If you look at the changes of carbon emission per country, the U.S. emission has decreased in the last 20 years. China's emission has tripled in the same period:
https://cdn.static-economist.com/sites/default/files/images/...
The main reason is the growth of Chinese economy. So if you want to make real impact, help the next emerging economies to grow without incurring huge growth in emissions. For example, if you look at the per-capital carbon emission by country:
https://en.wikipedia.org/wiki/List_of_countries_by_carbon_di...
China's is half of that of US, while India's is 1/8. Imagine if the India's number grow 4 times to that of China's, any saving in the U.S. will be completely cancelled out and some. In fact, U.S. can cut its emission to 0, and India only needs to grow its number to 75% of that of China's to cancel out the U.S. improvement.