downward vs downwards

downward is an adjective but downwards is not an adjective.

Word	Noun	Adjective	Verb	Adverb
downward	No	Yes	No	Yes
downwards	No	No	No	Yes

As adverbs, downward and downwards are synonyms defined as:

downward and downwards: spatially or metaphorically from a higher to a lower level or position

Other synonyms of downward include down, downwardly.

downward (adjective)	downwards (adjective)
extending or moving from a higher to a lower place
on or toward a surface regarded as a base

downward (adverb)	downwards (adverb)
spatially or metaphorically from a higher to a lower level or position	spatially or metaphorically from a higher to a lower level or position