Okay, a couple of quick notes. It’s simple surface Earth temperature. Since sunlight is simply falling on the surface and the Earth is radiating from the surface, this ignores any interactions with the very hot molten core.
Also, note that it’s a little colder here than in valid the average temperature of the Earth (13.9°C) is a difference of 27.1°C. This is because the Earth is not really bare rock. Instead, we have an amazing atmosphere that shields us from what life would be like if the world were actually that cold.
What would the Earth be like at that temperature? Well there is there are already places on Earth that are much colder, many of them in Antarctica, where temperatures can drop below -90 C. And although only a few scientists live on bases in Antarctica, there are other places in the Yukon, Alaska and Siberia where they are permanently inhabited and where temperatures can reach temperatures below – 60°C. Thus, we should not assume that a colder Earth would be uninhabitable for humans or other life forms based on temperature alone, but most of them are likely to be concentrated in the warmer parts of the globe. (Remember, the average is -13.2 Celsius, so just like Earth today, the poles would be colder than the equator. Antarctica and the Arctic would be much colder than they are now.)
And since an average of -13.2°C means that most of the planet will be below the freezing point (0°C), a much larger proportion of the water will be ice. However, slightly above average locations may have liquid water, depending on atmospheric pressure.
Surprisingly, you will still need a refrigerator for food and drinks. However, this refrigerator will not refrigerate the food – instead, it isolates it from the outside temperature so that the food does not freeze.
The earth would still be warmer than Mars, whose average temperature is around -63 C.. But Mars also has other problems that make it not so pleasant for people. The atmosphere is ultra-thin and contains very little oxygen. And without an atmosphere, the Earth would also have this problem. Life forms here must have evolved without the need for oxygen, and the humanoid visitors would need some sort of space suit for protection and breathing.
Luckily for us earthlings, we do have an atmosphere. Mostly nitrogen, but also high in oxygen and 0.04 percent carbon dioxide (by volume). Carbon dioxide, along with water vapor, absorbs infrared radiation emitted from the Earth’s surface and maintains a temperature that we can handle – for now.
But carbon dioxide is a growing problem. Too much means the atmosphere will reach even higher temperatures, leading to all the potential problems of climate change, including catastrophic glacier melt, sea level rise and extreme weather. That’s why the Paris Climate Agreement is the international goal of keeping the warming threshold 1.5 degrees Celsius above pre-industrial levels, and the bad news is that the past nine years have been the hottest on record. If temperatures continue to rise, the Earth will no longer be perfect for humans – or for many of the other creatures on it.