Should my superhabitable planet have ice caps?

Question

I'm designing an Earth-like planet and I'm concerned about whether or not it would have ice caps. Originally, I assumed it would be too warm, with a climate similar to Earth's during the Paleocene-Eocene Thermal Maximum. However, upon considering the rest of its parameters, I realized this might not be the case. They are as follows:

• Average surface temperature: 25°C

  As mentioned above, this is similar to PETM Earth. Then, polar conditions were too temperate for permanent ice caps. The global climate has been around these levels for the last 18 million years following a period of industrial global warming.

• Axial tilt: 6.2°

  Less seasonal temperature variation would create more consistently cold polar conditions, letting ice remain frozen there for longer periods. EDIT: I remember reading somewhere that planets with a lower tilt should have a warmer overall climate, but I can't find the source.

• Sea level pressure: 2.43 atm

  A thicker atmosphere would be able to circulate heat more effectively. This would create more even temperatures across different latitudes, potentially making polar temperatures warmer.

• Surface gravity: 1.28g

  I'm not sure how this would effect ocean and wind circulation. It could slow down currents near or at the surface, though I'd imagine a planet's topography would be a bigger factor. That being said, this would lower the maximum height limit for mountains and increase erosion from rainfall, creating a slightly flatter topography, so maybe it would even out?

• Ocean coverage: 80%

  Along with the higher surface gravity, this would create an overall flatter topography, allowing ocean and wind currents to flow more efficiently. It would also decrease the planet's albedo, though this could be counteracted by increased cloud cover from evaporation.

• Landmass Properties

  This planet's landmasses are concentrated around the equator and in a longitudinal great circle. This is due to the scorching equatorial climate as well as the tidal influence of a mars-sized moon it's locked to, which creates a permanent 800m tidal bulge. They tend to be smaller and more broken up with jagged coastlines and inland estuaries. Mountains are a bit shorter on average. While there are landmasses at the poles I'm not sure if they would be isolated enough to have a situation like Antarctica, in which thermal isolation would allow it to remain frozen even after the climate increases.

• Rotational Period: 43.125 hr @ 322 m/s

  On one hand, a slower rotation would create a greater temperature difference between the day and night sides, increasing the potential for ice to form at night. Furthermore, a reduced Coriolis force would slow down ocean and wind currents, lowering circulation to the poles.

• Surface area: 637.59 million km^2

  More surface area may reduce the effectiveness and distribution of solar heating.

• Apparent brightness of star: 1.0486L☉

  The star's true luminosity is only .168L☉, but it appears brighter based on the planet's semi-major axis, which is 0.4 AU.

Based on these provided factors, should my planet have polar ice caps to any capacity?

Answer 1

Short answer: No, it is too warm.

This is a bit beyond my expertise, but since no one else seems to be interested to answer, I will try my best:

If we look at the current climate, we have an average temperature of ~18°C and the North pole is almost ice-free in summer and most other ice sheets are melting at an alarming rate. I guess, seven degrees warmer would mean the total disappearance of all ice sheets, even if it takes millenia.

In addition, keeping an ice sheet stable is much easier than creating new ice sheets. Once the ice sheet exists, albedo is reduced due to the white color. The poles might still experience lower-than-zero temperatures over the whole respective winter, maybe even permafrost but no big ice sheets.

The low axial tilt is not really working in favor of the ice sheets. As mentioned before, forming the ice sheet is the hardest part and this is less likely, if the seasonal variability in temperature is low. Otherwise the ice might be able to form in the winter and can make it through the summer due to the lower albedo (as is it currently happening on the North pole but probably not through the next decades).

Answer 2

Frame challenge

You're not thinking in the long term.

I'm no expert on climate change, but I'm going to pose this thought anyways. What you've given us is a snapshot of your planet: "here's the current weather, here's the current axial tilt, here's the current surface temperature, etc." and then asked "should my planet have ice caps." Here's the thing: planets change.

Over the course of several millennia, the axial tilt might give rise to more drastic seasons or less so, depending on the change. In the 1990s, Earth underwent a major axial shift caused by melting glaciers. Speculation has it that a similar shift in axial tilt may be responsible for the climate change we're seeing today.

Surface temperature changes, too. Earth experienced some major heat spikes in its early years, as evidenced by the PETM period you are simulating. In recent times, Earth's temperature has risen by approximately .8 degrees Celsius per decade, and is currently rising by .18 Celsius per decade, a significantly increased rate of warming.

I could keep going, but I think I've made my point. If you want to determine whether your planet has ice caps, you need to determine not only its current state, but its long-term patterns. It's 25 degrees Celsius now, but is it recovering from an Ice Age? If that's the case, it probably has ice caps and they are probably melting fairly quickly. On the other hand, if it's coming off a molten period, ice caps are out of the question.

What you need to do is determine not just you're planets climate, but it's climate change. As I said, I'm no expert on climate change, but here's one basic pattern you can keep in mind:

Climate change works by positive feedback mechanisms.

Have you ever wondered how Earth got so hot or so cold (and we're talking extremes: the deep freezes of the Cryogenian period and the molten period known as The Hadean)? Many causes, obviously. For instance, the thaw after the Neoproterozoic Era may have been caused by erupting volcanoes. (Which, by the way, illustrates my point. If you asked if there should be volcanoes during a deep freeze, I would have said not. But there were, and that is what pulled Earth out of its period of frost. Because planets change.)

However, when it comes to extremes, one helpful thing to keep in mind is that climate change works by positive feedback mechanisms. That is, warm temperatures cause warmer temperatures, which cause even warmer temperatures. And likewise, cold temperatures cause colder temperatures, which cause... you get my drift (okay, pun intended).

Case in point: During PETM, there was a shift in ocean currents which brought warmer waters farther out into the sea, creating warmer conditions. A mechanism like this could trigger a more drastic change in ocean currents, bringing hotter water out into the sea... and the cycle continues. The opposite is true, too. When the Earth is covered in snow, more of the sun's energy is reflected into space. Hence, causing further cooling.

So in a nutshell: you need to determine where your planet came from, and where it is headed. But assuming a general trend in which the PETM conditions you laid out continue to exacerbate, causing even warmer conditions, should your planet have ice caps? My answer would be no, but that's an opinion.