Temperature records and CO2 concentrations graphed according to the age in time that the data represent. Age is plotted along the horizontal axis in thousands of years (kyr) before the present; each minor tic mark represents 1,000 years. The yellow dots give CO2 concentrations plotted on the yellow vertical line at the left, in parts per million by volume (p.p.m.v.). The horizontal bars with each dot represent the respective dating uncertainties. The red line and red shading give the Antarctic temperature proxies with the shading representing the error estimate of the measurement. The blue line and shading give the proxy global temperature in ºC with the shading representing the error estimate, plotted along the vertical blue line at the left, as the deviation from the average temperature prevailing from 11,500 to 6,500 years ago.
Increased CO2 levels are largely responsible for increased global temperatures. In order further to address causality, the authors modeled temperature evolution across the time scale ending the LIA, using a current climate model from the U. S. National Center for Atmospheric Research. Various factors potentially contributing to the global temperature evolution, such as greenhouse gases including CO2, the global level of solar irradiation, changes in reflectivity of the ice sheets as they melted, and freshwater fluxes into the ocean from the melting, were included in the modeling. Three model cases are presented in the graphic below,
Using a currently accepted climate model, the authors were able to identify increased greenhouse gases in the atmosphere as being a major factor causing the increase in the global temperature over the thousands of years considered. This conclusion is highly significant, for it shows that an increase in the atmospheric concentration of CO2 and other greenhouse gases, in and of themselves, can cause an increase in the long-term average global temperature.
The changes in temperature and CO2 levels tracked by Shakun and coworkers evolved over 14,000 or more years (each tic mark in their graphics represents 1,000 years). The CO2 concentration increased from about 190 p.p.m.v. to about 260 p.p.m.v., largely in about 7,000 years of this interval, and the proxy temperature changed by about 3.5ºC (6.3ºF) in this time period.
These are to be contrasted with changes associated with our present increase in the long-term global average temperature. This has occurred in only the last 150 or so years (or about 100 times faster), as contrasted with many thousands of years; has involved a much larger change in CO2 concentration from about 280 p.p.m.v. at the beginning of the industrial revolution to more than 390 p.p.m.v. presently; and an increase in the global average temperature of about 0.7ºC (1.3ºF).
Shakun and coworkers identified a lag of several hundred years between the time of an increase in CO2 concentration and the increase in the global average temperature. They mentioned possible factors such as high thermal inertia for absorbing heat by the oceans and the time taken in melting glaciers for this delay. It is possible that one or both of these factors is also at play today, although other climate-driving factors differ considerably between the end of the LIA and the present trends. If there be considerable time lags at play at the present time, it may be conjectured that the effects of warming of the planet may require times of one or more centuries to be fully felt.
© 2012 Henry Auer