The assumption here is that a UBI necessarily takes the form of a stand-alone policy rather than part of a package of reforms. While some libertarian supporters might prefer to see it take that form (i.e. a replacement for other social policies), most advocates see it as part of a package. For example, truly universal healthcare coverage and significant reductions in or the elimination of university tuition.
In addition, there is considerable research showing that much of our consumption is a byproduct of financial insecurity rather than the product of wealth. The far more egalitarian countries of Western Europe, for example, have much smaller per-capita carbon footprints than more or less equally wealthy (as measured by GDP) but inegalitarian societies like the United States. We borrow considerable amounts to maintain the appearance we are keeping up with the Jones’ in societies plagued with inequality. While it is almost certainly true that people will broaden their experiences with more money in their wallet, experiences generally have a much smaller carbon footprint than stuff. Therefore, it’s possible, even likely, that UBI would produce more spending yet have a lighter environmental impact overall. Experiences are also far more emotionally satisfying.
The existing practice of policing the poor to make sure they spend the money they receive through social welfare programs on the “right” things and requiring them to regularly submit paperwork to the government to prove they are still worthy of assistance is both costly to administer and demeaning to those these programs are intended to help. A UBI, especially when coupled with universal healthcare coverage, living wage laws, paid parental leave, etc., is far more efficient and humane.