Dear James,

can you explained what means topsoil for you? Indeed for pedologist topsoil represent the upper soil horizon enriched by organic material from decomposition of the litter on top of it by soil fauna.

Maybe topsoil for you represnts all the "soft material" covering hard bedrock can regolith.

What I mean by "top soil" in this case is the soft material, the outermost layer, that could potentially be used for farming, for example.

So, you are looking to the average regolith depth.

maybe you will find some responses here:

http://daac.ornl.gov/cgi-bin/dsviewer.pl?ds_id=1304

http://onlinelibrary.wiley.com/doi/10.1002/2015MS000526/full

If I'm interpreting the "Global 1-km Gridded: Average soil and sedimentary-deposit thickness" correctly then, the average thickness of regolith over the dry land mass of the earth is 15.5 meters (the data summary above the world image http://webmap.ornl.gov/wcsdown/wcsdown.jsp?dg_id=1304_1). Jerome, would you agree? Funny, because I made an internal wild guess at 50 feet (imperial unit system here in the USA). Thanks for the question and information. It was interesting.

Thanks Robert. Thanks Jerome. I knew it would not be an easy answer, but this information is indeed helpful. I am making some general characterizations in thermal comparisons of Earth and Mars.

Indeed, very interesting. I noticed that Antarctica appeared to not be included in the area considered in the data presented. Given the subject of your comparisons, perhaps this might make a difference to you.

@ Robert, yes I struggled to find the 15.5 m value, indeed thare is a rolling menu as you wrote above the image, so yes I agree with 15.5 m.

@ James, can you explained a bit more your project?

I see frequent references to estimated rates of topsoil loss from erosion which are usually reported as "at this rate we have X years left," but the stock estimate is not reported. I am curious as to how the "at this rate we have X years left" calculations are made and also as to why the current stock estimate is not reported as part of such findings.