A correlation is a relationship between two variables: as one changes, so does the other. In a positive correlation both increase together; in a negative correlation one increases as the other decreases. A causal relationship means that a change in one variable actually brings about the change in the other.A correlation does not prove causation. Two variables may be correlated because:
• one causes the other;
• both are caused by a third factor (a confounding variable);
• the link is due to chance.Ice cores show a strong positive correlation between atmospheric CO2 and temperature over hundreds of thousands of years. On its own this does not show that CO2 causes warming. In fact, at the end of some ice ages temperature began to rise a few hundred years before CO2. The warming, triggered by changes in the Earth's orbit, caused the oceans to release CO2, which then amplified the warming.To show causation, scientists need:
• a plausible mechanism: laboratory experiments show that CO2 absorbs infrared radiation (the greenhouse effect);
• controlled experiments where possible, in which only the independent variable is changed;
• predictions based on the cause that are tested and confirmed, for example that the lower atmosphere warms while the upper atmosphere cools.The strength of a correlation can be tested using Spearman's rank correlation coefficient, which gives a value between −1 and +1. Values close to +1 or −1 show a strong correlation. Even a significant correlation only shows an association, not a cause.
• one causes the other;
• both are caused by a third factor (a confounding variable);
• the link is due to chance.Ice cores show a strong positive correlation between atmospheric CO2 and temperature over hundreds of thousands of years. On its own this does not show that CO2 causes warming. In fact, at the end of some ice ages temperature began to rise a few hundred years before CO2. The warming, triggered by changes in the Earth's orbit, caused the oceans to release CO2, which then amplified the warming.To show causation, scientists need:
• a plausible mechanism: laboratory experiments show that CO2 absorbs infrared radiation (the greenhouse effect);
• controlled experiments where possible, in which only the independent variable is changed;
• predictions based on the cause that are tested and confirmed, for example that the lower atmosphere warms while the upper atmosphere cools.The strength of a correlation can be tested using Spearman's rank correlation coefficient, which gives a value between −1 and +1. Values close to +1 or −1 show a strong correlation. Even a significant correlation only shows an association, not a cause.