4 Billion Years On

Explainer

Climate Change Explained

A plain-English guide to the science behind climate change — what's happening, why it matters, and the key concepts you'll encounter in climate data.

Key Facts

Earth has warmed ~1.2 °C since pre-industrial times. The last decade was the hottest on record.

CO₂ levels are higher than at any point in at least 800,000 years — and rising faster than ever.

Global sea levels have risen ~21 cm since 1900 and the rate is accelerating — now ~4.5 mm/year.

Arctic summer sea-ice extent has declined ~13% per decade since satellite records began in 1979.

Extreme weather events — heatwaves, floods, droughts — are becoming more frequent and intense.

Glaciers worldwide are losing ~270 billion tonnes of ice per year, contributing to sea-level rise.

Roughly 1 million species face extinction risk, many driven by climate-related habitat loss.

At 1.5 °C of warming, coral reefs decline by 70-90%. At 2 °C, virtually all are lost.

How Climate Change Works

The sun's energy passes through the atmosphere and warms the Earth's surface. The surface radiates this energy back as infrared heat, but greenhouse gases — primarily CO₂, methane, and nitrous oxide — absorb some of that outgoing heat and re-emit it in all directions, warming the lower atmosphere. This is the greenhouse effect, and it's entirely natural.

The problem begins when human activities — burning coal, oil, and gas; deforestation; agriculture — release billions of extra tonnes of greenhouse gases. Since the Industrial Revolution, CO₂ concentrations have risen over 50%, intensifying the greenhouse effect and trapping more heat than the planet can radiate away.

This extra energy doesn't just raise the thermometer. It powers the entire climate system: warmer oceans fuel stronger storms, melting ice raises sea levels, shifting rainfall patterns cause droughts in some regions and floods in others, and ecosystems struggle to adapt to the pace of change.

Critically, the climate system contains feedback loops that can amplify warming. Melting Arctic ice, for example, exposes dark ocean water that absorbs more solar heat — accelerating further melting. Thawing permafrost releases stored methane, adding more greenhouse gas. These feedbacks mean that small temperature rises can trigger larger, self-reinforcing changes.

Scientists have identified several tipping points — thresholds beyond which changes become irreversible on human timescales. The collapse of the West Antarctic Ice Sheet, dieback of the Amazon rainforest, and disruption of Atlantic ocean circulation are among the most studied. The IPCC warns that some tipping points could be crossed between 1.5 °C and 2 °C of warming.

Glossary

Greenhouse effect
Certain gases in Earth's atmosphere trap heat from the sun, keeping the planet warm enough to support life. Without it, average surface temperature would be about −18 °C instead of +15 °C.
CO₂ (carbon dioxide)
The most significant long-lived greenhouse gas emitted by human activity, primarily from burning fossil fuels. Atmospheric concentration has risen from ~280 ppm (pre-industrial) to over 420 ppm today.
Methane (CH₄)
A potent greenhouse gas with roughly 80× the warming power of CO₂ over 20 years. Major sources include livestock, rice paddies, landfills, and fossil-fuel extraction.
Nitrous oxide (N₂O)
A long-lived greenhouse gas roughly 270× more warming than CO₂ per molecule. Mainly released from agricultural fertilisers & industrial processes.
Global warming
The long-term increase in Earth's average surface temperature, driven primarily by rising greenhouse gas concentrations. The planet has warmed approximately 1.2 °C since the late 1800s.
Climate change
Broader than warming alone — encompasses shifts in weather patterns, sea levels, ice coverage, ocean chemistry, and ecosystems caused by the energy imbalance from greenhouse gases.
Feedback loop
A process where warming triggers further warming (positive feedback) or counteracts it (negative feedback). Example: melting ice exposes darker ocean, which absorbs more heat, melting more ice.
Tipping point
A threshold beyond which a change becomes self-reinforcing and potentially irreversible. Examples include collapse of the West Antarctic Ice Sheet, Amazon rainforest dieback, and permafrost thaw.
Carbon budget
The total amount of CO₂ humanity can still emit while keeping warming below a given target (e.g. 1.5 °C). Current estimates suggest the 1.5 °C budget may be exhausted within this decade.
Net zero
The point at which greenhouse gas emissions released equal those removed from the atmosphere, through natural sinks or technology. Most climate targets aim for global net zero by 2050.
Paris Agreement
A 2015 international treaty where 196 parties agreed to limit warming to well below 2 °C, preferably 1.5 °C, above pre-industrial levels.
IPCC
The Intergovernmental Panel on Climate Change — a UN body that synthesises the latest climate science. Its Assessment Reports (AR6 is the latest) are considered the gold standard.
Carbon intensity
The amount of CO₂ emitted per unit of energy produced or GDP generated. A falling carbon intensity means cleaner energy or more efficient economies.
Albedo
The reflectivity of a surface. Ice and snow have high albedo (reflect sunlight); oceans and forests have low albedo (absorb more heat).
Radiative forcing
The difference between incoming solar energy and outgoing energy radiated back to space. Positive forcing (from greenhouse gases) warms the planet.
ppm / ppb
Parts per million / billion — units used to measure trace gas concentrations in the atmosphere. CO₂ is measured in ppm; methane in ppb.
Planetary boundaries
A framework identifying nine Earth-system processes (e.g. climate change, biodiversity loss) with safe limits. Crossing them risks abrupt or irreversible environmental change.

Explore Climate Data

See these concepts in action with real-time data on our dashboard pages:

Global & Local Climate

Temperature anomalies, CO₂ trends

Planetary Boundaries

Nine Earth-system thresholds

Greenhouse Gases

CO₂, methane & N₂O concentrations

Sea Levels & Ice

Sea level rise, Arctic ice extent

Extreme Weather

Disasters, storms & heatwaves

CO₂ Emissions

Country rankings & global trends

Further Reading

IPCC Sixth Assessment Report

The most comprehensive summary of climate science available.

NASA Climate

Real-time climate data, visualisations, and educational resources.

Carbon Brief

Clear, data-driven journalism covering the latest climate science and policy.

Met Office Climate Guide

Plain-English explainers from the UK's national weather service.

Our World in Data — CO₂ & GHGs

Interactive charts and data on global & country-level emissions.

Climate Action Tracker

Independent analysis of government climate pledges vs actual action.

Stockholm Resilience Centre

Research on the nine planetary boundaries framework.

Global Carbon Project

Annual carbon budgets and emissions datasets used by the IPCC.