The greenhouse effect is increasing because humans continue to add additional carbon dioxide to the atmosphere, chiefly by burning fossil fuels such as coal, oil and natural gas. Carbon dioxide is a greenhouse gas and so the higher the atmospheric concentration of carbon dioxide the greater the greenhouse effect. The direct effect of this is global warming. The follow-on effects include rising sea levels and climate change.
There is a great deal of issues still unknown in this very new science area. For example, it is quite possible that levels of CO2 over 100 PPM have almost no affect on temperature increases. We are also seeing NASA data to suggest that there is far less actual retention of heat then models projected. Sea level rise has also slowed to it's lowest levels in thousands of years according to NASA. We may well see less then 8 inches of rise in the next hundred years. The bottom line answer to your question is we still are trying to find out the answers.