Recall that photosynthetic rates remain relatively constant in regions near the equator. Imagine that tropical environments persist throughout Earth's northern and southern hemispheres; that is, that Earth's entire climate mirrors 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 Straight line sloping upward (atmospheric CO2 levels would not seasonally oscillate, but would have increased over time).

Explanation:

A straight line up the graph means it has a positive slope, meaning that even though atmospheric carbon dioxide levels would not oscillate through the seasons, in this case in tropical environments in the northern and southern hemisphere Earth, that is, the climate close to the equator, the positive slope indicates that carbon dioxide levels would increase over time.