Global Warming Science - A Sensible Voice (Part 1)
Rapp’s penultimate conclusion is that neither the “alarmists” nor the “naysayers” have proven their case, and there is a good deal more that we don’t understand about climate variability than we do understand. The balance of this column summarizes several of Rapp’s analyses and conclusions.
The Historical Context
Rapp begins with a concise discussion of the geo-scientific context for today’s debates. Based on recorded temperatures in the 20th century, we are quite certain that there was an early-century warming (1910-1940), followed by a mid-century cooling (1940-1975) and then a late-century warming (1975-2000). The big questions are: Were these temperature changes large compared to historical variations, and were they due to natural causes or human activities? Rapp declines to give a definitive answer to either question, citing data that is “not accurate enough” and models that are “difficult to test.”
Reaching back a bit further in history, most geologists believe the last millennium was characterized by a “Medieval Warm Period” (800-1200), a “Little Ice Age” (1300-1850) and the current warm period (1850-2000). Written histories of various European groups provide anecdotal evidence that confirm these climatic cycles. The author cites examples such as grapes growing in England during the MWP where none grow today and Iceland harbors blocked by sea ice well into summer during the LIA.
Ice Ages and Interglacials
Rapp provides a helpful summary of the geological evidence for cyclic climate change (over hundreds of millennia), drawing on an earlier textbook he published on ice ages. Based on isotope variations and radioactive dating of the hydrogen and oxygen in ice cores plus evidence of glaciation well south of the arctic, it is apparent that the quantity of snow and ice on Earth has cycled up and down approximately every 100,000 years for eons. Ice sheets up to two miles thick have been estimated to cover much of Canada, Russia, Europe and the U.S. during the coldest segment of the most recent ice age, which ended about 14,000 years ago. Evidence suggests that so much water was tied up in the glaciers that ocean levels dropped to the point where it was possible to walk from Sicily to North Africa on dry land.
Triggers and Feedback
The text highlights theories put forth by a number of scientists who have tried to find a causal link between the periodicity of the ice ages and cyclic variations in the Earth’s orbit or solar activity. The relatively small amplitude of some of these forcing functions (e.g., solar irradiance varies only about 0.14% over the course of an 11-year solar cycle) rules them out as direct causes of abrupt temperature change, but scientists are still exploring a number of indirect ways these cycles could influence climate.
Many of these hypotheses for the onset or culmination of an ice age suggest a solar “trigger” initiating a trend that is propagated by “positive feedback” from the Earth. Two candidate feedback mechanisms are the growth of polar ice caps (which increases reflectivity of sunlight and leads to cooling) and the absorption of CO2 into the oceans when water temperatures are colder (thereby diminishing the greenhouse effect). This oceanic reservoir is thought to be the primary source of observed cycles in atmospheric CO2 that closely track (lag) the oscillating temperature signatures in ice cores.
Due to the complexity of the physics, Rapp concludes that trigger and feedback theories are largely speculative, but he convincingly argues that global temperature changes prior to industrialization most certainly were not caused by human-generated CO2 emissions. However, he does confirm that current levels of atmospheric CO2 are about 35% higher than they have been in any of the last several interglacial periods. The implication of this is that regardless of whether atmospheric CO2 is ultimately found to be the cause of even a small portion of the recent temperature rise, which is still very much unproven, the most likely cause of the extra CO2, according to Rapp, is human emissions.
Next month’s column will include a discussion of proxies, the “hockey stick,” the reliability of climate models and the utility/wisdom of government intervention. IH