The Arctic and northern high latitudes are warming about 2–4 times faster than the global average, allowing ancient permafrost to thaw and release stored carbon. These permafrost soils currently store roughly one-third of the world's organic soil carbon, much of which has remained frozen for thousands of years. As the soils thaw, organic matter from dead plants and animals within them starts to decompose, and greenhouse gases, including carbon dioxide, are released.
Read the article its literally the second paragraph
Edit: if you can believe it, reading the whole article you get a complete and detailed picture of what the headline means and how they go from sink to source
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That doesn’t explain at all how a tree would get sequestered into solid permafrost.