Economics Externalities Article Review you need to leave at least two comments. 1.Greenhouse gases trap heat at the atmosphere and make the planet warmer. Carbon dioxide has the highest share of total greenhouse gas emissions by 79 percent which is 5981 million metric tons, while other gases have a little portion; Methane 11 percent, Nitrous Oxide 7 percent and Fluorinate gases 3 percent. 2. Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere. The largest source of greenhouse gas emissions from human activities in the United States is from the combustion of fossil fuels such as gasoline and diesel to transport people and goods, accounting for about 27 percent of total US. C02 emissions. 3. From 1990 to 2007, emission increased rapidly from around 6500 million metric tons to the highest point over 7000 million metric tons and the largest section is transportation. After that, the trend of emission decreased moderately from 2007 to 2009, due in part to a drop in US. economic production during this time. Emissions decreased again from 2010 to 2012 and continued to drop significantly, due to use of natural gas to generate electricity in place of more carbon- intensive fuels. From 2019 to 2020 only, emission decreased by 11 percent. This is due to the decrease of transportation by 13 percent because of Covid19. Compared 2020 with 2005, green house gas emissions were 21 percent below. From 1990 to 2019, greenhouse gas emissions per dollar of goods and services produced by the US. economy (the gross domestic product or GDP) declined by 50 percent. From the trend, we can assume that it will continue to decline as the increasing in both energy efficiency and structural changes in the economy. 4. In 2014, the highest carbon dioxide emitter is China which accounts for 30 percent. If we look only the US., the highest sector of emission is transportation. However, for global emission, electricity and heat production is the highest sector which is 25 percent of 2010 global greenhouse emissions