There is a great deal of interest in carbon emissions because of global warming. The World Bank produces data on carbon emissions by country in a variety of forms. It is found at https://data.worldbank.org/indicator/en.atm.co2e. pp.gd. One very interesting series is \(\mathrm{CO}_{2}\) emissions (kg per PPP \$ of GDP). Download this data set to a spreadsheet. There are estimates of \(\mathrm{CO}_{2}\) emissions per dollar of real GDP, a variable sometimes called the carbon intensity of production. At the time of writing this data started in 1990 and ended in 2014.

a. Find the values for the United States. How has the carbon intensity of output in the United States changed from 1990 to the most recent data?

b. Sort the data in the spreadsheet to find the countries with the five largest values of emissions per dollar of GDP in 1990; in the latest year. Has the list of high emission countries changed?

c. Now reconsider the five countries with the highest intensity in 1990. Have they reduced carbon intensity over this period?

d. Go back to the Penn World Tables and use the data on GDP in PPP dollars to measure if carbon emissions are higher in the United States in 2014 than in 1990? Are carbon emissions in China higher in 1990 or in 2014?