so that 的疑问The total annual output of new materal is estimat

so that 的疑问
The total annual output of new materal is estimated to be between one and two million separate texts,__ the amount of new literature which is readable only after language barriers have been surmounted is probably well onver half a million texts a year.
这里为什么要用‘so that 应该怎么分析?
这是节选自状语从句语法里的一篇完形选择填充，文章里的每个句子基本上都这么长。真是见鬼的练习！还有一些都没列出来，