Block reward is the incentive that makes people to mine. The bigger the incentive, the bigger is the sum that is spent to earn it. In the case of Bitcoin, if you want to earn the reward you must use lots of electricity. Basically the Bitcoin network is paying for people to waste electricity.
If we want to reduce the electricity consumption, we have to make the block reward smaller or raise the price of electricity (which will cause problems for the people around the world) or prevent the price of bitcoin going up.