Taiyou no Mokishiroku (2002)

In the early years of the 21st century, a series of massive earthquakes and volcanic eruptions have struck Japan. The worst possible scenario has occurred, and Japan is a devastated country, split in two down the middle. Tens of millions are dead, the economy is in shambles, and what is left is under occupation by "aid forces" from the U.S. and China. In the midst of this, Genichiro Ryu, an extraordinarily resourceful boy, manages to survive and spread his own brand of indefatigable optimism: "Above the clouds and ash, the sun still shines and the sky is still blue."

Source: www.anisearch.com/manga/2051