Trees are one of Earth’s largest banks for storing the carbon that gets emitted by natural processes and human activities. Forests cover about 30 percent of the planet’s surface, and as much as 45 percent of the carbon stored on land is tied up in forests. But did global forests hold more or less carbon in the past? And could they store more in the future? Does it matter where those trees are growing? Scientists really don’t know. But before they can find out, they’ll need a reliable inventory of what is growing today.