What is Climate Change?

How the Climate System Warms

The Sun warms Earth. Greenhouse gases (GHGs) like carbon dioxide (CO₂) and methane (CH₄) act like a thin blanket, trapping some of the outgoing heat and keeping the planet habitable. When humans burn fossil fuels, clear forests, or produce cement, we add more GHGs, thickening that blanket and raising temperatures. The Intergovernmental Panel on Climate Change (IPCC) concludes it is “unequivocal” that human influence has warmed the atmosphere, ocean, and land.

Since 1850–1900, global warming has accelerated alongside rising GHG concentrations. The IPCC’s 2023 synthesis confirms that observed increases in heat waves, heavy rainfall, ocean warming, sea level rise, and glacier loss are linked to this human-driven change.

The main greenhouse gases and how long they last

Carbon dioxide (CO₂) can persist for centuries in the climate system, methane (CH₄) lasts about a decade but warms strongly in the short term, and nitrous oxide (N₂O) endures for over a century. CO₂ is the largest driver and continues to set records, hitting ~430 ppm in June 2025 (parts per million).

Why scientists say warming is “unequivocal”

Independent datasets (surface thermometers, satellites, ocean buoys, and ice cores) all tell the same story. Physical fingerprints—like more ocean heat, shrinking glaciers, earlier spring, and rising sea levels—match model expectations of greenhouse warming, not natural cycles alone. The IPCC aggregates these lines of evidence across thousands of peer-reviewed studies.

The Evidence You Can Measure

Warming air and oceans

The year 2024 surpassed 2023 as the hottest on record. Each year from 2015–2024 ranks among the ten warmest ever observed. Oceans, which absorb over 90% of the extra heat, set new records for warmth, fueling marine heatwaves that stress coral reefs and fisheries.

Rising seas and accelerating rates

Sea level is rising because ice sheets and glaciers add water, and warm water expands. In 2024, the observed rate jumped to ~0.23 in/year (0.59 cm/year), above the typical ~0.17 in/year (0.44 cm/year) in recent decades—an acceleration linked mainly to unusual ocean warmth that year.

Ocean acidification and marine life

When CO₂ dissolves into seawater, it forms carbonic acid, lowering pH. The surface ocean has become about 0.1 pH units more acidic since pre-industrial times (≈ 26% increase in acidity), making it harder for corals and shell-builders to form skeletons.

What Causes Climate Change?

The biggest contributor is energy: burning coal, oil, and gas for electricity, heat, transport, industry, and buildings. Recent summaries attribute roughly three-quarters of global GHG emissions to energy-related activities, with the remainder from agriculture, land use, and waste.

Agrifood systems also matter. FAO estimates put their 2022 emissions near 16.2 Gt CO₂e (~30% of total), including farming, land-use change, and supply chains. Cutting food waste, improving rice cultivation, and reducing methane from livestock are high-impact options.

Impacts You Can Feel

Warmer air loads the dice for extreme heat, heavier downpours, and longer wildfire seasons. Rising seas raise baseline flood heights, so “once-in-a-century” coastal floods can occur far more often. Hotter oceans fuel stronger storms and harm marine ecosystems that supply food and jobs. These impacts already disrupt health, infrastructure, and economies across the world.

In 2024, record heat and ocean warmth coincided with accelerated glacier loss, reduced Antarctic sea ice, and a long-term sea-level rise averaging ~4.7 mm/year (0.19 in/year) over 2015–2024—faster than in the 1990s. This trend increases coastal risk from Miami to the Mediterranean and the Nile Delta.

What the Future Looks Like

Stabilizing temperature requires reaching net-zero CO₂, then net-negative later to lower warming. The IPCC shows that deep cuts this decade keep a path to limit warming close to 1.5 °C (2.7 °F), while current policies point to higher end-century warming and greater risks. The UNEP Emissions Gap Report (2024) details how far current pledges fall short and where rapid progress is possible.

Near-term choices matter because certain changes—like sea-level rise—lag behind temperature. Even with strong cuts, seas keep rising for decades due to past heat already stored in the ocean. Planning now for resilient coasts, water systems, and urban design saves money and lives later.

What We Can Do

Mitigation (cutting causes): scale up clean electricity; electrify transport and heating; improve efficiency; protect and restore forests; reduce methane in energy and food systems; and deploy low-carbon industry (green hydrogen, low-carbon cement and steel). Policies that price carbon, set clean-power standards, and fund innovation accelerate this shift.

Adaptation (managing risks): design heat-resilient cities (cool roofs, shade, early warnings), climate-smart agriculture (drought-tolerant crops, efficient irrigation), natural buffers (mangroves, wetlands), and flood-aware planning. Adaptation works best when paired with swift emission cuts that limit future extremes.

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