Changing the assumptions

Research Methods — Week 3 Application

Today’s plan

Goals

In the content session, you audited the assumptions behind biomass carbon accounting.

Now: test what happens when you change them.

1. Add transport emissions
2. Add supply chain emissions
3. Change the forest regrowth timeline
4. See how wide the range gets

• [5 min] Open WebR, recap. Today is scenario analysis — arithmetic with a purpose. The exercises provide the emission factors and distances; students focus on sensitivity, not data collection.

Questions?

Submit questions anonymously:

PollEv.com/geol

text geol to 07480 781235

• Open the Poll Everywhere Q&A activity before the session and leave it active throughout.
• Questions appear in your Poll Everywhere presenter view; upvoted questions rise to the top.
• Address during transitions or at the end of concept blocks, not mid-explanation.
• Reset (archive) the activity between sessions so questions don’t carry over from the previous week.

Guided walkthrough

🎓💻 Scenario analysis in R

Starting point: the “official” view

Biomass emission factor at combustion: 0 gCO₂/kWh

(That’s the carbon accounting rule.)

But what if we add the costs the rule ignores?

• [20 min] Walk through two additions live on screen.

Adding transport

# Pellets shipped ~6,000 km from SE United States
# Transport emission factor: ~15 gCO₂/tonne-km (bulk shipping)
# ~7.5 GJ per tonne of pellets → ~2,083 kWh per tonne

transport_co2 <- 6000 * 15  # gCO₂ per tonne of pellets
transport_per_kwh <- transport_co2 / 2083  # gCO₂ per kWh

How much does transport add?

• The answer is roughly 40–45 gCO₂/kWh — non-trivial but small compared to coal (~900 gCO₂/kWh).
• Ask: “Is that a big number?” Compared to what?

Adding the payback period

Trees absorb CO₂ as they grow. But how long does regrowth take?

• Fast regrowth (10 years): biomass is roughly carbon-neutral over the short term
• Slow regrowth (40+ years): biomass has a carbon debt for decades
• Net zero target: 2050 — only 25 years away

If the payback exceeds the policy horizon, biomass is not carbon-neutral when it matters.

• Show a simple cumulative emissions plot: biomass vs coal over time, under different payback assumptions. The lines cross at different points depending on the assumed regrowth rate.

Your scenario analysis

✏️💻 Independent exercises

Exercises

The WebR page has four scaffolded exercises:

1. Transport scenario — estimate CO₂ cost of shipping pellets
2. Supply chain scenario — add harvesting, processing, drying
3. Payback scenario — plot cumulative emissions under different regrowth timelines
4. Extension: Best case and worst case — combine all assumptions

How wide is the range? What does that tell you about certainty?

• [50 min] Main working block. Circulate. The emission factors and distances are provided in the exercises — students don’t need to research them.
• Key learning: the answer changes dramatically depending on what you include. The “official” zero is one end of a wide spectrum.

Discussion

💬 What did you find?

Key messages

• The answer depends on which costs you include
• The timescale matters — carbon-neutral over centuries, but not over decades
• The “official” answer (zero) is a policy choice, not a scientific measurement

• [15 min] Draw these out from the room. Students should feel the force of the ambiguity.

The ethical question

“If you were writing a policy briefing, which scenario would you present? Why?”

Cast your vote

PollEv.com/geol

text geol to 07480 781235

• Activate the poll before showing this slide.
• Most students will say “the full range” — but some will say “best case” or “worst case.” Both answers feed the traitor discussion in Week 4.
• Don’t resolve the tension. Let students sit with the disagreement.

Think about this. You’ll be writing that briefing next week.

• This is the pivot to Week 4. Some students will recognise this as the traitors’ dilemma — a faithful briefing presents the range; a misleading one cherry-picks. Don’t point this out yet.

Wrap-up

Commit your work

Save your code to week3.R. Commit and push.

Next week is the big one:

You’ll write your policy briefing and review each other’s work.

“What assumptions are we making?”

The question that determines the answer.

• Ensure everyone commits. Students should arrive at Week 4 with their scenario analysis code saved — they’ll reuse those figures.