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

Americans Are Losing Sight of Global Warming as an Important Issue

Summary.   Elisabeth Rosenthal in the New York Times on Oct. 15, 2011 has documented the decreased attention that the issue of global warming is receiving in the U. S., in contrast to the rest of the world.  The portion of Americans who believe the earth is warming fell from 79% in 2006 to 59% in 2010.  This issue breaks out along lines that follow differences between the Democratic (liberal) and Republican (conservative) parties.  This reduction in importance is so even though the U. S. is a major world emitter of greenhouse gases.  According to the article, many aspects of individual and corporate attitudes reinforce America’s reluctance to embrace global warming as an important issue.  In contrast, nations of the remainder of the world, including Europe, China, and other developing countries, accept that global warming is an important issue facing the world, and are implementing policies to reduce emissions of greenhouse gases.

Climate scientists from all over the world have set a goal of limiting overall warming of the planet to 2ºC (3.6ºF) above the temperature prevailing in pre-industrial times.  This important objective requires limiting the accumulated amount of carbon dioxide in the atmosphere, not simply reducing the annual rate that greenhouse gases are emitted.  One motivation to stimulate the U. S. to implement suitable policies could be the recognition that economic and societal harms brought about by extreme weather events are balanced by the economic and societal benefits arising from new investments in mitigation projects.

Introduction.  Climate scientists the world over, as a result of efforts going back several decades, have concluded overwhelmingly that our planet is undergoing a warming trend  due to the greenhouse effect arising from man-made carbon dioxide (CO₂) emitted into the atmosphere.  Our ever-increasing use of fossil fuels for mankind’s expanding needs for energy creates CO₂ in large quantities as the direct product of burning the fuels.  Other gases added by man to the atmosphere also add to the greenhouse effect.  The warming of the planet has resulted in several effects, many of which are detrimental to life on earth, and in extreme weather events that inflict severe harms to humans in their paths.

In spite of this situation, the public in the U. S. has grown less concerned about the dangers posed by global warming, as described by Elisabeth Rosenthal in the New York Times on Oct. 15, 2011.  This post addresses topics raised in Ms. Rosenthal’s article.

U. S. public support to address global warming is waning.  Ms. Rosenthal reports that the portion of Americans who believe the earth is warming has fallen from 79% in 2006 to 59% in 2010 (citing the Pew Research Group).

Political denial and backtracking on global warming.  Perhaps reflecting this changing attitude, at least one Republican presidential hopeful, Gov. Rick Perry of Texas has stated “the science is not settled” concerning man-made global warming.  On Aug. 17, 2011 (accessed Oct. 19, 2011) he stated "a substantial number of [climate] scientists … have manipulated data so that they will have dollars rolling in to their projects…..We're seeing weekly, or even daily, scientists who are coming forward and questioning the original idea that man-made global warming is what's causing the climate to change".

Further in seeming recognition of this changing sentiment among the public as well as in Congress, Ms. Rosenthal reports that President Obama is promoting the administration’s “green” energy project proposals in economic terms, omitting any mention of beneficial effect such projects would have in combating greenhouse emissions.  The administration appears to support final approval for construction of the TransCanada Keystone XL oil pipeline carrying oil from Alberta’s tar sands to the Gulf of Mexico (see the recent post on this subject).  It is also seeking an exception from Europe’s planned landing fee for air travel to the continent based on CO₂ emissions.

Nevertheless, the U. S. emits about 18% of the world’s greenhouse gases as of 2009, even though it has only 4.6% of the world’s population.  Until recently the U. S. was the nation with the highest annual emissions rate of all.  It was overtaken by the emissions originating from China.  Ms. Rosenthal identifies aspects of American culture, such as its preference for larger cars and homes, and the skepticism among many of its people toward the role of science and government policies in their daily lives, as factors that contribute to skepticism toward or rejection of global warming.

In addition, the article states American industries such as coal mining and oil and gas production oppose constraints on fossil fuels, since their financial well-being depends on continued production, if not expanded production.  The current protracted recession also makes it more difficult to accept higher costs for fossil fuels, or energy in general, that could accompany policies supporting renewable energy.

The rest of the world is moving to constrain greenhouse gas emissions.  As noted in Ms. Rosenthal’s article, a report on global energy by the London bank HSBC finds that the U. S. is the sole nation not implementing a program to reduce greenhouse gas emissions.  Other important emitters are actively pursuing such efforts.

Europe. The nations of the European Union have the significant goal of reducing greenhouse gas emissions by at least 80% by 2050, including an interim objective of a 25% reduction by 2020 (see this post).  According to the New York Times article, Europe is currently on track to achieve this interim goal. John Ashton, Britain’s special representative for climate change, points out that even in the face of economic hardship brought on by the global financial crisis, European countries view moving toward a “green” economy favorably, as an economic opportunity, not negatively due to the imposition of greater costs. “In the E. U. … despite the economic and financial crisis, the momentum on climate change has … continued”, Mr. Ashton states.

Developing countries.  China’s current 12^th Five Year Plan covering 2011-2015 projects continued major expansions of electric power generation based on coal (see this post).  Its energy consumption is scheduled to increase about 5% per year in this period.  Nevertheless, it is greatly expanding its small renewable energy sources.  China emphasizes energy intensity rather than overall emissions.  It plans to reduce its emissions intensity per unit of economic output by 17% by 2015, and by 40-45% with respect to the 2005 level by 2020.  India is also pursuing aggressive efforts to reduce greenhouse gas emissions, according to the Times article.

Extreme weather events.  As noted in the Times article, climate scientists predict that with increased levels of atmospheric greenhouse gases accumulating in the future, the resulting increased global temperatures will produce more, and more intense, episodes of extreme weather events.  These include, in different regions on the earth’s surface, excessive heat with drought, increased rainfall with severe flooding and intense storms, loss of ice cover and higher sea levels.  These occurrences inflict sudden and severe harms to populations and societies.  This blog has developed several posts dealing with this issue in recent months, summarized here.  Ms. Rosenthal points out that developing countries are less well equipped to deal with these events than are developed countries.  They therefore embrace worldwide efforts to implement mitigating strategies.  The article cites a 2010 Pew survey finding that 70% of people in China, India and South Korea were prepared to pay more for energy in order to mitigate the effects of global warming.

Discussion

The United States is alone among the major emitters of greenhouse gases not to have a national policy directed toward reducing the rate of emissions.  Without such a policy, annual emissions from the U. S. will increase at an accelerating pace, as economic development and its population continue to grow.  If anything, fiscal incentives from the federal government for promoting mitigation efforts in the private realm have been increased and reduced in fits and starts, hindering the ability of new enterprises to make long-term plans for development. 

In the absence of a single national policy, various state and regional greenhouse gas accords have been implemented (reported in this blog in the following posts: California’s Global Warming Solutions Act, The Western Climate Initiative, Midwestern Greenhouse Gas Reduction Accord, and The Regional Greenhouse Gas Initiative of the New England and Mid-Atlantic States).  These programs establish disparate emission reduction goals over the coming decades.  Most encourage or rely on a market driven cap-and-trade regime to bring about emissions reductions.

Changes in climate patterns, including increases in sea level and reduced glaciation that are predicted to worsen because of global warming, depend not on the annual rate of global greenhouse gas emissions, but rather on the absolute amount of greenhouse gases accumulated in the atmosphere.  Carbon dioxide, the major greenhouse gas, which originates from mankind’s burning of fossil fuels for energy, remains stable in the atmosphere for at least one century, once emitted into the air (remaining after absorption into the oceans and reincorporation into growing green plants).  Thus it is important to reduce emissions as drastically as possible as soon as possible, in order to keep the accumulated level of atmospheric carbon dioxide as low as possible.

The atmospheric CO₂ concentration can be conceived of as an atmospheric bathtub of CO₂.  Burning fossil fuels contributes more CO₂ through the “faucet”, but there is minimal “draining” of CO₂ from the bathtub because there are few natural mechanisms for removing CO₂ from the atmosphere.  Worldwide, mankind has to reach agreement on ways to turn the CO₂ faucet off as fast as possible.

Currently the global average CO₂ concentration is 392 parts per million (ppm, volumes of CO₂ per million volumes of the atmosphere).  It has been increasing, and increasing at an ever-accelerating pace, since the beginning of the industrial revolution, when the concentration was about 280 ppm.  The climate scientists of the world, at their meeting in Copenhagen in 2009, under the auspices of the United Nations Framework Convention on Climate Change, agreed to seek the objective of limiting the overall increase in the global average temperature above the temperature that prevailed prior to the industrial revolution to 2ºC (3.6ºF), estimated to require that the global average atmospheric CO₂ concentration be constrained to about 450 ppm.

In a series of recent posts this blog has shown that, on the one hand, economic and societal harms brought about by extreme weather events are balanced, on the other hand, by the economic and societal benefits arising from new investments in mitigation projects such as renewable energy programs and energy efficiency projects (summarized here, which contains further links to the remaining posts giving details of the analysis).  To the extent that U. S. investment in mitigation measures reduces the damages inflicted by extreme weather events, those investment costs are returned because the need to remedy disasters is reduced.  We conclude that the U. S. should implement a long-term national policy of developing mitigation measures intended to reduce greenhouse gas emissions, ultimately to near zero.  In this way the U. S. will join much of the planet in a world-wide strategy to stabilize atmospheric CO₂ at the lowest possible level.

© 2011 Henry Auer