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I find the IPCC report hard to read. It's peppered with unexplained terms and is generally cumbersome (no offence to its authors). Here's one example. What is the difference between top-down and bottom-up estimates?

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  • $\begingroup$ It's a good question, and a good answer by f.thorpe below: bottom-up=self-reported emissions (by companies, governments, ...), top-down=based on measurements. The difficulty of the top-down method is that you may measure CH4 at one point (say, in Germany), but it was actually emitted into the atmosphere elsewhere (say, the Netherlands) -- so it's part of the Netherlands' budget, not Germany's! Assigning the origin (=source) of a package of pollutants in the air requires you to run an "inverse" atmospheric transport model to estimate the likely source points that can explain your measurements. $\endgroup$
    – Erik
    Nov 13 '21 at 9:45
  • $\begingroup$ And the difficulty with the bottom-up inventories is that they can be wrong (either on purpose or not). See, for example, washingtonpost.com/climate-environment/interactive/2021/… . $\endgroup$
    – Erik
    Nov 13 '21 at 9:48
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A bottom-up emissions method is an inventory made by accounting for every known source (or category) and a best-guess estimate for each of their emissions. Bottom-up inventories are often based on government reports which are readily available (no model required) but can sometimes be incomplete or inaccurate. A top-down method is very different because it is derived from measurements at some location(s) on the Earth. In a top-down method, emissions are inferred from environmental measurements using models that link the relationship between emission rates (from the source) and concentrations measured out in the world. A top-down estimate is highly dependent on the model/method being used to infer emissions, but can give bottom-up inventories a good gut-check because top-down methods consider real-world conditions. Top down methods give very poor spatial definition (if any), because most environmental measurements are in select locations or have coarse resolution. Plus, it is nearly impossible to accurately attribute measurements at one location to all the emissions source locations. In contrast, bottom-up methods give very good spatial definition. However, bottom-up inventories are a patchwork that have varied uncertainty and may not be realistic for some particular sectors/locations.

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