1.5: Correlation and Causation
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Sugar consumption is up. Global temperatures are up. Is one causing the other?
Is the rise in sugar consumption in America causing the average global temperature to rise? Are rising temperatures causing people to eat more sugar? Both of these factors are rising, but how are they related? Are they related by correlation, causation, or both?
We made a few discoveries so far in this concept:
- Average global temperature has been rising for the past several decades.
- Atmospheric carbon dioxide levels have been rising for the past several decades.
- Carbon dioxide emissions into the atmosphere from fossil fuel burning have been rising for the past several decades.
A correlation is the mutual relationship between two or more things. We see here a correlation between three things: (1) CO2 emissions from fossil fuel burning are rising (2) atmospheric CO2 levels are rising, and (3) average global temperatures are rising. These three things exhibit positive correlation. They are all going in the same direction. If one factor rises while another sinks, they have negative correlation. For example, the amount of forest land is decreasing. This factor has a negative correlation with global temperature.
But correlation does not necessarily indicate causation. Causation refers to the factor that is producing the effect. If I push a toy car, I will cause it to move. To explain the difference between correlation and causation, let’s look at an example. Sugar consumption in the United States has been rising for decades (Figure below). There is a positive correlation between sugar consumption and rising average global temperatures.
American consumption of sugar, 1970-2009.
Is the rise in sugar consumption causing the rise in global temperatures? Is the rise in global temperatures causing the rise in sugar consumption? These two things aren't impossible, but they are extremely unlikely. There’s no mechanism for one to increase the other. Here there is correlation, not causation. To establish causation we need to know how one would cause the other.
Fossil Fuel Burning Causes Temperatures to Rise
Here is a brief outline of the way an increase in CO2 can increase global temperatures. This is a logical step-by-step argument. There is no part of this explanation that goes beyond the evidence.
- Greenhouse gases in the atmosphere trap heat. CO2 is a greenhouse gas.
- Burning fossil fuels releases CO2 into the atmosphere.
- The more greenhouse gases there are in the atmosphere, the more heat can be trapped.
- The more heat that’s trapped, the warmer average global temperatures are.
Because carbon dioxide is a greenhouse gas, increased CO2 in the atmosphere causes average global temperatures to rise. We found one cause for rising global temperatures. There are also others. Climate change science is dealt with extensively in the chapters Atmospheric Processes and Weather and Climate.
- Correlation is a comparison of two factors within a population. Correlation does not imply causation.
- If one factor is responsible for the change in another factor, there is causation.
- Establishing causation requires a mechanism to show how one factor can influence the other.
- Burning fossil fuels releases CO2 into the atmosphere. That CO2 traps heat, which causes global temperatures to rise.
- Describe correlation. Describe causation.
- Sugar consumption has been rising in the U.S. for decades. Can you think of something this might be positively correlated with? Could one be causal of the other?
- Outline the logical steps that link fossil fuel emissions to rising average global temperature.
Use the resources below to answer the questions that follow.
- Why did people fear polio?
- Why did people think there was a correlation between ice cream and polio?
- Why are there more police in Washington, D.C. than in Denver? Where is there more crime? Do police cause crime? Then why are there more police in Washington, D.C.?
- Explain why people often confuse correlation and causation.