The obligation is a long-dated liability discounted at a bond rate, so it swings violently with interest rates. A rate rise shrinks the obligation and can turn a deficit into a surplus without a single dollar changing hands.
Deficits are a claim ahead of shareholders, so they belong in net-debt for valuation purposes. The remeasurement gains and losses usually run through accumulated-other-comprehensive-income rather than earnings.
Example: Northwind Tools sponsors a closed scheme with a $310M obligation and $286M of assets, a $24M deficit. A one point rise in the discount rate would cut the obligation by about $40M and erase the deficit.
Related: net-debt, accumulated-other-comprehensive-income, contingent-liability, footnotes, discount-rate