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Understanding how the climate system responds to human influences is, unfortunately, a lot like trying to understand the connection between human nutrition and weight loss, a subject famously unsettled to this day. Imagine an experiment where we fed someone an extra half cucumber each day. That would be about an extra twenty calories, a 1 percent increase to the average 2,000-calorie daily adult diet. We’d let that go on for a year and see how much weight they gained. Of course, we would need to know many other things to draw any meaningful conclusions from the results: What else did they eat? How much did they exercise? Were there any changes in health or hormones that affect the rate at which they burn calories? Many things would have to be measured precisely to understand the effect of the additional cucumbers, although we would expect that, all else being equal, the added calories would add some weight. The problem with human-caused carbon dioxide and the climate is that, as in the cucumber experiment, all else isn’t necessarily equal, as there are other influences (forcings) on the climate, both human and natural, that can confuse the picture. Among the other human influences on the climate are methane emissions into the atmosphere (from fossil fuels, but more importantly from agriculture) and other minor gases that together exert a warming influence almost as great as that of human-caused CO2.
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Steven E. Koonin (Unsettled: What Climate Science Tells Us, What It Doesn’t, and Why It Matters)