The encyclical letter Laudato Si is available here on the Vatican website.
Pope Francis cites two psalms:
72. The Psalms frequently exhort us to praise God the Creator, “who spread out the earth on the waters, for his steadfast love endures for ever” (Ps 136:6). They also invite other creatures to join us in this praise: “Praise him, sun and moon, praise him, all you shining stars! Praise him, you highest heavens, and you waters above the heavens! Let them praise the name of the Lord, for he commanded and they were created” (Ps 148:3-5). We do not only exist by God’s mighty power; we also live with him and beside him. This is why we adore him.
Psalm 136 is one of the three common psalms for Easter. Psalm 148 appears in the Wedding Lectionary. I like Psalm 104–check Pentecost–for a significant litany from the natural world.