Dear Umbra,

Man-made greenhouse gases are blamed for recent global-warming trends. But man-made greenhouse gases account for only 5 percent of the greenhouse effect. Water vapor, over which civilization has virtually no control, accounts for some 95 percent of that greenhouse effect. Why has so much attention been focused on man-made gases when they comprise such a relatively small part of the problem?

B. Hawley
Ottawa, Ontario, Canada

Dearest B.,

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I’m going to pick apart your letter, and I truly do not mean to be snide. But the very language you use illustrates a common misunderstanding of our current situation, whether or not you yourself have this misunderstanding.

Greenhouse good. Global warming bad.

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You say human-made gases are 5 percent of the greenhouse effect. Then you ask why we focus on the 5 percent when they are a small amount of “the problem.” We must be clear on the distinction between two very different topics: the greenhouse effect and the problem.

The greenhouse effect is not a problem.

Without the greenhouse effect of heat-trapping gases, the sun’s energy would bounce back into space, and Earth would be a spinning, frozen ball. You and I would enjoy neither the bliss nor the misery that is life. We live in a greenhouse now, and we’re lucky.

The greenhouse in which we live is managed by a gardener who stays on top of venting the windows, watering, and buttoning things up on cold days. I may be losing you here because it seems I’m talking about God, but what I mean to say is that our pre-industrial greenhouse was a system at equilibrium — whatever force you believe to be responsible. “Energy balance” is another way to look at it. Earth absorbed and rejected the right amounts of energy from the sun to keep itself balanced, to keep the life inside it alive. We know a little about this amazing system. Oceans act as heat sinks; deciduous forests absorb CO2 when in leaf, release it when dormant; white surfaces such as ice caps are highly reflective and knock radiative heat back out into the atmosphere; periodic oscillations in climate are an adjustment from orbital wiggles or responses to volcanic eruptions.

So, greenhouse effect is good. The problem is that we are messing with the greenhouse. The unearthing and burning of sequestered carbon changes a balanced system. We are adding heat to the interior of the greenhouse in the form of quite powerful gases, mostly carbon dioxide but also the powerhouse methane, etc. We’re throwing the system out of whack and heating ourselves out of a home.

Although I disagree with the numbers you cite, we won’t quibble, we’ll just agree it’s true that water vapor is a powerful and highly prevalent greenhouse gas. I’ll talk more about water vapor in my next column. But it’s not the basis of the problem. We are the problem, we humankind and our human-made gases. The percentiles are relevant to scientific study, but in a way irrelevant to us as laymankind. Climate experts have determined that our carbon emissions are causing global warming. Let us not become too distracted by water vapor.