What makes biomes different from each other?
Temperature and rainfall/snowfall are the two main factors for land biomes.
Biomes on the land form as a direct result of temperature and moisture. Temperature is the most at the equator and drops as you go towards the poles (gets colder).
Rainfall is more complex but is essentially controlled by big atmospheric circulation cells that either produce a lot of rainfall (as in the tropics) or very little (as in the Sahara desert). See pic.
Marine biomes are controlled by temperature, depth, salinity, oxygen content, and pressure.
Global warming is now changing both of these factors, so biomes around the world are becoming stressed.