Carbon dioxide has gone up and down for hundreds of thousands of years without any help from us. So how do scientists know this rise is different? The answer is in the evidence — ice cores, tree rings, sediments and thermometers — and in the sheer speed of the change.
📚 What you need to know
Human activity has raised greenhouse gas concentrations sharply since the Industrial Revolution, above all carbon dioxide from burning fossil fuels.
Global warming is the rise in average temperature. Climate change is the wider set of consequences: shifted weather patterns, sea-level rise, ecosystem disruption.
Emissions accelerated sharply after 1950, driven by industrialisation, technology and population growth.
Ice cores, tree rings and sediments reveal past climates, and show a clear positive correlation between carbon dioxide and temperature.
For 800,000 years carbon dioxide stayed below about 300 ppm. It is now above 420 ppm, and average temperature has risen roughly 1.1 °C over the past century.
Positive feedback loops — melting ice lowering albedo, thawing permafrost releasing gas — amplify the warming already underway.
What changed, and when
Before the late 1700s, human societies burned wood and used muscle, wind and water for power. The Industrial Revolution replaced that with coal, then oil and gas. Every tonne of fossil fuel burned releases carbon that had been locked underground for millions of years, so it adds to the atmospheric store rather than cycling within it.
Industrial Revolution — began in Europe in the late 18th century and made fossil fuel use normal.
Technological change — the 20th century brought mass transport, electrification and rapid urbanisation, all energy-hungry.
Population growth — more people needing energy, food, materials and buildings multiplied the demand.
Together these explain why the rate of emissions did not just rise but accelerated after about 1950. Around 90% of global carbon dioxide emissions come from industry and burning fossil fuels, with deforestation and agriculture supplying much of the rest.
Deforestation counts twice. Cutting or burning a forest releases the carbon stored in the trees, and it also removes the photosynthesis that was taking carbon dioxide back out. A loss on both sides of the balance.
How we know what the climate used to be like
Thermometer records only go back to the mid-1800s. To go further we use proxies: natural records that preserve a signal of past conditions.
Evidence
How it works
What it tells us
Ice cores
Snow falls, compresses into ice and traps bubbles of air. Deeper ice is older, so a drilled core is a timeline you can read downwards
Direct samples of ancient atmospheres, giving carbon dioxide and methane concentrations going back hundreds of thousands of years
Tree rings
Trees add one ring a year, and the width depends on growing conditions. Wider rings suggest warmer, more favourable years
Year-by-year climate for the lifetime of the tree, and further back using timber and dead wood — the technique is called dendrochronology
Sediments
Lake and ocean beds accumulate layers containing pollen, shells and the remains of temperature-sensitive organisms
Indirect evidence of past temperatures, rainfall and vegetation over very long timescales
Instrumental records
Thermometers, and later satellites and ocean buoys, measuring directly
Precise measurements since the mid-1800s, confirming the sharp recent warming
Worth knowing: the air bubbles in an ice core are not a proxy at all — they are a genuine sample of ancient atmosphere. That is why the carbon dioxide record is so trusted, and why “we cannot know what the air used to be like” is not a good argument.
The carbon dioxide record
Put the ice-core data on a graph and the pattern is striking. Carbon dioxide swings up and down with the ice ages, in a cycle of roughly 100,000 years, but it stays inside a band — roughly 180 to 300 ppm. Then, in the last two centuries, it leaves the band entirely.
The natural cycles took tens of thousands of years to move 100 ppm. The modern rise has done more than that in about two hundred.
When you describe this graph, the mark is in the comparison, not the shape. Say the rise is larger and far faster than anything in the natural record, and that it starts when fossil fuel use starts.
Correlation, causation and why the case is strong
Ice-core data show carbon dioxide and temperature rising and falling together. On its own that is a correlation, and correlation does not prove causation. Examiners like this point, so make it — but do not stop there, because the full case rests on more than one line of evidence:
There is a known physical mechanism: carbon dioxide absorbs infrared radiation. This was measured in laboratories in the 1800s, long before the modern debate.
The timing matches. The sharp rise begins with industrial fossil fuel use and accelerates after 1950 with it.
The amount matches. The extra carbon dioxide is roughly what you would expect from the fuel we have burned.
Independent records — ice, trees, sediments, thermometers, satellites — all point the same way.
Feedback loops that make it worse
Warming does not act alone. It triggers processes that cause further warming, which is positive feedback: the original change is amplified. Two loops appear again and again in exams.
Feedback loops are why scientists talk about tipping points: past a certain stage, the system keeps warming even without further emissions.
The naming trap. With climate, the words work backwards from how they sound. Positive feedback amplifies a change and is usually bad news. Negative feedback counteracts a change and is usually good news, because it returns the system towards equilibrium.
Crossing a planetary boundary
Climate change is one of the nine planetary boundaries — thresholds which, once crossed, risk large and hard-to-reverse environmental change. The evidence suggests this boundary has already been crossed.
The Intergovernmental Panel on Climate Change (IPCC) assesses the published science and reports on it. Its findings are consistent: average global temperature has risen by roughly 1.1 °C over the past century with the fastest warming in recent decades, greenhouse gas concentrations have risen sharply because of fossil fuels, deforestation and agriculture, and the consequences already include more frequent heatwaves, storms and floods alongside long-term sea-level rise.
Worked examples
WORKED EXAMPLE
Using the ice-core record, explain how scientists can tell the recent rise in carbon dioxide is not part of a natural cycle. [4]
Step 1: describe the natural pattern
Over 800,000 years the concentration cycled between roughly 180 and 300 ppm, following the ice ages.
Step 2: compare the levelToday it is above 420 ppm — well beyond the top of that natural band.Step 3: compare the rate
Natural changes of about 100 ppm took tens of thousands of years. The recent change has been larger and taken about two hundred.
Step 4: link to the causeThe rise begins with industrial fossil fuel use and accelerates after 1950, matching the pattern of human emissions.Higher than natural, and far faster
WORKED EXAMPLE
Explain how melting Arctic sea ice acts as a positive feedback loop. [3]
Start the loop
Rising temperatures melt snow and sea ice, exposing darker ocean and land beneath.
The physical change
Ice has a high albedo and reflects most incoming sunlight; dark water has a low albedo and absorbs it instead.
Close the loopMore absorbed energy means more warming, which melts more ice, which lowers albedo further. The change amplifies itself.Melting → lower albedo → more absorption → more melting
💡 Exam tip
Learn three figures: 300 ppm natural ceiling, over 420 ppm today, about 1.1 °C of warming. They make any answer concrete.
Say correlation is not causation — then explain why the case is still strong. Half the point is the second half.
Always name the mechanism when describing feedback. “It gets worse” is not a loop; “lower albedo absorbs more energy” is.
Distinguish global warming (temperature) from climate change (the whole set of consequences).
For evidence questions, name the proxy and what it measures. Ice core and gas bubbles; tree ring and ring width.
⚠ Common mix-up
Positive feedback does not mean good. It means amplified. It is usually the more dangerous kind.
Ice cores are not just proxies. The trapped air is a real sample of the ancient atmosphere.
The greenhouse effect is not the cause of climate change. The enhanced greenhouse effect is. The natural one has always been here.
Thicker tree rings do not always mean warmer. Water, nutrients and shading also matter, which is why proxies are cross-checked against each other.
Carbon dioxide is not the only gas. Methane and nitrous oxide from agriculture and waste are a significant part of the picture.
“The climate has always changed” is not a counter-argument. The relevant point is the rate, and the fact that the mechanism this time is a known human one.
Up next: Climate Change and Ecosystems — coral bleaching, desertification, sea-level rise and the way whole biomes are being pushed towards the poles.
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