While startups race to build machines that can sequester CO2 from the air, the world's three trillion trees are already doing the same thing. A new platform, CTrees, is designed to track the carbon stored in every tree on earth. CTrees uses artificial intelligence to analyze satellite and aerial data along with local data from tree inventories. As trees grow -- or if they're lost to logging or fires -- CTrees can use new satellite images to keep a running estimate of total carbon. Its results are close to the gold standard of on-the-ground measurement, says Sassan Saatchi, a senior scientist at NASA's Jet Propulsion Laboratory. Such data is crucial for countries that rely on forests to capture a specific amount of carbon as part of their climate commitments. Companies and nonprofits selling carbon credits for protecting or restoring forests also need to confirm the value of their work.