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 disappear. If Earth's axis were not tilted (obliquity = 0°), the Sun would always sit over the equator and the geometry that produces spring/summer/autumn/winter would vanish. Key consequences:

What would stay the same
- There would still be a latitudinal temperature gradient: the equator would remain warm, the poles cold. Solar energy would still decline with increasing latitude, so climate zones (tropical, temperate, polar) would still exist as bands.

What would change
- No astronomical seasons: the subsolar point would always be on the equator, so solar declination would be constant. There would be no yearly movement of the Sun north and south.
- Day length nearly constant: everywhere (except the poles) would have about 12 hours of daylight and 12 hours of night every day all year (idealized; atmospheric refraction and the Sun’s apparent diameter add small differences). At the poles the Sun would circle the horizon day after day.
- No seasonal changes in solar heating at a given latitude: the seasonal swing in insolation that now produces warm summers and cold winters would be gone. Temperatures at a location would be steadier year‑round (subject to weather and ocean variability).
- Polar regions would be much darker on average: with no summer when the Sun climbs high at high latitudes, the poles would receive very little annual solar energy and would be much colder. Year‑round cold would favor permanent ice and sea ice and reduce seasonal melting.
- Weather and precipitation patterns would change: monsoons and other seasonally driven circulations, which depend on seasonal land–sea heating contrasts, would weaken or vanish. Storm-track intensity and seasonal shifts in jet streams would be much reduced; climates would be more zonal/steady.
- Ecosystems and human systems adapted to seasons would be strongly affected: migrations, breeding cycles, flowering and fruiting timed to seasons, deciduous forests that drop leaves for winter, seasonal agriculture calendars — many would be altered or disappear. Agricultural practices would have to change because photoperiodic cues and seasonal temperature windows would be absent (although small annual variations due to Earth’s orbital eccentricity would remain).
- Small residual “seasonality” would remain from orbital eccentricity (the Earth–Sun distance varies ~±1.7%), which changes global incoming solar power by a few percent during the year, but this is a weak, global effect and would not reproduce current hemisphere‑specific seasons.

Net climate effect
- The annual mean distribution of solar energy with latitude would be different from today (tilt redistributes sunlight toward the poles in summer); with zero tilt the poles would get less annual energy than now, so polar cooling and expansion of ice are likely. Mid‑latitude climates would see less variability but could be cooler in what are currently summer months and warmer in current winters, tending toward a more steady but possibly overall different climate regime.

In short: without axial tilt Earth would have no regular seasons—daily daylight nearly uniform, much colder poles with persistent ice, zonal (latitudinal) climate belts that change little through the year, and major consequences for weather, ecosystems and human agriculture that depend on seasonal cycles.