CO2 and Temperature Changes Are Correlated for 800,000 Years

Summary.  Global warming and cooling cycles over geological time scales are correlated with increasing and decreasing concentrations of atmospheric carbon dioxide, respectively.  This post documents correlated changes occurring  a) during the recent post-industrial period of global warming, b) over the last one thousand years, and c) over the past 800,000 years. 

Except for the industrial period, these changes reflect only global physical processes not involving actions by mankind, showing that the greenhouse effect is an established physical phenomenon.  Each geological warming or cooling trend, coupled with its change in carbon dioxide concentration, occurs very slowly, lasting thousands of years.  The current man-made warming trend, in contrast, is occurring at least 60 times faster, and its pace is accelerating.

 

Introduction. The previous post discussed variations over time in the atmospheric concentration of carbon dioxide (CO₂), the major greenhouse gas.  Recent changes in CO₂ concentration measured directly since 1958 were contrasted with a slightly broader record, extending back to 1700 CE, and finally with the geologic record of atmospheric CO₂ cataloging concentrations as far back as 800,000 years before the present. 

The post summarized the findings as

a)     geological changes in CO₂ levels have practically never exceeded 280 ppm, the level that existed just before humanity embarked on the industrial revolution;

b)     on a time scale relevant to human experience and lifetimes geological changes in CO₂ levels change extremely slowly, over periods of many thousands of years;

c)     physical properties of atmospheric CO₂ today show unequivocally that the excess CO₂ that has arisen in the past century comes from burning fossil fuels;

d)     virtually the entire increase in contemporary CO₂ levels has resulted in concentrations so high that they have never been found in the geological record for 800,000 years;

e)     contemporary CO₂ levels continue to increase unabated and at a rate 60 times or greater than in the geological record; and

f)      the rate of growth of contemporary CO₂ levels is accelerating.

Correlation between atmospheric CO₂ and temperature today.  CO₂, being a greenhouse gas, causes atmospheric temperatures to rise as the gas accumulates in the atmosphere.  Over the span of the industrial revolution the increase in the CO₂ concentration and the increase in the long-term global average temperature each follow the same trend with time.  This is seen in the graphic below.

Overlaid curve for annual global average temperature and atmospheric CO₂ concentration.  The scale for CO₂ concentration runs from about 280 to about 390 parts per million.  The scale for temperature runs from about 13.6 to about 14.6ºC (about 56.5 to about 58.2ºF). 

Source: Temperature data from http://www.cru.uea.ac.uk/cru/data/temperature/hadcrut3vgl.txt .  CO₂ data from ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt (1850-1958) and  ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_annmean_mlo.txt (1959-2009).

 

The graphic shows that the long-term global average temperature is closely correlated with the CO₂ concentration.  (The temperature trend shows many spiky bounces because, in addition to the atmospheric concentration of CO₂ and other greenhouse gases, the global temperature responds to other factors in the atmosphere, effects on land and especially effects occurring in the oceans.)

Climate science models show unequivocally that the temperature increase is caused by the additional CO₂ accumulating in the atmosphere.

Thousand-Year Correlation.  The CO₂-temperature correlation is even more striking when viewed for the time from the year 1000 CE to the present time, shown in the graphic below.

Correlation between CO₂ concentration and global average temperature (including geological proxy data) for the period 1000-2000 CE.  For CO₂ the blue points are from ice cores and the red points at the right are from direct atmospheric measurements.

Source: http://www.pir.sa.gov.au/pirsa/nrm/climate_change/a_guide_to_climate_change_and _adaption_in_agriculture_in_south_australia/chapter_1. 

 

It is seen that the trends for CO₂ concentration and global average temperature follow each other extremely closely over the last 1000 years.  Both are largely unchanged over the period from 1000 to about 1800 CE, then both change in correlated fashion as industrialization began at about that time.  Industrialization depended on burning fossil fuels as a source of energy, a process which did not occur prior to its beginning.  The consequence of industrialization has been the correlated rise in CO₂ and global average temperature seen in the two graphics above.

Correlation between atmospheric CO₂ and temperature over geologic time scales.  Climate scientists have aligned results obtained from ice cores for atmospheric CO₂ concentrations for the last 800,000 years with data for the temperature of the climate for the last 1 million years obtained from temperature proxies in the geological record.  The results are shown in the graphic below.

Atmospheric CO₂ concentration (orange line) and estimated global average temperature (navy blue line) from the present (right end of horizontal axis, not including information since industrialization) and as much as 1 million years ago (left end of horizontal axis). 

Source:  Slide 13 in Coursera course on Climate Literacy: Navigating Climate Change Conversations, https://class.coursera.org/climateliteracy-001/lecture/index; based on temperature proxy data in Zachos et al., 2001, Science Vol. 292, p. 686,  as transformed by Hansen and Sato, 2012 http://www.columbia.edu/~jeh1/mailings/2012/20120508_ClimateSensitivity.pdf; and CO₂ measurements in Luthi et al., 2008, Nature Vol. 453|doi:10.1038/nature06949. 

http://www.nature.com/nature/journal/v453/n7193/extref/nature06949-s2.xls.

 

The graphical presentation of the CO₂ concentration above is the same as was shown in the previous post.  It is important to note that each small division on the horizontal axis represents a very long time, 50,000 years.  For example, the minima in both temperature and CO₂ at the extreme right on the graph represent the last ice age.

The most important feature of the above graphic is that over the last 800,000 years, an extended time scale, the global average temperature and the atmospheric concentration of CO₂ have been highly correlated.  Many of the changes appear abrupt on the compressed time scale shown above. However, in the previous post one example of a sharp change in CO₂ concentration was expanded using the original data.  The expanded change for the instance selected, between 128,609 and 135,603 years ago, extends over 7,000 years, and cannot be considered abrupt on the scale that we as humans experience.  Over these 7,000 years the CO₂ concentration increased very slowly indeed.

The greenhouse effect from CO₂ causes changes in temperature over the millennial time scales shown in the graphic.  Over the entire time period shown the changes in temperature and the changes in CO₂ concentration are closely coupled.  In some cases the change in temperature occurs “first” whereas in others the change in CO₂ concentration occurs “first”, and on the compressed time scale shown, some changes in temperature and CO₂ appear to occur together.  Even if the onset of one of these changes was not caused by a prior change in the other, over the many thousands of years that the increases or the decreases occur, the changes are amplified by positive feedbacks of the processes under way, as explained in the next paragraph, inset below.  Therefore we may conclude that, once an increasing or decreasing trend begins, the greenhouse effect from atmospheric CO₂ reinforces that trend.

Over these millennia an “initial” increase in temperature, say, results in lowering the solubility of CO₂ in the warmer waters of the oceans.  This releases more CO₂ into the atmosphere, amplifying the warming process already under way due to the increased greenhouse effect.  Alternatively, an “initial” decrease in temperature, say, results in raising the solubility of CO₂ in the cooler oceans, removing more CO₂ from the atmosphere, thus amplifying the cooling process already under way due to the decreased greenhouse effect.  Likewise, an “initial” increase or decrease in CO₂ concentration raises or lowers the global average temperature, which amplifies the further release of CO₂ from, or increased absorption back into, the waters of the oceans.  These changes in atmospheric CO₂ levels amplify the processes already under way.

Conclusions

1.On the time scale that we directly experience as humans, excess injection of CO₂ into the atmosphere by burning fossil fuels is causing an increase in the long-term global average temperature.

2.During the last one thousand years the atmospheric CO₂ concentration and the global average temperature have been tightly correlated.

3.Over geological time scales extending back 800,000 years from the present, the atmospheric CO₂ concentration and the global average temperature are highly correlated.

4.For the period represented by industrialization these changes arise primarily because of the greenhouse effect due to mankind’s burning of fossil fuels for energy and creation of other greenhouse-active gases.  For geological time scales the close coupling between temperature and atmospheric CO₂ concentration shows that the greenhouse effect has operated to affect the temperature of the planet for hundreds of thousands of years, although it occurs at a far slower rate than presently with industrialization.

The emission of CO₂ since industrialization began has elevated the atmospheric CO₂ to levels never seen during the preceding 800,000 years in the geological record.  This is occurring at a rate 60 times or more faster than changes have proceeded in geological cycles.  The pace of adding new CO₂ is itself getting faster, as industrialization in developing countries requires burning more and more fossil fuels for energy. 

Global warming is just that, a global problem; once emitted, CO₂ becomes distributed in the atmosphere worldwide.  The harmful effects of global warming are widely known and becoming more apparent as time passes.  For this reason nations of the world must work to set aside differences in their perceived interests and reach agreement to limit worldwide emissions, as soon as possible. 

© 2013 Henry Auer

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