U.S. CO2 Emissions Gone Global

While Americans produce a disproportionately large amount of CO2, the contributions of the citizens of other nations are often comparatively small. We wonder: If all of the world's nearly seven billion people contributed as much CO2 as the average American, to what level would the world's population need to shrink to keep the current global level of CO2 unchanged?

A. 6 billion
B. 4 billion
C. 2.5 billion
D. 1.5 billion
E. Half a billion

A. 6 billion is not correct.

A world population of 6 billion, which was reached at the end of the 20th century, would produce a four-fold increase in today's global level of CO2 emissions — if each person contributed CO2 emissions at the rate of the average American (19.3 tons per person annually).

However, if the CO2 contributions of the 6 billion people were at the average Chinese rate — 4.6 tons per person annually — the total global amount of CO2 would be roughly the same as it is today.

B. 4 billion is not correct.

A world of 4 billion people emitting as much CO2 as the average American would nearly triple the current global CO2 level.

Even if the average U.S. CO2 level were cut in half, the worldwide CO2 level with 4 billion people would still be about 35% higher than its current level.

C. 2.5 billion is not correct.

Back in 1950, the population of the world was about 2.5 billion people. If the world declined to this smaller size with average U.S. CO2 emissions, the global production of CO2 would still be about 70% more than it is today.

D. 1.5 billion is correct.

In order to maintain the current global level of CO2 — 28.4 billion tons per year — the world population would need to be approximately 1.5 billion if everyone produced as much CO2 as the average American. This figure is about one-fifth of the current world population.

E. Half a billion is not correct.

A world of 500 million people emitting as Americans would produce roughly a third of the current level of CO2. However, this global population figure is believed to have been reached as far back as the end of the 15th century — 300 years before the start of the Industrial Revolution.

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