So does that mean the network is now demanding each ether be mined every 12 seconds (20% harder statistic mentioned in the linked article), or the difficulty has increased naturally? E.g. there has been a large spike in the number of ether miners lately, which has caused the network to naturally readjust the difficulty to maintain the 15s/ether mining rate? If so, these GPU miners leaving the field would readjust the difficulty back again towards the original levels, or is this difficulty change a more permanent thing due to some underlying change in the technology?