What was done
The author examines a number of theories of climate change over geologic time scales.  In addition, he introduces a new hypothesis that climate may in large part be influenced by the influx of extraterrestrial volatile compounds, such as H2O, CO2, CH4, NH3, CO, and NO2.

What was learned
According to the author, there are a number of weaknesses in the Milankovitch orbital theory, which is believed by many to account for earth's recurrent alternating episodes of glacial and interglacial climates.  Among these weaknesses, the author notes that (1) the Milankovitch theory does not explain why the earth was experiencing a geologic cooling trend prior to the Pleistocene era of ice ages, (2) it does not explain why some records have shown temperatures to begin increasing while insolation continued to decrease, and (3) it does not explain why the largest temperature changes have a 100,000-year periodicity when the 100,000-year Milankovitch cycle is the weakest of the three insolation peaks (the other peaks occurring at 23,000-year and 41,000-year intervals) and is "too small by a factor of ten" to explain the inferred temperature changes.

As an alternative explanation, the author introduces the hypothesis of extraterrestrial-volatile-accretion (ETV).  The ETV hypothesis contends that the ongoing accretion of volatile elements is "orders of magnitude higher" than previously believed, resulting in significant changes to the radiative balance of the planet.  The accretion of these volatile compounds "tends to warm the Earth, while the terrestrial processes cool the planet by absorbing these gases at a more uniform rate."  The radiative forcing of ETV, according to the author, is estimated to be three times the magnitude of forcing that results from the insolation change at the peak of the 100,000-year Milankovitch orbital cycle.  Support for the author's hypothesis is presented empirically and on the basis of correlative evidence.

What it means
This study highlights our lack of knowledge of the causes of global temperature changes throughout geologic time.  The introduction of the ETV hypothesis is intriguing and provides another explanation for climate change.  It also identifies a potential non-anthropogenic source of atmospheric CO2 that is of the same order of magnitude as that which was experienced during the last deglaciation, when the atmosphere's CO2 concentration rose approximately 80 ppm.  We will probably hear more on this subject as time progresses and other researchers test this hypothesis.