WASHINGTON—The icy region at the top of the globe, lashed by wildfire and pelted with increasingly heavy precipitation, has tipped into “uncharted territory,” scientists reported Tuesday. The Arctic tundra has shifted from storing carbon in the soil to becoming a carbon dioxide source, the National Oceanic and Atmospheric Administration (NOAA) and its partner researchers concluded in their 19th annual Arctic Report Card. As a result, the Arctic’s ability to help regulate Earth’s temperature is significantly compromised. Emissions from warming permafrost regions must be thought of as an increasing risk to a planet already being transformed by the overburden of fossil...