Carbon dioxide

In two centuries, carbon dioxide has become the most politically loaded molecule in chemistry. Atmospheric concentration rose from ~280 ppm before the industrial revolution to over 420 ppm in 2024 — a level not seen in the last 800,000 years according to ice cores. This accumulation, mainly due to fossil-fuel combustion and deforestation, is the principal driver of anthropogenic climate warming: CO₂ absorbs outgoing infrared radiation from Earth in several bands (notably 15 µm), re-emits part of it toward the ground, and amplifies the greenhouse effect.

At the molecular scale, CO₂ is a linear molecule with two short C=O bonds (1.16 Å). The overall dipole moment is zero by symmetry, but its asymmetric vibrational modes are IR-active — which is precisely what gives it its climate role. Under pressure and low temperature, it forms "carbon snow" (dry ice, sublimating at -78.5 °C at atmospheric pressure). Above 31 °C and 73 bar, it enters a supercritical phase: liquid/gas boundaries vanish and the fluid becomes an excellent green solvent for coffee decaffeination, perfume extraction and industrial cleaning.

In ecosystems, CO₂ plays a dual role: substrate of plant photosynthesis (CO₂ + H₂O → glucose) and product of aerobic respiration. The natural carbon cycle exchanges ~150 Gt of CO₂ per year between atmosphere, biosphere and oceans — anthropogenic emissions of ~37 Gt/yr, while a minority of the flux, suffice to unbalance the system because they accumulate with no fast natural counterpart. Oceans absorb roughly 30 % of this excess, gradually acidifying surface waters (pH dropping from 8.2 to 8.1 since 1850).