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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

Each post will begin with a capsule summary. It will then proceed with captioned sections to amplify and justify the statements and conclusions of the summary. I'll present images and tables where helpful to develop a point, since "a picture is worth a thousand words".

Tuesday, October 1, 2013

IPCC Fifth Assessment Report, Part 1: The Physical Science Basis

Summary.  The Intergovernmental Panel on Climate Change issued Part 1 of its Fifth Assessment Report, “The Physical Science Basis”, on September 30, 2013.  The Report first summarizes past changes in the climate system.  It states that the recent warming of the earth/s climate is “unequivocal”.  Since the 1950s changes in many climate parameters including concentrations of greenhouse gases; temperature; loss of snow cover, glaciers and ice sheets; and rising sea levels are “unprecedented” considering the last decades to thousands of years.  There are many recognized contributions to these effects but by far greatest single factor is the increase in the atmospheric concentration of the greenhouse gas carbon dioxide.

A variety of climate models of lesser or greater complexity successfully reproduce the earth’s recent climate history on global and regional scales and over longer periods of time, showing that the models adequately account for the major processes involved in evolving climate change. 

These same models are then employed together with four scenarios of decreasing stringency concerning greenhouse gas emissions. Imposing rigorous emission constraints will limit further warming to a low, but still more elevated, level than the present by 2100; whereas continuing “business as usual”, an essentially unconstrained scenario, will lead to drastic increases in temperature by 2100 and induce severe changes in climate and the consequences thereof.

Furthermore, since carbon dioxide remains in the atmosphere for centuries or thousands of years, humanity’s actions today and in the near future will lead to irreversible and persistently higher global average temperatures for long time periods, affecting the lives and wellbeing of mankind’s progeny for generations.  For this reason this writer believes it is now time for the member states of the United Nations to coalesce around a meaningful agreement to reduce GHG emissions toward zero in order to stabilize the climate at the lowest level possible of its  new, higher greenhouse-mediated temperature.

Introduction.  The Intergovernmental Panel on Climate Change (IPCC) is established under the United Nations Framework Convention on Climate Change.  Four Assessment Reports (ARs) have been issued previously beginning in 1990; they are summarized here. Part 1 of the IPCC Fifth Assessment Report (5AR), “The Physical Science Basis”, was released on September 30, 2013.  This post is based on the Summary for Policymakers. 

(Part 2,  “Impacts, Adaptation and Vulnerability”, is due in late March 2014; and Part 3, “Mitigation of Climate Change”, is due in early April, 2014.)

As explained below in the Details section, IPCC ARs are prepared by hundreds of eminent climate scientists selected from among all member nations of the U. N.  First and second drafts are prepared in succession, each reviewed by others before moving to the next stage, and finally reviewed by selected government officials.  For this reason the 5AR meets every reasonable standard for scientific rigor, objectivity and validity.  It represents a worldwide consensus on the current status of climate science and projections of future effects as the world continues to warm, and merits serious consideration by all in view of the process outlined here and in expanded form below.

Part 1 of 5AR is presented in four sections: a summary of data characterizing past and present trends, the sources for the heat energy that is warming the planet, providing an understanding of past climate patterns using models, and projections of possible future warming depending on assumptions of humanity’s behavior.  These are summarized here and expanded in the Details section.

Observed Changes in the Climate System.  The Summary characterizes the warming of the earth’s climate to date as “unequivocal”.  Since the 1950’s many climate parameters have changed to an extent that is “unprecedented” with respect to historical patterns going back from decades to thousands of years.  Particular findings include increased concentrations of greenhouse gases (GHGs) in the atmosphere, warming of the atmosphere and the waters of the oceans, decreased amounts of snow (such as found in high mountain regions) and ice (such as land-based glaciers and sea ice), and a higher average sea level.

Properties of the Earth System That Contribute to Global Warming.  Contributions from many climatic features must be considered in arriving at a final value for the rate that the earth absorbs energy from the sun or releases it back into space.  The result shows that the overall rate of energy absorption per unit area of the earth’s surface is in fact a warming contribution.  This rate of absorption was 43% higher in 2011 than the value given for 2005 presented in the IPCC Fourth Assessment Report (4AR).  The largest contributing factor is the increase in atmospheric concentration of CO2.

Providing an Understanding of Past Climate Patterns Using Models.  Humanity’s activities have been the dominant factor impacting global warming, both atmospheric and oceanic; changes in the global water cycle; reductions in snow and ice amounts; and the global mean sea level (95-100% likelihood).  Human activities affecting the earth’s climate are well understood, including adding to the atmospheric burden of GHGs. 

Historical data for the climate are well reproduced using climate models.  These have been improved greatly since 4AR.  More, and better quality, data have been acquired since then.  Computing power has greatly expanded.  Comprehensive models permit assessing effects with higher spatial and time resolution.  The Summary states with “very high confidence” that models now in use successfully provide temperature patterns at a continental scale of resolution and provide trends over many decades, capturing the dramatic warming since the middle of the 20th century as well as transient cooling effects of large volcanic eruptions.

Projections of Possible Future Warming.  An ensemble of climate models, all of which successfully reproduce past climate behavior, was used to project future trends.  Four possible pathways (termed RCPs) are considered, characterized by increasing rates of heating the planet due to continued manmade emissions of GHGs, up to the year 2100.  Based on these models and pathways, the Summary concludes that if humanity continues emitting GHGs the earth will warm further, and the other climate responses discussed above will likewise continue worsening along paths already under way.  Limiting further climate change, beyond the level already established, depends on “substantial and sustained” reductions of GHG emissions.  The extent and severity of most further changes foreseen in 5AR differ little from those already characterized in 4AR; sea level rise, however, is projected to be more pronounced.


The Summary ends by stating

“Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond…. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.”

The global average temperature increases essentially linearly with the amount of CO2 emitted into the atmosphere from 1870, through the historical period ending at 2010, and on into the projected emissions-temperature pathways through 2100.  The only difference in the projections for the four RCP cases is that for RCP 2.6 the trajectory ends in 2100 at a low level of total accumulated CO2, with an overall increase in temperature under 2ºC (3.6ºF); the intermediate RCPs extend to higher total accumulated emissions and correspondingly higher temperatures projected for 2100; and ending with RCP 8.5 at the highest level of total accumulated CO2 emitted by 2100 corresponding to a total temperature increase from 1870 of about 4.6ºC (8.3ºF).

Additional contributors to the rate of energy increase of the earth arise from sources other than CO2 such as other GHGs, warming feedbacks that affect the rate of energy accumulation and sources such as melting permafrost which emits new methane.  Accounting for these additional factors leads to lowering the projected amounts of manmade CO2 emissions permissible in order to remain below any given level of global warming.

The Summary further states “A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale” for the portion not absorbed into the ocean or taken up by photosynthetic plants.  There is no natural process operating within this time scale that removes CO2 from the atmosphere.  It is projected that 15 to 40% of emitted CO2 will remain in the atmosphere longer than 1,000 years.  Therefore the resulting warming of the earth will likewise persist at the higher temperatures projected in the models for many centuries, even after all new emissions of CO2 will have come to an end. At least partly for this reason global sea levels will continue rising beyond 2100, due to further thermal expansion and continued melting of ice sheets and glaciers.  Melting continues as long as the air temperature at the ice sheet surface remains above the melting point.  By 2300 the sea level could rise by 1 m (3.3 ft) under the RCP 2.6 scenario, or by as much as 1 m to more than 3 m (10 ft) under the RCP 8.5 scenario.  Sustained warm temperatures are thought to lead to complete loss of the Greenland ice sheet over 1,000 years, producing a sea level rise of up to 7 m (23 ft).
The IPCC, starting in 1990, has concluded that our planet is warming as the result of manmade emissions of GHGs including CO2.  In its First AR and thereafter, it has urged the nations of the world to reduce emissions drastically in order to minimize anticipated increases in the long-term average global temperature. Recent ARs corroborated the earlier ones as more, and more sophisticated, data has been accumulated and analyzed, and more robust modeling has permitted more detailed projections of future climate trajectories to be made.   As a result statements of the likelihood of potential outcomes have become more certain.

The present 5AR continues this progression, benefiting from robust newly gathered data and analysis, and elaboration of climate models of increasing sophistication, regional and spatial resolution, and time development.  It continues the trend of the earlier reports, presenting data analysis and projections that have not changed significantly in substance, but that are now presented with higher degrees of certainty in view of the newly acquired information and the specificity of new modeling forecasts.

If meaningful abatement steps were not undertaken, the ARs have warned, serious consequences to human welfare would occur.  These include rising sea levels carrying the danger of unprecedented storm surges, and region-dependent increases in heat and drought in certain areas or heavier precipitation and river flooding in others.  All these eventualities impact negatively on the socioeconomic wellbeing of affected populations.

These warnings are actually coming to pass with increasing regularity and ferocity in recent years; global warming contributes significantly to such extreme events.

Global warming is “unequivocal”; it is under way at the present time.  Climatic changes are due to manmade emissions of GHGs including CO2 released when fossil fuels are burned.  The harms arising from greenhouse effects, such as record high temperatures and heat waves, more and more intense storms including flooding, and sea level rise, cause widespread damage and human suffering.  Ultimately society pays for these emergencies through relief and adaptation measures.  Now is the time for the member states of the United Nations to coalesce around a meaningful agreement to reduce GHG emissions toward zero in order to stabilize the climate at its new, higher greenhouse-mediated temperature.


Significance of IPCC Assessment Reports.  The 5AR, like its predecessors, is produced by a large, ecumenical group of hundreds of  experts in their fields, and subjected to review by other experts and by appropriate governmental bodies before it is approved and accepted for release.  Technical details are based only on peer-reviewed journal articles and reports produced by renowned nongovernmental organizations or government agencies.  The exhaustive review assures that the released report both represents the current state of scientific and technical expertise, on the one hand, and the points of view of governments of the IPCC, on the other. 

The steps involved in preparing the reports are summarized here:

  1. Governments and organizations nominate authors, who are then selected by the organizers of the Working Groups (here called “Parts”)
  2. The selected authors prepare a first draft of the Part;
  3. The first draft is reviewed by others;
  4. Authors prepare a second draft considering reviewers’ comments;
  5. The second draft is reviewed by governments and experts
  6. A final draft is prepared considering reviewers’ comments;
  7. Governments review the final draft; and
  8. The final draft is approved and accepted by the IPCC, and released.
As a result of this thorough drafting and review process, the ARs are rigorously objective.  The reader cannot seriously believe that the ARs offer prejudiced or directed findings or opinions. Indeed, the approval and acceptance process likely leads to consensus positions on unresolved or contentious issues while minimizing the importance granted outlying results or evaluations.

Preparation of the first draft of Part 1 of the 5AR involved 659 experts and considered 21,400 comments; the second draft involved 800 experts and 26 governments, and considered 31,422 comments.

The following sections expand on the summaries outlined above.

Observed Changes in the Climate System.  As dates of observation advance from 1880 to the present, more data sets covering much of or all the globe have become available.   In recent decades satellite measurements have become significant. 

Considering 10-year averages, the last three decades have been successively warmer at the Earth’s surface than before, and warmer than any earlier decade going back as far as 1850.  For the period 1880 to 2012 the earth has warmed by 0.85ºC (1.5ºF), with a 90% confidence interval of 0.65 to 1.06ºC.   For the period 1901 to 2012, for which adequate regional data exist (this excludes the Arctic and Antarctica, and regions in the Amazon, central Africa and central China), a global map of temperature trends shows almost all regions on the earth’s surface of experienced warming (only a region of the North Atlantic south of Greenland became cooler).

The troposphere, the lowest part of the atmosphere closest to the earth’s surface, has warmed since the 1950’s (stated with virtual certainty, i.e. 99-100% likelihood).

The Summary specifically points out that short-term patterns are highly variable (speaking of periods as long as 10 to 15 years).  Variations on such a time scale cannot be taken to represent a change in long-term (periods of up to 60 years) trends.  It calls this phenomenon “natural variability”.  Short-term trends depend critically on the years that an interested observer might choose for the beginning and end of the period.  For example, the Summary cites the low rate of increase in temperature over the period 1998 to 2012, which began with a strong cyclical oceanic El Niño phase (warming of the ocean and atmosphere), was only 0.05ºC (0.09ºF) per decade.  The longer-term heating rate, however, from 1951-2012 was 0.12 ºC (0.22ºF) per decade. 

Heat energy is stored in the waters of the oceans to a far greater extent than on the surface of the earth or in the atmosphere.  Warming of the ocean accounts for more than 90% of the increase in heat energy experienced by the earth between 1971 and 2010, stored mostly in the upper 700 m (2,296 ft).  The upper 75 m (246 ft) warmed by 0.11ºC (0.20ºF) per decade over this interval.

The Greenland and Antarctic ice sheets and glaciers worldwide have decreased significantly.  The rates of loss due to melting have increased considerably over the past 20-30 years.  Arctic sea ice refers to permanent ice and peripheral seasonal ice frozen out of the Arctic Ocean.  The area of Arctic sea ice has decreased by 3.5 to 4.1% per decade over the period 1979 to 2012, and the summer minimum decreased by 9.4 to 13.6% (90-100% probability).  Northern hemisphere snow cover has decreased since the mid 20th century.  Areas of permafrost have experienced surface temperatures warmer by 2-3ºC (3.6-5.4ºF).

The rate of rise of sea level has increased since the mid 19th century to a rate larger than has been found for the last 2,000 years.  This rate of increase itself has grown from the late 1800’s and early 1900’s to considerably stronger increases since the early 20th century.  Sea level rise originates largely from contributions of a) the expansion of the volume occupied by ocean water as it warms, b) melting of glaciers and the polar ice sheets, and c) changes in storage of water by land areas.

Atmospheric concentrations of GHGs have increased to levels unseen in the geological record for at least the last 800,000 years.  These include carbon dioxide (CO2), methane (natural gas) and nitrous oxide.  Their atmospheric concentrations have increased since 1750 because of human activity associated with the Industrial Revolution.  CO2 has increased by 40% in this time, mostly from burning fossil fuels and cement production, and somewhat from emissions arising from changes in land use.  About 30% of atmospheric CO2 is absorbed by the waters of the oceans; since CO2 is an acid the increase from human activity has caused a surface zone of the oceans to become more acidic by 0.1 pH unit; this translates to an increase in the concentration of acid in the water by 26%. 

Properties of the Earth System That Contribute to Global Warming.  Processes affecting the energy balance of the earth are measured in terms of the rates of energy exchange per unit surface area. In order of decreasing rates  of heating the earth, the contributors are CO2, methane, ozone, fluorine-containing hydrocarbons (which originate from manufacture and recycling of refrigerators and air conditioners), nitrous oxide, carbon monoxide, and other lesser contributors. 

The role of aerosols has become better understood since 4AR.  Black carbon aerosols (originating from incomplete burning of fuels) contribute to net energy absorption, while white or light colored aerosols (from volcanic eruptions and byproducts of emanations from green plants) are negative, acting to reflect incoming sunlight back into space.  Aerosols figure importantly in the secondary effects of cloud droplet formation, and are evaluated here as having a net effect of increasing reflection of incoming sunlight back into space.  (Aerosols from volcanic eruptions and changes in net intensity of sunlight impinging on the earth are evaluated as being very small.  Volcanic aerosols are very transitory; even large eruptions, while having a cooling effect, last only a few years.)

Overall, the sum of the effects considered here result in a large increased rate of energy absorption per unit area by earth, almost double the rate evaluated in 1980.  The rate for 1980 in turn was more than double the rate evaluated for 1950.  (See the graphic below.)

Increase in the net rate of absorption of energy per unit area of the earth (Radiative Forcing, in watts per square meter) due to humanity’s activities for the years 1950, 1980 and 2011, relative to the value for 1750.  The horizontal black lines extending left and right from the tips of the red bars are estimates of the error in each value.  Values for the mean increase, and the lower and upper extents of the error (inside square brackets) are shown to the right of each bar in red.  These results are the sums of the contributions, positive and negative, described in the paragraph preceding this graphic.

Providing an Understanding of Past Climate Patterns Using Models.  Models successfully reproduce observed temperature trends over the long term from 1951 to 2012 with “very high confidence”.  But more short term effects are not matched in models.  For example, the reduced warming rate between 1998 and 2012, compared to the long-term trend from 1951 to 2012, is thought to arise equally from lower overall heating of the earth system, arising from volcanic eruptions and a weakening portion of the 11-year solar cycle, and a cooling contribution from internal variability such as larger removal of heat from the surface by the waters of the oceans.

Modeling global trends in extreme weather and extreme climate events has improved, as has continental scale modeling of precipitation.  Cyclical global patterns such as monsoons and the El Niño-Southern Oscillation have improved.

Modeling of positive and negative climate feedbacks has improved.  Surface warming, its effect on atmospheric content of water vapor, and dynamics of cloud formation are better.  The net feedback from these effects is “extremely likely” (95-100%) to be positive, amplifying warming effects on climate.

Climate sensitivity measures the extent of warming for a given change in overall energy absorption by the earth.  It is frequently characterized by the extent of warming expected from a doubling in atmospheric concentration of CO2.  A lower number means the earth warms only weakly with increases in GHGs, while a high number indicates stronger warming with increased GHGs.  In 5AR climate sensitivity is likely (66-100%) in the range 1.5 to 4.5ºC (2.7 to 8.1ºF).  The lower limit of 1.5ºC represents a decrease from the 2ºC lower limit in 4AR, but the upper limit is the same.  

More than half of the increase in global surface temperature from 1951 to 2010 is due to human emissions of GHGs and other manmade contributors to warming effects (95-100% likelihood). 

It is “very likely” (90-100%) that manmade contributions have led to increased heat content in the upper 700 m (2,296 ft) of the oceans.  They have also contributed to increased atmospheric moisture content, and consequent changes in precipitation including intensification of heavy precipitation over land. 

It is “very likely” (90-100%) that manmade effects have increased the frequency and intensity of extremes of temperature across the globe, probably doubling the occurrence of heat waves.  It is also “very likely” that manmade influences have contributed to loss of Arctic sea ice and the global rise in sea level.

Projections of possible future warming were carried out using a variety of models, ranging from simple climate models, to models of intermediate complexity, to comprehensive climate models, and Earth System Models.  These were run with four emissions scenarios termed RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5.  (The numbers refer to the rate of energy increase per unit area at the surface of the earth, in watts per square meter.)  RCP2.6 is intended to project a pathway to warming equilibrium within the guideline established earlier by the IPCC to limit global warming to 2ºC (3.6ºF) above the level before industrial times.  The remaining RCP scenarios reflect worsening, more severe warming originating from increasing rates of emission of GHGs.  RCP 8.5 approximates a “business as usual” pathway in which no significant policies are implemented to limit GHG emissions through 2100.

(Temperature changes in AR5 shown below are referenced to the starting period 1986-2005.  These are 0.61ºC (1.10ºF) above the preindustrial level.  This amount should be added to all values mentioned here to arrive at the full change since the beginning of the Industrial Revolution.)

The following table shows projected temperature increases over the 1986-2005 reference, and projected increases in global mean sea level over the 1986-2005 reference.

Changes in global mean surface temperature in ºC (top) and global mean sea level rise in m (bottom) for the two time periods shown, referenced to the period 1986-2005.  The “likely range” gives confidence limits for a 5%-95% interval. 
For temperature, corresponding values for ºF are exemplified as 1ºC =1.8ºF, 2.0ºC = 3.6ºF, and 3.7ºC = 6.7ºF.
For sea level, corresponding values for feet are exemplified as 0.24 m = 0.79 ft, 0.30 m = 1.0 ft, 0.40 m = 1.3 ft, and 0.63 m = 2.1 ft.

Global maps of warming are shown below for the mildest scenario, RCP 2.6, and the most severe scenario, RCP 8.5, referenced to the period 1986-2005.

Global grid of projected temperature changes by 2081-2100 referenced to the period 1986-2005.  The changes are color coded in ºC according to the heat bar at the bottom.  The number of models used is shown at the upper right of each map.  The stippling (dot pattern) is used to show high significance of the result for the given map location above internal variation (exceeds 13-87% deviation from the mean).
Source: IPCC 5AR Summary for Policymakers;

It is seen that temperatures are projected to be higher over land masses than over oceans.  Also, the size of the temperature increase is highest over the Arctic in both scenarios.  The changes in the Arctic would give rise to significant losses in sea ice and ice sheet masses, and would melt permafrost.

It is “virtually certain” (99-100% likelihood) that the frequency of hot temperature extremes on both daily and seasonal timescales will increase, and that of cold temperatures will diminish.

The models project regional changes in precipitation amount over the globe.  The range of differences between wetter and dryer regions will grow, and the contrast between wet and dry seasons will increase.  Extreme precipitation events are “very likely” (90-100%) to become more frequent and more intense.

It is “very likely” that Arctic sea ice will continue to decrease, that Northern hemisphere snow cover will be reduced, and the amount of glacier ice will decrease.  Permafrost will continue melting, and large fractions will be lost, depending on the scenario.

Global mean sea level will continue rising due to increased ocean temperature (thermal expansion, 30-55%) and increased melting from glaciers (15-35%) and the Greenland ice sheet.

Oceans will continue warming.  Heat absorbed from the atmosphere will be redistributed to greater depths, affecting the long-term ocean circulation. 

© 2013 Henry Auer

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