# Percentage of Oxgen left after burning all the available biomass

A documentry stated that during the Permian asteroid strike the atmosphere heated up to above 400C, potentially to 600C. As this would ignite most of the biomass, including us, I wondered if this fire would consume so much atmospheric oxygen that it would kill most animal life that survived the fire. Oxygen levels would only need to fall below about 10 percent for this to happen.

• Welcome to Stack Exchange. Are you asking how much oxygen would be left if all available biomass were burned, or how much was left after the Permian asteroid strike (where not all available biomass was burned, since obviously some life survived)? – Daniel Griscom Oct 23 '16 at 3:49

Estimates of earth's total biomass vary widely, from 0.5 to 4 trillion tons C, so instead of citing a source, I'll just go with an assumption of $1\times10^{15} \text{kg C}$. Measuring biomass in carbon is a convenient segue to the next assumption: assume that all biomass burns only in $\text{CX + O}_2\,\rightarrow\,\text{CO}_2 + \text{X}$.
Given these assumptions, $1\times10^{15} \text{kg C}\cdot \frac{1 \text{mol C}}{0.012 \text{kg C}} = 8.3 \times10^{16} \text{mol C}$. To react with so many mol's $\text{C}$ we will need the same number of mols of $\text{O}_2$, at molar mass $0.032 \text{kg O}_2$, which gives us a total of $2.7\times10^{15} \text{kg O}_2$ needed.
Oxygen is 20% of the atmosphere, which means there is $5.1\times10^{18} \text{kg atm} \cdot 0.2 \frac{\text{O}_2}{\text{atm}} = 1.0 \times 10^{18} \text{kg O}_2$ available.