Recall that photosynthetic rates remain relatively constant in regions near the equator. Imagine that tropical environments persisted throughout Earth's northern and southern hemispheres (i.e., Earth's entire climate mirrored that near the equator).
If Keeling had collected his atmospheric CO2 data on such an Earth, what would you expect the Keeling Curve to look like?

(A) a straight line without a slope (atmospheric CO2 levels would have remained constant over time)
(B) a sinusoidal curve sloping downward (atmospheric CO2 levels would fluctuate seasonally, but would have decreased over time)
(C) a straight line sloping downward (atmospheric CO2 levels would not seasonally oscillate, but would have decreased over time)
(D) a sinusoidal curve sloping upward (atmospheric CO2 levels would fluctuate seasonally, but would increase over time)
(E) a straight line sloping upward (atmospheric CO2 levels would not seasonally oscillate, but would have increased over time)

Keeling has collected various data over the last few decades regarding the earth's atmospheric CO₂, and with the help of the photosynthesis process, he tried to determine the Keeling curve in the graph. He also considers that the earth climatic condition is the same everywhere like in the equator.

In the tropical and equatorial areas, the temperature is relatively high and also the area receives a maximum amount of sunlight and rainfall. This region supports a large number of species. This region has a higher concentration of CO₂. In addition to the increasing industrial development and consumption of fossil fuels, the CO₂ content has drastically increased over the last few decades.

From the experiment, he observed that the CO₂ content is maximum during the spring season as the plant matters undergo decomposition it releases a large amount of CO₂ into the atmosphere. But this CO₂ content is minimum during the autumn season because plants absorb the lesser amount of CO₂ in order to carry out the process of photosynthesis.

Thus, the Keeling curve shows a straight line sloping upward in the graph, which describes that the CO₂ content will be high in the tropical and near-equatorial environment.

Hence, the correct answer is option (E).