With a sample that large you have to be careful not to draw conclusions too quickly. Just looking at the recent Launch HNs I see a bunch of startups that go way beyond just "execution risk":

Launch HN: Carbon Crusher (YC W22) – Carbon Negative Roads - https://news.ycombinator.com/item?id=30793076 - March 2022 (93 comments)

Launch HN: Charge Robotics (YC S21) - Robots that build solar farms - https://news.ycombinator.com/item?id=30780455 - March 2022 (79 comments)

Launch HN: AirMyne (YC W22) – Capturing CO2 from air at industrial scale - https://news.ycombinator.com/item?id=30712229 - March 2022 (192 comments)

Launch HN: Living Carbon (YC W20) – Trees that capture and store more carbon - https://news.ycombinator.com/item?id=30672841 - March 2022 (256 comments)

Launch HN: Micro Meat (YC S21) – Technology for scaling cultivated meat - https://news.ycombinator.com/item?id=30627418 - March 2022 (194 comments)

Launch HN: Phase Biolabs (YC W22) – Converting CO2 to Carbon-Neutral Chemicals - https://news.ycombinator.com/item?id=30557288 - March 2022 (88 comments)

These all happen to be climate-related but I'm pretty sure that's just a coincidence. One thing's for sure, YC is just as interested in funding high-risk-high-potential-value startups as it ever was.