Challenges to the CO2 Global Warming Hypothesis: (2) Questioning Nature’s Greenhouse Effect

A different challenge to the CO₂ global warming hypothesis from that discussed in my previous post questions the magnitude of the so-called natural greenhouse effect. Like the previous challenge, which was based on a new model for the earth’s carbon cycle, the challenge I’ll review here rejects the claim that human emissions of CO₂ alone have caused the bulk of current global warming.

It does so by disputing the widely accepted notion that the natural greenhouse effect – produced by the greenhouse gases already present in Earth’s preindustrial atmosphere, without any added CO₂ – causes warming of about 33 degrees Celsius (60 degrees Fahrenheit). Without the natural greenhouse effect, the globe would be 33 degrees Celsius cooler than it is now, too chilly for most living organisms to survive.

The controversial assertion about the greenhouse effect was made in a 2011 paper by Denis Rancourt, a former physics professor at the University of Ottawa in Canada, who says that the university opposed his research on the topic. Based on radiation physics constraints, Rancourt finds that the planetary greenhouse effect warms the earth by only 18 degrees, not 33 degrees, Celsius. Since the mean global surface temperature is currently 15.0 degrees Celsius, his result implies a mean surface temperature of -3.0 degrees Celsius in the absence of any atmosphere, as opposed to the conventional value of -18.0 degrees Celsius.

In addition, using a simple two-layer model of the atmosphere, Rancourt finds that the contribution of CO₂ emissions to current global warming is only 0.4 degrees Celsius, compared with the approximately 1 degree Celsius of observed warming since preindustrial times.

Actual greenhouse warming, he says, is a massive 60 degrees Celsius, but this is tempered by various cooling effects such as evapotranspiration, atmospheric thermals and absorption of incident shortwave solar radiation by the atmosphere. These effects are illustrated in the following figure, showing the earth’s energy flows (in watts per square meter) as calculated from satellite measurements between 2000 and 2004. It should be noted, however, that the details of these energy flow calculations have been questioned by global warming skeptics.

The often-quoted textbook warming of 33 degrees Celsius comes from assuming that the earth’s mean albedo, which measures the reflectivity of incoming sunlight, is the same 0.30 with or without its atmosphere. The albedo with an atmosphere, including the contribution of clouds, can be calculated from the shortwave satellite data on the left side of the figure above, as (79+23)/341 = 0.30. Rancourt calculates the albedo with no atmosphere from the same data, as 23/(23+161) = 0.125, which assumes the albedo is the same as that of the earth’s present surface.

This value is considerably less than the textbook value of 0.30. However, the temperature of an earth with no atmosphere – whether it’s Rancourt’s -4.0 degrees Celsius or a more frigid -19 degrees Celsius – would be low enough for the whole globe to be covered in ice.

Such an ice-encased planet, a glistening white ball as seen from space known as a “Snowball Earth,” is thought to have existed hundreds of millions of years ago. What’s relevant here is that the albedo of a Snowball Earth would be at least 0.4 (the albedo of marine ice) and possibly as high as 0.9 (the albedo of snow-covered ice).

That both values are well above Rancourt’s assumed value of 0.125 seems to cast doubt on his calculation of -4.0 degrees Celsius as the temperature of an earth stripped of its atmosphere. His calculation of CO₂ warming may also be on weak ground because, by his own admission, it ignores factors such as inhomogeneities in the earth’s atmosphere and surface; non-uniform irradiation of the surface; and constraints on the rate of decrease of temperature with altitude in the atmosphere, known as the lapse rate. Despite these limitations, Rancourt finds with his radiation balance approach that his double-layer atmosphere model yields essentially the same result as a single-layer model.

He also concludes that the steady state temperature of Earth’s surface is a sizable two orders of magnitude more sensitive to variations in the sun’s heat and light output, and to variations in planetary albedo due to land use changes, than to increases in the level of CO₂ in the atmosphere. These claims are not accepted even by the vast majority of climate change skeptics, despite Rancourt’s accurate assertion that global warming doesn’t cause weather extremes.

Next: Challenges to the CO2 Global Warming Hypothesis: (3) The Greenhouse Effect Doesn’t Exist