Final answer:
If Earth's axis were perpendicular to the orbital plane, the most obvious effect would be the elimination of seasonal variation.
Step-by-step explanation:
If Earth's axis were perpendicular to the orbital plane between Earth and the sun, the most obvious effect would be the elimination of seasonal variation. Currently, the 23.5° tilt of Earth's axis is what causes the variation in seasons. If the axis were perpendicular, the Sun's position in the sky would stay the same day after day, and the seasons would not change. This means that there would be no distinct summer, winter, spring, or fall.