The relationship between the concentration of atmospheric carbon dioxide and the planet’s global temperature is a fundamental concept in climate science. This connection, which governs the thermal balance of the Earth, is most clearly illustrated by historical graphs that chart these two variables across vast timescales. Understanding the data that forms these charts is central to grasping how human activity is currently altering the climate system.
Defining the Variables of the Graph
The historical graph linking the atmosphere and temperature uses two distinct metrics. Atmospheric carbon dioxide ($\text{CO}_2$) is measured in parts per million (ppm), representing the number of $\text{CO}_2$ molecules per million molecules of air. Pre-industrial concentrations hovered around 280 ppm, but modern measurements exceed 420 ppm, indicating a significant chemical shift.
Global temperature is typically represented by a “temperature anomaly” rather than an absolute temperature. An anomaly is the difference between an observed temperature and a long-term average, usually calculated over a 30-year period. Using anomalies filters out localized variations, allowing scientists to compare temperature trends across different regions and time periods more accurately. A positive anomaly indicates the temperature was warmer than the long-term average.
Collecting the Historical Data
The long-term picture of this relationship combines data from two different scientific methods. For a deep historical record, scientists rely on ice cores drilled from thick ice sheets in Antarctica. These cores preserve small bubbles of ancient air within the ice layers.
By analyzing the trapped air, researchers directly measure the concentration of greenhouse gases, including $\text{CO}_2$, from hundreds of thousands of years ago. Temperature history is reconstructed by examining the ratios of oxygen and hydrogen isotopes in the ice itself. The longest continuous records extend back more than 800,000 years, providing a baseline for natural climate variability.
To track recent changes, scientists use continuous, direct atmospheric sampling. The most famous example is the Keeling Curve, a record of $\text{CO}_2$ concentration started in 1958 at the Mauna Loa Observatory in Hawaii. This dataset captures the rapid, year-over-year increase in atmospheric $\text{CO}_2$ driven by the burning of fossil fuels.
The Physical Link Between $\text{CO}_2$ and Heat
The physical connection between atmospheric $\text{CO}_2$ and global temperature is explained by the greenhouse effect. This natural process involves certain gases that absorb and re-emit infrared radiation radiating away from the Earth’s surface. Without this effect, the planet’s average temperature would be far below freezing.
Carbon dioxide is an effective heat-trapping gas because its molecular structure allows it to vibrate when struck by infrared energy. $\text{CO}_2$ molecules capture outgoing heat, causing them to temporarily vibrate. The molecule then quickly releases this energy, re-radiating it in all directions, including back down toward the Earth’s surface.
An increase in atmospheric $\text{CO}_2$ adds more heat-trapping molecules to the system. This intensifies the natural greenhouse effect, leading to less heat escaping into space and a corresponding rise in the planet’s surface temperature.
Interpreting the Visual Correlation
When the ice core data is plotted, the graph shows a clear, correlated pattern over the last 800,000 years. This historical relationship appears as a series of repeating peaks and valleys corresponding with the planet’s alternating glacial (ice age) and interglacial (warm) periods. $\text{CO}_2$ levels naturally fluctuated between a low of about 180 ppm during ice ages and a high of about 300 ppm during warm periods.
In these past cycles, initial warming from changes in Earth’s orbit sometimes preceded the rise in $\text{CO}_2$. This small temperature increase caused the oceans to release stored $\text{CO}_2$, which then acted as a powerful natural amplifier, driving the majority of the subsequent warming. $\text{CO}_2$ functioned as a feedback mechanism, controlling the final extent of the warming.
The modern portion of the graph shows a significant departure from this natural pattern. Since the industrial era, the rate of $\text{CO}_2$ increase has been approximately 100 times faster than any natural increase observed previously. Atmospheric $\text{CO}_2$ has spiked far beyond the natural 300 ppm limit. This modern increase is driven by human emissions, meaning $\text{CO}_2$ is now the primary cause of warming, rather than a feedback.
Observed Consequences of the Recent Rise
The sharp, upward trajectory of the temperature line translates directly into global changes. Since the late 19th century, the Earth’s average surface temperature has increased by approximately 1.5°C. This warming is having widespread environmental effects.
The consequences include:
- Accelerated rise in global sea levels, caused by the thermal expansion of seawater and the melting of glaciers and ice sheets.
- Shifts in global precipitation patterns, leading to more frequent and intense droughts in some regions.
- Heavier rainfall and flooding events in other regions.
- Increased frequency and intensity of extreme weather events, including heatwaves and tropical storms.