A short primer on the greenhouse effect and climate change

The basic physics of the greenhouse effect is simple. The planet’s surface and atmosphere can be considered a system in steady state (incoming energy equals outgoing energy), with space being the ultimate heatsink.¹ The incident solar light peaks in the visible region (500 nm) of the electromagnetic spectrum, and is nearly balanced by infrared radiation emitted from high-altitude atmospheric gases (H₂O, CO₂, etc.). The atmosphere essentially acts like multiple bandstop filters that blocks most of the electromagnetic spectrum from reaching the surface, except for visible light and some ultraviolet and near-infrared, as well as certain bands in the IR range.

• Atmospheric window, Wikipedia
• Satellite and ocean data reveal marked increase in Earth’s heating rate, Geophysical Research Letters (2021), and a blog post about the paper

Greenhouse gases (GHG) in the upper troposphere form a spectral window that allows the planet surface and lower layers of the atmosphere to radiate excess energy with wavelengths in the IR into space. This requires emissions from the surface characteristic of temperatures roughly 10℃ to 15℃ warmer than the so-called grey body equilibrium surface temperature² [2]. An increase of GHG makes the atmosphere more opaque at IR wavelengths, thus reducing the planet’s heat radiation to space, and causing the planet to warm until the equilibrium of incident solar light to planetary heat radiation is restored (now at a higher equilibrium surface temperature) [3]. The importance of the atmosphere and the greenhouse effect to life on Earth cannot be understated. The greenhouse effect is the main reason why our surface temperature is not similar to that of the Moon (had it had an atmosphere).

Careful measurements of ocean heat content over six years (2005–2010) showed that the Earth is absorbing more energy from the Sun than it is radiating back to space, with an inferred mean planetary energy imbalance over that six-year period of (0.58±0.15) W m⁻². It is inferred, because of the two dominant causes of changes to Earth’s energy imbalance, greenhouse gases and changes of atmospheric aerosols, the latter remains unmeasured (on the other hand, the former is measured very precisely) [3].

• Physical theories and computer simulations in climate science, Inference Review, Kininmonth (2014)
• A review of ‘The physics of climate change’ by Lawrence Krauss, Inference Review, Socolow (2021)
• Beyond magical thinking: time to get real on climate change, E360, Vaclav Smil (2022)
• The Fermi problem of climate change, Branch Magazine, Anna Knörr (2022)

The long lifetime of CO₂ in the atmosphere³ (300 years) means that even if we deploy large-scale carbon capture and sequestration (which I believe is unfeasible), and even if we manage to arrest or even reverse that increase (which is not happening so far), the benefits will not be felt immediately. Our atmosphere, hydrosphere and biosphere are large and intricately interconnected systems, and the momentum of past emissions means that a global mean temperature rise is now assured. But we can still limit how large that rise will eventually be.

• Zero emissions isn’t enough. We need climate repair., Jacobin Magazine, Hudson (2022)

Solar energy collection and conversion

The challenge with energy conversion has always been a game of optimising collection and conversion. For coal, gas and oil collecting it was fairly straight-forward: just identify (and assert control of) the underground or undersea basins where it was naturally amassed. And once the internal combustion engine (ICE) was developed (at least 150 years ago), converting that fossil fuel to work was simple, but with large negative externalities (poor conversion efficency, pollution, noise) that everyone ignored because the fuel was so easy to collect and thus immensely profitable. Before the age of fossil fuels collection was often arduous, back-breaking manual work, and conversion was often even less efficient than with the ICE.

For solar power, collection and conversion works a little differently. The fuel (in the collection step) is no longer materia, but light. And the material itself is no longer broken down in a chemical reaction to produce heat, but rather facilitates (i.e., catalyses) the conversion of photons to electrons while itself remaining inert for decades. And since our primary source of light is the Sun, and sunlight does not arrive in concentrated beams but is rather evenly spread out everywhere, collecting sunlight requires installing photovoltaic (PV) panels on large areas of land⁴ or buildings.

• Agrivoltaics, Wikipedia
• Agrivoltaics: GWs of solar power from farmland using strategically placed panels (and raising crop yields), EnergyPost (2023)

Atmospheric carbon capture: inefficient, but may be unavoidable

We will most likely need to invest in the atmospheric capture of carbon dioxide, [5] not because it is particularly economical or efficient, but simply because it has now become imperative to quickly decrease the atmosphere’s CO₂ concentration.

This does open up interesting avenues for chemists to take that carbon up the value chain to carbohydrates and all kinds of compounds. But making any sort of fuel out of this captured carbon would defeat the whole point of the exercise, in my opinion.

Capturing carbon dioxide on a large scale from the atmosphere is technically possible, but would require massive amounts of renewable power; power which we would perhaps more wisely be using to supplant non-renewable power generation (there is an inherent conflict of priorities here) [5].

A more limited approach is to simply get the CO₂ out of the atmosphere and store it in bedrock or something, which is known as carbon capture and storage (CCS). Some authors have suggested methods that would combine the splitting of water with CO₂(aq) capture using ocean chemistry [6], and others have shown that carbon dioxide emissions could potentially provide 1570 TWh worth of electricity annually [7]. It is probably premature to pin any hope to such schemes.

We should rather commit to reduce and then eliminate our carbon emissions.

• Can sucking CO₂ out of the atmosphere really work? MIT Technology Review (2014)