[15 pts] The world’s forests influence the Earth’s climate through complex physical, chemical, and biological processes. Changes in forest area may disrupt such processes and lead to different feedback that can enhance or attenuate climate change. The figure shows an example of a feedback loop. 

In this context, answer the questions:

a. Describe the main biogeophysical process influenced by boreal forests using the feedback loop diagram. Focus on one process and remember to identify the feedback type.

b. Describe the main biogeophysical process influenced by tropical forests using the feedback loop diagram. Focus on one process and remember to identify the feedback type.

[10 pts] The figure shows a soil profile with 18 cm (depth) and two horizons. 

Horizon A is 6 cm deep and has 3% of carbon per mass of soil. 

Horizon B is 12 cm deep and has 1% of carbon per mass of soil. 

The bulk density of the soil is equal to 1.6 g/cm³.

Calculate the total soil carbon stock to 18 cm per area (in ton C/ha). 

Equations and conversion factors:

Stock per Area = Mass / Area

1 ton = 1,000,000 g = 10⁶ g

1 ha = 10,000 m² = 100,000,000 cm² = 10⁸ cm²

Anthropogenic modifications of the Earth system have significant impact on freshwater. Analyze the hydrologic cycle below and answer the questions for each area of interest.

a. [10 pts] Identify the anthropogenic modification indicated in A and explain how it disturbs blue water quantity and quality. Remember to define blue water in your answer. 

b. [5 pts] Identify the anthropogenic modification indicated in B and explain how it disturbs blue water quantity and green water quantity. Remember to distinguish blue and green water in your answer. 

Ozone (O₃) is a gas present in both the stratosphere and troposphere that plays a crucial role in the Earth system and is significantly influenced by anthropogenic activities. 

a. [10 pts] Explain how Stratospheric Ozone Depletion is quantified within the Planetary Boundary Framework, and describe its current status (i.e. whether we are within the safe operating space or have transgressed it). Discuss the importance of stratospheric ozone in your answer.

b. [10 pts] Explain how anthropogenic and natural factors influence stratospheric ozone and how we know that long-term changes in stratospheric ozone are primarily related to anthropogenic activities. 

c. [10 pts] Describe the approach used by current international policies (i.e. the Montreal Protocol and its amendments) to mitigate stratospheric ozone depletion. Discuss whether these policies have been successful and identify at least two assumptions or uncertainties that must be considered when analyzing the long-term effect of these policies. 

[3 bonus points] Conduct a self-assessment and attribute a score between 0 (minimum effort) and 3 (consistent and strategic effort) to yourself. Justify your self-assessment by explaining:

a. How much effort did you put into preparing for this exam? Explain what you did differently from Exam 1 and how that influenced your preparation.

b. We have worked in different groups during the last few weeks. How did interacting with different colleagues influence your learning? Did it help?

c. Do you expect the result of the exam to reflect your proficiency level in the course content? Were you able to demonstrate your learning? Explain what question was more challenging to you and how you think you should have done differently. 

[10 pts] The table shows the diameter at breast height (DBH) for eight hardwood, deciduous trees collected in a 5-meter radius area, as well as the aboveground dry biomass (bm) calculated with the equation from Jenkins et al. (2003). 

Calculate the total tree carbon per area (in ton C/ha). 

Assume that 50% of the aboveground dry biomass is carbon and the ration of 30% of root per aboveground dry biomass. 

Note that the last biomass value (tree #8) is missing in the table, so you will need to calculate that complete this question.

Equations and conversion factors:

For Hardwood (maple, oak, hickory, beech): 

Area of a circle = π r² 

1 ton = 1,000 kg

1 ha = 10,000 m²

Conduct a self-assessment and attribute a score between 0 (minimum effort) and 3 (maximum effort) to yourself. Justify your self-assessment by explaining:

a. How much effort did you put into preparing for this exam?

b. Did you follow any of the advice about successful preparation presented in the syllabus? If yes, what do you think worked well? What could you do to improve your preparation?

c. Do you expect the result of the exam to reflect your proficiency level in the course content? Were you able to demonstrate your learning?

The figure shows the En-ROADS model used during Lab 3 to study climate predictions, mitigation, and policies.

a. [5 pts] Calculate the CO₂ Emission from Energy (in Gton C/year) projected for 2050 using the Kaya Identity Model (see values for each variable in the figure).

Note that: 

Emissions = Population x GDP per Capita x Energy Intensity x Carbon Intensity

1 Mega (M) = million = 10⁶

1 Giga (G) = billion = 10⁹

1 Tera (T) = trillion = 10¹²

1 Exa (E) = billion billion = 10¹⁸

b. [5 pts] Should we act to reduce carbon emissions now to avoid a possible worst-case scenario or should we be certain that climate change is serious before taking action due to high costs of reducing emissions? Explain.

c. [5 pts] Describe one crucial action related to energy supply that should be part of a coherent climate policy to limit global warming.

Systems thinking is a way of understanding complex problems by looking at how different parts of a system are connected and influence each other. When we apply this approach to the Earth’s energy balance, we treat the Earth as a system that receives (input), stores (stock), and releases (output) energy.

a. [5 pts] How albedo and aerosols influence the Earth’s energy balance and temperature?

 b. [5 pts] The table shows the results for two simulations using the “Infrared Light in the Atmosphere” (MODTRAN) model (Lab 2), considering the concentration of atmospheric carbon dioxide (CO₂) in 2000 and 2025. Compute the difference in outgoing infrared radiation (IR) and explain the effect of CO₂increase between 2000 and 2025 influences the Earth’s energy balance and temperature.

Carbon is the chemical backbone of all life on Earth. While the total amount of carbon we have on Earth does not change, carbon moves between the atmosphere and other carbon reservoirs such as rocks (solid and interior Earth), land (biosphere), and oceans, greatly influencing the Earth’s climate. 

a. [5 pts] Describe the two ways carbon moves between the atmosphere and the rock reservoir.

b. [5 pts] Explain how weathering of rocks can act as stabilizing (negative) feedback on atmospheric CO₂, including two factors that influence the rate of weathering.

c. [5 pts] Can the weathering of rocks clean the CO₂ alone from the atmosphere solve the climate warming issue? Explain.