It was announced by Google China that to open AdSense links, now one have to click in a new browser window or browser tab.
The reason behind this change is the overwhelming request from publishers to improve user experience of Chinese web surfers. With the opening of a new window, users will not be required anymore to use the back button to continue browsing the publisherÃ¢â‚¬â„¢s site. This will minimize the chance of a user accidentally leaving a site.
For the Google AdSense network, the Chinese market plays an important role as the ever increasing demand of Chinese publishers cannot be ignored. This is what the rest of the post said: "The Chinese market and the demands of Chinese publishers is very important to Google AdSense network. We hope to being better experience to users, more revenue to publishers and more earnings to advertisers through constant improvement of our products. By this, we could achieve four-win situation: users win, publishers win, advertisers win, Google wins."
Currently, worldwide publishers cannot alter their AdSense codes to open ads in new window. This is what reported at the AdSense Help Center.