According the to the research performed by Corey J. A. Bradshaw, Xingli Giam, and Navjot S. Sodhi in 2010 on title ‘Evaluating the Relative Environmental Impact ofCountries’:
From a global perspective, the most populous and economically influential countries generally had the highest absolute environmental impact: Brazil, USA, China, Indonesia, Japan, Mexico, India, Russia, Australia and Peru were the 10 worst-ranked countries.
It also depends on how big the country is. For example if you are comparing ireland to america, it could clearly be america because they are bigger. So you need to make sure it is broken down to a comparable unit
I am not sure. We are all impacting the environment, and we can all make small changes to make it better, and slow down global warming. It is very easy for us to point the finger at other countries that impact the environment more than we do, but we should all make small changes that are easy to implement to save the environment for future generations