What could make polar areas of an earth-like planet habitable (non desert) and what side effect would it cause (i.e. how such planet would differ to contemporary earth)?
It is expected that the precipitations will rise significantly in the high latitudes regions. Also, it is the area (the Arctic but not Antarctica so far) with the fastest warming on the planet.
Source (page 859): https://www.ipcc-wg1.unibe.ch/publications/wg1-ar4/ar4-wg1-chapter11.pdf
I think you already know what the impacts would be on the whole planet.
- Increase in overall precipitations but spread differently (more concentrated),Because hotter air can carry more moisture. At most places, the increase in precipitation will not cover the increase of evaporation due to temperature.
- The deserts are pushed closer to the poles (meaning areas between 30 and 40 degrees of latitude tend to become drier
- More deserts and semi-arid climates
- Larger tropical forests but with more seasonal variability
A lot depends on what you mean by habitable. Various people's have inhabited the arctic regions over the millennia (the latest being the Inuit, or Eskimos), and Human species like the Neanderthals and Denisovans were known to live in Europe and Asia during the last Ice Age, with nothing more than neolithic technology.
All that seems to be required is the technology to make warm clothing (to retain heat), and enough ability to analyze the landscape for food and shelter, as well as being able to pass these observations down to your and your tribe's children. With enough time, various traits to maximize survival will become dominant (Inuit are short and "round" to minimize surface area and limit heat loss, while the Neanderthals had impressive nasal architecture to allow cold, dry air to be "pre warmed" before entering the lungs).
If you want the high arctic to become warm enough for unprotected agriculture, then a lot of high technology would be needed to raise the average temperature (In the European Warm Period, Vikings were able to do croft farming in Greenland; it is still too cold to do that in Modern Greenland even centuries after the Little Ice Age). For simplicity, I will envision a system of orbiting mirrors to increase the amount of insolation the poles receive.
The warmer weather will cause the permafrost to melt, making the ground cover much like a swamp of bog across much of the Asian and North American far north. Antarctica has been covered with glaciers for millennia, so the surface will be mostly exposed rock and gravel. Transportation and infrastructure will be tremendous undertakings in either place, and agriculture in the far north of the Northern Hemisphere will depend on a lot of drainage for the soil's moisture content to be low enough for the sorts of agriculture we practice.
The vast release of moisture will cause the water levels to rise and have strange effects on the overall global climate as the masses of cold water sink into the depths of the ocean and disrupt the deep currents. Interrupting the Gulf Stream would make Europe much colder, for example. Since the currents are very long term parts of the heat engine, effects might not be revealed for hundreds or even thousands of years.
More moisture in the atmosphere will make rain and precipitation more common, with both positive and negative effects (more agriculture, more flooding). Ironically, the weather will become much milder, since much weather is driven by temperature differentials between the warmer and colder parts of the Earth.
Some migratory species will no longer need to migrate, since their food sources do not disappear in winter, while others might continue to migrate based on the seasonable light (the six months of darkness will be illuminated by the mirrors, but the six months of light will be even brighter for the same reason). There will be other unpredictable changes in animal populations, as creatures migrate to take advantage of the newly opened ecological habitat, while other habitats (especially in the oceans) are disrupted due to the changing oceanic currents and long term climate changes (such as a colder Europe).
Overall, if you are going to inhabit the arctic or antarctic land masses with current people and technology, it might be better to treat it as "outer space" and make self contained habitations with minimal interaction with the outside environment.
The simple answer is that the planet needs to be closer to the sun, have a stronger greenhouse effect, or have a warmer sun. Any of these effects will warm up the entire planet compared to our own. By habitable I assume you mean similar to temperate areas of Earth, rather than people managing to live on the ice through hunting and fishing.
Warming up the entire planet by approximately 20 degrees through any combinations of these methods would turn the equator fairly uninhabitable (although life is adaptable, I expect it would find a way) whereas the poles would be much like our temperate areas are today.
That would give a temperature ranging over the year from -30 degrees to +30 degrees, which is enough to allow crops to be grown and agriculture to occur. This would especially be so if like our arctic there was enough ocean water around to moderate the temperature differences.
The extra heat would tend to make weather systems more powerful, the equator and middle latitudes would have massive violent storms and wide weather fluctuations. There would be more water in the air, hotter air, and tropical storms and hurricanes would be frequent.
Humidity and heat would both be very high, and adaptations in technology, physiology, or both would be required to survive.