Climate Change: Causes & Evidence
Climate Has Always Changed
Earth’s climate has never stood still. Over the past 2.6 million years — the Quaternary period — the planet has cycled in and out of long cold spells called glacials, separated by warmer interglacials. The last glacial peaked about 20,000 years ago, with ice sheets covering much of northern Europe, including most of Britain. We are currently in an interglacial. What is unusual today is the speed at which the climate is changing now.
Natural Causes of Climate Change
Natural climate change has four main drivers. Orbital changes — called Milankovitch cycles — alter how much sunlight different parts of the Earth receive on timescales of tens of thousands of years. Solar output varies in 11-year cycles and longer-term swings; cooler sunspot minima can drop temperatures slightly. Volcanic activity can chill the climate for several years by injecting sulphur dioxide into the upper atmosphere, where it reflects incoming sunlight. Asteroid impacts are rare but devastating, throwing dust into the atmosphere; the impact 66 million years ago is thought to have ended the age of the dinosaurs.
The four natural causes of climate change are orbital changes, solar output variations, volcanic activity and asteroid impacts. They explain past climate cycles but cannot account for the rapid warming since 1970.
Evidence For Past Climate
Scientists reconstruct past climates from natural archives. Ice cores from Antarctica and Greenland trap tiny bubbles of ancient atmosphere; analysing them gives us temperature and CO₂ data going back 800,000 years. Tree rings are wider in warm, wet years; they record annual climate going back several thousand years for some species. Historical sources — harvest records, frost-fair paintings of the frozen Thames, parish records of failed crops — show how UK climate has shifted since Roman times.
From around 950 to 1250 CE, much of the northern hemisphere was unusually warm — the ‘Medieval Warm Period’. Vineyards spread north into Britain. From about 1300 to 1850 CE, the ‘Little Ice Age’ brought colder conditions; the Thames froze often enough for ‘frost fairs’ to be held on it. Both periods were caused by a mix of solar output changes and volcanic activity. They prove climate is naturally variable, but the rate of change since 1970 is much faster than either swing.
The Enhanced Greenhouse Effect
The natural greenhouse effect keeps Earth about 33°C warmer than it would otherwise be. Greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) let in short-wave solar radiation but trap long-wave heat radiated from the surface. Since the Industrial Revolution, human activities have raised greenhouse-gas concentrations sharply: burning fossil fuels for electricity and transport; clearing forests for farming (releasing stored carbon); industrial farming (cattle release methane); and cement manufacture. Atmospheric CO₂ has risen from about 280 parts per million (ppm) in 1750 to over 420 ppm today — higher than at any point in the last 800,000 years of ice-core records.
Atmospheric CO₂ has risen by roughly 50% since 1750. Today’s rate of warming is about ten times faster than the natural warming that ended the last ice age.
Evidence It’s Happening Now
Multiple, independent lines of evidence point in the same direction. Global mean temperature has risen about 1.2°C since pre-industrial times, with most of the warming since 1970. Arctic sea-ice extent has declined by about 13% per decade. Mountain glaciers are retreating worldwide; sea levels have risen 21–24 cm since 1900 from ocean warming and melting ice. Extreme weather events — heatwaves, wildfires, intense storms — are increasing in frequency.
The IPCC projects 1.5°C to 4.4°C of warming by 2100. The wide range comes from three uncertainties. First, future emissions depend on political choices, economic growth and technology — we don’t know how quickly the world will switch from fossil fuels. Second, feedback loops are hard to model: melting permafrost releases methane that accelerates warming further. Third, the climate system has natural variability that masks short-term trends. Sea-level rise projections range from 28 to 101 cm by 2100 for similar reasons.
Consequences for People
Warmer climates are not all bad in every place — longer growing seasons in northern countries, less winter heating — but the dominant effects are negative. Coastal cities like Jakarta and parts of London face flooding from sea-level rise. Drier conditions in the Mediterranean and Sahel threaten food and water security. Tropical diseases such as malaria are spreading to new areas. Biodiversity is being lost as species cannot move fast enough to track shifting climate zones. Poor and developing countries are disproportionately affected because they have less capacity to adapt.