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Home · Sciences de la Terre

The Climate — What Is Established and What Is Debated

The point of this note is to clearly separate three things that are constantly conflated: the physics (solid), the projections (uncertain but bounded), and the policies (choices, and therefore debatable).

1. The physics: what is no longer up for debate

The greenhouse effect is a laboratory fact known since 1856. It was a woman, Eunice Foote, who established it first (a simple experiment: two cylinders in the sun, one filled with CO₂, the other with air; the CO₂ one heats up more). Tyndall measured it precisely in 1859. Arrhenius calculated in 1896 that doubling atmospheric CO₂ would warm the Earth by several degrees — and his estimate is close to today’s values.

The mechanism, in one sentence: solar radiation (visible light) passes through the atmosphere and heats the ground; the ground radiates heat back (infrared); certain molecules — H₂O, CO₂, CH₄ — absorb this infrared radiation and re-emit it in all directions, including downward. The atmosphere acts as a blanket.

Without the natural greenhouse effect, the Earth’s average temperature would be about −18 °C. It is not our enemy: it is what makes the planet habitable. The problem is solely its intensification.

The measured facts — and these are measurements, not models:

  • Atmospheric CO₂: ~280 ppm before the industrial era (measured in air bubbles trapped in ice cores), > 420 ppm today (measured continuously at Mauna Loa since 1958 — the “Keeling curve”). This is the highest level in at least 800,000 years, and probably in several million years.
  • Average temperature: +1.1 to +1.3 °C relative to 1850-1900.
  • Human origin is established through ISOTOPIC SIGNATURE, and this is the decisive point. Carbon from fossil fuels is depleted in ¹³C and contains no ¹⁴C at all (being radioactive, it disappeared millions of years ago). This exact depletion is observed in the atmosphere. This is not a correlation: it is a fingerprint. The additional CO₂ does indeed come from coal and oil.
  • And the test that definitively settles the solar hypothesis: if the Sun were heating the Earth, the entire atmosphere would warm. Yet what we observe is that the lower atmosphere (troposphere) is warming while the upper atmosphere (stratosphere) is COOLING. This is exactly the signature of an intensified greenhouse effect, and the opposite of what a more active Sun would produce. The Sun’s activity has moreover been measured, and it has been stable or in slight decline since 1980, while the temperature keeps rising.

👉 There is no longer any scientific debate on these points. Consensus surveys give > 97% of publishing climatologists, and a 2021 study (Lynas et al.) puts the figure at > 99%.

2. The projections: real, uncertain, bounded

Here, uncertainty does exist — and it’s important to know what it consists of.

“Climate sensitivity” (the warming produced by a doubling of CO₂) is estimated at between 2.5 and 4 °C, with the most likely value around 3 °C. This range has barely moved since 1979 — which is remarkable in itself: forty-five years of research have not shifted the estimate, they have narrowed it.

The IPCC scenarios (the SSPs) range from ~+1.5 °C (emissions cut very quickly) to ~+4 °C and beyond (continued reliance on fossil fuels) by 2100.

⚠️ And it’s important to be clear about what the IPCC is and isn’t. It is not a laboratory: it produces no research of its own. It is a synthesis of the existing literature, written by volunteer scientists, and its summary for policymakers is approved line by line by governments — including Saudi Arabia, Russia, and the United States. This procedure has a counterintuitive and well-documented consequence: it makes the IPCC CONSERVATIVE. Successive reports have underestimated ice melt and sea-level rise. The IPCC does not exaggerate: it smooths things out.

What is honestly poorly predicted: the precise behavior of clouds (the biggest source of uncertainty), tipping points (the Greenland ice sheet, permafrost, the AMOC — the Atlantic current), and regional effects — the global average is predicted well; what the weather will be like in Bordeaux in 2070 is predicted far less well.

3. The objections, taken seriously

“The climate has always changed.” True, and irrelevant. No one denies it; indeed, the causes of past variations are very well understood (Milanković cycles, volcanism, solar variations). The question is not WHETHER the climate changes, but at WHAT SPEED and WHY this time. Exits from glaciation take place over ~5,000 years; we are producing a comparable change in ~150 years, roughly 30 times faster. And it is speed that kills: ecosystems and societies adapt to a rate of change, not to a level. (Analogy: the human body can withstand a 200-meter fall — in an elevator. Not by falling.)

“CO₂ is only 0.04% of the atmosphere.” The argument from smallness. It is worthless: dose is not the same as potency. 0.04% cyanide in your blood kills you. Nitrogen and oxygen, which make up 99% of the atmosphere, are TRANSPARENT to infrared radiation — they play no part at all in the greenhouse effect. It is the trace gases that do all the work.

“CO₂ follows temperature in the ice cores, it doesn’t lead it.” This is correct, and it is a real argument — it must be taken seriously. During glacial exits, the Earth’s orbit triggers the warming, then the ocean releases CO₂, which amplifies the warming. CO₂ is an amplifier there, not a trigger. ⚠️ But this tells us nothing about today: this time, it is CO₂ that arrives first, and we know where it comes from (§1). That a match did not cause last year’s fire does not prove that a match cannot cause today’s.

“The models are wrong.” Verifiable, and verified. Hausen et al. (2019) tested 17 models published between 1970 and 2007 against the temperatures actually observed since. 14 out of 17 correctly predicted the warming. Hansen’s model, presented to the US Congress in 1988, has remained remarkably accurate over 35 years. Climate models are, empirically, one of the most successful predictive exercises in the history of science.

“It’s a conspiracy / scientists have a vested interest.” A researcher who disproved human-caused warming would win a Nobel Prize and instant fame. The professional incentive runs exactly the other way. Conversely, ExxonMobil’s own declassified internal documents show that its scientists had correctly modeled the warming as early as the 1970s-80s — and that the company funded public doubt while incorporating these very forecasts into its industrial planning. The strategy was explicitly copied from the tobacco industry’s playbook; the same firms, sometimes the same people, were involved (see Merchants of Doubt, Oreskes & Conway, 2010). Doubt was not a scientific position: it was a product, manufactured and sold.

4. What is TRULY debatable — and this is where the debate should be taking place

Here, there is no scientific truth, only choices. Confusing the two levels is the most widespread mistake.

  • Nuclear power. Low-carbon, dense, dispatchable; but expensive, slow to build, and it raises the question of waste and public acceptability. This is not a scientific debate, it is a trade-off.
  • Green growth or degrowth? Can GDP growth be decoupled from emissions? A relative decoupling has been observed in several countries; an absolute decoupling, fast enough on a global scale, has never been demonstrated. Both camps have arguments and data. The debate is open and legitimate.
  • Climate justice. Historical emissions are Western; the main victims are elsewhere. The wealthiest 10% of the planet emit roughly half of all CO₂. Who should pay, and how much? This is a question of justice, not physics. (See Justice, John Rawls.)
  • Adaptation or mitigation? Should we invest to reduce emissions, or to absorb the shock? The honest answer is “both,” and the budgetary trade-off is a political choice.
  • Geoengineering (injecting aerosols into the stratosphere). Technically plausible, and probably the most dangerous idea of the century: it does not address the cause, it requires indefinite maintenance on pain of a brutal rebound, and it raises the question of who, on Earth, would have the right to adjust the thermostat.

5. Orders of magnitude, so as not to say just anything

Much climate discourse fails on arithmetic. A few benchmarks:

  • A round trip Paris–New York ≈ 1 to 2 tonnes of CO₂ per passenger. The target compatible with the Paris Agreement is roughly 2 tonnes per person per year, ALL INCLUDED. A single transatlantic flight therefore consumes most of an entire year’s budget.
  • The average French footprint: ~9 tonnes/year. It must therefore be divided by ~4 or 5.
  • Distribution matters more than individual gestures: the trio of transport, housing (heating), and food (especially ruminant meat) dominates by far. Sorting your trash and turning off standby power is useful, and marginal. Saying otherwise is a way of shifting responsibility from the system onto the individual — and the very concept of the “individual carbon footprint” was popularized by a BP advertising campaign in 2004.

Symbolic dimension

Climate is the first truly global problem in human history. CO₂ knows no borders: what a person in China emits warms the Sahel; what an Englishman emitted in 1880 still warms us today. It is a problem no one can escape alone, even by acting perfectly.

It is therefore, structurally, a tragedy of the commons: everyone has an interest in others reducing their emissions, and an interest in not reducing their own. No human institution has ever been designed for this. The state is national; the market is short-sighted; elections last five years and the problem lasts three centuries. (See Microeconomics — Markets and Market Failures: the pure negative externality.)

And it forces us to think something we do not know how to think: an obligation toward people who do not yet exist. Our moral systems are built for our neighbor, our contemporary, our fellow citizen. They have never had to answer for the year 2200. Hans Jonas put it as early as 1979: “Act so that the effects of your action are compatible with the permanence of genuine human life on Earth.” It is a Kantian imperative extended across time — and no one knows how to make it binding.

The real problem with climate change is therefore not scientific. It has been established since 1896. The problem is that we have no institution, no morality, and no political horizon commensurate with what we know.

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