As Scientific American explains, climate science deniers have regularly tried to use Mars' changes as an explanation for patterns here on Earth. Mars saw a dramatic shift through strictly natural causes, so humanity can't be responsible for what's happening back home, can it? The problem, of course, is that Mars' changes likely occurred over billions of years. The increase in carbon emissions on Earth (and the resulting increase in temperature) on Earth has taken mere decades, aligning roughly with the rise of industry and fossil fuels. Bridenstine's willingness to study Mars' climate, but reluctance to study Earth's, suggests that he might be looking to other planets as justification for inaction on climate issues.
That's not helped by Bridenstine's own history. When he was a new politician, he asked President Obama to apologize for allegedly wasting money on climate research. He blamed the Sun for climate shifts and falsely claimed that temperatures had stopped rising. As recently as the start of 2017, Bridenstine had floated the idea of removing Earth science from NASA's scope and assigning it to another agency -- as if Earth science isn't inherently connected to NASA's studies.
There are signs that he may be changing his mind, if slightly. He writes in the questionnaire that NASA should still "advance both Earth science and planetary science," and that it's not right when these camps have to battle each other to get funding. The question is whether or not Bridenstine will let NASA study the full range of Earth science if he's approved in the coming weeks. Given his past and a White House that bristles at the mere mention of climate change, it won't be surprising if he's hesitant to support any science that contradicts the administration's agenda.