Evidence for climate change

Scientists reconstruct past climate from proxy records and measure recent change from the instrumental record:

• Ice cores — trapped air bubbles preserve past atmospheric composition (CO2 and methane), while oxygen isotopes indicate past temperature. Antarctic and Greenland cores span hundreds of thousands of years.
• Dendrochronology (tree rings) — ring width and density record year-by-year growing conditions.
• Pollen analysis — pollen preserved in peat and lake sediments shows which plants dominated, revealing past climate.
• Sea-level and glacial records — raised beaches, moraines and erratics mark former ice extent and sea levels.
• Instrumental record — thermometers, tide gauges and satellites give direct measurements since the mid-19th century.

Evaluate: proxies extend the record far beyond instruments, but each carries uncertainty and needs careful calibration. The strongest reconstructions combine several independent lines of evidence.

Reading the ice

Natural forcings

Climate has always changed naturally. The main drivers, acting over different timescales, are:

• Milankovitch cycles — regular variations in Earth's orbit that alter how solar energy is distributed:
  • Eccentricity — the shape of the orbit, cycling roughly every 100,000 years.
  • Obliquity (axial tilt) — the tilt varies between about 22° and 24.5° over roughly 41,000 years.
  • Precession — the wobble of the axis, roughly 26,000 years.
• Solar output — variations in the Sun's energy, linked to sunspot activity.
• Volcanic activity — large eruptions inject aerosols and ash into the stratosphere, reflecting sunlight and causing short-term cooling.
• Ocean circulation — changes in currents redistribute heat and can shift regional and global climate.

Which orbital cycle?

Your turn — mean annual rise

Natural vs enhanced greenhouse effect

The natural greenhouse effect keeps Earth warm enough for life: greenhouse gases absorb outgoing longwave (infrared) radiation and re-radiate it, warming the lower atmosphere. Without it, Earth would be frozen.

The enhanced greenhouse effect is the extra warming caused by rising greenhouse-gas concentrations from human activity, which increases the radiative forcing — the change in the atmosphere's energy balance.

• Carbon dioxide (CO2) — from burning fossil fuels, deforestation and cement; the largest human contribution.
• Methane (CH4) — from livestock, rice, landfill and fossil fuels; far more potent per molecule than CO2.
• Nitrous oxide (N2O) — mainly from fertilisers.
• Water vapour — the most abundant greenhouse gas and a key feedback: warmer air holds more of it.

Key term: radiative forcing is measured in watts per square metre. A positive forcing warms the planet; a negative forcing (e.g. reflective aerosols) cools it.

Enhanced, not natural

Your turn — rate of change

Feedbacks: amplifying and dampening

Feedbacks change how the climate responds to an initial forcing. Positive feedbacks amplify warming; negative feedbacks dampen it.

• Ice-albedo feedback (positive) — melting ice exposes darker ocean or land, which absorbs more solar energy, causing further warming and more melting.
• Permafrost / methane feedback (positive) — thawing permafrost releases CO2 and methane, adding to warming.
• Ocean outgassing (positive) — warmer water holds less dissolved CO2, releasing it to the atmosphere.
• Negative feedbacks — e.g. warming may increase cloud cover or plant growth that removes CO2, partly offsetting change. Their strength is uncertain.

Why it matters: positive feedbacks create the risk of tipping points, beyond which change becomes self-sustaining and hard to reverse.

Spot the positive feedback

Projected impacts

Projected impacts vary by region and depend on how much warming occurs. Broadly, scientists expect:

• Ecosystems — shifting climate zones, changing species ranges and timing, coral bleaching and biodiversity loss.
• Hydrology — altered rainfall, more intense droughts and floods, and shrinking glaciers that feed major rivers.
• Sea level — rising through thermal expansion and ice melt, threatening low-lying coasts and small island states.
• Agriculture — gains in some higher latitudes but losses across many tropical regions, affecting food security.
• People — risks to health, water supply and infrastructure, and the potential for displacement and migration.

Careful with numbers: the exact magnitude of future warming, sea-level rise and impacts is uncertain and depends on emissions pathways. Judge the direction and relative scale of change rather than memorising single figures.

Attribution and uncertainty

Because this is a geographical debate, you must weigh the arguments critically:

• Attribution — separating the human (enhanced) signal from natural variability. The scientific consensus is that recent rapid warming is very largely human-caused, based on multiple independent lines of evidence.
• Uncertainty — arises from the complexity of feedbacks, the behaviour of clouds and oceans, and future emissions. Uncertainty is not the same as ignorance: the direction of change is well established even where the precise magnitude is not.
• Models — climate models are tested against past climate; they are simplifications but skilful at reproducing large-scale trends.

Evaluative point: uncertainty cuts both ways - impacts could be milder or more severe than the central estimate, which is itself an argument for managing risk.

Mitigation vs adaptation

There are two broad strategies, usually used together:

• Mitigation — reducing the cause by cutting emissions or removing carbon: renewables (wind, solar, hydro), nuclear power, carbon capture and storage (CCS), afforestation, energy efficiency, and carbon pricing (taxes or emissions trading).
• Adaptation — managing the impacts we cannot avoid: flood defences, drought-resistant crops, water management, early-warning systems and managed retreat from vulnerable coasts.

Evaluate: mitigation tackles the root cause but needs global cooperation and has upfront costs; adaptation protects people now but does nothing to slow warming and can be costly for poorer countries. A balanced response uses both.

Forcing, feedback or response?

Tap a card, then tap the category it belongs to.

Your turn — percentage change

International governance

Because emissions and impacts cross borders, tackling climate change needs global cooperation:

• UNFCCC (1992) — the UN Framework Convention on Climate Change, the overarching treaty under which countries meet at the annual COP conferences.
• Kyoto Protocol (1997) — set legally binding emissions-reduction targets for developed (Annex I) countries but not developing nations; the USA did not ratify it.
• Paris Agreement (2015) — nearly all countries agreed to hold global warming well below 2°C above pre-industrial levels and to pursue efforts to limit it to 1.5°C, through nationally determined contributions.

Evaluate: Paris is near-universal and flexible, but its pledges are voluntary and not individually binding, so delivery depends on national action.

Match the agreement or strategy

Tap a description on the left, then its match on the right.

The Paris target

The big ideas to know

You have worked through the whole Climate Change debate. Press Finish to see your score.