A friend made a comment after he read the article:

"One thing I could argue with him is, in Chu's theory, he didn't take into consideration the effect of deforestation. North China used to be covered by forest, which helped to moderate temperature change. For instance, with forest, temperature may vary from 0 to 20 degree in a day; without forest, temperature may vary from -5 to 25 degree. Although both the average temperature is 10, the latter is more harsh to some species's survival."

IMHO this is a very good point.  We should also take into account that there used to be much more forest coverage 3000 years ago.  The forest should have made the climate milder.

This also brings another question.  According to the modern global warming theory, CO2 has a very significant role in terms of temperature change.  The more CO2, the higher the temperature of the earth.

3000 years ago, the density of CO2 in the atmosphere must be much lower than now, thanks to the vast amount of vegetation, which lowered CO2 through photosynthesis process.  Therefore, the temperature should be a lot lower then--according to the global warming theory.

So this is the question.  Which factor played a more superior role for the temperature of 3000 years ago?  Did the forest raise the low temperature in winter, or did the low CO2 make the earth colder in winter?