Human Impacts on the Critical Zone and Soils

Class Discussion (~25 min) - The Critical Zone as a systems model and human impacts and threats Define the term "system" as it pertains to the natural world, and describe the difference between quantitative and qualitative system modeling. Explain how (and when) humans have altered global erosion rates. Recognize some of the consequences of human domination of ecosystems. Discuss how human-induced climate change is expected to alter the hydrologic cycle. Describe what global-scale, human-induced changes can be observed in soils, the role of agriculture in these changes, and some of the consequences of changes to our soils. Formulate and evaluate any adaptive actions humanity can take to lessen negative impacts to the Critical Zone and soils.

Define the term "system" as it pertains to the natural world, and describe the difference between quantitative and qualitative system modeling.

Explain how (and when) humans have altered global erosion rates. 3 waves of soil erosion: initial ag, plows, industrialization (tropical forests)

Recognize some of the consequences of human domination of ecosystems Loss of biodiversity Loss of biomass of individual species Changes to water cycle Climate change Loss of fish Nitrogen fixation

Discuss how human-induced climate change is expected to alter the hydrologic cycle More water in higher latitudes. 10-30 more droughts. Sahara desert may be greener Rising sea levels More severe storms More severe droughts http://downloads.globalchange.gov/usimpacts/pdfs/water.pdf

Describe what global-scale, human-induced changes can be observed in soils, the role of agriculture in these changes, and some of the consequences of changes to our soils. Subsidence Erosion (lost of topsoil) & using unsuitable slopes Use of fertilizer to make up for nutrient loss Removal nutrient Compactions Loss of arable land

Formulate and evaluate any adaptive actions humanity can take to lessen negative impacts to the Critical Zone and soils. No till Use traditional farming Crop rotation Polycultures Population control Use fewer resources/capita

Unit 1.3 Earth System Modeling & Feedback Loops National Research Council. 2011. Graphic Concept by Madeline Ostrander as published in Yes! Magazine.

today Mental models Decision-making System dynamics & limits Feedback loops

Do you make mental models?

Thinking about the environment How do we make decisions? Is pushing harder by expanding a program the best decision?

System Dynamics Complex system: Ozone Control Ozone concentrations depend on initial VOC & NOx concentrations. Initial level NOx (ppbv) Initial level NOx (ppbv) Point A: What would be the best control? Initial level of VOC (ppmv carbon) Point B: What would be the best control? Kinetic modeling results of peak ozone concentrations. Taken from Environmental Engineering Science by Nazaroff and Alvarez-Cohen Engineering Science by Nazaroff and Alvarez- Cohen Kinetic modeling results of peak ozone concentrations. Taken from Environmental

Dynamic Behavior Differences? Dynamic: growth and decay: dP/dt = rP

Growth and Limits Overshoot: example? What model would produce this curve? Dwindling resources: example? What model would produce this curve? 1 = ( ) P t e + t 1

Feedback loops Feedback loops are when A causes or influences or determines B, and then B influences C, and then C in turn influences D, and so on and so on until eventually some X circles back around and influences A again. Mental models rarely contain feedback loops.

Examples of Feedback Loops Human influence on weather. Cerveny and Balling (1998) showed there is a 7-day cycle in aerosol pollutants on the eastern seaboard. Pollutants from automobiles & industry build up during the weekdays and dissipate during the weekends. (There is no natural 7-day weather pattern) Probability of tropical cyclones varies on a 7-day cycle, with chance of rain highest on weekends. Weekly cycles of temp, cloud cover, & other variables have been documented in many parts of the world

Reinforcing feedback loops + R Eggs Chickens + A systems feedback structure Generates its dynamics Chickens Eggs Modified from J. Sterman Sustainability Science: the emerging paradigm & the urban environment

Balancing feedback Structure: + B Chickens Road Crossings + Behavior: Road Crossings Chickens Time

Modeling steps Draw a graph of an important variable that changes over time: e.g. growth, decay, overshoot, or combinations Identify key variables & interconnections Draw a graph of what you expect: reference mode. If you cannot draw what you expect, then you do not have dynamic problem

Stocks and Flows Variables: stock or reservoir (e.g. lake), flux (e.g. flow added) Flow out, mass Flow in, mass time time Stock (mass) Residence time = = ???? ???? At steady state, flow in = flow out

Where to start? Stocks: Stock (mass) Add flows & check units Flow direction: Balance flows

Critical Zone Box Models (Blitz 2006) Reservoirs These are the stocks & can take many forms. E.g. forms in which carbon resides within the earth system--usually expressed in terms of the mass of carbon in gigatons (Gt), (billions of metric tons) Transfer mechanisms or fluxes The flow of energy/matter across a given area. E.g. processes that move carbon between reservoirs. Usually involve physical process and chemical reaction. Transfer rateor fluxes -- The rate of flow of energy or matter across a given area, usually expressed in terms of Gt per year (mass/time) Residence time -- The average length of time during which a substance, a portion of material, an object, etc., is in a given location or condition. (T = Mass/flux) E.g. for carbon in a reservoir -- estimated by dividing the amount of carbon in that reservoir by the transfer rates in and out of it. For example, the residence time for atmospheric carbon dioxide is 760 Gt divided by 60 Gt per year or ~13 years. Biomass -- The amount of organic material in a reservoir.

Lets create a box model for the C cycle. There are three important carbon cycles in the Earth System: The short term organic carbon cycle, with emphasis on the interactions between the atmosphere and the biosphere. It has terrestrial (land) and marine (ocean) components. The long term organic carbon cycle, with emphasis on the formation and destruction of fossil fuels and other sediments containing organic carbon The longterm inorganic carbon cycle with emphasis on calcium carbonate (CaCO3, limestone), by far the largest of the carbon reservoirs. This cycle is linked to the carbonate-silicate cycle, supplying the calcium ions necessary for the formation of limestone. Note: We are separating the cycles by residence time!!!

What are the C reservoirs? The terrestrial biosphere is much more massive than the marine biosphere, largely because of the presence of trees. Soils also contain a large amount of organic material. The influence of the land biosphere is evident in Fig. 8-4. Some portion of the organic matter (CH2O) is eroded from land to the sea. The marine biosphere operates like a 'biological pump'. In the sunlit uppermost 100 meters of the ocean, photosynthesis serves as a source of oxygen and a sink for carbon dioxide and nutrients like phosphorous. Fecal pellets (waists) and dying marine organisms decay as they sink. Their organic content (C-H and C-C bonds) decomposes in the upper (1 km or so) ocean, consuming dissolved oxygen and giving off (dissolved) carbon dioxide. Hence, the upper ocean has much higher carbon dioxide concentrations and lower oxygen concentrations than the waters below 1km, as shown on p. 155. Remember this is ON AVERAGE because the marine biosphere is active only in those limited regions of the ocean where upwelling is bringing up nutrients from below.

What are the C reservoirs? Cont. The long term organic carbon cycle Only a tiny fraction of the organic material that is generated by photosynthesis each year escapes the decay process by being buried and ultimately incorporated into fossil fuel deposits or sediments containing organic material. Through this slow process, carbon from both terrestrial and marine biosphere reservoirs enters into the long term organic carbon cycle. The rate is so slow as to be virtually unmeasurable. Weathering of these same sediments releases carbon back into the other reservoirs.

Diagram of the organic carbon cycle (Bitz 2006) http://www.atmos.washington.edu/2006Q1/211/293xNxWallace_002.jpg.pagespeed.ic.fWr9VQKZBy.jpg Short term C cycle: Processes or fluxes P: Photosynthesis incorporation of C into plants R: Respiration & decay oxidation of organic matter B: Burial of organic matter W: Exposure and weathering of buried organic matter (involves oxidation)

Why do we care about the C cycle? Does it matter which part of the cycle we use C? Human society is burning fossil fuels at a rate many orders of magnitude faster than they were created. The fossil fuel reservoir in is 4700/760 = 6 times larger than the atmospheric reservoir, so if it were all added to the atmospheric reservoir (by the burning of fossil fuels) without any of it being taken up by the other reservoirs, the atmospheric concentration of carbon dioxide would increase by a factor of 6.5. This, of course, is an upper limit and not an actual prediction.

Summary We can look at processes systematically by using a systems approach. By understanding Earth s cycles (e.g. C cycle) we can develop a qualitative systems model graphic. This model can be made quantitative by adding fluxes and stocks. A systems approach can constrain behaviors & provide insights A systems approach can approximate the fluxes, stocks, residence time (e.g. average of each) but it does not give you physical insight into the system. Humans have altered many Earth processes by using a systems approach we can learn the consequences of human actions.