Lesson Plan

Officially Implausible

1944 to 1971, Mexico, India, Pakistan, Stanford and Oslo • Van Vuuren et al, Geoscientific Model Development, 7 April 2026: the architects of the SSP framework formally retire RCP8.5, SSP5-8.5, and SSP3-7.0 as 'implausible' from the CMIP7 framework that will inform IPCC AR7. • May 7, 2026

Before using this lesson plan: Read the full story or listen to the 5-minute narration (works as a bell-ringer).

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Lesson Plan: Officially Implausible

Subject area

STEM: ecology, agronomy, climate science, history of forecasting, philosophy of science.

Recommended audience

High-school biology and environmental science (Years 11-13 / Grades 10-12)

First-year undergraduate environmental studies, agronomy, or atmospheric science

General STEM enrichment for ages 14+

Lesson length

60-75 minutes (one period plus optional follow-up).

Learning objectives

By the end of the lesson, students will be able to:

Distinguish between a "worst-case scenario" and an "expected outcome" in scientific forecasting.

Describe Paul Ehrlich's central prediction in The Population Bomb (1968) and the policy and cultural reaction it produced.

Describe Norman Borlaug's semi-dwarf wheat and explain how the Green Revolution caused observed food production to bend below Ehrlich's predicted famine trajectory.

Distinguish between the IPCC, the CMIP framework, and the SSP / RCP scenarios.

Explain why RCP8.5 / SSP5-8.5 was designed as an upper-bound scenario and identify three reasons it has become implausible (renewables cost, climate policy, observed emission trends).

Read and summarise the central argument of Hausfather and Peters' January 2020 Nature commentary, and Van Vuuren et al's April 2026 Geoscientific Model Development paper.

Discuss how scientific institutions retire flagship predictions when the underlying assumptions break: by quiet replacement rather than formal retraction.

Key terms

Forecast, scenario, projection, prediction

Worst-case scenario, business-as-usual, baseline

Green Revolution, semi-dwarf wheat, CIMMYT, demographic transition

IPCC, CMIP, SSP, RCP

Climate sensitivity, emissions pathway

Implausible (in the technical sense used in scenario science)

Pre-class preparation

Bring or display a copy of The Population Bomb (1968) if available, or a screenshot of its cover.

A printed graph showing world cereal production from 1960 to 2025 (FAO data).

A printed graph showing global CO2 emissions 1990-2025 with the original RCP8.5 trajectory overlaid as a dashed line.

Lesson outline

1. Hook (5 min): "the battle to feed all of humanity is over"

Read aloud the opening sentences of The Population Bomb. Pause. Ask: when do you think this was written? (1968.) When do you think the predicted hundreds of millions of starvation deaths were supposed to happen? (1970s and 1980s.) Did they? (No.)

2. The Population Bomb in context (10 min)

Paul Ehrlich, Stanford butterfly biologist, 35 years old.

Commissioned by David Brower of the Sierra Club to write a paperback in time for the 1968 presidential election.

Published May 1968. ~2 million copies. Tonight Show appearance February 1970 sent it to the bestseller list.

Endorsed by Walter Cronkite, cited by Lyndon Johnson.

Central prediction: hundreds of millions starve in 1970s and 1980s. India, Pakistan, parts of Africa especially.

Why was the prediction widely believed? Population was indeed rising at unprecedented speed (world population grew from 3 billion in 1960 to 4 billion in 1974). Existing wheat varieties were yield-limited.

3. The trajectory bends: Norman Borlaug (15 min)

Norman Borlaug, born Iowa 1914, agronomist, working at the Rockefeller Foundation's Mexican wheat-research station from 1944.

Bred semi-dwarf wheat varieties (Lerma Rojo, Sonora 64) in the 1950s and early 1960s. Why semi-dwarf? Tall wheat collapses ("lodges") under fertiliser-driven heavy heads. Short wheat does not.

Mexico's wheat: 1950 imported half its grain. 1963 became a wheat exporter.

1965: Borlaug ships ~250 tonnes of seed to Pakistan and ~18,000 tonnes to India. Both governments initially resist.

India 1965: 12.3 million tonnes wheat. India 1970: 20.1 million tonnes. Pakistan 1965: 4.6 million. Pakistan 1970: 7.3 million.

India stops importing grain 1971. Pakistan stops importing grain 1968.

Borlaug Nobel Peace Prize 1970. Credited with averting "a famine of unprecedented proportions." Estimated to have saved up to one billion lives.

Discussion: what assumptions did The Population Bomb's prediction depend on? (No new agricultural technology, no policy reform, no change in fertiliser use.) When those assumptions broke, the prediction broke with them. There was no need to declare it "wrong" because the world simply did not follow the projected path.

4. The retirement that wasn't (5 min)

Ehrlich never formally retracted The Population Bomb.

Continued to teach at Stanford until well past his retirement.

Continued (and continues) to argue the bomb will go off, just later than predicted.

The book is still in print and on undergraduate reading lists, increasingly as a sociology of forecasting rather than as a forecast.

Discussion: why might it be hard for an author to retract a famous prediction? Why might the academic community find quiet replacement easier than public correction?

5. The IPCC scenario architecture (10 min)

The IPCC was established in 1988 by the WMO and UNEP. Issues an Assessment Report roughly every 7 years.

Each Assessment Report uses a small number of named emissions scenarios produced by the upstream modelling community (CMIP: Coupled Model Intercomparison Project, run by the World Climate Research Programme).

Scenario sequence:

RCP8.5 and SSP5-8.5 are technically equivalent: both produce ~8.5 W/m² of additional radiative forcing by 2100.

The "8.5" scenario was designed as an upper bound, asking: if no climate policy is enacted and coal use expands roughly fivefold over the century (larger than some estimates of recoverable coal reserves), what happens?

Discussion: a worst-case scenario has a specific epistemic role. It is meant to bracket the upper end of plausible outcomes for stress-testing. It is not meant to be the "expected outcome."

6. How the worst case became the headline (10 min)

Approximately two-thirds of climate-impact studies between 2010 and 2020 used RCP8.5 / SSP5-8.5 as the headline scenario.

Headlines repeated the worst-case projections as if they were forecasts: cities under water by 2100, x degrees of warming, etc.

January 2020: Zeke Hausfather and Glen Peters publish in Nature "Emissions: the business-as-usual story is misleading." Argue:

IPCC AR6 (2021) acknowledges this on page 1-110 of WG1 chapter 1: "the likelihood of high emissions scenarios such as RCP8.5 or SSP5-8.5 is considered low." Then the same report cites SSP5-8.5 a further 1,359 times.

7. The retirement (10 min)

7 April 2026: Detlef van Vuuren et al, in Geoscientific Model Development, publish "The CMIP7 ScenarioMIP design."

Van Vuuren is a senior scientist at the PBL Netherlands Environmental Assessment Agency, professor at Utrecht University, and was the lead architect of the original RCP framework.

The paper formally removes RCP8.5, SSP5-8.5, and SSP3-7.0 from the next round of model intercomparison. The CMIP7 framework will inform IPCC AR7 (synthesis 2029).

Verbatim from the paper: "the CMIP6 high emission levels (quantified by SSP5-8.5) have become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends."

The other scenarios (the moderate ones) remain in place. The architecture stands. Climate-sensitivity ranges are unchanged. What has been retired is the worst case.

The IPCC has not retracted anything. The retirement is by the upstream scenario-modelling community in an academic paper, not by an IPCC press release.

Discussion: why is this retirement structurally similar to the quiet retirement of Ehrlich's prediction? In both cases:

This is not a finding that climate change is not happening, or that the IPCC was lying, or that the moderate scenarios are wrong. It is a finding about the worst-case bracket of the projection space.

8. Wrap and assignment (5 min)

Pose: "Find one example of a scientific worst-case prediction in current public discussion. Identify the assumptions on which the worst case depends. Identify the policy or technological developments that would cause those assumptions to break. Estimate the probability you assign to those developments occurring."

Optional homework: write a 1000-word essay comparing The Population Bomb (1968) and The CMIP7 ScenarioMIP design (2026) as institutional retirements of worst-case predictions. What did each prediction get right? What did each get wrong? What did each prediction usefully accomplish even though its central scenario did not occur?

Discussion questions

Was The Population Bomb wrong? Defend or refute, distinguishing between "the prediction did not come true" and "the prediction was unreasonable to make in 1968."

RCP8.5 was designed as an upper bound, not as a forecast. How should the climate-science community communicate this distinction to journalists and policymakers? What is lost if a worst case is presented as the expected future? What is lost if a worst case is omitted from the literature entirely?

Norman Borlaug never explicitly attacked Paul Ehrlich. Ehrlich never explicitly engaged with Borlaug. What does this tell you about how scientific debates are actually resolved?

The CMIP7 paper retires RCP8.5 because of "trends in the costs of renewables, the emergence of climate policy and recent emission trends." If global climate policy weakened or coal use bent back upward, would the worst case be reinstated? What would that say about the philosophy of scenario forecasting?

The IPCC AR6 cited SSP5-8.5 1,359 times after declaring its likelihood "low." Why might an institution cite a scenario it has explicitly down-weighted? How should a careful reader respond?

The lesson plan above describes the climate-science worst case being retired by its own architects. Suppose you are a science journalist in May 2026. Write three different headlines for this story: one for the Daily Sceptic, one for Nature News, and one for Time Rhymes. Explain the difference.

Extension activities

Data exercise: download FAO global cereal-production data from 1961 to 2024 and plot it alongside the population growth curve. Identify the inflection points around 1965 (Borlaug seeds released to India and Pakistan) and 1970 (Nobel Peace Prize).

Reading exercise: read the abstract and conclusions of Hausfather and Peters (Nature 2020) and Van Vuuren et al (Geoscientific Model Development 2026). Write a 500-word memo summarising the argument of each.

Modelling exercise: using a simple spreadsheet, build a population-vs-food-supply model with two assumptions: (a) constant 1965-vintage wheat yields; (b) Borlaug semi-dwarf yields. Vary the population growth rate and the fertiliser availability. Identify the conditions under which famine occurs.

Writing assignment: write a 1000-word piece arguing either that scientific forecasting institutions should issue formal retractions when worst-case predictions are retired, or that quiet replacement is the appropriate institutional response. Address both the philosophical and the practical considerations.