Climate Change
Explainer
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.
Natural Climate Patterns
Earth's climate isn't driven by greenhouse gases alone. Several large-scale ocean–atmosphere cycles shift weather patterns around the globe on timescales of months to decades. Understanding these patterns is essential for interpreting year-to-year swings in temperature, rainfall, and extreme weather.
ENSO – El Niño / La Niña
The El Niño–Southern Oscillation (ENSO) is the most influential natural climate pattern on Earth. It describes a recurring shift in sea-surface temperatures across the tropical Pacific Ocean, typically cycling every 2–7 years.
El Niño (warm phase)
Trade winds weaken, allowing warm water to spread eastward across the Pacific. This releases extra heat into the atmosphere, temporarily boosting global temperatures by 0.1–0.2 °C. El Niño years often bring drought to Australia and South-East Asia, heavier rainfall to the Americas, and milder winters in northern Europe.
La Niña (cool phase)
Trade winds strengthen, pushing warm water west and bringing cool, nutrient-rich water to the surface in the eastern Pacific. La Niña temporarily masks global warming, and is associated with wetter conditions in Australia, drier weather in the southern US, and more Atlantic hurricanes.
Why it matters for climate data: Record-warm years (like 2016 and 2023) often coincide with strong El Niño events. When interpreting any single year's temperature, it's important to consider whether ENSO gave it a boost or applied the brakes.
NAO – North Atlantic Oscillation
The NAO describes the pressure difference between the Icelandic Low and the Azores High. It is the dominant driver of winter weather across Europe and eastern North America.
Positive NAO
A strong pressure gradient steers the jet stream northward, bringing mild, wet, and windy winters to northern Europe and drier conditions to the Mediterranean.
Negative NAO
A weaker gradient lets the jet stream meander south, allowing Arctic air to plunge into Europe and the eastern US. This brings cold snaps, snow, and blocking high-pressure systems.
Other Key Oscillations
AMO – Atlantic Multidecadal Oscillation
A 60–80 year cycle in North Atlantic sea-surface temperatures that influences hurricane activity, Sahel rainfall, and European summer temperatures. Currently in its warm phase since the mid-1990s.
PDO – Pacific Decadal Oscillation
Like a slow-motion ENSO, the PDO shifts Pacific temperatures on 20–30 year timescales. Its warm phase tends to enhance El Niño effects, while its cool phase amplifies La Niña impacts.
IOD – Indian Ocean Dipole
A temperature gradient across the Indian Ocean that strongly affects rainfall in East Africa, India, and Australia. A positive IOD can compound drought conditions in Australia when paired with El Niño.
MJO – Madden-Julian Oscillation
A 30–60 day tropical weather pattern that moves eastward around the equator, modulating monsoon strength, tropical cyclone formation, and even mid-latitude weather patterns.
The bigger picture: These natural oscillations redistribute heat around the planet – they don't create or destroy it. While El Niño can temporarily push global temperatures to record highs and La Niña can temporarily suppress them, the long-term warming trend from greenhouse gases continues regardless. In climate data, separating the signal (human-caused warming) from the noise (natural variability) is one of the core challenges.
Why Some Places Warm Faster Than Others
The global average hides huge regional variation. Finland and Sweden are warming at twice the global rate. Svalbard has already warmed nearly 3.5°C. Tropical regions warm slowly in absolute terms but are already close to the limits of human heat tolerance. These are the mechanisms that explain the pattern - the same terms you'll see highlighted across this site will link back here.
Arctic amplification
The Arctic has warmed roughly 3–4× the global mean rate since 1979 — the fastest-warming region on Earth. As sea ice and snow disappear, dark ocean and land are exposed, absorbing more sunlight. Warmer, moister air masses now penetrate further north, and changes in atmospheric and ocean circulation trap extra heat at high latitudes. The effect is strongest in winter and autumn.
Source: NOAA Arctic Report Card 2024Albedo feedback
Fresh snow reflects up to 80% of incoming sunlight; bare soil and open ocean reflect less than 15%. When warming shrinks snow or ice cover, the newly exposed dark surface absorbs far more solar energy, driving further warming and further melt. This positive feedback amplifies warming wherever seasonal or permanent ice is retreating — the Arctic, mountain ranges, Scandinavia, Canada, and even mid-latitude regions losing their winter snowpack.
Source: NASA — Arctic Sea Ice & AlbedoLand warms faster than ocean
Oceans absorb most of the excess heat from greenhouse gases, but water has an enormous heat capacity and mixes heat into the deep. Land has a much lower heat capacity and dries out when warmed, losing its evaporative cooling. As a result, global land-surface temperatures have risen about 1.6°C since pre-industrial times, while ocean surface temperatures have risen about 0.9°C. Large continental interiors — central Asia, central North America, the Sahel — warm fastest.
Source: IPCC AR6 WGI — Chapter 2Latitude effect
Warming is not evenly distributed. Polar and sub-polar regions warm much faster than the tropics because of Arctic amplification, snow-albedo feedback, and dry winter air. The tropics warm more slowly — around 0.8× the global rate — but even small increases push temperatures into ranges that are dangerous for health, agriculture and ecosystems, because tropical life is adapted to a narrow temperature band.
Source: Copernicus Climate Change Service — Global Climate HighlightsAerosol reduction
Sulphate and other pollution aerosols have a cooling effect: they scatter incoming sunlight and seed brighter, more reflective clouds. Since the 1980s, clean-air legislation has dramatically cut aerosol emissions across Europe, North America and (more recently) East Asia and global shipping. The result is excellent for human health but accelerates the warming that was previously masked. Europe, in particular, is now the fastest-warming continent partly because of this effect.
Source: Copernicus — European State of the ClimateHeat domes
A heat dome is a persistent, slow-moving ridge of high pressure that compresses the air beneath it, prevents clouds from forming and pins hot, dry air in place. Recent summers have seen record-breaking heat domes over western Canada and the US Pacific Northwest (2021), Europe (2022, 2023) and India (2024). A wavier, more sluggish jet stream appears to make these blocking patterns both more frequent and longer-lasting.
Source: World Meteorological Organization — State of the ClimateJet-stream shifts
The mid-latitude jet stream is driven by the temperature contrast between the tropics and the poles. As Arctic amplification weakens that contrast, the jet becomes slower and more meandering, forming large stationary waves. This allows heat domes, cold outbreaks and stalled rainfall patterns to persist much longer over the same region, making weather extremes — rather than the average warming itself — the most acute impact in many mid-latitude countries.
Source: Royal Meteorological Society — Jet Stream PrimerDry-soil amplification
Moist soil cools itself (and the air above it) as water evaporates. When soil dries out — through drought, deforestation or extended heat — that evaporative brake is lost, and nearly all the incoming solar energy goes into heating the surface and air. The Mediterranean, the western US, the Sahel and the Middle East are particularly vulnerable; heatwaves and droughts now frequently reinforce each other in a feedback known as compound drought-heat events.
Source: IPCC AR6 WGI — Chapter 11 (Weather Extremes)Permafrost thaw
Arctic permafrost holds roughly twice as much carbon as currently sits in the atmosphere, locked up in frozen organic matter. As it thaws, microbes decompose that carbon and release CO₂ and methane — a self-reinforcing loop often called the permafrost carbon feedback. Parts of northern Canada, Alaska, Siberia and Scandinavia are already transitioning from carbon sinks to carbon sources.
Source: NOAA Arctic Report Card — PermafrostUrban heat island
Dense built surfaces absorb sunlight during the day and release it slowly overnight, while reduced vegetation means less evaporative cooling and shade. Large cities can run 3–7°C hotter than nearby countryside, and that baseline magnifies the health impact of every heatwave. The effect is strongest in rapidly urbanising regions of the Middle East, South Asia and Africa where summer temperatures already exceed 40°C.
Source: EPA — Learn About Heat IslandsElevation-dependent warming
Peer-reviewed syntheses show mountain regions — the Alps, the Rockies, the Andes, the Himalayas — warming roughly 1.5–2× faster than nearby lowlands. The same snow-albedo feedback as the Arctic is at work: as the snowline retreats uphill, bare rock absorbs more solar energy. Glacier loss and changes in cloud cover amplify the effect. Switzerland, for example, has already warmed more than 2.8°C since pre-industrial times.
Source: IPCC — Special Report on the Ocean & Cryosphere, Chapter 2Deforestation
Mature forests cool the land through shade and by pumping water into the air (transpiration), which forms clouds and rainfall. When forests are cleared, the exposed land heats up, rainfall patterns shift, and decades of stored carbon are released as CO₂. The Amazon, Congo Basin and South-East Asia show the starkest local effects, and Amazon deforestation now contributes measurable regional warming above the global-average signal.
Source: MIT Climate Portal — DeforestationSeasonal shifts
In the Northern Hemisphere, winter average temperatures are rising 1.5–2× faster than summer averages because snow and sea-ice loss unlock the albedo feedback most strongly in the cold season. Spring is arriving roughly 2 weeks earlier than in the 1950s across much of Europe and North America, and autumn is running later. These shifts disrupt wildlife, agriculture and water supply, and mean a region’s annual-average warming can hide much bigger seasonal extremes.
Source: EU Copernicus — European State of the Climate, SeasonsOcean-current changes
Ocean circulation moves vast quantities of heat around the globe. The Atlantic Meridional Overturning Circulation (AMOC) carries warm surface water north and returns cold deep water south, making north-west Europe unusually mild for its latitude. Freshwater from Greenland ice melt is slowing this circulation, and many studies find a multi-decadal weakening trend. Weakening could bring cooler, wetter UK/Ireland winters but hotter European summers, and raise sea levels along the US East Coast.
Source: Met Office — AMOC ExplainerMonsoon disruption
South and South-East Asia, West Africa and northern Australia depend on the seasonal monsoon to cool the land and deliver most of their annual rainfall. Global warming and El Niño events are making monsoons less reliable: later onsets, longer mid-season dry spells, and more intense bursts of rain. When the monsoon fails or arrives late, cooling rains are absent, soils stay dry, and pre-monsoon heatwaves stretch further into the year — a major driver of the record-breaking 45–48°C temperatures seen across Myanmar, India, Pakistan and the Philippines in recent years.
Source: WMO — State of the Climate in AsiaENSO
ENSO is the single biggest source of year-to-year variability in global temperature. During an El Niño, unusually warm water piles up in the eastern tropical Pacific, releasing heat to the atmosphere and lifting global mean temperature by ~0.1–0.3°C for 6–12 months. La Niña does the opposite. ENSO also shifts rainfall and storm tracks worldwide — driving drought in Australia, flooding in Peru, wetter Californias and milder UK winters. Record-warm years almost always coincide with El Niño stacked on top of the long-term warming trend.
Source: NOAA Climate.gov — ENSONorth Atlantic Oscillation
The NAO measures the pressure difference between the Icelandic Low and the Azores High. When it is positive, the jet stream is strong and steers mild, wet Atlantic air into the UK and Scandinavia while the Mediterranean stays dry. When it is negative, the jet weakens and buckles, allowing cold Arctic air to flood south into Europe and the eastern US — often bringing Britain its coldest winters. A persistently positive NAO has been a major driver of recent mild UK/Irish winters.
Source: Met Office — NAO ExplainerArctic Oscillation
The Arctic Oscillation describes the strength of the polar vortex — a ring of winds high in the stratosphere that keeps cold air locked over the Arctic. In its positive phase, the vortex is strong and cold air stays north. In its negative phase, the vortex weakens or splits, allowing frigid Arctic air to pour south — responsible for the US “polar vortex” cold snaps and many of Europe’s coldest winter outbreaks. Climate change may be weakening the vortex more often by warming the Arctic.
Source: NOAA Climate.gov — Arctic OscillationIndian Ocean Dipole
The IOD is to the Indian Ocean what ENSO is to the Pacific. When the western Indian Ocean is unusually warm and the east is cool (positive IOD), Australia and Indonesia turn dry and fire-prone while East Africa faces heavy rains and flooding. Negative phases flip these patterns. A strong positive IOD combined with El Niño drove Australia’s catastrophic 2019–2020 Black Summer bushfires.
Source: Bureau of Meteorology — IODThe bigger picture: most regions experience several of these at once. Finland combines Arctic amplification, snow-albedo feedback and seasonal shifts; the Mediterranean combines dry-soil amplification, heat domes and a weakening jet stream; tropical cities combine urban heat islands with the narrow thermal tolerance of life at low latitudes.
Glossary
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
El Niño / La Niña
Live ENSO state, Niño 3.4 & forecast
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.