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

Public Attitudes about Extreme Weather and Increased Global Average Temperatures

Summary.  Yale University and George Mason University released their latest survey, Extreme Weather, Climate & Preparedness in the American Mind, concerning attitudes of the American public over long-term world-wide average temperature and extreme weather events, in April, 2012.  A clear majority of the American public finds that unusual weather events have occurred in the preceding year.  By more than 2-to-1, people feel that over the past several years the weather has gotten worse rather than better.  Americans associated the recent extreme weather events with long-term increases in global temperature.  The New York Times reports that other recent surveys have provided similar results.

This survey appears to represent a shift toward greater recognition of the consequences arising from long-term increases in global temperature, compared to earlier recent Yale-George Mason surveys.

We conclude by urging American federal policymakers to recognize this shift in attitudes among the (presumably voting) population of the country.  Our policymakers should enact policies that address long-term global temperature increases and their consequences, as well as develop policies that help the country adapt to changes already under way.
 
Introduction.  Concern about an increase in the long-term world-wide average temperature ascribed largely to release of greenhouse gases by mankind has been expressed by climate scientists for several decades.  Harmful effects on climate, extremes of weather, and human consequences from this temperature increase have also been predicted over the same time scale.  Corresponding degrees of concern among the peoples of the world has varied with time and by nation or region of the world.

In the United States, several survey results have tracked details of the public’s recognition of, and degree of concern over, long-term average temperature increases and their effects.  This post discusses a survey of Americans (Leiserowitz, A., Maibach, E., Roser-Renouf, C., & Hmielowski, J. D. (2012) ExtremeWeather, Climate & Preparedness in the American Mind. Yale University and George Mason University. New Haven, CT: Yale Project on Climate Change Communication.)  that was released April 18, 2011 by the Yale Project on Climate Change Communication and the George Mason University Center for Climate Change Communication.  The survey was conducted the second half of March 2012, interviewing 1,008 people.

Climate and Extreme Weather.  The survey notes that in 2011 the U. S. had 14 serious weather and climate disasters each causing US$1 billion or more in physical damage, more than in any previous year.  Total damage from these events has been evaluated as US$53 billion, not to mention human costs of life and disruption of lives for survivors which are not readily calculable.  Disasters included severe drought and an extended heat wave in Texas and Oklahoma, widespread flooding along the Mississippi River valley, the remnants of Hurricane Irene which brought rain deluges and flooding along the east coast, and numerous damaging tornados.  From January to March 2012 temperatures across the continental portion of the U. S. were 6.0ºF (3.3ºC) higher than normal.

This blog reported on extreme weather events and damage assessments in the U. S. over the preceding ten years in this post , and selectively around the world in this post.

Highlights of the Survey. 
 
More than half of Americans surveyed recognized that, for the preceding year, unusual weather events have occurred (56% for their local area, and 62% elsewhere in the U. S.).

82% personally experienced one or more types of extreme weather in the preceding year; extreme weather includes extreme high winds, extreme rainstorms, extreme heat waves, drought, extreme cold temperatures, extreme snowstorms, tornadoes, floods, hurricanes and wildfires.

More than one-third reported that in the past year they were personally harmed (harm to property, financial status, or physical or mental health) by at least one extreme weather event, and separately, more than one-third reported knowing another person who was harmed.

Americans surveyed stated that over the past several years the weather in the U. S. has become worse rather than better, by 52% to 22%.

About half of those surveyed say that, over the past several decades, the extreme events of heat waves, droughts and very heavy rain storms have become much more common or somewhat more common.  Slightly less than half had the same sense with respect to harm to crops, floods, air quality, forest fires, water quality and transportation.

In the survey group, Americans agree, strongly or somewhat, with the notion  that global warming contributed to the severity of several newsworthy extreme events, including the unusually warm winter of December 2011 and January 2012 (72%), record high summer temperatures in the U.S. in 2011 (70%), the drought in Texas and Oklahoma in 2011 (69%), record snowfall in the U.S. in 2010 and 2011 (61%), the Mississippi River floods in the spring of 2011 (63%), and Hurricane Irene (59%).

Analysis

More than half of Americans recognized that unusual or extreme weather events have occurred in the past year, with five-eighths recognizing that such events occurred broadly nation-wide.  Four-fifths of Americans experienced extreme weather themselves, and one-third suffered personal harm as a result.  Significantly, Americans sense that over the past several decades, extreme weather events have become more frequent.  Depending on the event, roughly two-thirds of Americans sense that global warming has contributed to the severity of extreme weather events that occurred in the year preceding the survey.

These results represent a measurable change from previous survey results.  The same consortium published “GlobalWarming’s Six Americas in May 2011” (Leiserowitz, A., Maibach, E., Roser-Renouf, C., & Smith, N. (2011) Global Warming’s Six Americas in May 2011. Yale University and George Mason University. New Haven, CT: Yale Project on Climate Change Communication) .  In that survey, which was conducted with a different focus than the present one, only 37% of those surveyed were either alarmed or concerned about global warming (on a scale of six categories ranging on the negative side to “dismissive”).  These two categories remained essentially constant back to January 2010, but had been as high as one-half in November 2008.  Only 47% of those surveyed agreed strongly or somewhat that the record snowstorms of the winter of 2010-2011 made them question whether global warming is occurring, and 54% agreed strongly or somewhat that record heat waves in the summer of 2010 in the U. S. strengthened their belief that global warming is occurring.  Although the present survey asked no questions concerning the cause(s) of global warming, the May 2011 survey found that 49% of respondents thought that it is caused mostly or partly by human activities.

The New York Times, reporting on the present survey, noted that other recent survey results also show increased concern by the public about climate change and its effects.  In its report, the New York Times quotes A. Leiserowitz, the lead author of the survey report, as saying “People are starting to connect the dots”.

Weather extremes are one consequence predicted by climate scientists for the effects of an increase in the long-term worldwide average temperature.  The Intergovernmental Panel on Climate Change (IPCC) released a “Summary for Policy Makers of its Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation” [Field, C. B., Barros, V., Stocker, T.F., Qin, D., Dokken, D., Ebi, K.L., Mastrandrea, M. D., Mach, K. J., Plattner, G.-K., Allen, S. K., Tignor, M. and P. M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA on November 18, 2011, just ahead of the then-upcoming Conference of the Parties on climate change to convene in Durban, South Africa.  This blog reported on the special report in this post.

In an earlier tutorial post, we presented a basis for understanding that a rise in long-term global average temperature could worsen extreme events.  Very briefly, this is because warmer air holds more moisture than cooler air.  As moisture passes from the vapor to the condensed state (clouds and fog), it releases heat; and in reverse, as clouds evaporate to water vapor, heat is absorbed from the surrounding air.  These localized exchanges of heat lead to temperature gradients that can make winds more intense, and the higher water content of the air affected means that more moisture can fall as rain or snow.   Furthermore, rigorous climate models predict that certain regions of the earth’s surface will become more arid, leading to drought and possible famine, while other regions will experience more precipitation.

Policy for a regime of increasing global average temperatures

The survey reported in this post, and other recent surveys as well, show that the American public is becoming more attuned to the notion that increasing global average temperatures lead to more, and more severe, extreme weather events.  These events lead to damage and harms, physical, mental and societal, that carry with them significant financial losses, losses that ultimately are borne by the taxpayers of the U. S. 

Nevertheless, policymakers responsible for setting priorities and for implementing programs in the U. S. have steadfastly ignored public sentiment concerning this issue.  The U. S. has not been able to put in place a statutory policy program to address contemporary problems related to increasing global average temperatures, nor to develop alternative energy sources that avoid the release of greenhouse gases into the atmosphere.  As the American public becomes more persuaded that increasing global average temperatures lead to severe weather events and their consequences, such as floods, droughts and shortages of food staples, U. S. policymakers should recognize the significance of these shifts in attitudes among the electorate.  Our elected representatives should act on the imminent dangers posed by increasing global average temperatures in order to mitigate their severity, and to adapt to the resulting changed environment.


© 2012 Henry Auer

The Environmental Protection Agency Proposes to Limit GHG Emissions

Summary.  The U. S. Environmental Protection Agency has issued a new proposed rule to limit greenhouse gas emissions from large electric generating plants.  The rule provides a strong incentive to migrate from coal-fired power plants to combined cycle gas-fired plants, and to commercialize carbon capture and storage technology.  Implementing this rule may provide an incentive to all parties of the world to coalesce around a new international agreement to limit greenhouse gas emissions.

Introduction.  The U. S. does not have a nation-wide legislated policy governing emissions of greenhouse gases that lead to warming of the planet.  Certain American states and regions do have such policies at various stages of progress and achievement; they generally employ a cap-and-trade market-driven mechanism to limit emissions.  Separately, the U. S. Environmental Protection Agency (EPA) has put forth a regulation limiting greenhouse gas emissions from new large electric generating plants, described here.

EPA issued a proposed regulation (the “rule”) limiting greenhouse gas (GHG) emissions (primarily carbon dioxide (CO₂)) from electric generating plants on March 27, 2012.   As a proposed regulation, it is available for comment on its provisions by the public, including affected parties, for 60 days.  EPA then assesses the comments and makes any appropriate changes to the regulation, which is then issued as a final regulation.

The rule proposes to limit GHG emissions from new large fossil fuel-fired power plants, those with a generating capacity greater than 25 megawatts by placing an upper limit on their CO₂ emissions.  The rule is issued under the authority of the Clean Air Act, as upheld by the U. S. Supreme Court in Massachusetts v. EPA in April 2007.  The decision specifically granted the EPA this authority provided that, upon suitable scientific inquiry, GHGs qualify as “air pollutants”.  EPA did in fact reach this determination after thorough scientific analysis of the question.  The rule proposes to combine new coal-fired electric generating plants (steam boilers and Integrated Gasification Combined Cycle plants) and combined cycle plants into a new single class whose performance will be limited to GHG emissions of 1,000 lb CO₂/megawatt-hour of electricity generated.  This limit is based on the current performance characteristics of natural gas-fired combined cycle (NGCC) generating plants, already widely used and which already meet this limitation.  (Natural gas combined cycle plants have two stages of power generation.  In the first, the gas is burned in a turbine not unlike a jet engine that powers airplanes.  The hot exhaust turns the turbine which is used to generate electricity in this first stage.  The hot exhaust then is used to heat water to steam, and the steam drives a second generating turbine in the second stage.  These plants have very high efficiency levels.)

The rule notes that generating plants not using NGCC, such as those powered by coal, coal refuse, and oil or petroleum can meet the new standard if they remove CO₂ from the exhaust gas stream using carbon capture and storage (CCS).  CCS entails capturing CO₂ from a utility-scale source that burns a fossil fuel such as coal or natural gas; transporting the purified CO₂ to a remote geological storage site, typically a deep subterranean repository, and injecting or piping the CO₂ into the storage or sequestering formation.  In a previous post , we noted that many problems remain to make CCS industrially viable for utility-scale facilities.  If successful, this technology would make a significant contribution to limiting GHG emissions into the atmosphere, thus constraining the increase in the long-term global average temperature.  The rule suggests that a CCS installation that removes at least 50% of the CO₂ in the exhaust gas would meet the constraints.  As noted in the earlier post, CCS is generally thought to be capable of removing more CO₂ than that.  While noting that CCS could help non-natural gas-fired plants meet the proposed limits on emissions, the rule recognizes that at present the technology is more expensive than constructing new NGCC plants, but offers the expectation that costs will decrease as the technology is implemented and becomes mature.

EPA supposes that, by issuing this rule, most new electric generating plants will in fact use NGCC to provide the power, rather than using coal with CCS.  The rule exempts power plants already operating, as well as those already permitted and for which construction begins within 1 year of the date of this rule.

The Coal Industry Forecasts Growth in Usage.  Shortly after the rule was issued, GBI Research issued a report, Coal Mining Market in North America to 2020 – Carbon Emissions and Skilled Labor Shortages Likely to Limit Production  (summarized here ), which projects that after a slight decrease in coal production and consumption in the U. S. in 2012, mining production is projected to increase to 940 million metric tons by 2020.  This is ascribed largely to coal plants already under construction, and/or those that will be completed by 2015.  As such, they do not fall under the proposed rule.  The new plants will add 11.5 gigawatts of generating capacity.

The U. S. and Canada produced about 14.1% of the all the coal in the world in 2011, of which 93% was devoted to electric power generation.

Analysis.  The U. S. emits a large amount of CO₂ and other greenhouse gases into the atmosphere; a few years ago China overtook the U. S. as the leading nation in emissions of the world.  Yet, in distinction to many nations and the European Union , the U. S. has no national legislated energy policy in place that sets forth a plan to limit greenhouse gas emissions. 

As an alternative, a U. S. executive agency, EPA, has issued a first proposed rule to limit emissions from large fixed point sources involved in electricity generation.  Once issued as a final rule, the plan should limit power plants using any fossil fuel to a long-term emissions rate less  than 1,000 lb CO₂/megawatt-hour of electricity generated.  EPA recognizes that the effect of this rule will be at least two-fold.  First, it will provide an incentive to move away from use of coal as a fuel for new electric generating plants.  Second, it will provide an incentive for further research, development and deployment of CCS installations around the country.  The U. S. Department of Energy already supports such efforts.

In a related development, President Obama and his administration have raised the Corporate Average Fuel Efficiency (the average target for miles per gallon for cars made by each manufacturer) to 54.5 mpg by 2025.  This comes on top of a previous increase by the Administration to 35.5 mpg by 2016.  The second stage will lead to significant decreases in CO₂ emissions when implemented.

Increased long-term global average temperatures induced by adding higher and higher levels of CO₂ and other GHGs to the atmosphere lead to harmful effects around world.  These include more numerous and more severe extreme weather events and a rising mean sea level that threatens low-lying areas during coastal storm surges.  For these reasons it is encouraging that the proposed EPA rule takes a first step in limiting emissions from the U. S.  We may hope that this action encourages all nations of the world to reach a global agreement on limiting emissions in the near future.

© 2012 Henry Auer