The climate has always changed. That is true, and it is also the sentence people use to dodge the question. The real question is whether the change happening now looks like the natural ones. It does not — and the evidence for that comes from bubbles of ancient air, rings of wood and layers of mud.
📚 What you need to know
Human activity, mainly burning fossil fuels, has raised the concentration of greenhouse gases sharply since the Industrial Revolution.
Emissions accelerated hard after 1950 because of industrialisation, transport, urbanisation and population growth all at once.
Ice cores, tree rings and sediments are proxies: natural records that let us reconstruct climate long before thermometers existed.
These records show carbon dioxide staying between roughly 180 and 300 ppm for hundreds of thousands of years. It is now above 425 ppm.
Carbon dioxide and temperature move together in the record. Correlation is not proof on its own, but combined with the known physics it is very strong evidence.
The enhanced greenhouse effect is the natural effect made stronger by extra gas. Positive feedback loops then amplify the warming further.
The change we are causing
Since about 1750 humans have been digging up carbon that took hundreds of millions of years to bury, and releasing it in a couple of centuries. Every tonne of coal, oil or gas burned puts carbon dioxide into the atmosphere. Clearing forests makes it worse twice over: burning or rotting trees release their stored carbon, and the forest that would have absorbed future carbon dioxide is gone. Agriculture adds methane from cattle and flooded rice fields, and nitrous oxide from fertiliser.
The result is two linked things that students often blur together. Global warming is the rise in average global temperature. Climate change is the wider set of consequences: shifted rainfall belts, rising seas, stronger storms, moving ecosystems. Warming is the cause; climate change is the package of effects.
Careful with wording in exams. “Global warming” is one measurable quantity going up. “Climate change” is everything that follows from it. If a question asks for impacts, they want the second.
Why 1950 is the turning point
The Industrial Revolution began in Europe in the late 18th century and started the fossil fuel habit, but the graph really bends after 1950. Three things arrived together:
Technology and industry spread worldwide. Cars, aircraft, cement, steel and chemicals became normal in far more countries, not just a few.
Cities grew fast. Urban living raises energy use per person: buildings to heat and cool, roads, freight, electricity grids.
Population grew steeply. More people, each using more energy than their grandparents did, multiplies out very quickly.
That is the point to make in an exam answer. It is not just “there were more people”. It is more people and higher energy use per person and a wider spread of industry — three multipliers stacking on top of each other.
How we know what the air used to be like
Thermometers only go back to the mid-1800s. Everything before that comes from proxies — natural things that recorded the climate as they formed, and that we can read back afterwards.
🧩 How an ice core gives you ancient air
Snow falls on an ice sheet in Antarctica or Greenland, with air trapped in the gaps between the flakes.
More snow buries it and compresses it into ice. The trapped air is sealed into bubbles.
Layers stack up year on year, so depth becomes a timeline: the deeper you drill, the older the ice.
Scientists drill out a core and count or date the layers to work out the age at each depth.
The bubbles are crushed open and the gas is analysed, giving a direct sample of the atmosphere from that year.
Oxygen isotopes in the ice itself reveal how cold it was, so you get temperature and carbon dioxide from the same core.
Tree rings work on a similar idea. A tree adds one ring a year, and the ring is wider in a warm, wet, favourable year. Counting back through the rings of an old tree gives a year-by-year record of local growing conditions, and cores taken from very old trees or preserved timber can reach back many centuries. Sediments on lake and ocean floors build up in layers too, and the pollen, shells and remains of temperature-sensitive organisms in each layer tell you what the environment was like when that layer formed.
Why use several proxies? Each one has weaknesses. Tree rings also respond to rainfall and soil, ice cores only exist near the poles. When separate methods with different weaknesses agree, the conclusion gets much harder to argue with.
What the record actually shows
The point of this graph is not the wiggles — those are natural ice ages coming and going. The point is that the modern value sits completely outside the range the planet has used for the whole of human existence.
Temperature reconstructed from the same cores rises and falls in step with the carbon dioxide. Instrumental records since the mid-1800s then take over and show a clear upward trend, with short dips along the way but no doubt about the direction. Roughly 1.1 °C of warming has happened over the past century, and most of it recently.
A correlation on its own would be weak. What makes this convincing is that we also know, from laboratory physics done in the 1850s, exactly why carbon dioxide absorbs infrared. The graph agrees with the mechanism, and the mechanism predicted the graph.
The enhanced greenhouse effect
Nothing new is happening chemically. The gases absorb infrared exactly as they always did. There is simply more of them, so a larger share of the outgoing infrared is absorbed and sent back down, and the surface has to warm until enough heat escapes to balance the incoming sunlight again. Around 90% of global carbon dioxide emissions come from industry and fossil fuel burning, and human activity accounts for essentially all of the increase over the last 150 years.
The difference in one line
natural greenhouse effect = necessary • enhanced greenhouse effect = the same process pushed too far
Feedback loops make it worse
Once warming starts, the climate system responds — and some of those responses feed straight back into the warming. A positive feedback loop amplifies the original change. A negative feedback loop damps it down and pushes the system back towards where it was.
The same loop runs in reverse during an ice age: cooling grows the ice, the brighter surface reflects more sunlight, and the cooling deepens. Positive feedback does not mean “good” — it means “self-reinforcing”.
The other loop you must know is permafrost. Frozen ground across the Arctic holds huge amounts of carbon in organic matter that froze before it could rot. As the ground thaws, that material decomposes and releases carbon dioxide and methane, which warms the atmosphere further, which thaws more ground.
Evidence source
What it records
Roughly how far back
Instrumental thermometer records
Directly measured air and sea temperature
Mid-1800s to now
Ice cores
Trapped air bubbles giving past greenhouse gas levels, plus isotope temperatures
Hundreds of thousands of years
Tree rings
Yearly growth conditions: warmth and water availability
Centuries to a few thousand years
Lake and ocean sediments
Pollen and remains of climate-sensitive organisms, layer by layer
Thousands to millions of years
WORKED EXAMPLE
Explain how ice cores provide evidence for the relationship between carbon dioxide and global temperature. [4]
Point 1 — how the record forms
Snow falling on ice sheets traps air; further snow compresses it into ice and seals the air into bubbles, so each layer holds a sample of the atmosphere from that time.
Point 2 — how it is datedLayers accumulate annually, so depth in the core corresponds to age — deeper ice is older.Point 3 — the two measurementsGas from the bubbles gives past CO₂ concentration, while oxygen isotope ratios in the ice itself give past temperature.Point 4 — the conclusionBecause both come from the same core at the same depth, they can be plotted together, and they show a clear positive correlation across many glacial cycles.4 / 4Point 3 is the one most students miss. Say where each variable comes from.
WORKED EXAMPLE
A student says: “Carbon dioxide and temperature are correlated, so carbon dioxide causes warming.” Evaluate this statement. [3]
Step 1 — concede the weakness
Correlation alone does not prove causation; two variables can rise together because a third factor drives both, or by coincidence.
Step 2 — give the supporting evidenceHowever, the warming mechanism is independently known: CO₂ absorbs outgoing long-wave radiation, which is measurable in a laboratory.Step 3 — reach a judgementCorrelation across many independent proxies, plus a known physical mechanism, plus modern direct measurements, together make the causal claim well supported.3 / 3“Evaluate” means give both sides and then commit to an answer. Sitting on the fence loses the last mark.
💡 Exam tip
Learn two numbers: the natural range of roughly 180–300 ppm, and today’s value above 425 ppm. A number in an answer is worth a lot.
When asked for causes, name the activity and the gas: fossil fuel burning gives CO₂, livestock and rice give CH₄, fertiliser gives N₂O.
For “how do we know” questions, always name at least two different proxies and say why using several matters.
Explain feedback loops as a circle, and finish by returning to the starting box. Half a loop is half the marks.
Say enhanced greenhouse effect when you mean the human-caused part. It is a mark-scheme word.
Do not just say “population growth”. Say population growth combined with rising energy use per person.
⚠️ Common mix-up
Positive feedback means good. It means self-amplifying. In climate it is usually the dangerous kind.
Thinking proxies measure temperature directly. They record something that depends on climate, which we then interpret.
Claiming the climate never changed before. It did, repeatedly. The argument is about the speed and cause of the current change.
Mixing up warming and climate change when a question asks specifically for one of them.
Forgetting deforestation counts twice — released carbon plus lost future absorption.
Writing “the ozone hole causes global warming”. Two separate problems with two separate causes.
Up next: Climate Change and Ecosystems — what all this actually does to coral reefs, forests, deserts and the species living in them.
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