To understand the transition, a few simple physical notions are needed: how heat is trapped, how much carbon we can still emit, and how energy is measured.
2.1The greenhouse effect, in one picture
The Sun heats the Earth's surface, which re-emits this energy as infrared. Greenhouse gases (CO₂, methane (CH₄), nitrous oxide (N₂O), water vapour) absorb part of this radiation and send it back towards the ground. Without them, the Earth would be at −18 °C: the greenhouse effect is natural and vital. The problem is its amplification by our emissions.
2.2Radiative forcing and the carbon budget
Radiative forcing measures the energy imbalance (in W/m²) added by greenhouse gases. The carbon budget is the total amount of CO₂ humanity can still emit while staying below a warming threshold. For 1.5 °C, only about 130 GtCO₂ remain from 2025, barely three to four years of emissions at the current rate.
2.3+1.5 °C: a climate average, not weather
2024 was the first calendar year to exceed +1.55 °C above pre-industrial levels. But a single year is not the climate: the underlying trend is around +1.3 °C. This distinction is essential: crossing 1.5 °C in one year does not mean the Paris goal is lost for good, but that we are getting dangerously close to it.
2.4The carbon cycle
The oceans and the biosphere absorb about half of our CO₂ emissions. The rest accumulates in the atmosphere, for centuries. This is why the CO₂ already emitted will keep warming the planet long after we stop producing it: there is inertia.
2.5The basics of energy
- Primary energy (extracted: crude oil, uranium, wind) vs final energy (consumed: electricity, petrol).
- Power (the kW, instantaneous) vs energy (the kWh = power × time). 1 kWh = 3.6 MJ.
- Capacity factor (actual / maximum output): solar ~11-15% in France, onshore wind ~25%, offshore ~40%, nuclear ~70-90%.
- EROI (energy returned on energy invested): how much energy a source produces compared with the energy it cost.
The "100%" trap and intermittency
A 1 GW solar plant does not produce as much as a 1 GW reactor: the capacity factor changes everything. Comparing installed capacity without talking about actual output is the most common mistake in the energy debate.
Sources
IPCC, AR6, Working Group I; Indicators of Global Climate Change (carbon budget); WMO/Copernicus (2024 temperatures); RTE and ADEME for capacity factors.
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