infiltration of water into soil, runoff into streams and rivers, and snowmelt determine soil moisture, snow pack, and saturated areas within the watershed - conditions that vary with a timescale of days to months and that influence surface fluxes.

Terrestrial ecosystems are an expression of an ecological system. All ecosystems have structure - the arrangement of materials in pools and reservoirs - and function - the flows and exchanges among these pools. For example, the carbon cycle is commonly described by pools such as foliage, stem, and root biomass and decomposing soil organic matter. Functions include carbon uptake during photosynthesis and carbon loss during respiration. A variety of ecological processes operating at timescales of days to months influence ecosystem function. The amount of leaf area is an important determinant of photosynthesis, absorption of solar radiation, heat and momentum fluxes, evapo- transpiration, and interception. In many plant communities, the presence of leaves varies seasonally in relation to temperature or moisture stress. Other processes such as litterfall, decomposition, mineralization of organically bound nutrients, nutrient uptake, and the allocation of resources to growth influence carbon storage. Short-term functioning of terrestrial ecosystems is seen in the fluxes of photosynthesis and respiration in relation to the diurnal cycle of solar radiation, temperature, and humidity and day-to-day variability arising from the passage of weather systems. Interannual variability in these functions is seen in the interannual variability of atmospheric CO₂. Long-term functioning manifests in the relationships of climate with net primary production and soil carbon turnover.

Ecosystems are not just static elements of the landscape; they are dynamic. The abundance and biomass of plant species change over periods of years to centuries. Disturbances such as floods, fires, and hurricanes initiate temporal change in ecosystems known as succession. The life history patterns of plants have evolved in part as a result of recurring disturbances. Many plant species are ephemeral members of the landscape, adapted to recently disturbed sites. Others dominate old-growth ecosystems in the late stages of succession. Changes in climate and atmospheric composition affect ecosystems. Long-term changes in temperature, precipitation, atmospheric CO₂ , the chemistry of rainfall, and the deposition of chemical elements onto leaves and soil alter the conditions for vegetation growth. Resulting changes in species composition, ecosystem structure, and nutrient availability each feeds back to affect climate. In particular, forest growth absorbs carbon from the atmosphere while deforestation releases carbon to the atmosphere. Human activities also alter ecosystems through clearing of land for agriculture, farm abandonment, and introduction of invasive species.

The outcome of these physical, chemical, and biological processes can be seen in the influence of the world’s forests on climate , shown in Figure 1.7 (Bonan 2008). Tropical rainforests provide a negative climate forcing that cools climate. High rates of carbon storage reduce the accumulation of anthropogenic CO₂ emissions in the atmosphere (reducing greenhouse gas warming). Evaporation of the plentiful rainfall augments this with strong evaporative cooling.

Fig. 1.7. Climate services of (a) tropical, (b) temperate, and (c) boreal forests in terms of albedo, evapotranspiration, and carbon storage. Symbols indicate a positive climate forcing (+, warming) or negative climate forcing (–, cooling). text boxes indicate other key processes with uncertain climate infl uences. adapted from Bonan (2008)

In boreal forests, strong absorption of solar radiation (low surface albedo) may outweigh carbon sequestration so that the boreal forest warms global climate (positive climate forcing) compared with removal of the forest. The net climate forcing of temperate forests is more uncertain. Reforestation and afforestation sequester carbon, but biogeophysical processes augment or diminish the negative biogeochemical forcing of climate. Low surface albedo, especially during winter in snowy climates, contributes to warming, while rates of high evapotranspira- tion during summer contribute to cooling.

These forest-atmosphere interactions can dampen or amplify anthropogenic climate change. However, an integrated assessment of forest influences entails an evaluation beyond albedo, evapotranspiration, and carbon to include other greenhouse gases, biogenic aerosols, and reactive gases. Forests, in addition to being carbon sinks, also act as sources for aerosol particles. The combined carbon cycle and biogeophysical effect of tropical forests cools climate, but fires, biogenic aerosols, and reactive gases in these forests also affect clouds and precipitation. Biogenic aerosols are also important in boreal forests, where the net forcing from fire must also be considered.

An emerging research frontier is to link the biogeophysical and carbon cycle influences of terrestrial ecosystems with a full depiction of biogeochemical feedbacks mediated through atmospheric chemistry. Terrestrial ecosystems are sources of CH₄ and N₂O. Both are powerful, long-lived greenhouse gases. Additional reactive nitrogen increases N₂O emissions, can produce tropospheric ozone, increase the oxidation capacity of the troposphere (OH radical), decrease CH₄, form aerosols, and increase the deposition of nitrogen onto land (Figure 1.6f). Biomass burning during wildfires injects black carbon (soot) and primary organic aerosols into the atmosphere and also many short-lived gases that affect atmospheric chemistry and air quality (Figure 1.6g). Mineral aerosols (dust) are another important type of aerosol. Plants emit numerous biogenic volatile organic compounds (mostly as isoprene and monoterpenes) that increase the lifetime of CH₄ in the atmosphere and that produce ozone and secondary organic aerosols (Figure 1.6h). Tropospheric ozone is a greenhouse gas that warms climate. The effects of aerosols on climate are complex. Many aerosols scatter solar radiation back to space and cool climate; black carbon absorbs solar radiation and warms climate. Aerosols can also increase cloud brightness (reflecting more solar radiation to space) and suppress rainfall.