There are some older ETFs with longer track records have targeted low-carbon stocks and doing well attracting investors.
The SPDR MSCI ACWI Low Carbon Target ETF (LOWC) has been raking in assets. The 1,318-stock fund tracks stocks from developed and emerging markets companies with lower carbon footprints than the broader market and has a similar top holdings list to State Street's newer Fossil Fuel Free ETF. Top holdings include Apple, Microsoft and Johnson & Johnson.
The iShares MSCI ACWI Low Carbon Target ETF (CRBN) follows the same index. Both funds also have low 0.20 percent fees.
"These funds have sizable assets," said Mishra. "And you get exposure to well-known companies, like Apple."
But low carbon does not mean no carbon, and the same caution is warranted even for the fossil fuel–free ETFs. For example, low-carbon ETFs own oilfield services companies, including Schlumberger. And the new State Street Fossil Fuel Free ETF includes the same oil service companies, as well as refiners and pipeline companies, though at a total portfolio holdings level that is only 2 percent of assets.
Overall, these ETFs have lots of good, large stocks that have good valuations, Rosenbluth said. But that also means that when the entire stock market is crashing, performance for these ETFs can suffer. Right now the oil rout is pressuring all stocks. The State Street Fossil Fuel Free ETF, for example, is down by 10 percent since its launch last December.
That said, avoiding the energy sector and oil stocks rather than waiting for the bounce may look like a good prospect to more investors who haven't given up on stocks as a whole, and for more reasons than just fighting the long-term effects of climate change.
— By Constance Gustke, special to CNBC.com