Mid-21st century crisis

The mid-21st century crisis, often simply called the Crisis, was a period in the history of humanity marked by unprecedented death and chaos. During this period, which began in the late 2020s and ended with the Unification Wars, Earth's population dropped by over seven billion, or well over two thirds of the global population. It was marked by wars, famine, natural disasters and political instability across the planet. 
The main cause of the Crisis is considered to have been climate change. Carbon emissions exploded following the rapid industrialisation of Africa and South Asia in the 2010s and 2020s. Even the most pessimistic estimates of the time proved to be too optimistic, and by 2035 the average temperature of Earth had risen by 2°C. As a result, massive immigration from the countries most affected caused worldwide political instability. The lack of basic resources, most notably water and food, led to many armed conflicts between states, whilst others collapsed into anarchy. The leading superpowers of the early century, the United States of America and the People's Republic of China, both collapsed during the 2030s as a result of military conflict, debt and political instability
The Crisis was felt differently across the globe. Europe was the continent which was least affected by the crisis. During this period the European population grew by around 20%, with most of the growth being attributed to immigration. The prior population did not however decrease significantly. Other regions of the world saw drastic falls in population due to emigration and famine. Africa's population dropped by over 75%, and in some regions of Central Africa the population dropped more than tenfold. Regions such as Latin America, South and East Asia were similarly affected by the Crisis. The year 2044 was the most lethal in these regions, when over 400 million died from starvation and thirst. 
Terran scholars have come to label the Crisis "the greatest turning point in human history". The Crisis made possible the Unification and set up a political climate in Europe which allowed for a man like the Custodian to rise. The population decrease as well as the raising of awareness of the period allowed for drastic measures to be taken by the subsequent world government to reverse most of the effects of climate change.