Ecosystems and Climate. Common Science

Chapter Summary. When viewed from space, Earth is seen as a blue marble. The dominant features of the planet are the blue of the oceans and the white of the clouds traversing the atmosphere. It is an image of fluids - water and air - in motion. Indeed, the study of Earth’s climate is dominated by the geophysical principles of fluid dynamics. With closer inspection, however, one can discern land masses - the continents - and the plants that grow on the land. The blue of the oceans gives way to the emerald green of vegetation. Weather, climate, and atmospheric composition have long been known to determine the floristic composition of these plants, their arrangement into communities, and their functioning as ecosystems.

Earth system scientists now recognize that the patterns and processes of plant communities and ecosystems not only respond to weather, climate, and atmospheric composition, but also feedback through a variety of physical, chemical, and biological processes to influence the atmosphere. The geoscientific understanding of planet Earth has given way to a new paradigm of biogeosciences. Ecological climatology is an interdisciplinary framework to study the functioning of terrestrial ecosystems in the Earth system through their cycling of energy, water, chemical elements, and trace gases. Changes in terrestrial ecosystems through natural vegetation dynamics and through human uses of land are a key determinate of Earth’s climate.

Common Science. 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 the Greek scholars Aristotle (ca. 350 bce) and Theophrastus (ca. 300 bce) and their books Meteorologica and Enquiry into Plants, respectively, but their modern beginnings trace back to natural history and plant geography. Naturalists and geographers of the seventeenth, eighteenth, and nineteenth centuries 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 temperature creates latitudinal zones of tropical, temperate, and arctic vegetation and in 1874 proposed formal vegetation zones with associated temperature limits. This provided an objective basis to map climatic regions, and in 1884 Wladimir Koppen used maps of vegetation geography to produce climate maps. His five primary climate zones shared similar temperature delimitations as de Candolle’s vegetation (Table 1.1) .

The close correspondence between climate and vegetation is readily apparent, and many secondary climate zones such as tropical savanna, tropical rainforest, and tundra are named after vegetation. Although vegetation is no longer used to map the present climate, it is a primary means to reconstruct past climate from relationships of temperature and precipitation with tree-ring width, pollen abundance, and leaf form.

Despite shared origins, twentieth-century advancement of ecology and climatology proceeded not as an integrated and unified science, but rather in the typical disciplinary framework of science into specialized fields of study that favored reductionism. Plant ecology splintered into topical studies of physiology, populations, communities, ecosystems, landscapes, and biogeochemistry. The study of the atmosphere became organized around spatial scales of micrometeorology, mesoscale meteorology, and global climate and topical fields such as boundary layer meteorology, hydrometeorology, radiative transfer, atmospheric dynamics, and atmospheric chemistry.

With lack of communication across disciplines, ecologists and climatologists can draw different insights from the same observations. Pieter Bruegel the Elder’s painting “Hunters in the Snow” exemplifies this (Figure 1.1) . The painting has been used in climatology textbooks to illustrate climate change (Lamb 1977, pp. 275-276; Lamb 1995, pp. 233-235). Bruegel painted this scene in the winter of 1565 and it depicts, from a climatologist’s perspective, Bruegel’s impression of the severe winters of that era. It was the beginning of prolonged artistic interest in Dutch winter landscapes that coincided with an extended period of colder than usual European winters. Ecologists have similarly used this painting to illustrate the ecological concept of a landscape (Forman and Godron 1986, pp. 5-6). Instead of a visual record of an unusually cold climate, the ecological perspective perceives an expression of the core tenets of landscape ecology. From an ecological point of view, the painting depicts heterogeneity of landscape elements, spatial scale, and movement across the landscape.

Fig. 1.1. “hunters in the Snow” (Pieter Bruegel the elder). reproduced with permission of the kunsthistorisches museum (Vienna)

Earth, too, has long been viewed differently by ecologists and climatologists. Ecologists have historically seen weather, climate, and atmospheric composition as external forcings that shape plant communities and ecosystem functions. The manner in which ecosystems influence weather, climate, and atmospheric composition was not examined in classic ecology textbooks. Similarly, climatology textbooks emphasized the physics and fluid dynamics of the atmosphere, not the vegetation at the lowest boundary of the atmosphere.

The advent of global models of Earth’s climate in the 1970s and 1980s altered the disciplinary study of ecology and climatology. These models require a mathematical representation of the exchanges of energy, water, and momentum between land and atmosphere. These processes are regulated in part by plants, which with their leaves, stomata, and diversity of life do not conform to the mathematics of fluid dynamics. Atmospheric scientists developing climate models had to expand their geophysical framework to a biogeophysical framework (Deardorff 1978; Dickinson et al. 1986; Sellers et al. 1986) .

The ongoing evolution of climate models to models of the Earth system is marked by recognition of the central role of terrestrial ecosystems in regulating Earth’s climate through physical, chemical, and biological processes and the critical influence that human appropriation of ecosystem functions has on climate (Pitman 2003; Bonan 2008; Arneth et al. 2010, 2014; Levis 2010 ; Seneviratne et al. 2010 ; Mahowald et al. 2011). Models of Earth’s land surface, including its terrestrial ecosystems, for climate simulation have expanded beyond their hydrometeorological heritage (with emphasis on surface energy fluxes and the hydrologic cycle) to include carbon, reactive nitrogen, aerosols, anthropogenic land use, and vegetation dynamics. These models are important research tools to study land-atmosphere interactions and climate feedback from ecological processes.