It depends on whether carbon dioxide, methane and nitrous oxide have the same proportion of the atmosphere or the same partial pressures.
If they were the same proportion of the atmosphere, Earth would be 0.8C colder if the climate sensitivity were 1C and 3.5C colder if the climate sensitivity were 4C. The reason why the temperature difference would be less than the climate sensitivity is because a thinner atmosphere would lead to a relative humidity closer to 100%.
If the partial pressure of carbon dioxide, methane and nitrous oxide were the same, Earth would be warmer, because the relative humidity would be closer to 100%. There would be less air to suppress the evaporation of water.
Antarcticice
No. Air does not dissolve water. The vapor pressure of water is a function of temperature, only, not of atmospheric composition.
The atmosphere has a great effect on the life on the planet, long ago oxygen level rose (this was not an increase in density but just oxygen) and that allowed many insects to grow much larger spiders 2 ft long and centipedes 7ft long as examples (this is a model)
http://cdn9.quochoi.org/files/2013/01/co...
If you lowered the density of the entire atmosphere by 50% then there may be no life on the planet, for several reasons
1. A much lower density would significantly lower water vapor making things colder and drier
2. The Alpine tree line is the point above which trees no longer grow, several factors contribute to this as well as air pressure temperature also plays a part, this can vary between 2000m to 4800m the later is in the tropics (Bolivia) due to the warmer tropical conditions.
But for the 50% you are talking about that is the equivalent of a mountain the height of Mt McKinley in Alaska which is very close to the air pressure you are talking about ~48%, at 6800m this is well above any treeline.
People climbing Everest can develop altitude sickness without oxygen at these altitudes and ~50% of people going to just the lower base camps get altitude sickness at levels of ~4000m.
Lets see O2 @ 21/2=10.5 % , would probably mean no or minimal life at high altitudes. Smaller life forms/dwarf would be common place in the mid to lower terrestrial regions. Only the oceans would have the capacity to maintain any mass as expressed on the species level. That as strange as it seems would almost be ideal for reptiles based on calories, temperature, and humidity. Bulk mammal mass wouldn't exist unless they crossed into the amphibian threshold.
Hotter in the day and colder at night on land. The oceans would boil and evaporate thus increasing the gaseous atmosphere.
The density of atmosphere of a planet is determined by gravity, which is determined by the planets mass, so your question could not occur, hypothetically though it would be colder.
If Earths atmosphere was half as thick what would our climate be like? How would it be different from today?
