Answer: A third variable problem
Explanation: A third variable problem is said to exist if an observed correlation between variables aren't caused by any sort of causal relationship between both variables but rater due to an relationship which exist between each of the variable in the experiment and another variable which isn't part of the study. The Third variable in the context above as discovered by Dr. Richardson is temperature, whereby rising temperature affects eac of the variables in the earlier experiment, hence, leading to th me perceived relationship between them.