In the US, the Senate needs to pass a Resolution of Ratification with a two-thirds majority vote. The Resolution of Ratification authorizes and advises the President to ratify the treaty. Without the approval of the Senate, the treaty would not be ratified. Even if the treaty is ratified, it's largely up to the states to implement the agreement.
Agreements like the Kyoto Protocol and the Convention on Biological Diversity have been opposed by both Democrats and Republicans. Regardless, these agreements have been implemented through various other international agreements.
The treaties are also implemented through various United Nations-accredited Non-governmental Organizations, publicly traded corporations, and other members of civil society, without approval by the Senate. Organizations like the International Union For The Conservation of Nature, which consists of politicians, governments, lobbyists, environmental groups, activist groups, and other special interest groups, claim to be "the global authority on nature and all things to safeguard it" and incrementally implement the agreements in the U.S..
Collectively, these groups implement the treaties, regardless of Senate approval, through legislative and executive actions by federal, state, county, and city governments.
The Convention on Biological Diversity, for example, authorized the production of the Global Biodiversity Assessment, which provided the scientific basis for implementing the agreement. The Global Biodiversity Assessment emphasizes fundamentally changing land use control of private property, agriculture, mining, and recreation.
Section 126.96.36.199 of the Global Biodiversity Assessment states:
“Property rights are not absolute and unchanging, but rather a
complex, dynamic and shifting relationship between two or more
parties, over space and time.”
The legal approach to the UN's view of property rights is discussed in Section 188.8.131.52 (pages 786-787):
“Plants and animals are objects whose degree of protection depends on
the value they represent for human beings. Although well intentioned,
this specifically anthropocentric view leads directly to the
subordination of biological diversity, and to its sacrifice in spite
of modern understanding of the advantages of conservation. We should
accept biodiversity as a legal subject, and supply it with adequate
rights. This could clarify the principle that biodiversity is not
available for uncontrolled human use. Contrary to current custom, it
would therefore become necessary to justify any interference with
biodiversity, and to provide proof that human interests justify the
damage caused to biodiversity".
The Convention on Biological Diversity is a legally binding agreement that establishes international law that requires the abolition of trade restrictions and subjects American citizens and institutions to judgment by the International Criminal Court. The Convention on Biological Diversity was therefore considered a threat to national sovereignty and security by both sides of the political aisle.
Private ownership of land, due process of law, and local representation are fundamental to the U.S. Constitution and national sovereignty. Therefore, the reason these treaties were never ratified is simple. They did not garner the votes necessary to become law.
See: Global Biodiversity Assessment, Convention on Biological Diversity Resolution of Ratification: Senate Consideration of Treaty Document 103-20