Final answer:
The two consequences of the decline in mortality during the twentieth century are significant population growth and an aging population that affects workforce sustainability and increases demand for healthcare and social services.
Step-by-step explanation:
The decline in mortality during the twentieth century has led to two significant consequences. The first consequence is dramatic world population growth due to falling death rates accompanied by high birth rates, especially in developing nations where birth rates showed little change. The second consequence is the aging of the population, which has increased the average age of the overall population, creating economic implications such as labor shortages and increased demand for medical and social services.
Developments in public health, sanitation, and medical innovations such as the use of antibiotics and vaccines have been key factors in reducing death rates. Clean drinking water and proper sewage disposal, combined with immunizations for various diseases, have played a substantial role in elevating life expectancy and decreasing mortality rates.
On the economic front, nations are grappling with the challenges posed by a demographic shift, whereby an aging population requires more resources for healthcare and welfare, leading to a strain on state spending. Moreover, as the population ages and fertility rates drop, there is a concern for the sustainability of the workforce, particularly in physically demanding industries.