A team of environmental scientists believes that global warming will have a negative impact on tropical fish populations. They design an experiment to model the effects of global warming. First, they fill two aquariums with salt water and put a dozen tropical fish in each. The water temperature in both aquariums begins at 25 degrees C. Aquarium A is kept at a consistent 25 degrees C, but Aquarium B's water temperature is gradually raised by 1 degree C each week of the experiment. At the end of week 8, 8 of the 12 fish in Aquarium A survived and 9 of 12 fish in Aquarium B survived. The scientists conclude that global warming does not pose a significant threat to tropical fish. What was the independent variable?