By Dr. Robert Thorson

This is a cautionary tale from climatology. Beware of simplistic explanations, especially when they involve clouds, the most challenging part of climate modeling. A good example of the so-called cloud problem is associated with the bunny fence in the Australian outback.

The rabbit fence is continent-dividing barrier built in 1907 to keep rabbits away from agricultural crops. It seems as though the fence also keeps the clouds away, but it doesn’t. Rather, it coincides with the boundary between two types of terrain with very different weather.

To the east is native vegetation with a frequent cloud cover and considerable rain. To the west is cropland, an area of drier, clearer skies where the rainfall has dropped by about 20 percent from previous conditions. These are the facts confirmed by ground measurements and satellite observations. The question is why the difference in moisture? The short answer is we don’t know.

The long answer comes from a study by Tom Lyons from Australia’s Murdoch University and Udaysankar Nair from the University of Alabama’s Huntsville Earth System Science Center. They offer two possible explanations for the difference in cloud cover having to do with the darker vegetation absorbing more heat. In one version, the warmer air just above the water-transpiring native vegetation lifts the local humidity to the height necessary to produce cloud cover. A second version of the differential heating hypothesis suggests that the rising air lowers the pressure and draws cooler air in from the cropland, which then rises to form clouds.

Two other hypotheses have to do with aerosols: fine solid particles of dust, salt and organic matter suspended in the air. Aerosols are both more frequent and much smaller on the cropland side. There, they create ultra-small droplets which are less likely to gather as clouds and therefore to produce rain.

The cropland side has more dust devils — local vortexes resembling transient mini-tornadoes — because it is less uniform in its reflectivity and has a lower surface roughness. In the first aerosol hypothesis, the intensified dust devils may be responsible for the greater aerosol content of the cropland side by lifting aerosols to higher elevations. Alternatively, the aerosols come from the saltier soils on the cropland side. This resulted from a rise in the groundwater tables there after the local, deep-rooted vegetation was cut and replaced by crops.

My point is not that you care about these detailed arguments. What I want you to realize are two important conclusions about climate science. First, the simple act of clearing native vegetation can change the climate for the worse. Second, the net effect of cloud cover on global warming will depend on many more fundamental factors, in this case six processes or attributes — surface reflectivity, air convection, evapo-transpiration, groundwater movement, soil salinity and surface turbulence.

Rain follows the plow. That’s what gullible Australians were told in the late 19th century to induce them to clear semi-arid land and plant crops. Americans were fed the same propaganda to encourage them to settle the great American West. In both places, true believers moved into the semi-desert, turned over the soils with great hope, planted seeds and waited for the lushness to follow. It never came. Instead they got drier soils and less rain.

I have another reason for bringing up the bunny fence example. Scientists usually reason in one of two basic directions. Climate modeling is an example of forward reasoning. It answers the question “Given this, what will happen?” Understanding present climates is an example of inverse reasoning.

It answers the question “Given this, what happened?” This is another way of asking: “What’s going on now?”

Most of the legitimate scientific objections to climate modeling come from the built-in uncertainties associated with the more complicated case of forward modeling. Only rarely do we find an example where the simpler inverse problem is still too complex to understand without serious thinking and more data than currently available. This case of the rabbit fence demonstrates just how complex climate science is, even under seemingly simple circumstances.

Given our limited comprehension of Earth’s climate, it seems prudent to halt our self-inflicted experiment with greenhouse climate change before it’s too late.