Scale in the Earth System

On the importance of perspective.

"All things have their season, and in their times all things pass under heaven. A time to be born and a time to die. A time to plant, and a time to pluck up that which is planted. A time to kill, and a time to heal. A time to destroy, and a time to build. A time to weep, and a time to laugh. A time to mourn, and a time to dance. A time to scatter stones, and a time to gather. A time to embrace, and a time to be far from embraces. A time to get, and a time to lose. A time to keep, and a time to cast away. A time to rend, and a time to sew. A time to keep silence, and a time to speak. A time of love, and a time of hatred. A time of war, and a time of peace." Ecclesiastes 3:1-8

I was first introduced to the concept of scale while studying geology during my undergraduate studies. Geology is the science that catalogs how the Earth system has evolved throughout its 4.54 billion years of existence using clues gained from the planet's esoteric, paradoxical, and dreadfully incomplete rock record. The immensity of geologic time is difficult to grasp. Scale plays a major role in how I conduct my graduate research. Practitioners in the field of biogeochemistry study the chemical cycles of biologically important elements like carbon, nitrogen, and phosphorus in the biosphere and try to understand how these elements weave their way through the complex web of feedback loops that characterize Gaia. An appreciation for scale in space and time is essential when meditating on Earth system processes.

Geological actions operate on timescales measured in the thousands, tens of thousands, hundreds of thousands, millions, tens of millions, hundreds of millions, and billions of years. They manifest themselves spatially, in the form of geological structures, at scales ranging from millimeters to hundreds of kilometers. The Earth marches to a different beat than what we are used to. These rhythms are not easy to conceptualize. What does a million years mean when our lives are measured in the tens of years? This isn't an easy question to answer. I don't have a good response. Regardless of how we feel about it, long timescale processes are the ones most characteristic of geology. The movement of plates and resulting displacement of continents, which occure on hundreds of million and billion year timescales, and the volcanic release of carbon dioxide and its draw down by interaction with terrestrial minerals, which occurs on tens of millions of year timescales, are examples of such processes.

I study the carbon and oxygen cycles. These cycles are quantitatively linked to one another through photosynthesis, in the creation of biomass and oxygen gas alongside the drawdown of carbon dioxide, and respiration, in the breakdown of biomass into carbon dioxide alongside the drawdown of oxygen. Timescales are important to keep in mind in this work. Changes in Earth's climate modulated by shifts in the Earth's orientation in space relative to the sun are a primary driver of glacial-interglacial cycles in atmospheric carbon dioxide levels, which occur on tens of thousands to hundreds of thousands of year cycles, while the balance between photosynthetic production of oxygen and the respiratory consumption of oxygen in on short timescales, measured in years, decades, and centuries, control atmospheric oxygen levels in the modern. Both of these processes can be explored using measurements from ice core, tree, and lake, oceanic records, as well as from modern Earth surface samples, which is the domain of paleoclimatology.

Spatial scales are also important to keep in mind. Last year I learned about the nitrogen cycle for a collaboration I contributed to. It is the most complex biogeochemical cycle on Earth. Diazotrophs, nitrogen-fixing bacteria, convert inert atmospheric nitrogen into reactive forms of N2, which is metabolically available to photosynthesizers, who need it to create biomass. Reactive nitrogen is in turn cycled among various nitrogen-bearing chemical species by a wild assortment of microbial metabolisms, and is ultimately converted back into inert, unreactive N2 by denitrifiers. Metabolisms involved nitrogenous compounds are most widespread in marine anoxic zones, parts of the ocean where the water has very little oxygen. The spatial distribution of these regions is controlled by physical and biological factors. The movement of ocean currents, the bathymetry of the seafloor, as well as ubiquitous temperature and salinity gradients, alter the equilibrium oxygen content in seawater. Microbial communities in turn alter this underlying distribution of oxygen by metabolizing in response to the chemical gradients of other bio-essential nutrients at local, regional, and global scales. To a first approximation, physical processes control the global and mesoscale distribution of oxygen while microbial processes control it at local scales.

A crucial question to ask when looking at any Earth system process or material, be it in the geosphere, hydrosphere, biosphere, atmosphere etc., is the following - on what spatial and temporal scales does it vary relevant to the question we are trying to answer? Are we dealing with a process that primarily acts at extremely small spatial and temporal scales, like changes in aqueous chemistry mediated by the microbial metabolic processes? Or are we dealing with a process that acts on extremely large spatial and temporal scales, like changes in the magnitude and orientation of Earth's magnetic field? Or does the process act on various scales at once and reflect the balance of short, medium, and long spatial and temporal scales? The answer to this question determines the scope of the study and the results that will be uncovered.

Often we aren't interested in all of the processes that affect a given observation, since only a few are relevant to the question at hand. Here's a concrete example. I am currently developing a computational chemical model of the Earth's atmosphere that explores the isotopic composition of oxygen given different boundary conditions. Broadly, atmospheric O2 is regulated on geologic timescales by the balance between organic matter burial, which constitutes a source of O2, and the oxidation of minerals at the Earth's, which constitutes a sink of O2. While this process is important if you want to understand how the isotopic composition of atmospheric O2 changes on long timescales, it isn't an important factor on glacial-interglacial or millennial-decadal time scales. Since I am interested in the isotopic composition of relatively modern atmospheric oxygen, this geologic cycle is described in less depth relative to other mechanisms incorporated into the model.

To understand the world at the scale relevant to your question, much of its complexity will be left out. Be mindful that this has occurred, and do not ignore what you do not see out of convenience. Truly, the map and the territory are not the same. Keep in mind that every "observation" has an implicit model of the world associated with it. Figure out what it is and change your perspective if need be.