For the purposes of environmental hazard, it isn't number of transactions that matter but number of miners. That in turn is going to depend on the expected payo

For the purposes of environmental hazard, it isn't number of transactions that matter but number of miners. That in turn is going to depend on the expected payout to miners (either in value of new coin or in transaction fees). New coin mining rate is fixed for bitcoin, so the only parameters one can change to reduce the environmental impact are value and transaction fees.