My world is at the outer edge of the habitable zone. There is enough heat and geological activity to start life, but a snowball earth is looming and threatens to freeze it all. There is some spring thaw at the equator, allowing some simple algae to multiply. The carbon dioxide is not sufficient to make an earth-like warm planet, though.

Then comes the pitch-patch: a pitch-black patch of single-celled algae that thrives in the the pools of molten ice in spring.

The pitch-patch manages to venture out of the pools and populate the thin film of water that makes ice slippery. It has antifreeze ethanol in its membranes. Most importantly, it is black, therefore absorbing enough sunlight and heat to raise the temperature around it. This creates a positive feedback that melts more ice and promotes more growth.

Eventually, the pitch-patch creates large icefields of algae. Its effect is noticeable from outer space, just as we see forests from orbit around Earth.

So, can the pitch-patch, along with carbon dioxide, make the difference and melt larger fields of ice? Can it create a more hospitable environment at-least around the equator?

EDIT: To point this out, there is sufficient carbon dioxide for algal growth and some greenhouse effect. However, the planet is too far to allow liquid water all-over the planet. For now, it is present in liquid form in a narrow band around the equator, and some life thrives there. Polar ice caps are much larger than those on Earth, but not exceedingly so. However, the chances of the snowball-earth to occur, are much higher than those on our planet, because of the distance from its sun.

  • $\begingroup$ If your planet has less CO2 than Earth, that would be your limiting factor. $\endgroup$
    – Alexander
    Jan 31, 2019 at 19:27
  • $\begingroup$ If there is significantly less CO2 than in the present atmosphere of Earth then Earth-like photosynthetic plants (including algae) cannot survive, or, even if they manage to scrape a living near natural sources of CO2, they cannot proliferate. That biomass of black algae? That's concentrated CO2. $\endgroup$
    – AlexP
    Jan 31, 2019 at 20:06
  • $\begingroup$ Have a look at Daisy World and the Gaia Hypothesis which deals with this mathematically - en.wikipedia.org/wiki/Daisyworld $\endgroup$ Jan 31, 2019 at 20:32
  • $\begingroup$ Did you just call water Molten Ice? $\endgroup$
    – Shadowzee
    Feb 1, 2019 at 2:02

3 Answers 3


The answer is not easy, because black algae will influence the energetic balance of the planet in two, opposite ways:

  • their black surface will absorb more stellar radiation, locally increasing the temperature
  • their photosynthetic activity will remove $CO_2$ from the atmosphere, thus lowering the green house effect.

It is therefore plausible that the planet will have a feedback mechanism of the following sort:

  1. Algae population increases, more ice melts
  2. as a consequence of algae increase, the $CO_2$ content in the atmosphere decreases
  3. with a weaker greenhouse effect the temperature lowers
  4. lower temperatures inhibit algae growth
  5. as a consequence, $CO_2$ in the atmosphere increases
  6. back to 1.

This would obviously result in small oscillations of the temperature, not dramatic changes.

For a better answer I think more details would be needed, to better model the climatic interactions of the world.

  • 1
    $\begingroup$ However, if the algae sink to the bottom of their ice pool in step 4 and are not decomposed (think of oil or coal deposits), lots of CO2 is permanently bound and the greenhouse effect grows weaker and weaker until the snowball is complete... So the climate does not only depend on those algae, but also on micro-organisms that would decompose dead algae. $\endgroup$
    – Elmy
    Feb 1, 2019 at 7:01
  • $\begingroup$ I was trying to work-around that problem. I may include two parameters: (1) The black algae creates a warmer environment by heat absorption, therefore competing over single-celled green algae and gradually outnumbering it. Population shifts to more black algae but less green one. (2) The black algae may "spread thin" over a wide area, therefore tarnishing a large surface area compared to its mass, so it will use less CO2 than seaweeds and trees. Will that help to create a stronger greenhouse effect? $\endgroup$ Feb 1, 2019 at 9:59

If large patches of the planet are darkened, then yes, that will lower the planet's albedo, raising the average temperature.


Albedo is a non-dimensional, unitless quantity that indicates how well a surface reflects solar energy. Albedo (α) varies between 0 and 1. Albedo commonly refers to the "whiteness" of a surface, with 0 meaning black and 1 meaning white. A value of 0 means the surface is a "perfect absorber" that absorbs all incoming energy. Absorbed solar energy can be used to heat the surface or, when sea ice is present, melt the surface. A value of 1 means the surface is a "perfect reflector" that reflects all incoming energy.


So, can the pitch-patch, along with carbon dioxide, make the difference and melt larger fields of ice? Can it create a more hospitable environment at-least around the equator?

  • there is sufficient carbon dioxide for algal growth and some greenhouse effect.
  • the planet is right on the "snowline" of the star system: too far to allow liquid water all-over the planet, but close enough to allow a band around the equator.

I think in order for your algae to work, it cannot interfere with CO2... or else CO2 levels aren't as important as (for example) being on the snowline. It sounds to me like your algae will have to be a KEY part of the solution, but not all of it: it requires a small ecosystem of symbiotic relationships.


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