When fossil fuels are burned for energy use, gases are emitted from the reaction. Typical gases released include carbon dioxide, methane, nitrous oxides, among other gases, depending on the type of fossil fuel and burning environment. Due to the increased burning of coal and other fossil fuels since the Industrial Revolution, Earth's atmosphere has been continuously absorbing an increasing amount of these gases. The huge increase in the amount of these gases has created drastic changes to the Earth.
One of the most widespread and destructive impacts of greenhouse gas emissions is global warming. While natural, cyclical changes in temperature are common and expected on Earth, the rate of the planet's heating due to humans in the short amount of time since the Industrial Revolution has never been quicker in recorded history, and 2014 was the hottest year on record.  Therefore, the effects of the increase in greenhouse gases can already been seen in Earth's temperatures. The gases help heat Earth's surface by working as a greenhouse would, allowing light from the sun to come in to the surface, but then blocking the infrared radiation from escaping the atmosphere, thus trapping the radiation and thus heat near Earth's surface and heating it up in the process.
The CO2 levels measured right now are very high, and only growing. CO2 is a very potent and abundant greenhouse gas, and it the levels are currently around 400 ppm as observed at the Mauna Loa Observatory. Anything other 350 ppm is considered unsafe.  There are many effects of increased carbon dioxide and GHG emissions on our planet. One effect of the increase of GHG emissions is the melting of sea ice, most notably in the Arctic and Greenland, where glaciers and sea ice have been melting quick recently.. This leads to a sea level rise, which has been occurring at a rate even 10x faster than was previously believed. This increase in sea level has many effects, including habitat loss, erosion, and flooding. Storm surges will reach even further up onto land, and lower-level areas will be flooded completely. This is an example of just one of the impacts GHG emissions can have. 
The increased emissions of GHG by humans have created an environment which may not be able to be altered. Methane, CO2, nitrous oxides, and other gases have begun trapping heat near Earth, causing possibly irreversible damage to our planet. What we do is up to us, but one thing is for sure: something needs to be done.
© Brandon Sutter. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. Jayalakshmi, "Earth Day 2015: Global Warming Rate 'Moderate', While March Beats Heat Records," International Business Times, 22 Apr 15.
 R. Meyer, "Not Doomed Yet: A New Newsletter About Climate Change," The Atlantic, 14 Sep 15.
 T. Folger, "Rising Seas," National Geographic, September 2013.