Dear Umbra,

The many articles on global warming conclude with something about the inherent complexity and uncertainty of the issue. So exactly what is the evidence for (and against) arguing that the current warming trend is inside the scope of normal fluctuations? What is the evidence for (and against) arguing that the trend is caused by human activities, and is not just part of a “natural cycle”? Is it true that the U.S. is just about the only country where scientists seriously debate the reality of global warming? Who are these people writing articles telling us we don’t have anything to worry about? What are their credentials?

Kevin Carroll
Chicago, Ill.

Dearest Kevin,

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Yours is a very long question, I have received many like it, and in the next few weeks I’m going to harp on climate change in honor of Earth Day, probably until we’re all terrified of the future and sick of me.

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Who nose the truth?

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I’ll answer your question’s last part quickly: no credible scientific body in the United States continues to debate whether or not human activity contributes to climate change. I don’t tend to read the articles telling us we have nothing to worry about, but I assume some people writing such articles have jobs at stake.

What is uncertain about global warming is what the effects will be. Not “will they be bad” or “will they be severe” — rather, “how bad and severe will they be?” What is complex is the situation. I like to use the Intergovernmental Panel on Climate Change and the United Nations Environment Program as data sources. Today’s fun page from the IPCC site: PowerPoint charts showing the past, present, and future of our climate. Let’s open a new wwwindow to “Temperature Change (1760-2100)” (chart TS22), where we can see the relatively flat line that is global temperature through the recent past, and the sharp incline that is global temperature in the near future. There are different ideas, as you’ll see by the “temperature range in various scenarios” bars on the right, about how much the temperature will rise. That’s what’s complex and debated. Feel free to cruise other charts and see things like the millions of people expected to be hit by coastal storm surges, etc.

OK, but to the meat of your question: how we know. Apparently there are at least four ways we know that our fossil-fuel burning is a problem. One is through testing the nuclei of carbon atoms in tree rings. Naturally released carbon from sources on the earth’s surface still has a radioactive nucleic portion, while fossil-fuel nuclei have lost their radioactivity over the millennia. By looking at the radioactive content of carbon in tree rings, actual scientists have found the radioactivity of carbon in the atmosphere to be diluted since we started intensively burning fossil fuels.

The evidence is in.

Photo: iStockphoto.

Second, actual observations of carbon dioxide in the atmosphere since the 1950s match the predictions of anthropogenic effect on atmospheric carbon. Third, carbon dioxide in ice-core sampling, which can check air bubbles back hundreds of thousands of years, shows little variance for the 10,000 years prior to industrialization — then, ta da! Variance. Fourth, short-term measurements of CO2 release show that more CO2 is emitted over the area where the most fossil fuels are burned: Bhutan. (Ooh, just checking to see if you were paying attention. The Northern Hemisphere is where the most fossil fuels are burned, and where the most CO2 is emitted.)

Also, we know because we are smart. Actual scientists study the planet’s climate system, study greenhouse gases such as carbon dioxide, and do what scientists do: play with what might happen in various scenarios. Such as, for example, humans unearth buried carbon and burn it as if there is no tomorrow, releasing CO2 into the atmosphere. Scientists ask: Will this knock our climate system off balance? The scientists make the models, and then check the results of the models against observed changes.

One way you can reduce the argument over anthropogenic cause is that it is an argument over what proves humans are dumb: are we dumb because we can’t figure out climatic patterns, or are we dumb because we ignore our own conclusions?

I’ve run out of room, so I’ll answer the bit about “normal fluctuations” in an upcoming column.