You can use either. 'That' means that what follows is a fact. The writer offers (whether' in brackets as an alternative because some people may argue that 'humans are warming the planet' is not a fact. 'Whether' means that this is a question - are humans warming the planet? The writer offers both options so the reader can choose.

"whether" expresses doubt

Claire answered your question. If you use "that", it means debate will probably center on what we should do moving forward in light of the fact that we are warming the planet.