Personally, I believe the UN should deal with disaster sites of this scale and effects to neighboring countries (presently, that means Chernobyl and Fukushima). Both sites are way too much to handle for the local government (Chernobyl) or the local government just can't be trusted to cut corners (Fukushima, where the Japanese government plans to release the contaminated water into international sea).

The problem is: the UN countries can't even be bothered to put up the small sum of money for feeding refugees (delinquent payments and resulting food cuts were the major trigger for the 2015 migration) or for relinquishing patents for COVID-19 vaccines... it will end up in a similarly disastrous state as it already is - and possibly even worse, given the UN's reputation as an useless paper tiger.

The obvious fix would be a permanent funding stream for the UN via its own taxes - say, for example, a 0.5% tax on all individual incomes (no matter if from employment or capital gains) of all membership countries. That could fund stuff like handling refugees, fights against pandemics, restructuring after disasters, or peacekeeping interventions without nations dooming operations (cough USA vs WHO) for political "gain".