1. You would have to take into account the increasing urban heat islands that are affecting the climatological temperature records. Also, you would have to take into account technology over those years and how they may have affected the readings when it comes to temperature.
2. There are different types things that will affect the climate in either a positive or a negative way, this is what is meant by a positive and a negative mechanism. A positive feedback would be the now-albedo feedback. This increases global surface air temperature and might cause snow and ice to melt in the polar latitudes. This melting would reduce the albedo of the surface, then allowing more solar energy to reach the surface of the Earth. A negative feedback would be if there were too much melting of the snow and ice it could cause excessive flooding in certain areas of the world.
3. This is significant because a 3 degree increase would put the carbon cycle in reverse. What this means is that instead of vegetation absorbing the carbon dioxide it would start to release it. This would cause more droughts in some areas and also increase hurricanes and the sea level which could eliminate cities around the world.
4. Arid climate areas are hotter because they skies are clear and the sinking air. You put this together with low humidity and high summer sun beating down on a barren landscape it will produce extreme heat. There is also the fact that they end up getting less rainfall, rainfall