(Part 2 in the “Think Like a Scientist” Series)
Last time, we explored how to evaluate scientific evidence -
asking questions about reliability, accuracy, validity, and bias.
The steps in this post follow that same way of thinking: questioning, testing,
weighing evidence, and drawing reasoned conclusions.
🌍 The Big Question
Now we’re going to apply those skills to one of the biggest
scientific debates of our time:
“Is human activity really responsible for climate
change?”
Some people say it isn’t. Others argue the evidence
overwhelmingly shows that it is.
In science, we don’t choose sides - we test ideas against evidence.
🧠 Before We Begin
Climate change is an enormous and complex topic - entire
university degrees are devoted to it - so this post can’t cover everything.
Instead, it focuses on the question from a GCSE Science point of view:
- what
you need to know to evaluate evidence,
- how
to recognise reliable data, and
- how
to think like a scientist when faced with a big, controversial
question.
Think of it as a chance to practise exam-style skills
while seeing how those same principles work in real science.
💡 A Quick Note
This post is designed to stretch your thinking.
Don’t worry if you can’t follow every detail on your first read - focus on how
scientists think, not just what they know.
(There’s a Student Summary Sheet at the end to help you review the key
ideas.)
🔍 Step 1: If human activity isn’t responsible for climate change, what is?
Before scientists accept any conclusion, they ask what other
causes could explain the data.
Here are some of the main natural or alternative explanations often discussed -
including one that came from a recent statistical paper in Norway.
|
Evidence or Claim |
What It Suggests |
How Scientists Evaluate It |
|
Natural cycles (Milankovitch cycles) |
Earth’s climate has changed before without humans. |
True - but those cycles happen over tens of thousands of
years, not decades. The current rate of warming is far faster. |
|
Volcanic activity |
Volcanoes release CO₂, so they could cause warming. |
Volcanoes emit less than 1 % of annual CO₂ compared
with humans. Major eruptions often cause cooling because of dust and
sulphur aerosols. |
|
Human CO₂ is only 3–5 % of total CO₂ |
Natural sources produce far more CO₂, so human output
seems too small to matter. |
True for gross (total) emissions, but natural CO₂
is mostly re-absorbed each year. Human emissions are an extra, unbalanced
addition, causing the steady rise seen in the atmosphere. |
|
Solar output changes |
Maybe the Sun has become stronger. |
Satellite data show solar output has slightly decreased
since the 1970s while global temperatures rose sharply. |
|
Urban heat effect |
Cities are warmer, so global data might be biased. |
Scientists correct for this by using rural stations, ocean
buoys, and satellites. The overall warming trend remains. |
|
Short-term variation (El Niño / La Niña) |
Natural patterns might explain temperature swings. |
They create short-term ups and downs, but the long-term
global trend keeps rising. |
|
Statistical study (Dagsvik & Moen, 2023) |
A discussion paper from Statistics Norway argued that
man-made CO₂ might not strongly affect temperature. |
Not peer-reviewed; based on statistical correlation, not
physical modelling*. The authors note their results don’t disprove
human influence, and Statistics Norway clarified it isn’t their institutional
view. |
* What does that mean?
Statistical
correlation, not physical modelling
A statistical correlation looks for patterns in data - for example, when
CO₂ levels go up, do temperatures also go up? But it doesn’t explain why.
Physical models, on
the other hand, use real-world science, such as how greenhouse gases absorb
infrared radiation - to show how temperature changes happen.
Scientists prefer physical
models because they are based on tested laws of physics, not just numbers that
seem to move together.
Scientists test each of these ideas carefully, looking for
patterns that fit all the data.
Sometimes alternative explanations work for part of the evidence - but not for
everything.
When the natural factors don’t match the scale or speed of current warming,
scientists look for another explanation that does.
🌡️ Step 2: Following the Evidence
Now let’s see what the wider body of evidence shows.
|
Type of Evidence |
What It Shows |
Evaluation |
|
CO₂ data (Mauna Loa Observatory) |
Atmospheric CO₂ rose from ≈ 315 ppm* (1958) to >
420 ppm today. |
Reliable long-term dataset verified by multiple labs. |
|
Carbon isotopes* (¹²C / ¹³C) |
The extra CO₂ has a fossil-fuel chemical signature. |
Valid link confirming human source. |
|
Temperature records |
Global temperature + 1.2 °C since 1880; most rapid rise
after 1950. |
Highly consistent across NASA*, NOAA, Met Office
and JMA. |
|
Climate models |
Only reproduce observed warming when human emissions are
included. |
Peer-reviewed, tested, and validated over decades. |
|
Scientific consensus |
≈ 97 % of publishing climate scientists agree humans drive
recent warming (source NASA). |
Based on thousands of independent, peer-reviewed studies. |
* What does that mean?
ppm (parts per million)
ppm stands for parts per million. It’s a way of measuring very small
amounts of gas in the air.
For example, when
scientists say atmospheric CO₂ is about 420 ppm, it means that out of
every one million air molecules, about 420 are carbon dioxide
molecules.
That might sound tiny, but
those few hundred molecules trap enough heat to make a big difference to
Earth’s climate.
Fossil fuel chemical
signature (¹³C and ¹⁴C) - How can scientists tell CO₂ comes from fossil fuels?
First, what’s an
isotope?
An isotope is a form of the same element that has the same number of
protons but a different number of neutrons in its nucleus, and therefore
different masses.
For carbon, the main
isotopes are ¹²C, ¹³C, and ¹⁴C. They all behave like
carbon, but they have slightly different masses:
¹²C – the lightest and most common
¹³C – a bit heavier and rarer
¹⁴C – radioactive and unstable; it decays over thousands of years
¹⁴C is made naturally in the upper atmosphere when cosmic rays hit nitrogen atoms. Living things constantly take in ¹⁴C while alive, keeping their ratio of ¹⁴C to ¹²C roughly constant. When they die, the ¹⁴C slowly decays away.
Living plants prefer to
absorb ¹²C during photosynthesis, so they contain less ¹³C. Fossil fuels
are made from ancient plants, which means they’re also rich in ¹²C and have no
¹⁴C left (it has long since decayed).
When scientists measure
today’s atmosphere, they find that the extra CO₂ being added contains less
¹³C and almost no ¹⁴C - exactly the pattern expected if the carbon
is coming from burning fossil fuels, not volcanoes or oceans.
- Deep
inside Earth, some carbon is stored in rocks. When volcanoes erupt, they
release CO₂ along with ash and gases.
- The ocean both absorbs and releases CO₂. When water warms, some CO₂ escapes into the air; when it cools, it takes CO₂ back in. This acts like a natural “breathing” system that keeps carbon levels steady.
Who are NASA, NOAA,
Met Office and JMA?
These are major scientific organisations that collect and analyse global
climate data:
NASA – National Aeronautics and Space Administration (USA)
NOAA – National Oceanic and Atmospheric Administration (USA)
Met Office – The UK’s national weather and climate service
JMA – Japan Meteorological Agency
Each organisation runs its
own climate monitoring systems, but all four produce very similar results,
which makes their findings more reliable.
🧠 Step 3: What the Evidence Shows Overall
A good scientist doesn’t ignore alternatives - they test
them.
It’s also important to remember that correlation doesn’t always mean causation.
Just because global temperatures and CO₂ levels rise together doesn’t automatically prove one causes the other - scientists test this by comparing data from different time periods, natural events, and climate models.
When those tests consistently show that temperatures only rise when human CO₂ emissions increase, the evidence for causation becomes much stronger.
When natural causes (Sun, volcanoes, cycles) are included in
models, they can’t fully reproduce the rapid warming observed since 1950.
When human greenhouse-gas emissions are added, the models fit the observations
much more closely.
When all the evidence is considered together, the pattern
seems to point towards human influence as the main driver of recent change.
However, scientists continue to test new data and models to check whether other
factors might still play a role - that’s how scientific understanding develops.
The combination of CO₂ records, isotope data, and
temperature measurements currently provides the most consistent explanation:
that extra carbon in the atmosphere is mainly from burning fossil fuels, which
increases heat trapped by the Earth.
In science, no single answer is ever final - the best
explanation is simply the one that fits all the available evidence so
far.
Ask yourself:
• Which evidence is most direct or reliable?
• Which explanations fail or succeed when tested?
• How could new data change our confidence in current conclusions?
Scientific confidence grows when multiple lines of evidence
point the same way - but scientists keep testing to see if that picture still
holds.
That’s why consensus matters: it shows where the evidence
currently leads, not where the discussion ends.
⚖️ Step 4: Bias and Transparency in Science
When scientists publish research, they must declare who
funded it and any possible conflicts of interest.
This doesn’t automatically mean their results are biased, but it allows others
to judge independence and repeat the work.
Different groups may have different motivations - energy
companies, governments, or environmental organisations.
What matters is whether the methods and data are open for checking.
The same rule should apply to anyone making public claims
about climate change - journalists, influencers, or politicians.
Being open about funding and methods helps build trust; hiding it damages
credibility.
Tip:
When evaluating a claim, ask:
- Who
funded or benefits from this statement?
- Has
it been peer-reviewed or reproduced by independent teams?
- Would
the conclusion hold if another group repeated the study?
Transparency is a key part of good science - it keeps
research honest and self-correcting.
🧾 Step 5: Model GCSE-Style Question & Answer
Question:
Evaluate the claim that human activity is not responsible for climate
change. (6 marks)
💡 Exam Tip:
In “evaluate” questions, always:
1️. Present evidence for and against,
2️. Comment on the quality or reliability
of that evidence, and
3️. Finish with a clear, balanced judgement backed by
data.
Indicative Content
Arguments suggesting human activity is not responsible:
- The
climate has changed naturally before (e.g. ice ages, Milankovitch cycles).
- Solar
output, volcanic activity, and ocean patterns can affect temperature.
- Some
studies (e.g. Dagsvik & Moen, 2023) question the strength of the link
between CO₂ and temperature rise.
- Human
CO₂ emissions make up only about 3–5 % of total annual CO₂ - some
interpret this as too small to cause major change.
Arguments suggesting human activity is
responsible:
- Global
CO₂ concentrations have increased from ≈ 315 ppm (1958) to > 420 ppm
today.
- Carbon-isotope
evidence (¹³C / ¹⁴C ratios) shows the extra CO₂ comes from fossil fuels.
- Climate
models only reproduce observed warming when human emissions are included.
- Satellite,
ocean and surface data from NASA, NOAA, Met Office and JMA all show the
same long-term warming pattern.
- The
scientific consensus (≈ 97 %) supports human-driven warming, based on many
peer-reviewed studies.
🎯 Mark Scheme (AO3 Evaluate)
|
Level |
Marks |
Descriptor |
|
Level 1 |
1–2 marks |
Makes simple statements about climate change; may mention
human or natural causes but with little or no evaluation. Little use of
evidence or scientific terminology. |
|
Level 2 |
3–4 marks |
Gives arguments both for and against with some supporting
evidence. Begins to weigh up reliability or validity of data (e.g. mentions
that one explanation doesn’t fit all observations). Some use of key terms
such as CO₂ or temperature trend. |
|
Level 3 |
5–6 marks |
Evaluates both sides clearly using several pieces of
accurate evidence. Judges which explanation is best supported by reliable
data and justifies reasoning with reference to scientific principles (e.g.
isotopes, models, consensus). Communicates ideas logically and precisely. |
🔬 Step 6: Reflection
Science isn’t about proving someone right or wrong - it’s
about finding the explanation that best fits the evidence available right now.
That means staying open to new data, questioning methods, and being honest
about uncertainty.
When scientists disagree, it isn’t a weakness - it’s part of
how science improves. Every new experiment, dataset, or model helps refine our
understanding.
For students, that’s the same mindset you’re practising in
your exams.
When you evaluate evidence in a 6-mark question, you’re not just revising facts
- you’re learning how to think scientifically: to weigh data, recognise
bias, and build conclusions that make sense.
So whether it’s a climate question or a classroom practical,
remember that science isn’t only about what we know.
It’s about how we think - the careful, curious, questioning way that
moves knowledge forward.
“Science is a way of thinking much more than it is a body
of knowledge.”
- Carl Sagan
Stay tuned: next time, I’ll be exploring how teachers
and tutors can teach evaluation effectively - the challenges, scaffolds, and
strategies that help students think like scientists, whether lessons are online
or in person.
🧩 Student Summary Sheet – Think Like a
Scientist: Evaluating Climate Change Evidence
🧠 Key Takeaways
- Science
is about testing explanations, not defending opinions.
- Always
look at both sides: natural causes and human causes.
- Reliable
evidence is:
- Repeated
and measured accurately,
- Reviewed
by other scientists,
- Free
from bias (not all funded or promoted by one group’s agenda).
- The
best explanation is the one that fits all the data.
🌍 What the Evidence Shows
|
Type of Evidence |
What It Shows |
Why It’s Reliable |
|
Natural factors |
Volcanic eruptions, solar cycles, and ocean patterns
affect climate. |
True - but these changes are too small or too slow to
explain modern warming. |
|
CO₂ measurements |
Levels rose from 315 ppm in 1958 to 420 ppm today. |
Continuous, precise global data. |
|
Isotopes (¹³C and ¹⁴C) |
The carbon in the air matches that from fossil fuels. |
Clear chemical “fingerprint.” |
|
Models and data |
Climate models match observations only when human
emissions are included. |
Tested and peer-reviewed. |
|
Consensus |
Around 97 % of scientists agree humans are the main cause. |
Based on decades of independent research. |
⚖️ How to Answer a 6-Mark
“Evaluate” Question
Question example: Evaluate the claim that human
activity is not responsible for climate change.
- State
both sides: mention natural and human causes.
- Use
evidence: quote data, examples, or model results.
- Comment
on reliability: how good or trustworthy is the evidence?
- Make
a judgement: which side fits all the data and why.
Sentence starters:
- “Some
evidence suggests that…”
- “However,
this may not fully explain…”
- “The
most reliable evidence shows that…”
- “Overall,
the explanation that fits best is…”
💬 Remember
- Evaluation
means weighing evidence, not guessing or choosing sides.
- Good
scientists - and good students - keep questioning, test every idea fairly,
and stay open to new evidence.
📚 Sources and Further Reading
These sources represent a mix of primary scientific data
(NASA, NOAA, IPCC), peer-reviewed studies, and example discussion papers.
If you use information like this in your own work, always:
- Check
whether it’s peer-reviewed,
- Note
who funded or published it, and
- Use
more than one source when evaluating a claim.
🌍 General Climate Data
and Evidence
- NASA
Goddard Institute for Space Studies (2024). Global Temperature Data.
Available at: https://data.giss.nasa.gov/gistemp/
- National
Oceanic and Atmospheric Administration (NOAA) (2024). Climate at a
Glance: Global Time Series. Available at:
https://www.ncdc.noaa.gov/cag/
- Met
Office (UK) (2024). State of the UK Climate. Available at:
https://www.metoffice.gov.uk/research/climate
- Japan
Meteorological Agency (JMA) (2024). Global Temperature Anomalies.
Available at:
https://ds.data.jma.go.jp/tcc/tcc/products/gwp/temp/ann_wld.html
🔬 CO₂ and Atmospheric
Measurements
- NOAA Global Monitoring Laboratory - Trends in Atmospheric Carbon Dioxide (CO2) - Moana Loa Observatory. Available at: https://gml.noaa.gov/ccgg/trends/
- Keeling,
C.D. et al. (1958–2024). Mauna Loa Atmospheric CO₂ Record. Scripps
Institution of Oceanography. Available at: https://scrippsco2.ucsd.edu/
- IPCC
(2021). Sixth Assessment Report (AR6): The Physical Science Basis.
Intergovernmental Panel on Climate Change. Available at:
https://www.ipcc.ch/report/ar6/wg1/
⚗️ Isotopic Evidence and Carbon
Sources
- Tans,
P. and Keeling, R. (2023). Trends in ¹³C/¹²C Ratios in Atmospheric CO₂.
NOAA Global Monitoring Laboratory.
- Schmitt,
J. et al. (2012). Carbon isotope constraints on the role of CO₂ in
glacial–interglacial climate change. Science, 336(6082),
pp.711–714.
- Levin,
I. and Hesshaimer, V. (2000). Radiocarbon – A Unique Tracer of Global
Carbon Cycle Dynamics. Radiocarbon, 42(1), pp.69–80.
☀️ Natural Factors and
Alternative Explanations
- Lean,
J.L. (2018). Estimating Solar Irradiance Since 1600. Geophysical
Research Letters, 45(16), pp.9529–9537.
- US
Geological Survey (2023). Volcanic Gases and Their Effects.
Available at: https://www.usgs.gov/volcanoes
- Trenberth,
K.E. et al. (2014). Natural variability and climate change:
Observations and model analysis. Climate Dynamics, 42(5–6),
pp.1385–1403.
📈 Climate Models and
Consensus
- Cook,
J. et al. (2016). Consensus on consensus: a synthesis of consensus
estimates on human-caused global warming. Environmental Research
Letters, 11(4), 048002.
- Hausfather,
Z. et al. (2020). Evaluating the performance of past climate model
projections. Geophysical Research Letters, 47(1),
e2019GL085378.
📊 Contrary or Minority
Evidence Discussed
- Dagsvik,
J.K. and Moen, S.H. (2023). To what extent are temperature levels
changing due to greenhouse gas emissions? Statistics Norway Discussion
Paper No. 1009.
- Lomborg,
B. (2001). The Skeptical Environmentalist: Measuring the Real State of
the World. Cambridge University Press.
🧠 Science and Thinking
- Sagan,
C. (1996). The Demon-Haunted World: Science as a Candle in the Dark.
Ballantine Books.
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