Monday, October 29, Coming up: Short-term organic carbon cycle (p ) Marine organic carbon cycle and nutrient limitation (p )

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1 Monday, October 29, 2017 Assessment available until Friday Topic - The Carbon Cycle (Chapter 8) Overview of the carbon cycle (p ) Reservoirs (p ) Steady State (p ) Residence Time (p 154) Oxidized and reduced carbon (p 154) Coming up: Short-term organic carbon cycle (p ) Marine organic carbon cycle and nutrient limitation (p )

2 Several interesting stories this week Tesla provides Puerto Rico Children s Hospital with electricity Big tropical Philippe storm moving up the East Coast tropical-storm-phillipe-forms-cuba-heading-florida Update 1.2 million without power thousands-without-electricity-in-northeast-storm/

3 Back to the Carbon Cycle

4 Statement of the problem How much carbon is there? Where does it come from and where does it go? How much do humans contribute to the changes in carbon in the atmosphere? Remember CO 2 and CH 4 are greenhouse gases, and (by definition), they are a large factor in global warming! Later this term we ll examine how predicted changes compare to actual changes (Chapter 15)

5 Recall ice core records show that CO 2 in the atmosphere has oscillated from 200 to 280 ppm over four major ice ages CO 2 = 400 ppm. higher than any value found in any ice core To understand the past, present, and future, we need to know how carbon is cycled through the Earth System

6 Bold = size of reservoir Arrows mean fluxes in or out

7 Flow diagram depicts how a carbon atom flows from one reservoir to another

8 Carbon Reservoirs

9 Oxidized vs. Reduced Carbon The oxidation state of carbon refers to the nature of the atoms bonded to that carbon. The more hydrogens atoms that are attached to carbon the more reduced it is The more oxygen atoms that are attached to carbon the more oxidized it is. CO 2 is a highly oxidized form of carbon CH 4 is a highly reduced form of carbon To oxidize carbon, one typically only has to add oxygen and burn it (e.g., combustion or respiration)

10 Reservoirs Scientists have various ways to express the timescales for exchange of nutrients between reservoirs Definition of reservoir a place (or form) where nutrients are stored. We depict these as boxes in our diagrams. Example: The main two forms of carbon in the atmosphere are carbon dioxide (CO 2 ) and methane (CH 4 ). There is 400 parts per million of CO 2, or about 870 Gtons. First, a few definitions we use metric ton (i.e., tonne), which is 2240 lbs. Also, note that the book shows a figure with 760 Gt of carbon as CO 2. This was when the CO 2 mixing ratio was 340 ppm. It s now over 400 ppm and still rising. There is 2 parts per million of CH 4, or about 4.5 Gt of carbon as CH 4. It s unclear why the book says 10 Gtons I ll check on this.

11 Respiration and aerobic decomposition: CH 2 O + O 2 CO 2 + H 2 O Photosynthesis CO 2 + H 2 O CH 2 O + O 2

12 In summer, plants and trees take up CO 2 In winter, plants and trees lose their leaves, and those leaves that have fallen off in previous years become buried and decay, reforming CO 2 We expect to see an annual cycle in CO 2 decreasing values in summer and rising values in winter But we also expect the value at the end of the year to return to the same value as the previous year that is, if the system is in steady state We addressed these exchange processes in Homework 1 earlier this term

13 Which hemisphere dominates the annual CO 2 cycle?

14 CO 2 decreases in summer CO 2 increases in winter

15 Carbon Reservoirs and Steady State

16 Is Atmospheric CO 2 in Steady State?

17 Steady State and Residence Times

18 A note on residence times, lifetimes, exchange times, and doubling times. These terms are used loosely in the geosciences. They essentially mean the same thing, in that one calculates them by taking the size of a reservoir say 760 Gton of carbon and dividing by a rate of loss (also called a sink ), such as photosynthesis. If a reservoir is in steady state, one gets the same answer if they use the size of the reservoir and the rate of increase (also called a source ), such as respiration and decomposition. The doubling time is the same because one takes as the numerator the reservoir size and assumes it doubles (i.e., add another 760 Gtons to get 1520 Gtons, or a doubling, in our example above).

19 We interchange these terms only because we are trying to convey different kinds of properties or concepts. For example, when we ask how long would it take to double the reservoir? we would loosely call this the doubling time. If we asked how long does a carbon atom spend in the atmosphere? we would talk about the residence time. And finally, if we said How quickly does CO2 exchange between the atmosphere and the surface? we might use the term exchange time. The key is recognizing that whe a term is used, it implies a particular ratio of a reservoir size (usually in Gtons) and a flux (usually in Gtons yr -1 ). You will do some example calculations (all simple, if you just follow the words and logic) in Homework 4.

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