What is the evidence that the climate is changing, and how do we distinguish natural variability from human-driven warming?
Evaluate the evidence for recent climate change and assess the relative roles of natural and anthropogenic causes
A focused answer to the H2 Geography outcome on climate change. The lines of evidence for recent warming, natural forcings, the enhanced greenhouse effect, and how attribution distinguishes human from natural causes.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SEAB wants you to evaluate the evidence that the climate is changing and to weigh the natural and human causes of recent warming. The central insight is that climate has always varied for natural reasons, but the rapid warming of the past century carries a distinctive human fingerprint, identified by attribution science.
The answer
Lines of evidence for recent warming
The case for warming rests on several independent lines, which is what makes it robust:
- Instrumental records: global mean surface temperature has risen by roughly 1.1 degrees Celsius since pre-industrial times.
- Cryosphere: glaciers and ice sheets are shrinking and Arctic sea-ice extent is declining.
- Sea level: rising through thermal expansion of warming oceans and added meltwater.
- Oceans: warming and acidifying as they absorb heat and carbon dioxide.
- Biological indicators: species ranges shifting poleward and to higher altitudes, and earlier spring events (phenology).
- Palaeoclimate proxies: ice cores, tree rings and ocean sediments reconstruct past climate and place current warming in a long context.
Natural causes of climate change
Climate varies naturally over many timescales:
- Milankovitch cycles: slow variations in the Earth's orbit and tilt drive the ice-age cycles over tens of thousands of years.
- Solar variability: changes in the Sun's output alter the energy received.
- Volcanic eruptions: large eruptions inject reflective aerosols, causing short-term cooling.
- Internal variability: the El Nino Southern Oscillation and similar oscillations redistribute heat between ocean and atmosphere year to year.
These explain past climate change and short-term fluctuations.
Human causes: the enhanced greenhouse effect
Burning fossil fuels, deforestation, cement production and agriculture have raised atmospheric carbon dioxide from about 280 parts per million before industrialisation to over 420 today, alongside rising methane and nitrous oxide. These gases absorb more outgoing longwave radiation, enhancing the natural greenhouse effect and trapping additional heat.
Attribution: separating human from natural
Several fingerprints point to human causes rather than natural forcing:
- The warming is concentrated since about 1950, faster than orbital or solar changes can produce.
- The stratosphere has cooled while the surface warmed, the signature of greenhouse forcing (solar forcing would warm both).
- Solar output has been flat or slightly declining recently while temperatures rose.
The conclusion is that natural forcings explain long-term and short-term variability, but the recent rapid trend is attributed mainly to human emissions.
Examples in context
Example 1. The Keeling Curve and Mauna Loa. Continuous measurements of atmospheric carbon dioxide at Mauna Loa since 1958 produce the Keeling Curve, showing a steady rise from about 315 to over 420 parts per million with a clear seasonal saw-tooth. It is among the most direct demonstrations that human emissions are accumulating in the atmosphere, anchoring the attribution of recent warming.
Example 2. Climate change pressures on Singapore and the region. Singapore has documented warming of over one degree Celsius since the 1950s and projects further rises in temperature, more intense rainfall and sea-level rise. Its National Climate Change Study and adaptation planning illustrate how a low-lying equatorial city-state translates global evidence into local risk, linking the science to coastal protection and urban cooling.
Try this
Q1. State three independent lines of evidence that the climate is warming. [3 marks]
- Cue. Rising instrumental surface temperatures; shrinking glaciers and Arctic sea ice; rising global sea level (also warming oceans, shifting species, palaeoclimate proxies).
Q2. Explain one natural cause of long-term climate change. [2 marks]
- Cue. Milankovitch cycles: slow changes in the Earth's orbit and axial tilt alter the distribution and amount of insolation, pacing the glacial and interglacial cycles over tens of thousands of years.
Q3. Explain one piece of evidence that recent warming is human-caused rather than solar. [3 marks]
- Cue. The stratosphere has cooled as the surface warmed; an enhanced greenhouse effect traps longwave radiation low down, cooling the upper atmosphere, whereas increased solar output would warm both layers.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original12 marksAssess the relative importance of natural and human causes in explaining recent global warming.Show worked answer →
Argument: although natural forcings influence climate over many timescales, the rapid warming since the mid-twentieth century is best explained by the human-driven enhancement of the greenhouse effect, with natural factors playing a secondary role.
Natural causes to set out: variations in the Earth's orbit (Milankovitch cycles) drive ice ages over tens of thousands of years; changes in solar output vary energy received; large volcanic eruptions inject aerosols that cause short-term cooling; and internal variability such as the El Nino Southern Oscillation redistributes heat year to year. These explain past climate change and short-term wobbles.
Human causes: burning fossil fuels, deforestation, cement production and agriculture have raised atmospheric carbon dioxide from about 280 to over 420 parts per million, plus methane and nitrous oxide, enhancing the greenhouse effect and trapping more longwave radiation.
Attribution: the timing and pattern point to human causes, warming concentrated since 1950, faster than orbital or solar changes can explain, with stratospheric cooling alongside surface warming (the fingerprint of greenhouse rather than solar forcing), and solar output flat or slightly declining recently while temperatures rose.
Evaluation and marks: a strong answer weighs the two, concluding that natural forcings explain long-term and short-term variability but not the recent rapid trend, which is attributed mainly to human emissions. Markers reward distinguishing natural from human forcings, the carbon-dioxide rise, and the attribution reasoning.
Original10 marksEvaluate the strength of the evidence that the global climate is warming.Show worked answer →
Argument: multiple independent lines of evidence converge on warming, which makes the conclusion robust, though each line has limitations.
Lines of evidence: instrumental records show global mean surface temperature rising by roughly 1.1 degrees Celsius since pre-industrial times; shrinking glaciers and ice sheets and declining Arctic sea-ice extent; rising global sea level from thermal expansion and melt; warming and acidifying oceans; shifts in species ranges and earlier spring events (phenology); and palaeoclimate proxies such as ice cores, tree rings and ocean sediments that place current warming in a long context.
Evaluation of reliability: instrumental records are precise but short and need correction for changing station coverage and the urban heat island; proxies extend the record but have wider uncertainty; satellite data give global coverage but only since 1979. Because independent methods agree, the conclusion is strong despite individual limitations.
Markers reward a range of evidence types, a comment on the reliability of each, and the argument that convergence of independent lines makes the case robust.
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