You can pretty much trace the incredibly sharp rise in CO2 levels right along with the Industrial Revolution. Humans can definitely effect the environment that much. For example:
- CFC's burned a hole in the ozone layer over Antarctica in the mid 90's. CFC's come from human made products and industries. We banned them in the US (we were and still are the largest producers of them), and guess what, the ozone hole is slowly repairing itself. The hole is letting in large amounts of UV light, which is adversely effecting all life under its shadow. Plankton for instance has been decreasing in these areas under the hole. Knocking out the bottom of a food chain is no good. For further insight, look at the mass extinction event that killed most plankton 2 million years ago - it coincided with a large supernova relatively close to our solar system that apparently blasted off much of the Earths ozone. We're doing the same thing right now, ever so slowly.
- West African desertification. Bad farming techniques have effected many places around the globe, but this is one of the most stark examples of what our species is capable of wreaking on the earth. West Africa used to support many different cultures and societies. That was until livestock were introduced to the region. Pounding hoofs trampled the grass and upset the ecosystems on the plains. Now the Sahel is being eaten by the Sahara at a rate of 10 km a year in some places.
- The Aral Sea. Once an enormous lake between the boarders of Khazakstan and Uzbekistan, the Aral was used for irrigation by the Soviets to make the region its "breadbasket" to compete with the United States's Midwest. Over the next 40 years, the lake shrank. Once the fourth largest lake in the world, its now only the eighth. Salinity in the lake is so high most of the fish have died. The sandy and polluted lakebed now kicks up incredibly large dust storms, which ravage crops and have given the people in the region some of the highest rates of lung disease in the world. Ecosystems around the lake are dead. Efforts are being focused at saving the northern part of the lake, but it looks like it will be too little too late. The Aral will most likely be gone by the end of the century.
- Easter Island. Man arrived at this small pacific island around 900 AD. A small civilization quickly sprang up on the densely forested island. Humans started cutting the trees to build their towns, make their boats and stoke their fires. However, the island was small and the population the trees had to support was large. By the time Europeans has arrived, Easter Island was devoid of large trees, only supporting a few species of small palms. The natives had by that time exhausted the island of other resources as well; all nesting birds were absent from the isle, and all the dogs they brought with them were eaten. Deforestation led to the collapse of their civilization, and even though the island has had centuries to recover, it remains treeless and windswept. Lately, its been suffering from soil erosion with the introduction of livestock, much as has been occurring in the Sahara and around the globe.
I can bring up another ten examples off the top of my head of how humans have affected the environment. We're the most destructive species this planet has ever seen. Its completely within our ability to cause a global climate shift, and thats precisely what our recent activity on this Earth has produced. I'm still amazed that there are people out there that maintain that we could never cause something that colossal. Its perfectly withing our capabilities as a species, and even follows our repeating history of ecological disasters we have produced.