Methane.

CH4

What does Methane and Carbon Dioxide have in common?

They are compounds with a common atom of carbon. Carbon has four bonding electrons. Methane uses single bonds to claim four hydrogen atoms.

Below is CO2. It has a central carbon atom double bonded to two oxygen atoms.

Both are greenhouse gases with a central carbon, four bonding electrons, but, different bonding atoms. Methane has four hydrogens and carbon dioxide has two oxygens. All the bonding electrons are full and stable.

Methane (CH4) is the second most prevalent greenhouse gas (click here) emitted in the United States from human activities. 

In 2013, CH4 accounted for about 10% of all U.S. greenhouse gas emissions from human activities. Methane is emitted by natural sources such as wetlands, as well as human activities such as leakage from natural gas systems and the raising of livestock. Natural processes in soil and chemical reactions in the atmosphere help remove CH4 from the atmosphere. Methane's lifetime in the atmosphere is much shorter than carbon dioxide (CO2), but CH4 is more efficient at trapping radiation than CO2. Pound for pound, the comparative impact of CH4 on climate change is more than 25 times greater than CO2 over a 100-year period.
Globally, over 60% of total CH4 emissions come from human activities. Methane is emitted from industry, agriculture, and waste management activities,...

In thinking back to these two graphs, what are the chances there were there were natural sources of methane such as wetlands and dinosaur farts? 

The point is land use has changed, the planet is full of people, some with an economy that requires large amount of fossil fuels to burn for energy. 

Earth has always had methane emitted naturally. That is not in question. But, the escalating amount of greenhouse gases has become ridiculous and speaks to human nature in being the top predator on Earth. There is not stopping human beings from destroying the biotic content of Earth than Earth itself. 

Earth has no way of communicating it's dominance over life, except, to act out extremely high greenhouse gas levels in the way of drought, floods and the sixth extinction. 

Human beings are the top predators on Earth, but, they sure don't seem to be the smartest predators on Earth.