Core Vocabulary

These are the terms you need to read an article, a job posting, or a company’s climate report without stopping to look things up. Definitions are deliberately short. The ★ marks the roughly ten terms that appear in nearly every conversation — internalize those first; treat the rest as reference.

Climate and carbon

★ Greenhouse gas (GHG) — a gas that traps heat in the atmosphere. The main ones are carbon dioxide (CO₂), methane, and nitrous oxide; methane traps far more heat per tonne than CO₂ but breaks down within decades.

Carbon dioxide equivalent (CO₂e) — a common unit that converts every greenhouse gas into the amount of CO₂ that would cause the same warming, so mixed emissions can be added up as one number.

★ Net zero — the state where the greenhouse gases you emit are balanced by an equal amount removed from the air, so your net addition to the atmosphere is zero. Unlike “carbon neutral,” which often leans on cheap offsets, credible net zero means deep actual cuts first and removals only for the stubborn remainder.

Carbon sink — a reservoir that absorbs more carbon than it releases: forests, soils, and especially the ocean. The opposite is a source.

Radiative forcing — the measure of how strongly a factor pushes the climate to warm or cool, in watts per square meter. More greenhouse gas means more positive forcing.

★ Mitigation — action that reduces the cause of climate change: cutting emissions or removing carbon. Contrast with adaptation.

★ Adaptation — action that reduces the harm from climate change we can no longer avoid: sea walls, drought-resistant crops, early-warning systems. Mitigation treats the disease; adaptation manages the symptoms.

Carbon offset — a credit for a tonne of emissions avoided or removed somewhere else (e.g., a protected forest), bought to cancel out your own. Quality varies enormously, and many offsets don’t represent real, additional, lasting reductions.

How we measure and know

Parts per million (ppm) — the unit for gas concentration in the air. Atmospheric CO₂ has risen from about 280 ppm before industrialization to over 420 ppm today.

Baseline — the reference point a change is measured against (e.g., pre-industrial temperature). Choose a different baseline and the same data can tell a different story, which is why baselines are argued over.

Emission scopes (1, 2, 3) — a way of dividing an organization’s emissions: Scope 1 is what it burns directly, Scope 2 is the electricity it buys, and Scope 3 is everything up and down its supply chain. Scope 3 is usually the largest and the most conveniently ignored.

Life-cycle assessment (LCA) — accounting for a product’s total environmental impact across its whole life: raw materials, manufacture, use, and disposal. It’s how you avoid being fooled by a “clean” product with a dirty supply chain.

Attribution science — the field that estimates how much a specific event (a heatwave, a flood) was made more likely or intense by climate change. It’s why scientists can now say a given disaster was, say, three times more likely because of warming.

Living systems

★ Biodiversity — the variety of life at every level: the number of species, the genetic variety within them, and the range of ecosystems they form. More diversity generally means more resilience.

★ Ecosystem services — the free benefits nature provides to people: pollination of crops, water purification, flood protection, carbon storage, fisheries. Naming them makes nature’s value visible in decisions that would otherwise treat it as worthless.

Keystone species — a species whose presence holds an ecosystem’s structure together, out of proportion to its numbers; remove it and the system unravels (e.g., sea otters keeping urchins from destroying kelp forests).

Eutrophication — when excess nutrients (usually nitrogen and phosphorus from fertilizer or sewage) overfeed algae in water; the algae bloom, die, decay, and strip the water of oxygen, creating “dead zones.”

Habitat fragmentation — the breaking of a continuous habitat into isolated patches by roads, farms, or cities. The total area lost understates the damage, because small isolated patches support far fewer species.

Pollution and materials

★ Externality — a cost (or benefit) of an activity that falls on others and isn’t reflected in its price. Pollution is the textbook negative externality; it’s why markets overproduce harm. See the mental models.

Particulate matter (PM2.5) — airborne particles smaller than 2.5 microns, small enough to lodge deep in the lungs and enter the blood. It’s the pollutant that does the most damage to human health worldwide.

Persistent organic pollutant / “forever chemicals” (PFAS) — synthetic chemicals that don’t readily break down, so they accumulate in the environment and in bodies over decades. PFAS, used in nonstick and waterproof coatings, are the best-known modern example.

Bioaccumulation — the buildup of a persistent substance in an organism over time, concentrating as it moves up the food chain — which is why top predators (and people) carry the highest doses of mercury and PFAS.

★ Circular economy — an economy designed so materials are reused, repaired, and recycled in loops rather than used once and discarded. The opposite of the “take–make–waste” linear model; see Waste, Materials & Toxics.

Energy and the transition

Fossil fuel — coal, oil, and natural gas: ancient carbon dug up and burned, releasing CO₂ that had been locked away for millions of years. The dominant source of energy and of emissions.

★ Renewable energy — energy from sources that replenish on human timescales: sunlight, wind, water, geothermal heat. Contrast with fossil and nuclear, which draw down a finite stock.

★ Decarbonization — the broad project of removing carbon emissions from an activity or an economy, usually by switching from fossil fuels to clean energy.

Intermittency — the fact that solar and wind produce power only when the sun shines or wind blows, not on demand. Managing it — with storage, grids, and flexible demand — is the central technical challenge of a renewable grid.

Carbon capture and storage (CCS) — technology that captures CO₂ (from a smokestack or the open air) and stores it underground. Real but expensive and small-scale today; useful for emissions that are otherwise hard to cut, risky as an excuse to keep burning.

Governance, economics, and justice

★ Carbon pricing — putting a price on emitting CO₂ so the cost of the damage shows up in decisions. The two main forms are a carbon tax and cap and trade.

Cap and trade — a system that sets a hard limit (cap) on total emissions, issues that many permits, and lets firms buy and sell them; the market sets the price. A carbon tax fixes the price and lets the quantity float — cap and trade fixes the quantity and lets the price float.

Polluter-pays principle — the idea that whoever causes pollution should bear its costs, rather than society. It’s the moral logic behind most environmental regulation.

Precautionary principle — when an action risks serious or irreversible harm, a lack of full scientific certainty shouldn’t be used as a reason to delay protection. Act on strong evidence rather than waiting for proof.

Environmental justice — the recognition that environmental harms fall disproportionately on the poor and marginalized, who did least to cause them, and the movement to correct that imbalance.

★ Greenwashing — marketing that makes a company or product seem more environmentally responsible than it is: vague claims, cherry-picked metrics, or offsets standing in for real change. Learning to spot it is a core practitioner skill.

Sustainability — meeting present needs without undermining the ability of future generations to meet theirs. Overused to the point of meaning almost anything, which is exactly why you should ask what someone means by it.