Increasing CO2 Caused Global Temperature Increase as the Last Ice Age Receded

Summary.  Shakun and coworkers (2012) examined atmospheric CO₂ concentrations and proxies for local temperature at 80 global locations for the period in which the last ice age was receding.  They found a very strong correlation between increased CO₂ levels and global temperature increases, with a lag in temperature change of several hundred years with respect to CO₂ changes.  Modeling of the climate during this period showed that changes in CO₂ levels alone were sufficient to explain most of the warming of the planet, providing a causative explanation for the warming.
 

The present trends of increasing atmospheric CO₂ concentrations and rising long-term average global temperatures are occurring at a rate about 100 times faster than happened during the melting of the glaciers of the last ice age.

Introduction.  The long-term global average temperature (i.e., temperature as measured over the entire surface of the world averaged over time periods of a year or longer) has been increasing in recent decades.  This increasing trend began as the industrial revolution got under way in the 19^th century, and has coincided with an increase in the atmospheric concentration of carbon dioxide (CO₂) and other greenhouse gases over the same time period.  As a way of lending credence to the causative correlation between atmospheric CO₂ levels and the global temperature rise that we are currently experiencing, climate scientists are studying correlations of atmospheric CO₂ and global temperatures on geological time scales.  Such work has indeed shown that correlations are found on these long-term times, going back as far as 800,000 years, based on captured contemporaneous air bubbles entrapped in ice cores bored in Antarctic glaciers.

Changes in CO₂ and temperature as the Last Ice Age receded.  Jeremy D. Shakun and coworkers (Nature vol. 484, pp. 49-54; 5 April 2012; doi:10.1038/nature10915 ; free abstract available) have reexamined these issues and extended measurements of temperature and CO₂ during the disappearance of the last ice age (LIA; 22,000 to 6,500 years before the present).   Their work addressed a number of interrelated questions, both general and specific, that they felt remained unresolved from previous work.  Importantly, as alluded to above, much work had focused on only a few sites in Antarctica, and this had led to ambiguity concerning the sequential occurrence of changes in CO₂ and temperature. 

Shakun and coworkers accumulated temperature records previously obtained by others from 80 locations with a wide range of northern and southern latitudes, both oceanic (67) and terrestrial (13), from the end of the LIA.  Each entry had to be dated with acceptable accuracy (200 years resolution).   Of course, humans were not there to measure the temperature; over the last several decades climate scientists have identified and calibrated “proxy” physical or chemical parameters as diagnostic measures of temperature.  In this work, the authors considered proxies derived from seven different parameters.  Whereas the temperature records reflect geographic distinctions across the globe, measurements of atmospheric CO₂ need not, since CO₂ rapidly disperses uniformly in the atmosphere.  CO₂ concentrations at various time points were obtained from ice-entrapped air bubbles from glacial ice cores.  

Increasing CO₂ concentrations are correlated with, and occur before, global temperature increases.  The proxy temperature results and the values for the ambient atmospheric CO₂ concentrations graphed over a 15,000 year span beginning with the maximal time for the LIA are shown in the graphic below.

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. 

© Macmillan Publishers Limited.

Source: Shakun and coworkers; http://www.nature.com/nature/journal/v484/n7392/full/nature10915.html?WT.ec_id=NATURE-20120405.

  

Shakun and coworkers report a very strong statistical correlation between the data for the CO₂ concentration and the global proxy temperature results (correlation coefficient = 0.94, on a scale in which 0 indicates lack of any correlation whatsoever and 1.00 represents perfect correlation between two sets of data).  In a more detailed analysis of these data, the authors evaluate that in the Southern Hemisphere, the temperature curve (red line) leads the CO₂ concentration curve by 620 years with a standard error of 660 years, in accord with the Antarctic anomaly that these authors identified in the introduction.  However, as readily seen in the graphic, and as further analyzed by the authors, the global temperature curve (blue line) lags the CO₂ concentration by 460 years with a standard error of 340 years, and the Northern Hemisphere temperature proxies (not shown above) lag the CO₂ concentration by 720 years with a standard error of 330 years.  The authors used detailed modeling of the oceanic Atlantic meridional overturning circulation, a known current prevailing in the Atlantic Ocean, to show that heat from the depths of the ocean contributed non-CO₂ driven warming in the Southern Hemisphere, to help explain the Antarctic anomaly.

Thus, considering the overall global temperature results, the authors conclude “the overall correlation and phasing of global temperature and CO₂ are consistent with CO₂ being an important driver of global warming during deglaciation (melting of the LIA glaciers), with the [hundred-year] scale lag of temperature behind CO₂ being consistent with the thermal inertia of the climate system owing to ocean heat uptake and ice melting”.

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, ALL factors, CO₂ (including all greenhouse gases) only, and ORB (including solar irradiation only).

Portion of a graphic image taken from Shakun and coworkers showing the time evolution of certain climate parameters.  c, The same CO₂ data (yellow dots) as in the first graphic above; d, the same proxy global temperature deviation in ºC (blue line) as in the first graphic above; and e, modeled temperature evolution based on three simulations: ALL (deep violet line) including all factors considered, CO₂ (rose-pink line) including only greenhouse gases, and ORB (green line) including only solar irradiation.

© Macmillan Publishers Limited.

Source: Shakun and coworkers; http://www.nature.com/nature/journal/v484/n7392/full/nature10915.html?WT.ec_id=NATURE-20120405.

The modeled temperature evolution curves in e in the graphic above show that including ALL climate factors (deep violet line) reproduces the observed temperature record based on 80 global observation locations very well, although the amplitude is slightly less (correlation coefficient = 0.97).  Similarly, including only greenhouse gases (CO₂ (rose-pink line)) reproduces the ALL model temperature trend exceptionally well (correlation coefficient = 0.98).   In contrast, the ORB model (green line) including only solar irradiation fails to reproduce the observed trend, indicating that this factor, which includes changes in the earth’s orbit around the sun, plays only a “modest role” in causing global temperature change.  In view of these modeled results the authors conclude that “greenhouse gases can explain most of the mean warming [observed] at these 80 sites” around the globe.

Factors contributing to increased CO₂ concentrations.  The authors further analyzed earlier data from others and carried out additional modeling themselves to understand the source(s) of the additional CO₂ vented into the atmosphere over these many thousands of years.  Detailed modeling of the oceanic Atlantic meridional overturning circulation, responding to the new thermal gradients, the melting of Antarctic sea ice cover, and addition of freshwater to the oceans from melting glaciers (sea level rose by 120 m (390 ft.) over the full time interval considered) as factors contributing to release of CO₂ from the ocean depths.

 

Analysis

 

Shakun and coworkers have analyzed experimental results on the time course of atmospheric CO₂ concentrations and proxies for global temperature values during the period in which the LIA came to an end.  They showed that these two parameters are highly correlated throughout this time period, and demonstrated unequivocally that global temperatures lagged atmospheric CO₂ concentrations by several hundred years throughout this period.  In doing so, they resolved earlier ambiguities in the data apparently due to sampling error, because the earlier results had been based on observations from only a few sites which were not geographically representative of the planet as a whole.

 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