What it is
Climate change is the long-term shift in Earth’s average weather patterns, driven overwhelmingly by human activity since the Industrial Revolution. The mechanism is the greenhouse effect: certain gases in the atmosphere trap heat that would otherwise radiate back into space, warming the planet’s surface. These are greenhouse gases (GHGs), and trapping heat is a natural process — it’s why Earth is habitable at all. The problem is volume: burning fossil fuels, clearing forests, and industrial agriculture have pushed GHG concentrations far above pre-industrial levels, intensifying the effect well beyond its natural baseline.
The main greenhouse gases
Roughly in order of how much warming they cause today:
- Carbon dioxide (CO2) — from burning fossil fuels (coal, oil, gas) and deforestation. Long-lived: a molecule emitted today can still be warming the atmosphere centuries from now.
- Methane (CH4) — from livestock, agriculture, and fossil fuel extraction. Shorter-lived than CO2 but far more potent per molecule while it’s in the atmosphere.
- Nitrous oxide (N2O) — mostly from fertilizer use.
- F-gases (fluorinated gases) — synthetic and industrial (refrigerants, some manufacturing). A small share of total emissions but very potent.
CO2-equivalent and GWP
Because these gases don’t trap heat equally, scientists convert them to a common unit: CO2-equivalent (CO2e), using each gas’s global warming potential (GWP), a measure of how much warming a given mass of that gas causes relative to the same mass of CO2, usually over a 100-year period. That’s why you’ll see emissions reported in “tons of CO2e” even when the actual gas is methane or N2O — it’s a shared currency for comparing and adding up different gases.
Think of GWP as an exchange rate: one ton of methane counts as roughly 28–30 tons of CO2 on this scale, because pound-for-pound it traps far more heat while it’s in the atmosphere, even though it breaks down faster.
Weather vs. climate
Weather is short-term (today’s forecast); climate is the long-term average and pattern (decades). A cold week doesn’t disprove climate change any more than one hot day proves it — the signal is in the trend, not any single data point.
Where we are
Global average temperature is running roughly 1.3–1.4°C above pre-industrial levels on a multi-year average (the exact figure depends on the reference period and dataset used, and individual years have already spiked above 1.5°C). The Paris Agreement’s targets, “well below 2°C” and pursuing 1.5°C, describe how much additional cumulative warming, measured from pre-industrial times, the world is trying to avoid. Every fraction of a degree matters because impacts scale non-linearly: damages don’t rise steadily, they can accelerate sharply past certain thresholds.
Tipping points and feedback loops
Some parts of the climate system don’t respond gradually — they can flip into a different state once pushed far enough (e.g., large-scale ice sheet collapse, permafrost thaw releasing stored methane and CO2). A feedback loop is where a change reinforces itself: melting Arctic ice exposes darker ocean water, which absorbs more heat than reflective ice did, which melts more ice. These dynamics are a core reason scientists emphasize acting early rather than waiting for damage to become obvious and reversible.
The IPCC
The IPCC (Intergovernmental Panel on Climate Change) is the UN body that synthesizes climate science into periodic Assessment Reports. Governments approve the Summary for Policymakers line-by-line in plenary sessions; the fuller underlying report is written and reviewed by scientists and isn’t negotiated word-for-word the same way. Either way, it’s the closest thing to a single authoritative summary of scientific consensus, and the source most policy and corporate targets ultimately trace back to.
Scope 1, 2, and 3 emissions
The rest of this hub leans on one more framework constantly: Scope 1, 2, and 3 emissions (the GHG Protocol), which categorizes an organization’s emissions by where they physically come from:
- Scope 1 — direct emissions from sources the company owns or controls (its own vehicles, on-site fuel combustion).
- Scope 2 — indirect emissions from purchased electricity, heat, or steam.
- Scope 3 — everything else in the value chain: suppliers, business travel, employee commuting, and, often the largest share, how customers use and dispose of the product.
Scope 3 is why “carbon accounting” is hard, and why an entire software category (see the Climate SaaS topic) exists largely to estimate it.
Why it matters
Every other topic in this hub borrows this one’s vocabulary. Climate policy’s carbon pricing and disclosure rules are built around measuring and pricing these same gases. Climate SaaS companies exist largely to help other companies measure and report their Scope 1/2/3 emissions. Climate risk analysis distinguishes physical risk (the direct physical effects of a warming climate) from transition risk (business risk from the shift to a low-carbon economy), a distinction that only makes sense once the underlying science does. Skip this topic and the rest of the hub reads as vocabulary without context.