Since temperature records only go back to the invention of Fahrenheit’s thermometer (1724) and globe-wide averages are only available for the past 150 years, earlier temperatures must be inferred from various types of geologic data. Other things equal, these “proxies” can act as a substitute measure of historical temperature. Scientists have analyzed hundreds of such proxies, and many provide a basis for reconstructing temperatures. Unfortunately, all are affected by factors other than temperature, some quite strongly. For example, tree-ring thicknesses track well with temperature, but, unfortunately, they also track with soil nutrients, moisture, etc.
Ice cores taken from the polar caps provide a basis for temperature reconstruction based on isotope abundance in the ice and analysis of gases enclosed in bubbles. Unfortunately, ice-core proxies also suffer from confounding effects related to (a) the amount of precipitation, which causes a variable depletion of heavy-isotope water vapor, and (b) the long time required for snow to compact into ice, during which trapped gases can diffuse vertically up or down, “smearing” the results.
The Hockey Stick
In 1998, the “MBH” team of researchers attempted a comprehensive analysis of the past 1,000 years using a multi-proxy network. Their analysis, which incorporated more than 1,000 proxies (some going back to the year 1000 A.D.), used a particular type of regression analysis to compute a time-varying global average temperature over the time period of interest.
Their work was hailed by many scientists, but their analysis suffered from a major flaw that the MBH team still has not conceded. Problematically, the MBH research used the mean temperature of the 20th century as the baseline for their analysis, instead of the mean for the entire 1,000-year period. As a result, the historical proxy data was skewed such that the only significant temperature change in the millennium was in the 20th century (“the hockey stick”). There was hardly any little ice age and no medieval warm period.
In 2005, the “M&M” team studied the MBH results and found this analytical error. Their work showed that the MBH analysis had inadvertently placed a huge weighting on a small number of proxies that had upward trends in the 20th century. M&M later showed that the MBH algorithm creates a hockey stick even when random noise is entered as input! When the raw data are re-evaluated using the M&M method and one particularly inaccurate proxy is removed, both a medieval warm period and a little ice age are evident.
Rapp’s conclusion is to acknowledge that there has been significant temperature rise in the 20th century, but there were comparable high temperatures earlier in the millennium as well.
Leading or Lagging?
One of the biggest popular misconceptions exposed by Rapp is how CO2 correlates with global temperature. In the film “An Inconvenient Truth,” ice-core data is shown wherein CO2 and temperature track each other fairly well over the past several cycles of ice ages and interglacials. What the film fails to state is that the CO2 change lags the temperature change by hundreds of years.
Further, Rapp points out that while CO2 concentrations today (~380 ppm) do exceed maximum levels found in all of the most recent ice-age cycles (~280 ppm), CO2 levels sometimes exceeded 1,000 ppm in more distantly past interglacials.
Reliability of Climate Models
While acknowledging accuracy in some aspects of the climate models used to predict climate change, Rapp is very critical of current researchers’ willingness to draw broader conclusions than are justified. “The fact that the models indicate significant warming due to greenhouse gases has nothing whatever to do with their reliability,” he states, while pointing out the researchers’ inability to accurately predict cloud formation, ocean currents and other natural phenomena that affect climate. Even if climate models accurately predict that additional heat is trapped when CO2 levels increase, they fail to predict the degree to which this heating is mitigated or augmented by clouds and oceans and whether net temperature changes are catastrophic or irrelevant.
While the author agrees with climate scientists that CO2 increases appear to be human caused and probably contribute in small measure to global warming, he puts little faith in the ability of scientists to predict the global consequence of these findings.
In conclusion, Rapp ponders the role of policymakers who suggest actions to counteract these iffy physical consequences and is reminded of a famous quote by Harry S. Truman. Lamenting the fact that his economists always included “on the other hand” scenarios in their analyses, Truman exclaimed, “Give me a one-handed economist!”IH
Global Warming Science - A Sensible Voice (Part 2)
February 1, 2010