Methane (CH) in the Earth's atmosphere is a powerful greenhouse gas with a global warming potential (GWP) 84 times greater than CO over a 20-year time frame.  Methane is not as persistent as CO, and tails off to about 28 times greater than CO over a 100-year time frame. 
Radiative or climate forcing is the scientific concept used to measure the human impact on the environment in watts per square meter (W/m). It refers to the "difference between solar irradiance absorbed by the Earth and energy radiated back to space" The direct radiative greenhouse gas forcing effect of methane relative to the year 1750 was estimated at 0.5 W/m in the 2007 IPCC "Climate Change Synthesis Report 2007".
In their May 21, 2021 173-page "Global Methane Assessment", the UNEP and CCAP said that their "understanding of methane's effect on radiative forcing" improved with research by teams led by M. Etminan in 2016, and William Collins in 2018, which resulted in an "upward revision" since the 2014 IPCC Fifth Assessment Report (AR5). The "improved understanding" says that prior estimates of the "overall societal impact of methane emissions" were likely underestimated. 
Etminan et al. published their new calculations for methane's radiative forcing (RF) in a 2016 Geophysical Research Letters journal article which incorporated the shortwave bands of CH in measuring forcing, not used in previous, simpler IPCC methods. Their new RF calculations which significantly revised those cited in earlier, successive IPCC reports for well mixed greenhouse gases (WMGHG) forcings by including the shortwave forcing component due to CH, resulted in estimates that were approximately 20–25% higher. Collins et al. said that CH mitigation that reduces atmospheric methane by the end of the century, could "make a substantial difference to the feasibility of achieving the Paris climate targets", and would provide us with more "allowable carbon emissions to 2100".
In addition to the direct heating effect and the normal feedbacks, the methane breaks down to carbon dioxide and water. This water is often above the tropopause, where little water usually reaches. Ramanathan (1998) notes that both water and ice clouds, when formed at cold lower stratospheric temperatures, are extremely efficient in enhancing the atmospheric greenhouse effect. He also notes that there is a distinct possibility that large increases in methane in future may lead to a surface warming that increases nonlinearly with the methane concentration.
Mitigation efforts to reduce short-lived climate pollutants like methane and black carbon would help combat "near-term climate change" and would support Sustainable Development Goals.