Saturday, February 17, 2007

Why climate needs humidity

Climate is a set, at each point and every instant, of the four physical variables from the last posting: temperature, humidity, water state, and wind velocity. Actual measurements are samples of this set taken to be representative of some small box of space and short time interval. Forecasts are projected similarly over boxes of space and time.

A consequence is that humidity and the water state are independent variables that cannot be reduced to one of the others. When people talk about climate change, they often mean just temperature, which is wrong. Climate change means all of those variables changing. When we consider the precipitation of rain and snow and the growth and decay of living things dependent on that precipitation, it is especially important to keep this fact in mind.

Not doing so gets you into common climate fallacies. Even experts commit them. (More details can be found here.)

The "hockey stick" temperature graph. This graph was the result of a fatally flawed study of a few years back that purported to show that the Earth's temperature had not changed appreciably for the last thousand years - until 1980, when it supposedly started to rise dramatically. There were many faults in this study - ethical, methodological, technical, historical - each one of which was enough to invalidate it. I want to just mention one (technical) mistake, the misuse of temperature as the sole climate variable.

This study's temperature index was a weighted composite of a bunch of biogeological proxies for local climate at various points on the Earth's surface at various times over the last millenium. That's a standard way to gauge past climate, before their were accurate human records. One in particular was based on California pine cones, whose growth is somewhat sensitive to temperature and very sensitive to the water available from precipitation. The study wrongly attributed all of the changes over time of these pine cones to just temperature - which is not even the most important factor controlling the growth.

Obviously, a proper treatment would require at least two variables, temperature and "wetness." Through another flaw (in the statistical methodology), the misattributed variations in the pine cones were magnified into a large recent temperature increase. This study by now has been completely discredited.

The disappearing snows of Kilimanjaro. This is sometimes trotted out as a "smoking gun" of "global warming." Comparisons are made between snow cover at the mountain peak, say, fifty years ago and recently. The snow has receded. It's just not due to rising temperatures.

In fact, temperatures at Kilimanjaro have been falling, more or less, during those fifty years. But the climate around the mountain is also drying out. (The two things are related: as temperatures fall, less evaporates from nearby bodies of water, and thus less is available in the air to precipitate out.) The snow is disappearing, not because it's melting away (some of it melts every year), but because what melts isn't being replenished as it once was.

There's another lesson here: static "things" (like snow cover) are really snapshots of processes. The snow cover at any time is the cumulative result of snow falling and melting over many prior years. The exact amount is a balance of the competing forces that put snow down and cause it to melt, and that balance constantly shifts, every day and every season.

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