What is an increase in atmospheric greenhouse gases most likely to cause?

An increase in atmospheric greenhouse gases are likely to cause many different things. Here are a few examples: 

- Greenhouse gases (such as carbon dioxide, methane..) will cause the earth to get hotter. A hotter earth basically means global warming.
- The increased temperature will cause the polar ice caps to melt, which in turn increases sea levels (making some low-lying areas prone to flooding) and destroys habitats for many species (polar bears, for example). 
- Extreme weather will become more commonplace, droughts will occur more often.

There are many more things that I can include but if I were you I'd go check out BBC Bitesize, it's a website that includes lots of different subjects including Geography. I'm not sure if you're in the UK but even if you aren't it will be worth to give it a look. Hope this helps!