Is Global Warming Caused by Human Activities?

We can answer this question by looking at the earth’s actual temperature record.

Since 1900, greenhouse gases in the atmosphere have increased by an amount equivalent to about a 50% rise in carbon dioxide alone. This gives us a way to anticipate what can be expected for manmade global warming in the next century, when the increase in greenhouse gases is expected to be the equivalent of a 100% increase in CO2.

The earth has warmed by approximately 0.5 degrees C since 1900, presumably in response to this 50% effective increase in CO2. If a 50% increase in the last hundred years produced a half-degree rise, a 100% increase in CO2 in the next hundred years will produce twice that rise, or one degree.

However, a closer look at the timing of the CO2 and temperature increase shows that the warming will be much less than one degree. Most of the half-degree temperature rise since 1900 occurred prior to 1940, but more than half of the additional CO2 entered the atmosphere after 1940. Greenhouse gases produced by human activities cannot be the cause of a global warming that occurred before these gases existed. Natural factors, such as an increase in the sun?s brightness, must have produced a large part of that earlier warming of 0.5 degrees C. Human activities, such as burning coal and oil, can only account for at most a few tenths of a degree.

But if a 50% increase in CO2 produced a rise of a few tenths of a degree, the 100% increase expected in the next century will produce only about a half a degree. A warming of half a degree, spread over a century, would be indistinguishable from the natural climate variations that have occurred throughout climate history.

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