Question

How would seasons be affected if the Earth's axis were not tilted?
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Answer 1

Seasons as we know them would basically disappear. The axial tilt (obliquity) is the main reason different latitudes get different amounts of sunlight at different times of year. If Earth's axis were exactly perpendicular to its orbital plane (tilt = 0°):

What would change
- No seasonal shift in the Sun’s declination. The Sun would always be over the equator at local noon. The Sun’s maximum height in the sky at a given latitude would not change through the year.
- Day length would be essentially constant: about 12 hours of daylight and 12 hours of night at every latitude (away from the exact poles), all year round.
- There would be no “summer” with higher solar angles and longer days at high latitudes, and no “winter” with low solar angles and short days.

Climate and weather consequences
- Stronger north–south (latitudinal) climate contrast: the equatorial/tropical belt would stay warm year-round; high latitudes would stay cold year-round because they never receive high-angle summer sun.
- Polar regions would be much colder on average and would lack a warm summer to melt winter snow/ice. That favors much larger, more permanent polar ice sheets and higher planetary albedo, likely producing a cooler global climate.
- Seasonal phenomena driven by the Sun’s north–south migration would vanish: large-scale monsoons and seasonal shifts in the storm tracks would be greatly weakened or reorganized because the seasonal heating contrast between land and ocean would be much smaller.
- Weather patterns would be more zonal and stationary (latitude-band-like), with precipitation belts and circulation cells less seasonally mobile.

Biological and human impacts
- Ecosystems that depend on seasonal cycles (deciduous forests, temperate agriculture, many migratory species, breeding cycles of insects and birds, etc.) would be severely altered. In high latitudes many species would either disappear or evolve very different life histories.
- Agriculture in mid- and high-latitudes would be disrupted: there would be no warm growing season as we now have; many current farming regions would become too cold for crops that need a summer; cropping patterns and food production would shift toward the tropics/subtropics or require artificial heating/greenhouses.

Residual “seasonality”
- You would not have tilt-driven seasons, but a very small annual variation would remain because Earth’s orbit is slightly elliptical. Insolation at Earth varies with distance to the Sun by roughly ±3.4% around the mean (about a 6.8% difference peak-to-peak between perihelion and aphelion). That would produce only weak, global-scale annual changes — nothing like the strong, latitude-dependent seasons produced by axial tilt.

In short: with no axial tilt, latitude would determine a nearly unchanging climate banding (hot at the equator, cold at the poles), day length would be steady, and the dramatic seasonal changes in temperature, daylight and ecosystems that we experience today would largely vanish.