I'm hopeful that future cryptocurrencies won't be so energy-intensive to mine. Bitcoin is already a non-negligible contributor to global CO2 emissions, believe it or not. EDIT: Reversible computing ( http://en.wikipedia.org/wiki/Reversible_computing ) is a possible way to have computationally difficult proof-of-work while minimizing energy consumption.
Define non-negligible and show us some numbers. Mining is supposed to be difficult.
I like DogeCoin (much shibe) but this is still a Bad Thing. It would be much better if the problem being solved to create a blockchain were also useful in some other context, eg protein folding. Of course, some such problems are handled better than others by computers; protein folding is something that humans seem to do well but for which we haven't managed to develop great algorithms yet.
But if part of the work can be done in the human brain rather than in a CPU/GPU/ASIC, is that bad? The current computation model basically means that mining rewards go to whoever invests the most capital, ie has money to throw at the problem. This is inherently inequitable, because it means people with more money or a willingness to appropriate the equipment of others will mine/dig/discover more coin. Of course, the argument is that since mining is rather random, in theory you can start mining on your small CPU tomorrow and hit some coin, or get a random amount of coin for a fixed amount of mining. But random rewards devalue expertise and experience in favor of an unnaturally even probability distribution. I really think the computational abstraction is a barrier to cyptocurrency adoption, because ultimately your economic incentive is only as good as people's adoption of a sunk cost fallacy.