Does the world have enough lithium? It depends who you ask.

A 2008 study by French researcher William Tahil found there were just 3.9 million metric tons of recoverable deposits globally in mineral ores and Andean salt lakes. That's little enough that the world would risk running out as demand for lithium-ion car batteries and utility-scale storage ramps up over the coming decades.

A survey the following year by consultants Gerry Clarke and Peter Harben, though, concluded there was about 10 times that amount. Depending on the other parameters applied, those numbers suggest deposits could provide lithium for anything from a further 100 million cars -- about 10 percent of the global auto fleet -- to 10 billion or more.

That vast range of estimates is inevitable given compounding uncertainties around the quantities available and the amounts needed. Still, a glimpse of how the sausage is made may help to demonstrate why, as we argued in Wednesday's column, fears of peak supply for battery materials should be taken with a pinch of altiplano salt.

Calculating mineral reserves is a little like working out the contents of a lucky dip by sticking knitting needles into a box. Geologists must drill through rock to produce kilometers of core samples at a cost of several hundred dollars a meter, and then analyze what they've dug up to work out the rough shape and concentration of an irregular ore deposit deep below ground -- a mineral resource.

After that, engineers assess the cost of building and running a mine and processing plant, while economists estimate future prices. Put that financial and economic data together with the geological information and you have a mineral reserve, the gold standard for working out supply availability.

As should be evident, though, that estimate contains multiplying layers of variables. A movement in just one can, for example, make 3.5 billion barrels of oil disappear. That's one of the reasons people invest in mineral exploration companies: There's always the chance that a few extra bits of data could produce a dramatic change, as when a 2007 update increased the estimated lithium content of Argentina's Salar del Rincon brine lake from 253,000 tons to 1.4 million tons. Geologists' estimates of Rincon's lithium concentration range from 0.033 percent to 0.05 percent, a difference that on its own would be enough to increase the resource base by 50 percent or reduce it by a third.

Then come demand issues. The amount of lithium used per kilowatt-hour of battery power is changing rapidly as manufacturers improve efficiency. One 2015 study by Citigroup Inc. estimated about two kilograms of lithium carbonate equivalent are needed for each kWh; more recent appraisals by Deutsche Bank AG and Advantage Lithium reckon it's about a third as much.

On top of that, there's the question of how powerful batteries will be. A hybrid Toyota Motor Corp. Prius has a 4.4kWh battery; a Tesla Inc. Model S can have one as large as 85kWh. Most analysts base their estimates on the 30kWh to 40kWh range of Nissan Motor Co.'s all-electric Leaf, but in truth it's anyone's guess which size will dominate.

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