How do we know that humans have triggered the warming?

Call it law and order: climate change. Scientists used detective work to pinpoint the main suspect of Earth’s warming: us.

They proved it could be nothing but carbon dioxide and other greenhouse gases from burning fossil fuels.


EDITOR’S NOTE: This story is part of an ongoing series that answers some of the most fundamental questions about climate change, the science behind it, the effects of global warming, and how the world is dealing with it.


For more than 30 years, top scientists from around the world have worked together every several years to report on climate change and its causes, and with each report – and the rise in global temperatures – they have become increasingly certain that change climate is caused by human activities. In the latest version of their report they stated: “It is unequivocal that human influence has warmed the atmosphere, the ocean and the earth.”

Scientists, including the late Ralph Cicero, the former president of the National Academy of Scientists, told The Associated Press that their belief that climate change is a man-made problem equates to their certainty in understanding that cigarettes are deadly.

One way to show that humans caused warming “is to eliminate everything else,” said Gabe Vecchi, a climate scientist at Princeton University.

Scientists can calculate the amount of heat trapped by different suspects, using a complex understanding of chemistry and physics and putting it into computer simulations that have generally been accurate in portraying the climate, the past and the future. They measure what they call radiative forcing in watts per square meter.

The first and most frequent natural suspect is the sun. The sun is what warms the Earth in general by providing around 1,361 watts per square meter of heat, year after year. This is the baseline, the delicate balance that makes the Earth livable. The changes in energy from the sun have been minimal, about one-tenth of a watt per square meter, the scientists calculate.

But carbon dioxide from burning fossil fuels is now trapping heat at 2.07 watts per square meter, more than 20 times that of changes in the sun, according to the U.S. Environmental Protection Agency. Methane, another powerful heat-trapping gas, is 0.5 watts per square meter.

The sun’s 11-year cycle goes through regular but small highs and lows, but that doesn’t seem to change Earth’s temperature. And if anything, the ever so slight changes in 11-year average solar irradiance are shifting downwards, according to NASA’s calculations, with the space agency concluding “it is therefore extremely unlikely that the Sun caused the warming trend in temperature. global observed over the last century “.

In other words, the sun had an alibi.

The other natural suspects – volcanoes and cosmic rays – have had even less influence during the past 150 years of warming, the scientists conclude.

The other way to show that carbon dioxide causes warming is to build what Vecchi calls “a causal chain.”

National Oceanic and Atmospheric Administration records measured on a Hawaiian volcano show rising carbon dioxide levels, as do ice records that date back thousands of years. But the key is what kind of carbon dioxide.

There are three types of carbon-containing material. Some contain light carbon or carbon-12. Some contain heavy carbon or carbon-13 and still others contain radioactive carbon-14.

In the last century or so, there is more carbon-12 in the atmosphere than carbon-13 and less carbon-14 in the past few decades, according to NOAA. Carbon-12 is essentially fossil carbon from a long time ago, such as in fossil fuels. So the change in the ratio of carbon-12 to carbon-13 tells scientists that carbon in the air comes more from burning fossil fuels than from natural carbon, Vecchi said.

This is the fingerprint of burning coal, oil and natural gas.


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Follow Seth Borenstein on Twitter at @borenbears


The Associated Press’s climate and environmental coverage receives support from several private foundations. Find out more about the AP climate initiative here. The AP is solely responsible for all content.

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