The seasonal contrast grows as the axial tilt rises, such that winters are colder and summers are warmer in both hemispheres. More tilt indicates more severe seasons—warmer summers and harsher winters; less tilt means milder summers and winters. About half of Earth's surface is covered by water, so this affects how much energy can be absorbed from or released into the planet's atmosphere.
If the tilt was completely flat, then there would be no seasonal variation at all. The frozen poles would never see sunlight and the equator would always be scorching hot with no ice at either end. This is not quite true: There would still be a difference between night and day, but it would be very small -- 12 hours instead of 24. The tilt has no effect on astronomical objects such as stars or planets because they don't change position relative to Earth's axis of rotation.
In fact, the tilt has no effect at all for things like oceans which flow around any changes in orientation caused by the rotation. For land masses, though, the amount of daylight changes depending on whether you're looking north or south. In the Northern Hemisphere, the tilt causes spring to come earlier and fall later, which prevents winter storms from forming over land. In the Southern Hemisphere, winter comes later and summer arrives early, which allows antarctic explorers to search for life beneath the ice.
Because seasons are created by the shifting angles at which sunlight strikes the Earth, a decrease in tilt would result in fewer severe seasons (due to its tilted axis). Less tilt, on the other hand, would result in less of a shift in angle around the orbit, resulting in colder summers and warmer winters. There would be significant changes throughout the world, with different regions experiencing different effects depending on how much change there is overall. For example, areas near the equator that experience warm temperatures all year round would likely see very little change, while regions closer to the poles would be affected more heavily.
The amount of land area covered by ice is greatest in Antarctica, where it accounts for 40% of the continent. The rest is water, mostly ocean. If the tilt decreased significantly, some of this ice would flow toward the north and south poles until it melted. Most of it, however, is so far away from the rest of Europe or North America that it would have no impact on these continents even if it did melt. But what would happen if it all did melt? That depends on who you ask. Some say that it would cause an increase in sea level because most scientists agree that the Antarctic ice sheet acts as a brake on our planet's rotation. However, others say that because the weight of the ice is spread out over a large area, it would not cause much change at all.
Because of the axial tilt, the days are longer than the nights in the summer and shorter in the winter. The days and nights would be the same length if the tilt angle was zero, and there would be no seasons. The higher latitudes would not experience the weather extremes as they do presently. There would still be wind and rain, but not as much variation as there is now.
The current average tilt angle of the Earth's axis of rotation is 23.5 degrees. If this angle were to remain constant, there would be no change in the length of the day or season over time. However, due to gravitational forces acting on the rotating planet, the axis does shift slightly from moment to moment. This leads to the phenomenon of seasonal change, with regions at high latitude experiencing less sunlight during part of the year and more daylight during the other parts.
At its most extreme, the tilt can reach 90 degrees from north to south. In this case, we are talking about a completely different planet from the one we know today. All water would be found underground because the sun's rays would not be able to reach the surface. Antarctica would be frozen solid all the time because there would be no nightfall when the polar region receives no sunlight.
The existence of a stable axis makes us capable of imagining a world without seasons.