How would seasons on earth differ if earth's axis was at a 90 degree angle relative to its orbit?

seasons on earth would not be affected.

the entire planet would experience the same season at any given time.

there would be no seasons anywhere on earth.

it would always be summer at the equators and winter at the poles.

There would be no seasons anywhere on Earth

Explanation:

If the Earth's axis was not tilting and it was at a 90 degrees angle relative to its orbit, then the seasons on Earth would disappear. The climate on the planet would be defined by the latitude. The equator and the areas close to it would always have summer, as they will receive direct sunlight throughout all of the year. The mid-latitudes will always have temperate climate, reminding of spring and autumn, as the sunlight will not be very direct, but it will also not be very dispersed, jut enough to create a constantly pleasant climate. The higher latitudes will always have winter, as the sunlight will come at a very low angle throughout all of the year, thus it will be very dispersed, not enough to ever warm this parts enough, so they will constantly be under ice.

The entire planet would experience the same season at any given time.

Explanation:

Due to the tilt, some parts of the earth will be tilted towards the earth at a particular point in the orbit while others will be tilted away from the sun. This means the amount of sunlight reaching these points is differential. This also changes dynamically, more pronounced in the poles, as the earth revolves around the sun. Without the tilt, this phenomenon would not be therefore and there would be no changes as the earth revolves around the sun.