Mining gold is environmentally problematic, but no new gold needs to be mined when gold is transferred from one person to another. Bitcoin transactions require new blocks to be added to the block chain, and therefore more "mining" must take place.
There is a bigger problem, however. If a gold miner finds a more energy-efficient mining process, they will use it because it is more profitable. No technological improvements make Bitcoin mining more energy efficient (miners are incentivized to expand their operation rather than to mine at the same rate using less power), because the whole point of Bitcoin mining is to prove that electricity was utilized. That means that there is no real solution to Bitcoin's pollution problem, other than scrapping Bitcoin entirely and replacing it with something more energy-efficient (like a distributed ledger operated by a consortium of banks).
Bitcoin's energy consumption is a function of the number of miners, not the numbers of users. Just because more people transfer and use bitcoin doesn't mean more energy is used.
Every transaction requires at least one block (typically several) to be added to the block chain, and the block size limit means that, in fact, energy consumption in Bitcoin is proportional to the number of transactions. Beyond technical details, there is also the economic incentive: miners are paid to include transactions in new blocks, and if there are more transactions (thus higher demand) the fee will rise and miners will consume more power in order to collect the higher fees.
There is a bigger problem, however. If a gold miner finds a more energy-efficient mining process, they will use it because it is more profitable. No technological improvements make Bitcoin mining more energy efficient (miners are incentivized to expand their operation rather than to mine at the same rate using less power), because the whole point of Bitcoin mining is to prove that electricity was utilized. That means that there is no real solution to Bitcoin's pollution problem, other than scrapping Bitcoin entirely and replacing it with something more energy-efficient (like a distributed ledger operated by a consortium of banks).