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Greenhouse Gas
Evidence

Analyze how data links greenhouse gases to rising global temperatures, and learn to interpret the scientific evidence behind climate change.

Earth & Space Science

What Are Greenhouse Gases?

Greenhouse gases (GHGs) are molecules in Earth's atmosphere that absorb and re-emit infrared radiation: heat energy radiating from Earth's surface. Without them, Earth's average temperature would be about \( -18^\circ \text{C} \) instead of the livable \( +15^\circ \text{C} \) we enjoy today.

\( \text{CO}_2 \)
Carbon dioxide: released by burning fossil fuels, deforestation, and respiration.
\( \text{CH}_4 \)
Methane: emitted from livestock, landfills, and natural gas systems.
\( \text{N}_2\text{O} \)
Nitrous oxide: from agriculture, industrial processes, and combustion.

The Greenhouse Effect Explained

Earth's Surface Atmosphere (GHG layer) Sun Solar radiation IR heat Re-emitted Escapes to space G G G

Why Does This Matter?

A natural greenhouse effect is essential for life: it keeps the planet warm enough for liquid water. The problem arises when human activities add extra greenhouse gases, strengthening the effect and causing temperatures to rise beyond natural levels.

1
Burning fossil fuels (coal, oil, gas) releases billions of tons of \( \text{CO}_2 \) every year.
2
More GHGs trap more infrared heat, raising global average temperature.
3
Data from ice cores, satellites, and weather stations all confirm this warming trend.

Core Evidence: \( \text{CO}_2 \) and Temperature

280 320 360 400 420
\( \text{CO}_2 \text{ (ppm)} \)
 1880 1920 1960 2000 2024
\( \text{CO}_2 \) (ppm)
Temp. Anomaly

Ice Core Evidence: 800,000 Years

800k 600k 400k 200k Now Years before present 420 ppm Natural range: 180-280 ppm
\( \text{CO}_2 \)
Temp.

Correlation vs. Causation

Seeing two variables rise together does not automatically prove one causes the other. So how do scientists move from correlation to causation for greenhouse gases and temperature?

Correlation
Two variables change together in a pattern. \( \text{CO}_2 \) and temperature both rise over time: a strong positive correlation.
Causation
One variable directly produces a change in the other. Lab experiments confirm \( \text{CO}_2 \) absorbs infrared radiation: a physical mechanism.

Multiple Lines of Evidence

Ice Cores
Ancient air bubbles show \( \text{CO}_2 \) and temperature tracked together for 800,000 years.
Direct Measurements
Mauna Loa Observatory has measured rising \( \text{CO}_2 \) continuously since 1958 (Keeling Curve).
Satellite Data
Satellites measure outgoing infrared radiation and confirm less heat is escaping at GHG wavelengths.
Climate Models
Computer simulations only match observed warming when human GHG emissions are included.

How to Analyze the Data

1
Identify the variables: What is being measured on each axis? (e.g., \( \text{CO}_2 \) in ppm, temperature anomaly in \( ^\circ \text{C} \)).
2
Determine the time scale: Does the graph span decades, centuries, or millennia?
3
Look for correlation: Do the two lines rise together, fall together, or show no pattern?
4
Check for a mechanism: Is there a known scientific reason why one variable would affect the other?
5
Consider alternative explanations: Could solar cycles, volcanic activity, or other factors explain the pattern instead?

Example 1: The Keeling Curve

310 330 350 370 420
\( \text{CO}_2 \text{ (ppm)} \)
 1958 1975 1992 2008 2024 420 ppm 315

Example 2: Natural vs. Human Factors

-0.2 +0.3 +0.8 +1.2
\( \text{Temp. Anomaly } (^\circ\text{C}) \)
 1900 1930 1960 1990 2020 Natural only Natural + Human Observed

Example 3: \( \text{CH}_4 \) Outsized Impact

Not all greenhouse gases are equal. Methane (\( \text{CH}_4 \)) is present in much smaller quantities than \( \text{CO}_2 \), but it traps heat far more effectively per molecule.

\( \text{CO}_2 \) \( \text{CH}_4 \)
Concentration ~420 ppm ~1.9 ppm
Global Warming Potential (100 yr) 1 (baseline) 28
Atmospheric Lifetime 300-1000 years ~12 years
Main Source Fossil fuel combustion Agriculture, landfills

The Carbon Isotope Fingerprint

Scientists can distinguish natural \( \text{CO}_2 \) from fossil fuel \( \text{CO}_2 \) by examining carbon isotopes. Fossil fuels are enriched in the lighter isotope \( ^{12}\text{C} \), while the atmosphere's ratio of \( ^{13}\text{C} \) to \( ^{12}\text{C} \) has been declining: a clear human fingerprint.

\( ^{13}\text{C} / ^{12}\text{C} \) Ratio
Declining since industrialization: proves the extra \( \text{CO}_2 \) comes from burning ancient carbon, not volcanoes or oceans.
Why It Matters
This isotopic evidence rules out natural sources as the primary cause of rising \( \text{CO}_2 \), strengthening the case for human causation.

Common Mistakes to Avoid

✕ Confusing Weather and Climate
"It snowed today, so global warming is fake." Weather is short-term; climate is the long-term average over 30+ years.
✕ Correlation = Causation
Citing only the graph without mentioning the physical mechanism (infrared absorption) or lab evidence.
✕ Ignoring Multiple Evidence
Relying on a single data source. Strong claims need ice cores, direct measurements, satellite data, and models together.

Summary of Evidence Types

Evidence Type What It Shows Time Scale
Ice Cores \( \text{CO}_2 \) and temp track together over glacial cycles 800,000 years
Keeling Curve Continuous rise in atmospheric \( \text{CO}_2 \) 1958-present
Carbon Isotopes Extra \( \text{CO}_2 \) comes from fossil fuels 200 years
Satellite Data Less infrared escaping at GHG wavelengths 1970-present
Climate Models Only human + natural factors match observations 1900-present

Question 1 of 5

The Keeling Curve shows atmospheric \( \text{CO}_2 \) rising from about 315 ppm in 1958 to about 420 ppm today. What does this data directly demonstrate?

Question 2 of 5

A student says: "Ice core data proves that \( \text{CO}_2 \) causes temperature change." What is the most accurate response?

Question 3 of 5

Methane (\( \text{CH}_4 \)) has a Global Warming Potential (GWP) of 28. If a farm releases 5 tons of \( \text{CH}_4 \), what is the \( \text{CO}_2 \)-equivalent impact over 100 years?

Question 4 of 5

Climate models that include only natural factors (solar variation, volcanic eruptions) fail to reproduce observed warming since 1950. What does this evidence support?

Question 5 of 5

Scientists observe that the ratio of \( ^{13}\text{C} \) to \( ^{12}\text{C} \) in atmospheric \( \text{CO}_2 \) has been decreasing over time. What does this indicate?