Humans cause global warming: The evidence

Scientists use typical detective tools to determine who is the main suspect in global warming: Us.

They concluded that the cause could not be other than carbon dioxide and other greenhouse gases generated by the burning of fossil fuels.


EDITOR’S NOTE: This dispatch is part of a series on the most serious problems associated with climate change, the role of science, the impact of global warming and what is being done to deal with this issue.


For more than 30 years, some of the world’s leading scientists have collaborated to prepare reports on climate change and its causes. And with each report — as with each increase in the planet’s temperature — the certainty that climate change is caused by human activity increased. In their latest version they said: “It is indisputable that human influence warmed the atmosphere, the oceans and the land.”

Several scientists, including Ralph Cicerone, former president of the US National Academy of Scientists, told the Associated Press that they are as sure that climate change is man-made as they are that cigarettes can be deadly.

One way to prove that humans are causing warming “is to rule out other possible factors,” said Gabe Vecchi, a Princeton University climate professor.

Scientists can calculate how much heat each suspect traps using elements of chemistry and physics, as well as computer simulations that generally produce accurate pictures of weather, both past and into the future. They measure the radioactive force in watts per square meter.

The most obvious natural suspect is the sun, which warms the Earth and produces 1,361 watts per square meter year after year. It generates a delicate balance that is what makes life possible on Earth. Changes in energy from the sun have been minimal, about a tenth of a watt per square meter, scientists estimate.

But carbon dioxide from burning fossil fuels is trapping heat at a rate of 2.07 watts per square meter, 20 times more than the changes associated with the sun, according to the US Environmental Protection Agency. Methane is another powerful heat-trapping gas, 0.5 watts per square meter.

The sun’s 11-year cycle makes for small ups and downs, but that doesn’t seem to affect Earth’s temperature. In any case, the trend has been downward, not rising temperatures, according to NASA calculations. “It is very unlikely that the Sun caused global temperatures to warm in the last century,” NASA said.

I mean, the sun has an alibi.

The other natural suspects — volcanoes and cosmic rays — played even less part in global warming over the past 150 years, according to the scientists.

Another way to corroborate that carbon dioxide is the cause of warming is through a “causal chain”.

Records from the US National Oceanic and Atmospheric Administration indicate that a Hawaiian volcano shows rising levels of carbon dioxide, and so do ice archives. The key here is the type of carbon dioxide they produce.

There are three types of materials that contain carbon: Some contain light carbon, or carbon 12. Others have heavy carbon, or carbon 13, and there are those that contain a radioactive carbon, or carbon 14.

Over the last century there has been more carbon 12 in the atmosphere compared to carbon 13, and less carbon 14 in recent decades, according to ANOA.

Carbon 12 is basically a fossil carbon from a long time ago, like what appears in fossil fuels. Changes in the carbon-12 and carbon-13 ratios reveal that the carbon in the air is mostly a product of fossil fuels and not natural carbon, Vecchi said.

This is like a fingerprint that reveals the burning of coal, oil and natural gas.


The Associated Press’ climate and environment coverage is supported by several private foundations. The HPD is solely responsible for the content.


Seth Borenstein is on Twitter, a @borenbears

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