Climate Change: A Physics Perspective
Two Lessons, Two Disciplines
Climate change appears in two different parts of your course — once in chemistry (about the chemical composition of the atmosphere and the evidence for rising CO₂ from human activities) and once here in physics. The physics angle focuses on how radiation and energy balance explain why CO₂ traps heat. Understanding both together gives you the most complete picture and the tools to answer questions from either discipline.
Earth’s Radiation Balance
The Earth receives energy from the Sun as shortwave radiation — predominantly visible light and some UV. It also loses energy by emitting its own longwave radiation (infrared). For the climate to be stable, the energy arriving must equal the energy leaving. This balance is called the energy balance.
Not all the solar radiation that arrives at Earth’s surface is absorbed. Some is reflected back to space. The fraction of incoming radiation that is reflected is called the albedo. Surfaces with high albedo (ice, snow, clouds) reflect more solar energy and absorb less. Surfaces with low albedo (dark ocean, forests) absorb more. The average albedo of Earth is roughly 0.3, meaning about 30% of incoming solar energy is reflected without heating the planet.
High albedo surfaces (ice, snow, clouds) reflect solar radiation; low albedo surfaces (ocean, forest) absorb it. As Arctic ice melts, it is replaced by darker ocean water — lower albedo means more absorption and more warming. This is a positive feedback loop.
The Greenhouse Effect
Earth’s surface warms because it absorbs solar radiation and re-emits this energy as infrared radiation. The atmosphere contains gases — principally water vapour, carbon dioxide and methane — that absorb this outgoing infrared and re-emit some of it back towards Earth’s surface. These are called greenhouse gases. Without them, Earth’s average surface temperature would be far colder than it is — the greenhouse effect is natural and essential to life.
The problem is not the greenhouse effect itself but its enhancement. As human activities release more CO₂ and methane into the atmosphere, more infrared is absorbed and returned to the surface. The energy balance is disrupted: more energy arrives at the surface than leaves to space, and the planet warms until a new equilibrium is reached at a higher temperature.
A molecule absorbs radiation when the energy of the photon matches a vibrational mode of the molecule. CO₂ and H₂O have molecular structures with vibrational modes (bending and stretching of bonds) that correspond to infrared wavelengths. Nitrogen (N₂) and oxygen (O₂), which make up most of the atmosphere, are symmetrical diatomic molecules with no such vibrational modes at infrared wavelengths — they are essentially transparent to outgoing infrared. This is why a small increase in CO₂ concentration has a disproportionately large effect on the greenhouse effect: the main atmospheric gases already let infrared through; adding CO₂ closes the gaps.
Earth’s surface emits infrared. Greenhouse gases (CO₂, CH₄ = methane, water vapour) absorb this infrared and re-emit some back to the surface. More greenhouse gas → more infrared returned → higher equilibrium surface temperature.
The Electromagnetic Spectrum and Climate
Understanding why the atmosphere behaves differently towards incoming and outgoing radiation requires placing both on the electromagnetic spectrum. Incoming solar radiation peaks in the visible and near-infrared region (short wavelengths). Earth, at its surface temperature, emits radiation that peaks in the mid-infrared region (longer wavelengths). The atmosphere is largely transparent to the incoming shortwave radiation but absorbs strongly in specific infrared wavelength bands — precisely where greenhouse gases have their vibrational modes.
UV radiation from the Sun is mostly absorbed by ozone in the stratosphere before reaching the surface. Visible light passes through the atmosphere largely unaffected. The key action is at the infrared end: shortwave in, longwave out, with the greenhouse gases acting as an asymmetric filter.
Climate scientists use computer models to predict how rising greenhouse gas concentrations will affect global temperatures. These models are built on well-established physics (the radiation balance principles covered in this lesson) and tested against historical climate data. Where models from different research groups using different assumptions agree, scientists have greater confidence in the prediction. Where they disagree, this signals genuine uncertainty about feedbacks and timescales.
The key evidence that the current warming is human-caused rather than natural includes: the rate of warming is faster than any natural cycle detected in ice-core records; rising CO₂ concentrations in the atmosphere correlate with industrial activity; the isotopic signature of atmospheric CO₂ shows it comes from burning fossil fuels; and the stratosphere is cooling while the troposphere warms — the pattern expected from greenhouse gas forcing, not from increased solar output. This is an example of how multiple independent lines of evidence converge to support one explanation — a hallmark of strong scientific reasoning.
Consequences and the Need for Action
A disrupted energy balance has physical consequences that flow directly from the physics. Warmer oceans expand (thermal expansion) and melt land ice, raising sea levels. Warmer air holds more water vapour, intensifying rainfall events and droughts. The extra energy in the climate system drives more intense weather. These effects are not uniform globally, but the physics of the energy imbalance means they are unavoidable until the atmosphere returns to a new equilibrium.
Reducing greenhouse gas emissions directly addresses the physical cause: fewer greenhouse gas molecules in the atmosphere means less infrared is absorbed and returned to the surface, allowing the energy balance to stabilise at a lower equilibrium temperature. Switching from fossil fuels to renewable energy sources reduces the rate at which CO₂ is added to the atmosphere without removing CO₂ already there — hence why scientists also discuss carbon capture technologies that directly reduce atmospheric CO₂ concentrations.