Geology
read this article (http://www.pbs.org/wgbh/nova/space/contrail-effect.html) from the PBS NOVA series (fantastic TV show if you haven’t seen it) about jet contrails and their surprising impact on surface temperatures.
When you finish reading the article, I want you to summarize in your own words the basic mechanism at play in the jet contrail-temperature relationship, and how it is that we determined that relationship.  How do contrails alter the Earth’s energy balance?  No need to rewrite the article, just a couple of paragraphs will do.