text: From 1996 to 2004, researchers in the European Project for Ice Coring in Antarctica (EPICA) project were able to drill and analyze gases trapped in the ice cores in Antarctica to reconstruct GHG concentrations in the atmosphere over the past 800,000 years". They found that prior to approximately 900,000 years ago, the cycle of ice ages followed by relatively short warm periods lasted about 40,000 years, but by 800,000 years ago the time interval changed dramatically to cycles that lasted 100,000 years. There were low values of GHG in ice ages, and high values during the warm periods. 
  This 2016 EPA illustration above is a compilation of paleoclimatology showing  methane concentrations over time based on analysis of gas bubbles from EPICA Dome C, Antarcticaapproximately 797,446 BCE to 1937 CE, Law Dome, Antarcticaapproximately 1008 CE to 1980 CE Cape Grim, Australia1985 CE to 2015 CE Mauna Loa, Hawaii1984 CE to 2015 CE and Shetland Islands, Scotland: 1993 CE to 2001 CE
  The massive and rapid release of large volumes of methane gas from such sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the Earth's distant past, such as the Paleocene–Eocene Thermal Maximum, and the Great Dying.
  In 2001, NASAs Goddard Institute for Space Studies and Columbia Universitys Center for Climate Systems Research scientists confirmed that other greenhouse gases apart from carbon dioxide were important factors in climate change in research presented at the annual meeting of the American Geophysical Union (AGU). They offered a theory on the 100,000-year long Paleocene–Eocene Thermal Maximum that occurred approximately 55 million years ago. They posited that there was a vast release of methane that had previously been kept stable through "cold temperatures and high pressure...beneath the ocean floor". This methane release into the atmosphere resulted in the warming of the earth. A 2009 journal article in Science, confirmed NASA research that the contribution of methane to global warming had previously been underestimated.
  Early in the Earths history carbon dioxide and methane likely produced a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. During this time, Earths earliest life appeared. According to a 2003 article in the journal Geology, these first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.