Complex sentence: One narrative describing the causes of the crisis begins with the significant increase in savings available for investment during the 2000--2007 period when the global pool of fixed income securities increased from approximately $36 trillion in 2000 to $70 trillion by 2007.
Split into simpler sentences: One narrative describing the causes of the crisis begins with the significant increase in savings available for investment during the 2000--2007 period. During this time, the global pool of fixed income securities increased from approximately $36 trillion in 2000 to $70 trillion by 2007.