Chapter summary

This chapter examines the biogeophysical coupling between terrestrial vegetation and climate. “Daisyworld” is first introduced as a simple model of coupled climate–vegetation dynamics. The Daisyworld model illustrates the potential for regulation of climate by vegetation. More realistic examples of climate–vegetation interactions occur regionally in response to gradients in soil water or temperature. The gradient from tropical rainforest to tropical deciduous forest to savanna to desert represents increasing aridity. This gradient is not only a response to soil moisture but also feeds back to affect climate, especially precipitation. An example of this is in northern and western Africa, where climate model simulations demonstrate that the expansion of vegetation into desert in response to increased rainfall feeds back to increase rainfall. Another example is the boreal forest–tundra ecotone. The transition from forest to tundra is related to cold temperature. Numerous studies show that the northward migration of trees in response to climate warming feeds back to accentuate the warming; loss of tree cover with a cold climate reinforces cold temperatures. These studies indicate widespread changes in vegetation structure and biogeography in response to climate change are likely to themselves change climate. Biogeochemical feedbacks are considered in Chapter 29.

Chapter summary

This chapter gives an overview of the various climates found on Earth at the macroscale, mesoscale, and microscale. Macroclimate is the large-scale climate over 2000 km or more resulting from geographic variation in net radiation, the resultant transport of heat by the atmosphere and oceans, and high and low surface pressure belts. Macroclimate zones are often distinguished based on temperature and precipitation. One such classification scheme is that of Köppen, which illustrates the major climate zones found on Earth. In contrast, mesoclimates and microclimates are regional and local climates, respectively. Microclimates are climatic features typically smaller than 2 km. A forest has a different microclimate than an adjacent clearing. Mesoscale is between microscale and macroscale, covering atmospheric processes at scales of 2–2000 km. Regional (mesoscale) climates are illustrated in terms of the effect of topography on solar radiation, temperature, and precipitation in mountains. Lakes and oceans also greatly influence local and regional climate, with generally mild temperatures and reduced temperature variability compared with inland climates. Differential heating between land and ocean also results in a local circulation known as a sea breeze.

Global climate zones

Although no two places experience exactly the same climate, several different generalized climate zones can be recognized. Figure 2.1 (color plate) illustrates one such climate classification – the Köppen classification as modified by Trewartha (Finch et al. 1957; Trewartha 1968; Ahrens 1998). This scheme utilizes five major climate zones based on temperature and precipitation.

Chapter summary

The structure and composition of vegetation and the functioning of terrestrial ecosystems, which at a local scale are shaped by environmental factors such as temperature and moisture, are also influenced by global climate patterns. This is seen in the biogeography of vegetation and in the global carbon cycle, especially net primary production. The geographic distribution of biomes is closely correlated with measures of temperature, precipitation, and evapotranspiration. So, too, are annual net primary production and decomposition rates. Long-term changes in climate alter the biogeography and functioning of Earth's vegetation. Global models of terrestrial ecosystems provide a quantitative framework to understand planetary ecology and the role of terrestrial ecosystems in the climate system.

Biogeography

The broad influence of climate on macroscale ecology is evident when the complexity and diversity of terrestrial communities and ecosystems are reduced to a few biomes, or broad classes of vegetation (e.g., forest, grassland, shrubland, or desert). The natural vegetation of Earth has a distinct geographic pattern that corresponds to climate zones (Fig. 2.5). The close correspondence between climate zones and biomes is readily apparent as climate zones such as tropical savanna, tropical rainforest, and tundra are named after vegetation (Table 6.1).

Tropical evergreen forests (tropical rainforests) are the dominant vegetation in hot, wet equatorial regions of South America, Africa, Southeast Asia, and Indonesia. In these regions, monthly temperatures are warm year-round, precipitation is abundant, and there is little seasonal variation in temperature or rainfall. Plants grow rapidly and continuously.

Chapter summary

The principles that determine the temperature, energy balance, and the photosynthetic rate of a leaf also determine those of plant canopies when integrated over all leaves in the canopy. These processes are greatly affected by the amount of leaf area, quantified by leaf area index. The vertical profile of leaf area in the canopy affects the distribution of radiation in the canopy and the absorption of radiation by leaves. With low leaf area index, plants absorb little solar radiation, and the overall surface albedo is largely that of soil. The absorption of radiation increases with greater leaf area index, and surface albedo responds more to the optical properties of foliage rather than soil. The integration of leaf processes over the light profile is central to understand the scaling of leaf processes to the canopy. The carbon uptake by a canopy is the integration of the photosynthetic rates of individual leaves, accounting for variations in light and microclimate with depth in the canopy. Similarly, canopy conductance is an aggregate measure of the conductance of individual leaves. The profile of leaf area in the canopy also greatly affects turbulence within the canopy. Counter-gradient fluxes and intermittent turbulence are common and preclude the use of simple models based on diffusion along a concentration gradient. The effects of vegetation on surface fluxes can be modeled by treating the soil–canopy system as an effective bulk surface, or “big leaf,” or by separately modeling soil and vegetation, the latter often represented as several layers of sunlit and shaded leaves.

Chapter summary

This chapter continues the discussion of the preceding chapter, focusing on the arrangement of individual plants in populations and multiple species in communities. The concept of a niche is central to the understanding of the organization of species across the landscape. A niche represents the components of the environment to which a species is adapted. Evolutionary pressures have led to niche differentiation in which species differ in preferences for resources and have different functional roles in communities. This is seen in the dispersion of species along resource gradients. Though vegetation can be classified into distinct communities of species, most communities intergrade continuously and exist within a continuum of populations. Species are not grouped along environmental gradients in distinct natural associations, but rather are distributed individualistically according to their own physiology and life history patterns. The manner in which plant populations are arranged into recognizable communities is critical to understanding the response of vegetation to climate change. A plant community that exists today may have no analog under different climate. Particular species enter and dominate a given locale based on prevailing climate and other environmental conditions, the occurrence of disturbance such as fire, and their own life history patterns.

Niche and species abundance

The environment is spatially heterogeneous, varying in light, temperature, soil water, nutrients, and other conditions. Just as physiological processes vary depending on the specific environmental conditions encountered by a plant, so too do plant species thrive over a specific range of environmental conditions.

Chapter summary

The fluxes of sensible and latent heat occur because turbulent mixing of air transports heat and moisture, typically away from the surface. These fluxes are described in terms of transport by mean motion (typically a small term) and by turbulence (the dominant term). Turbulent transport is quantified by the covariance of temperature and moisture fluctuations with vertical velocity fluctuations. Turbulent fluxes of sensible heat, latent heat, and momentum are related to logarithmic profiles of temperature, humidity, and wind near the surface. Monin–Obukhov similarity theory describes profiles and fluxes in the surface layer of the atmosphere and is used to derive the aerodynamic resistances that regulate turbulent fluxes. These resistances and the profiles of temperature, humidity, and wind in the surface layer depend in part on roughness length and displacement height, which vary greatly among land cover types.

Turbulence

The exchanges of sensible and latent heat between land and atmosphere occur because of turbulent mixing of air and resultant heat and moisture transport. The flow of air can be represented as discrete parcels of air moving vertically and horizontally. These parcels have properties such as temperature, humidity, and momentum (mass times velocity). As the parcels of air move, they carry with them their heat, moisture, and momentum. Turbulence creates eddies that mix air from above downward and from below upward and transports heat and water vapor in relation to the temperature and moisture of the parcels of air being mixed (Fig. 14.1).

Chapter summary

Earth's climate must be understood in terms of a system of several interacting spheres and the energy, water, and biogeochemical cycles that link these spheres. The main components of the Earth system are: atmosphere, air; hydrosphere, water; cryosphere, frozen portion of Earth; biosphere, living things; pedosphere, soil; and humans. People are important agents of environmental change through land use and land cover change and co-option of the hydrologic cycle and biogeochemical cycles. Greater understanding of Earth and its climate requires that all components of the Earth system – physical, chemical, biological, socioeconomic – be considered.

Atmosphere

The atmosphere is the air that surrounds Earth. It is comprised primarily of nitrogen (N2) and oxygen (O2), which together account for 99% of the volume of the atmosphere (Table 2.1). Many other gases occur in trace amounts that when combined comprise less than 1% of the volume of the atmosphere. Although they occur in minor quantities, some of these gases play an important role in Earth's radiation balance through the greenhouse effect.

Greenhouses gases are poor absorbers of solar radiation, but are strong absorbers of longwave radiation. As a result, the Sun's radiation passes through the atmosphere and heats the surface, but the longwave radiation emitted by the surface is absorbed by greenhouse gases in the atmosphere. The majority of this longwave radiation is emitted back to the surface, warming the surface. This re-emission of longwave radiation back to the surface is the greenhouse effect that warms the surface.

Chapter summary

Whether residential or commercial, infill development, or on the urban fringe, urban land uses represent an alteration of the natural landscape. The vast tracts of impervious roads, sidewalks, driveways, parking lots, roofs, and walls alter the surface energy budget and the hydrologic cycle. The most prominent characteristic of the urban climate is the urban heat island, by which the air temperature in cities can be several degrees warmer than that of rural landscapes. The heat island arises due to the reduced emission of longwave radiation by the city surface, much of which is trapped by tall buildings, reduced latent heat flux and increased sensible heat flux because of greater impervious surface area, and from the storage of heat in urban materials during the day that is released at night. Cities also generate more runoff compared with rural landscapes because of greater impervious surface area. Vegetated parks within cities ameliorate the urban heat island by reducing the impervious surface area and allowing for infiltration and evaporation of rainfall.

Urban morphology

Large cities in the United States have a distinct physical morphology (Table 30.1). Single-family residential housing is the dominant land use in all cities, ranging from 49% to 78% of total land area. Apartment housing is generally modest (about 5% of area) except in Baltimore and Philadelphia, where row houses are abundant. Industrial zones range from 10% to 22% of total area while commercial zones range from 7% to 17%.

Chapter summary

The change in land cover from human uses of land is an important forcing of climate. The conversion of natural vegetation to agricultural land, through the effects on net radiation, the partitioning of this energy into sensible and latent heat, and the partitioning of precipitation into soil water, evapotranspiration, and runoff, can alter climate. Among the surface characteristics altered by land-cover change are albedo, surface roughness, leaf area index, stomatal conductance, root depth, and soil texture and structure. Land degradation in arid and semi-arid climates increases surface albedo, reduces evapotranspiration, and may contribute to low rainfall in these regions. Extensive deforestation and land clearing have altered the climate of vast regions of Australia. The clearing of tropical forests for agriculture creates a warmer, drier climate. The clearing of temperate forests and grasslands to cultivate crops is generally thought to cool climate, primarily because of higher albedo. However, the climate signal associated with crops is complicated and related to the timing of crop planting, growth, and harvesting relative to the phenology of natural vegetation. Irrigation leads to a cooler, moister climate. The influence of historical land-cover change on climate needs to be considered as a climate forcing in addition to traditional forcings such as greenhouse gases, aerosols, solar variability, and ozone. Future projected land-cover changes due to human land uses are also likely to alter climate.

Ecological climatology – concepts

Scientific origins

Ecology is the study of interactions of organisms among themselves and with their environment. It seeks to understand patterns in nature (e.g., the spatial and temporal distribution of organisms) and the processes governing those patterns. Climatology is the study of the physical state of the atmosphere – its instantaneous state or weather, its seasonal-to-interannual variability, its long-term average condition or climate, and how climate changes over time. These two fields of scientific study are distinctly different. Ecology is a discipline within the biological sciences and has as its core the principle of natural selection. Climatology is a discipline within the geophysical sciences based on applied physics and fluid dynamics. Both, however, share a common history.

The origin of these sciences is attributed to Aristotle (circa 350 BC) and Theophrastus (circa 300 BC) and their books Meteorologica and Enquiry into Plants, respectively, but their modern beginnings trace back to natural history and plant geography. Seventeenth, eighteenth, and nineteenth century naturalists and geographers saw changes in vegetation as they explored new regions and laid the foundation for the development of ecology and climatology as they sought explanations for these geographic patterns. Alexander von Humboldt, in the early 1800s, observed that widely separated regions have structurally and functionally similar vegetation if their climates are similar. Alphonse de Candolle hypothesized that latitudinal zones of tropical, temperate, and arctic vegetation are caused by temperature and in 1874 proposed formal vegetation zones with associated temperature limits.

Chapter summary

Atmospheric and oceanic processes and their coupling dominate much of the study of seasonal-to-interannual climate variability. However, land surface processes contribute significantly to climate variability. Soil water is an important determinant of seasonal precipitation forecasts. The recycling of precipitation in evapotranspiration can lead to a positive feedback by which wet soils pump more moisture into the atmosphere, which enhances rainfall and further wets the soil. Conversely, dry soils, with low rates of evapotranspiration, can reduce rainfall. The retention of precipitation by soil and the influence of soil water on subsequent evapotranspiration contribute to and amplify interannual precipitation variability over tropical and middle latitudes. The presence of snow is also an important initial condition required for accurate forecasts. The high albedo of snow-covered surfaces prevents the surface from warming during the day. On warm days, a large portion of net radiation at the surface is used to melt snow. By cooling the surface and reducing the land–ocean temperature contrast, snow can influence summer precipitation in monsoon climates. The seasonal emergence of leaves in spring imparts a discernible signal to air temperature. Greater latent heat flux with leaf emergence cools air temperature.

Soil water

Atmospheric model simulations have routinely demonstrated the importance of soil water, through its effect on evapotranspiration, for climate simulation. These simulations typically manipulate soil water or more generally soil wetness (the effect of soil water on evapotranspiration).

Chapter summary

The previous chapters focused on the mean state of the atmosphere and climate zones. However, the realized temperature and precipitation in any year can deviate markedly from the long-term climatology. Some years are warmer or colder than normal; some are wetter or drier than normal. This is a realization of climate variability at seasonal-to-interannual timescales. Major modes of climate variability include the El Niño/Southern Oscillation and the North Atlantic Oscillation. El Niño is a large-scale warming of sea surface temperature in the eastern tropical Pacific. It is associated with a reduced Walker Circulation that weakens the trade winds, with below normal rainfall in the western tropical Pacific and above normal rainfall along the equator east of the dateline. The changes in tropical sea surface temperature and atmospheric circulation affect temperature and precipitation worldwide. The North Atlantic Oscillation is associated with changes in the strength of the Icelandic low and subtropical high in the North Atlantic during winter. This affects the strength of middle latitude westerlies, with consequences for winter temperature and precipitation from eastern North America to Europe and Asia.

Floods, droughts, and heat waves

Climate is often described in terms of climatic means, typically defined by 30-year averages. For example, the mean temperature for the month of July in central England during the period 1971–2000 was 16.5 – (Fig. 7.1). However, the monthly temperature varied considerably from this 30-year mean in any given year.