isn't the universe expanding at an accelerating rate? if the problem is excess planetary heat, you can use thermosynthesis to accelerate novel thermogenesis in pseudo-living species - imagine endothermic sea sponges cooling the oceans and helping to repopulate at risk ocean animals.
"this thing humans do cannot continue" is a weak argument. everything humans do is some expression of the natural state of conditions in their immediate environment, technology is just another convolutional cycle on top of the pre-existing ones, squeezing more efficiency from the raw resources existing there. the planet is not in balance, and ecosystems are not fixed. things are in a state of glacial flux with periods of extreme instability due to various cycle renewals - volcanic, tectonic, asteroid/comet impactor, solar, geomagnetic symmetry breaking, viral.
'man made causes' are a misnomer, they are just accelerated selection events, man happens to be a global selector for almost all living things, and man activity on the planet can be a mass extinction event due to many scaling factors, the most obvious one being the technology cycle mentioned previously, the efficiency it provides can be simplified to 'making more heat'.
it's easy to fall prey to goldilocks thinking - due to limited information and the tyranny of the present. try to think in terms beyond a human lifespan. look at the progress achieved in the past 300 years. if we wanted to we could spend the next 300 years returning earth back to a pre industrial epoch and wipe from the surface of the planet all traces of the modern advanced civilization we created, not by a cataclysm or destructive war but by planned intentional decolonisation. given that its possible and very likely to happen (perhaps a catalyst is required, like a new global religion) over the past 300k-1my this maybe happened multiple times. humans are still here in some form or another and they will continue to be here into the foreseeable future x10 my's, because they have shown capable of surviving (milankovitch scales, 100k, 41k, 23k years).
technology is dominant and makes the smartest humans think in brittle ways. the people shaping rocks for millennia were not stupid, they were just incapable of thinking beyond their stone paradigm. pretend to be a godlike alien silently watching the world from the largrangian point in the earth-moon system. millions and billions of years go by. once life begins on earth it continues into the present, the idea that humans, in the next few thousand years, will somehow pose an unconscious risk to a process that has survived and morphed for billions of years is a misreading of the story. what COULD happen is another mass selection event, where a human bottleneck eliminates 99.9% of all living things. that happened before and it's part of some larger cycle that humans are necessarily a part of (as living things) and what is considered causal could also be simply an expression of a deeper reason for living things to exist - to out compete other living things and monopolize the space available to them.
is the physicist a finitist? seems like he substituted growth which is the economists mantra, for change which is his. predicting the future is a waste of time, as an agent in the world you will either cause the future or be slave to it despite your best efforts either way. if it is knowable it will be unchangeable, given all your free actions will lead to the known outcome. it is clearly unknowable and therefore ununchangeable - since the capacity to measure any deviation or change does not exist. what will occur is as likely to occur or not occur had you participated or not.
my pet theory is that any time you see people attempting a cross-disciplinary leap of faith their parachute often fails to open. good 20th century examples being james watson and william shockley - both making the tempting leap from (bio)electrical systems to socio-communication ones, leading to self-embarrassment. the only interesting problem for the 21st century is how do you transform knowledge from one domain to another and retain it, the only way humans will survive their own extinction without reverting back to barbaric primitivism is to plasticize expertise and mass produce it.