Impact of atmospheric radiocarbon and stable isotope measurements on understanding the global CH4 budget over 1850&#8211;2015
Authors: Fujita Ryo , Graven Heather 
Published: March 2020

&lt;p&gt;Measurements of stable isotope ratios of atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;(&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;) have been utilized to evaluate contributions of individual CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;sources and sinks to global atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;budget. However, given the uncertainty of both the source isotope signatures and kinetic isotope effects, recent estimates of the global atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;budget using stable isotope observations are still inconclusive&lt;span&gt;.&amp;#160;&lt;/span&gt;Radiocarbon measurements (&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;) could provide &lt;span&gt;stronger&amp;#160;&lt;/span&gt;additional&amp;#160;&lt;span&gt;constraint&amp;#160;&lt;/span&gt;&lt;span&gt;on&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;fossil-fuel&amp;#160;&lt;/span&gt;CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;&lt;span&gt;sources &lt;/span&gt;(i.e.,&lt;sup&gt;14&lt;/sup&gt;C-free)&lt;span&gt;, but the uncertainty of &lt;sup&gt;14&lt;/sup&gt;CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;emissions from nuclear power facilities and a lack of data have limited such utilization. Here we describe&amp;#160;&lt;/span&gt;&lt;span&gt;a&amp;#160;&lt;/span&gt;&lt;span&gt;new&amp;#160;&lt;/span&gt;&lt;span&gt;approach&amp;#160;&lt;/span&gt;&lt;span&gt;to&amp;#160;&lt;/span&gt;&lt;span&gt;estimate&amp;#160;&lt;/span&gt;&lt;span&gt;plausible&amp;#160;&lt;/span&gt;global&amp;#160;&lt;span&gt;CH&lt;sub&gt;4&amp;#160;&lt;/sub&gt;&lt;/span&gt;&lt;span&gt;emissions&amp;#160;&lt;/span&gt;&lt;span&gt;and&amp;#160;&lt;/span&gt;&lt;span&gt;sinks&amp;#160;&lt;/span&gt;&lt;span&gt;scenarios&amp;#160;&lt;/span&gt;&lt;span&gt;over&amp;#160;&lt;/span&gt;&lt;span&gt;1850&amp;#8211;2015 using&amp;#160;&lt;/span&gt;observations and &lt;span&gt;one-box&amp;#160;&lt;/span&gt;&lt;span&gt;model&amp;#160;&lt;/span&gt;&lt;span&gt;simulations of atmospheric &lt;/span&gt;CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;, and &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;. A&lt;/span&gt;s inputs to the model,&amp;#160;&lt;span&gt;we prepare a priori &lt;/span&gt;bottom-up CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;emission inventories, total atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;lifetime, source and sink isotope signatures, nuclear power &lt;span&gt;facility&amp;#160;&lt;/span&gt;database, and atmospheric &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;and &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;observations and their uncertainties. We then run a Monte Carlo simulation of atmospheric CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;, and &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt; over the period using the inputs with the uncertainties. By using the observational CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;and three isotope constraints, we derive the best combinations of biogenic, anthropogenic fossil-fuel, natural geologic, biomass-burning, and &lt;span&gt;nuclear power facility&amp;#160;&lt;/span&gt;emissions and total CH&lt;sub&gt;4&amp;#160;&lt;/sub&gt;lifetime. We find that reconciling CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;, and &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4 &lt;/sub&gt;observations indicates that (1) natural geologic emissions are likely smaller than the recent bottom-up estimate 43&amp;#8211;50 Tg CH&lt;sub&gt;4&lt;/sub&gt; yr&lt;sup&gt;-1&lt;/sup&gt;&amp;#160;reported by Etiope et al. (2019), (2) biomass burning and anthropogenic fossil emissions are larger than current bottom-up estimates, and (3) biogenic emissions are somewhat smaller than current bottom-up estimates. Our finding suggests multiple isotope measurements, including &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, have a strong potential to evaluate the current and future bottom-up global CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;emission inventories.&lt;/p&gt;