Well, I will throw a few random ideas that I have gotten from my environmental science class.
-Global climate change is causing winter to start later and end earlier in many areas which is effecting a lot of species such as bears, migratory birds, and fish. Bears hibernate for shorter periods of time and have messed up their diets, migratory birds are having to travel to different places, and many fish are dying off because of the climate change.
-You could research the effects of Damming the Colorado River. Although I am not sure if you need something that is a global issue. But, the Colorado River is fed primarily through the melting of the snowpack, but the snowpack is reducing because of global climate change.
-The Three Gorges Dam in China is a very interesting subject. It is the worlds largest hydroelectric powerstation and has had a ton of environmental consequences. You could talk about the general consequences of building dams such as sedimentation, disruption of fish migration, etc. You could also talk about the benefits, such as the clean energy it provides, the ability to control flooding, etc. Then you could talk specifically about the Three Gorges Dam and how it displaced about 1 1/4 million people and destroyed a lot of agricultural and cultural sites. Also the dam lead to the extinction of a species of dolphin and is impacting other fish species. There is a lot of information on this topic and it is fairly interesting.
-Of course you could always do a paper on the global population
That's all that I could think of... I know theres a TON of topics you could choose from... I'll try and think of some more.