The United States emits a quarter of the world's carbon dioxide, and its per-capita emissions are also the highest in the world.
Literal
America [topic-は] world [of-の] 1/4 [of-の] carbon-dioxide [object-を] emit-[progressive-ており], per-one-person [of-の] emission-amount [also-も] world [in-で] most many [explanatory-のです].
Formal news/scholarly register. 排出しており is the conjunctive ~ており form, a formal/literary variant of ~ていて that chains clauses in written Japanese. 二酸化炭素 'carbon dioxide' is a transparent Sino-Japanese compound (二 'two' + 酸化 'oxidation' + 炭素 'carbon'). 一人当たり 'per person, per capita' uses the suffix ~当たり 'per.' のです at the end frames the statement as explanation. The sentence is typical of environmental policy discourse.