Aren't they contributing to global warming?
In theory yes, but only by being alive and having natural body functions. The waste would decompose anyway. Realistically, by collecting it in one place, it would be that much easier to capture the methane, which would then constitute a carbon-neutral process. The amounts we are talking about, however are probably too small to be economical.
Although you question prompts me to ask alaskamomma how far north she is and how much things have warmed up for her over the past decade or so?
Probably a great deal, by the Greenies' reckoning. The substance that makes the composting work is wood chips, which are very high in carbon. That very carbon is oxidized in the composting process to create the high temperatures that make the system work. When carbon is oxidized you get (OH, HORRORS!!!) CO2.