Karma is simply "cause and effect". The theory is most scientific. Unfortunately this term is abused all the time.
There is a Chinese saying, "If you plant melons, you will harvest melons. If you plant beans, you will harvest beans." Karma is just as simple as that.
In the old days, when there was natural disaster, the emperor would reflect on what he did and blame himself as if the natural disaster was a punishment from Heaven for his wrong doing. Well, this might be just a show to tell people he was, in fact, a conscientious emperor. Through that, the people were brought together to reconstruct the country. However, it was abuse of the term "Karma" unless what the emperor did actually caused the natural disaster.
During the "Great Leap Forward", Chairman Mao ordered farmers to turn the soil three feet under to cover up the top soil. His theory was that the soil on the surface had been used for thousands of years while the soil below was "virgin soil" and must be very fertile. This was the cause of the "Great Famine". But, he never had regrets for his crazy policies. Yet, we cannot relate the wrong doings of the Chinese government in those days to the huge earthquake in Tangshan near Beijing in 1976 because these were unrelated things.
The chief editor of a popular magazine in Hong Kong also claimed that the recent Sichuan earthquake to be punishment from Heaven on what the Chinese government did to the Chinese people. Shame on him.