# Plausible reasons for a whole planet to have a temperate oceanic climate?

In my book series, I’m worldbuilding a planet whose culture is based on Celtic mythology and history, and in order to be consistent with real life I feel like the whole planet’s environment needs to mirror that in which the celts (particularly the insular ones) lived in irl: forests and grasslands with a temperate oceanic (Cfb) climate dominating the vast majority of the landscape like it does in Britain. I do allow small exceptions for mountain ranges, peaks, and slightly hotter and drier areas (which can be given subpolar oceanic, tundra, and warm-summer mediterranean climates respectively) but what circumstances could plausibly cause an earth-sized planet to be this temperate and wet all around?

• The peoples who spoke Celtic languages lived in a large variety of climates, from the perennially rainy British Islands all across Europe to the warm and sunny shores of the Mediterranean and the harsh continental hot summers and freezing winters of Transylvania and Asia Minor. Very many of them lived in Central Europe, which most definitely does not have an oceanic climate. (And anyway, most of Gaul has a temperate oceanic climate, like the British Islands, with the difference that it enjoys a much healthier dose of sunshine.) Mar 20 '20 at 4:50
• @AlexP continental hot summers ... of Transylvania you are either kidding or you don't know the meaning of hot summers. Or I don't know anymore how the summers are in Transylvania nowadays, with the global warming. Mar 20 '20 at 7:04
• Write to your politicritter to repel that pesky cosine law first. Otherwise you can't have it, the poles will always be as cold as the atmosphere allows them to be. Mar 20 '20 at 7:11
• Mono-climate planets are hugely unrealistic. Your best bets are pretty much all magic (of either a Tolkien or Clarkian style). Mar 20 '20 at 16:27
• Just a reminder, all you actually want is that all the land area of the planet accessible to your culture has that climate. That is entirely different from needing entire planet to have that climate. A bunch a small island continents that all just happen to be located so that they have the correct climate is just as good. So is having all the land with wrong climate being too distant or full of hostile firbolg. Mar 21 '20 at 0:11

A planet with a heavy greenhouse effect can have it.

Think about a planet which is at the far ranges of goldilocks zone, like Mars. It compensates the low insolation with high greenhouse effect, like Venus. As a result, planet's climate is much more uniform than Earth's. Difference between equatorial and polar regions is minimal. However, this planet should have a heavy cloud cover, and sunshine would be rarely seen on the surface.

• "heavy cloud cover, and sunshine would be rarely seen on the surface" yup, sounds like GB or Ireland to me! Mar 21 '20 at 18:45
• Those storms in the Northern Sea? Would be gentle breezes in comparison with the winds required to carry/distribute the heat from equator/tropics to the poles. But, yeah, let's skip that reality-check, cause we can't check the planets in the whole Universe. Mar 22 '20 at 6:24

In general, you can't have the same climate on whole planet.

• There are always areas at equator which would got more sunlight and therefore have different climate.
• Britain enjoys warm Gulf stream which makes climate more warm. Compare climate at London at 51°30′ N and other side of Atlantic ocean: coastal Saint-Augustin at 51°25′N with subarctic(!) climate. The Quebec at 46°48′ N is much closer to the equator and also has colder (Dfb class) climate than London.
• Far away from sea climate is more arid and summer is warmer. In other words, you have continental climate away from sea :)

If you really want to do so, you could describe really strange world

• Without continents but with big islands.
• With mountain area near equator (the area would got more sunlight but when altitude grows, climate become colder) you could spread desired climate to one direction.
• With warm oceanic streams near poles you could spread desired climate to another direction.

But I suppose that such world is so artificial that it's hard to believe in it.

On the other hand you have another options:

• If your setting is close to our middle ages then people do believe that (relatively small in terms of planet) area 1000x1000 km (which is big enough for many countries) is entire world and nobody cares what goes on somewhere else.
• If your setting about high-tech then terraforming was performed (either by people or by an Ancient civilisation) and nobody cares about details.
• If your setting allows magic then ... it's a magic! Very strong and arcane magic, of course.