Energy, Entropy, and Change

Change is a fundamental aspect of the Universe. For anything to happen, something has to change: in position, form, energy state, composition, etc., etc., etc.

Energy is the amount of work that can be performed by a force. There are many different ways of categorizing energy. For example, by the medium of storage or the source: chemical, electrical, magnetic, gravitational, mechanical, sound, etc. Alternatively, we can consider potential (stored) and kinetic (moving, being used) energy.

The flow of energy produces change. This can range from an electron moving from one state to another, though the chemical changes in a cell, through the transfer of heat energy from the core of the Earth to the mantle and from the lithosphere to the atmosphere (together driving geology), or between the gravitional energy and fusion energy at balance in a star.

Physicists in the 19th Century worked out the Laws of Thermodynamics describing the transfer of energy and heat through physical systems. We won't deal with all the aspects, but we should highlight two:

It is easy to go from potential energy to kinetic to waste entropic energy. Consider that a spark can light a 30 m (100') tall tree, which releases its stored chemical potential energy as kinetic energy (heat and a little gravitional when it falls), which as the fire dies down dissipates out as waste entropy. But it takes MUCH longer (i.e., it is much more difficult) to take the kinetic energy of sunlight to build up and grow a 30 m tall tree.

The 2nd Law is the reason that perpetual motion machines are impossible. However, there is a common misunderstanding about entropy. Even though total entropy of the Universe must increase, local pockets of the Universe can actually decrease their entropy by doing work (which, consequently, adds to the total entropy of the rest of the Universe.)

Colloquially we tend to equate increasing entropy with increasing disorder. After all--as we all experience in our dorm rooms or offices--we have to expend energy to organize a room but it is easy to disorder it. However, this colloquial view can be misleading. After all, many "orderly" (regularly patterened) structures in the Universe represent the result of systems settling down from a higher energy state to lower energy (and thus more entropic) conditions, such as ripple marks of sand at a beach, snowflakes (higher entropy than energetic but "disorderly" water vapor), ocean currents, and spiral galaxies.

Complexification: complex entities can emerge locally by drawing up free energy available in a system, using and dissipating that energy to generate their own complex structures. Life itself being the best example: using energy flowing through a system to build itself and to do things. Complex entities (organisms, atmospheric systems, plate tectonics, etc.) can exist so long as there is a throughput of energy; if that throughput of energy were to stop (e.g., loss of sunlight for organisms and atmospheres; loss of heat from the core-mantle boundary for plate tectonics) these systems would fail and break down. Some factors (for example, competition and natural selection in living systems; repeated partial melting and cooling in geological systems; etc.) can favor an increase in complexity over time. In these cases, those parts that are better at taking advantage of the throughput of energy become more common than other parts. Or, in other words, the more dissipative structures are favored.

Emergent Properties: In Nature, many phenomena show up on larger scales that are not necessarily predictable from the individual components:

Emergent properties happen from the systems with many similar small components. These properties emerge from the various new ways that the individual components are put together.

Some changes occur through a series of continuous conditions, with no two consecutive steps more or less a dramatic change than any other. But in some systems there are major discrete changes from state to a tremendously different one: a phase transition. For example, liquid water transforms directly into water vapor or into solid ice; it doesn't gradually go through more and more "airy" water grading insensibly into vapor, nor firmer and firmer gel before it solidifies into ice. (Colloquially, the phrase "tipping point" has been introduced for the same concept.) Within each phase the conditions stay largely the same, operating under their own rules: they are metastable.

The scale of time is also important in changes. Some events might happen in nanoseconds, others might take millions of years to occur. Different systems operate in different ways over different scales of time. More about this next semester.

Some changes (events) are rare single discrete happenings, and may be unrelated in any meaningful way to the ordinary operation of a system. For example, the asteroid that hit the Earth at the end of the Cretaceous Period (and thereby bringing the world of the dinosaurs to an end) represents an event: on biologically-meaningful time scales, asteroid impacts are too rare to be considered a normal part of the ecosystem.

Other changes are parts of cycles: the repeated flow of energy and/or matter through a system. Most ongoing natural processes are involved in cycles at some time scale or another. Each cycle involves various components:

Many systems have feedback loops: processes whose continued operation modify the future operation of that same process. We can think to two types of feedback:


(Note that this is contradictory to our colloquial use of the terms: we prefer positive to negative feedback in our personal emotional life, but positive feedback in physical systems can tend to destroy that system!).

A famous case of positive feedback:

Another case of runaway positive feedback is the climate of the planet Venus. Venus may have originally had liquid water oceans. As the early Sun increased in brightness, evaporation increased. H2O is a greenhouse gas, so the atmosphere got hotter, leading to more evaporation. Water vapor high in the atmosphere got split into H2 and O2; the former was lost to space and Venus dried out. On Earth one major sink for CO2 is chemical weathering and deposition: water breaks down minerals in the rocks, using up carbon dioxide in the process, and that weathered sediment (containg carbon) gets transported and sequestered from the atmosphere. On dry Venus the CO2 could not be removed from the atmosphere via weathering, etc., so it built up in the atmosphere, ramping greenhouse effects up even higher. Because of this, average temperatures on Venus are 460°C; 40 C° hotter than Mercury even though it receives only 1/4 as much sunlight!

Finally, one last concept: Homeostasis. Processes in a system can operate together to maintain a stable condition. Often if there are a set of counteracting feedback loops in a system, these work to prevent extreme conditions from overpowering, and thus a "middle of the road" condition can be automatically maintained. (A standard thermostat operates this way.) Homeostasis is very common in biological systems, and are known to operate to some degree in other physical systems.

What is a System? Components of the Earth System
A system is a set of interacting and interdependent entities that form an integrated whole. In the case of the Earth system we are talking about the planet Earth and its various interacting components: living or abiotic (non-living); gaseous or liquid or solid; and so on.

It is common to talk about different "spheres" of the Earth. In most cases, this is a good literal description: overall, the different components of the Earth are essentially a set of concentric spheres of varying density and composition (densest at the core). Only a few of these are not complete spheres, but these happen to be the ones with which we have very direct contact.

Different researchers might recognize various subdivisions of the spheres. Here is a list, running from the most interior (deepest and densest) outwards:

Each sphere has its own particular properties and compositions. They interact with each other in various ways: sometimes as sources, sometimes as sinks, sometimes as fluxes. Residence times and flux rates vary greatly amongst these. We'll be examining some of the key features of each today and in future lectures and projects.

Deep Earth: Core and Mantle
The deep interior of the Earth interacts with the parts in which climate happens and we live, but generally only very slowly. Our knowledge of the interior comes almost exclusively from various forms of remote sensing: despite movies the contrary, we do not have the means to drill deep into the mantle or to the core.


The innermost part of the earth is the core, comprised largely of the metals iron and nickel. The inner core is solid, despite having temperatures over 5700 K: with pressures of 330-360 gigapascals the metals are compressed into a solid crystalline structure. The radius of the inner core is 1220 km. The inner core is surrounded by the 2260 km thick outer core, which is liquid. Motions of this vast inner sea of molten metal generates the magnetic field of our planet. The core is hot because of heat left over from the initial formation of the planet, but also (far more importantly) from radioactive decay of various isotopes and the heat of crystallization of the growing inner core.

Surrounding the core is the 2890 km thick rocky mantle. The mantle represents 85% of the Earth's volume. It is basically solid, but because it is hot and under pressure it can flow like tremendously dense silly putty. The mantle rock is very dense: much denser than the typical rocks found on the continent or the ocean floor.

Heat from the core-mantle boundary is dissipated by the formation of vast convection cells in the mantle:

This motion (moving at rates comparable to finger nail growth: a few cm per year) drives the action of shallower geology. The mantle plays a role in the long term carbon cycle, but is otherwise mostly isolated from climate actions.

Shallow Earth: Lithosphere and Crustal Processes
Technically speaking, the lithosphere is a dynamic subdivision of the Earth, whereas the core and mantle are compositional subdivisions. The mantle is covered by the brittle rocky part of the Earth: the crust (which ranges from about 5 km to 50 km deep). Functionally, however, the outermost mantle shell and the crust move as a single unit, collectively the lithosphere. The lithosphere is divided into various plates, which move relative to each other as a result of the mantle convection cells below. Interaction between plates results in nearly all of geological phenomena:

Such action results in the widening of oceans; the motion of continents; the loss (subduction of older oceanic crust back into the mantle; the driving of volcanoes and earthquakes; the uplift of new mountains; and more.

The lithosphere rides along a mobile asthenosphere, a portion of the mantle where temperature-pressure conditions support the presence of many molten droplets within the rock.

Compositionally, the crust is phenomenally diverse. All sorts of rocks are formed and deposited here. The lithosphere is also a region of various types of activity, continuous or episodic, small-scale to catastrophic. Some of the major ones to consider are:

Lithospheric processes thus both add and subtract material from surficial Earth systems, and these might be as slow as the erosion of a mountain range or as rapid as the eruption of a volcano.

Pedosphere and Cryosphere
These spheres represent the interactions between various other spheres. The pedosphere is the collective term for the world of soils, comprised of parts of the lithosphere modified by the hydro- and biospheres. The pedosphere adds new material fairly rapidly compared to the lithosphere (thousands of years), but these materials (soils) can also be exhausted quite quickly. Different parts of the pedosphere comprise fairly sizeable sinks (and potential sourcss) of greenhouse gasses.

The cryosphere is the collective term for the Earth's ices. The cryosphere contains sea ice (ice floating on the surface of the ocean) as well as glaciers (masses of ice sitting on solid ground), as well as the far more ephemeral snow packs, lake ice, etc. Among glaciers, some are alpine glaciers (that is, found in mountains) and others are continental glaciers (spread over larger surfaces of land). In the present world there are only continental glaciers of any scale in Greenland and Antarctica, but in the past (ice ages) they covered vast regions of the continents.

Glaciers move and grow by the addition of new snow. As snow falls on a region and does not melt entirely, it will build up to eventually form glacial ice. The glaciers move out from their origin (downhill for apline glaciers; spreading out like cold crystalline molasses for continental glaciers). Depending on the rate of ablation (melting plus calving) and the rate of accumulation the front of the glacier may advance, retreat, or stand still.

Glaciers and sea ice can grow or shrink quite rapidly, and their presence or absence are highly important factors in Earth's climate, as we will see in future lectures.

Hydrosphere

Earth's hydrosphere--the realm of liquid water--is dominated by the salty oceans. Ground water represents the vast majority of the liquid freshwater (although less than half the mass of the cryosphere), and surface fresh water is only a tiny fraction:

Ground water represents water which is percolating through rock and soil, while surface may be flowing as streams or puddled into lakes.

As you might imagine, because of its volume and its area (currently about 71% of the surface) the oceans dominate the hydrosphere's interactions with the rest of the Earth's systems. The oceans are actually not of uniform structure. Instead, like the deep earth and the atmosphere, there are different layers with different properties. The shallow ocean contains the vast majority of the living part of the seas; has currents driven largely by winds; exchanges oxygen and carbon dioxide with the atmosphere; and so forth. A large fraction of weather processes has to do with interaction between the atmosphere and the oceans.

The deep ocean is typically saltier, colder, moves more slowly, and represents a vast store of nutrients. Interactions between the shallow and deep ocean tends to be relatively slow. One of the biggest drivers of climate--and a long-term means by which carbon dioxide will be removed from the shallow seas and thus the atmosphere--is the thermohaline oceanic conveyor belt about which we will learn more in a few weeks.

Atmosphere
Earth's atmosphere is mostly N2 (78%), O2 (21%), and Ar (0.93%) in terms of the dry fraction of the atmosphere (in the troposphere (lower levels of the atmosphere) H2O vapor can be up to 4%). None of the three main atmospheric gasses are greenhouse gasses (as we will see later on). Of the remaining dry fraction of the atmosphere, CO2 dominates:

The atmosphere forms a series of shells of decreasing density and (at first) unexpected variation of temperature as you move upwards towards space. The boundaries between the various layers are called "pauses":

The troposphere is where weather happens. In fact, 90% of the atmosphere by mass is in the troposphere:

It is in the troposphere that a lot of mixing goes on: we will see more about this later on.

Differential heating of different latitudes and of land and sea result in the generation of atmospheric convection cells and smaller weather patterns.

However, the higher regions are signifigant, too. Temperature drops with height in the troposphere (as you know when going up mountians, for instance.) But this shifts at the tropopause, when temperatures start increasing. This is due to the re-radiation of UV as infrared (heat) in the ozone layer, which is part of the stratosphere. The stratosphere is relatively dry, and does not mix much with the troposphere. Gasses or particles that make it up to the stratosphere tend to stay there for long periods of time.

The higher layers are less significant for ust: the mesosphere, thermosphere, and exosphere are not significant contributors to the global climate system.

The Living World: Biosphere
Life exists throughout the hydrosphere, in the lower parts of the atmosphere, within the pedosphere and upper lithosphere, and of course living at the interface of all of these. For our purposes here we aren't going to dwell on the details of the biosphere too much, other than to indicate the ways in which it interacts with the other parts of the system.

One major way is photosynthesis. Here is the process greatly simplified:

Or, in other words, photosynthesizers (plants and algae) take in water and carbon dioxide and using sunlight convert these into sugars and waste oxygen gas. Photosynthesizers are the major form of autotroph: organisms that don't eat other organisms.

In constrast, most other life forms (animals, fungi, most "protists", etc.) are heterotrophs: we get nutrients from digesting other life forms. In this reaction, we take in sugars and oxygen and release waste water and carbon dioxide. In general, the actions of the autotrophs and heterotrophs balance each other out, but we will see that there are cases when they reach different equilibrium states.

Other major actions of the biosphere is the creation of the pedosphere and (in the form of broken down organic materials of various sorts) contributions to the lithosphere.

The Technological World: Anthroposphere
Finally, human activity is a very recent but very profound part of the Earth system (as per last week's lecture.) Human actions have affected all the shallow spheres to varying degrees. We will be examining these over the next two semesters.