What, Again? Global Warming Continues Unabated

Summary: This series of posts tabulates important findings from the six Assessment Reports (ARs) that the United Nations Intergovernmental Panel on Climate Change (IPCC) has released since 1990.  The previous post, “What, Again? Greenhouse Gases Accumulate in the Atmosphere", summarizing the six ARs, presents past greenhouse gas (GHG) emission rates, discusses future emission scenarios and proposes principles to minimize further emissions to keep accumulated GHG levels to as low a level as possible.

This post presents the direct consequences of rising GHG emissions: the increase in global average temperatures and its consequences, as foreseen in differing emission scenarios over the three decades that ARs have been issued.  Extreme weather and climate leads to significant economic and social harms and damages.  A final post will deal specifically with the effects of warming on our water environment: extremes of precipitation or its failure, melting ice domains and rising seas.

Some may feel this series repeats refrains, looping like broken records; such people may suffer from “climate fatigue”.  Humanity, however, has not responded to the worsening climate documented in the AR series. The critical, dire climate projections summarized in these posts should provide powerful incentives to take meaningful action at this time.

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The United Nations Intergovernmental Panel on Climate Change (IPCC) released the first of three volumes of its Sixth Assessment Report (AR6) in August 2021.  ARs have been issued at intervals of 6-7 years since 1990. They document the history of the annual rate of global emissions of the principal GHGs, arising from human activity, and of the total amount of GHGs accumulated in the atmosphere since the industrial revolution (Ind. Rev.) began.  Using climate models and a range of scenarios of GHG emission rates they present projections for each scenario of future climate characteristics and effects to the end of this century.  They also discuss general goals (but not specific policies) for limiting future emission rates.

The results and projections presented in ARs 1-6 are broadly consistent with each other across the AR series, but have greater specificity and stronger assessments of likelihood as the series progresses.  They record the profound increase, due to human activity, of accumulated atmospheric GHGs, especially carbon dioxide (CO₂), across the years.  Most GHGs do not dissipate in the atmosphere; they continue accumulating to higher levels as long as their annual emission rates continue.  It’s as if a tub fills higher and higher with water (representing GHGs), as long as the faucet is open.  When the emissions (the faucet) are turned off, the level in the tub doesn’t get lower; the water/GHG content stays at the level accumulated up to that time.  Being greenhouse gases, they retain more and more heat as their level increases.  The result is that global average temperatures continue rising, causing damaging extreme events the world over.

The graphic below shows the coupled relationship between atmospheric CO₂ levels from 1880 to 2019 and corresponding yearly global average temperatures.  The correlation

 

Overlaid graphs of atmospheric CO₂ concentration in parts per million (ppm; orange line, orange numerical scale on the right vertical axis) and yearly values of the global average temperature in °C (white line, white numerical scale on the left vertical axis; for corresponding °F multiply by x1.8).  Temperature values shown are the deviations from the 30-year average temperature from 1881-1910. Source: Climate Central https://www.climatecentral.org/ using data from the National Aeronautics and Space Administration Goddard Institute for Space Studies (NASA GISS), National Oceanic and Atmospheric Administration National Centers for  Environmental Information (NOAA NCEI), and NOAA Earth System Research Laboratories (ESRL).

 

is remarkable.  Elsewhere in the ARs (but not included in this series of posts) the IPCC has shown that it is only due to the CO₂ and other GHGs that have been added to the Earth system from human actions that the global temperature has risen as shown in the graphic, and not because of any other potential factor. 

The topics selected for this post, tabulated below in the Details section, are Past Temperature Rise, Projected Further Rise in Global Average Temperature, and Projected Warming Patterns.  As seen in the table, the authors of AR1 recognized already in 1990 that because of the accumulation GHGs the Earth system was warming (Column 2).  The global average temperature has continued rising, by 2021 (AR6) reaching 1.07°C (2.06°F) above the 1850-1900 average value, early in the Ind. Rev.

In future decades the temperature is projected to continue increasing (Column 3), depending on the emission scenario, from stringent to relaxed annual emission rates.  In AR6, compared to 1850-1900 (early in the Ind. Rev.), stringent emission scenarios project 1.0-1.8°C (1.8-3.2°F) warming by 2100, whereas the relaxed scenario projects 3.3-5.7°C (5.9-10.3°F) warming. Finally, the need to reduce annual emissions was recognized beginning with AR1 and reiterated, with increasing urgency, throughout the series. Current goals are to keep warming to 1.5°C (2.7°F; optimal) or 2°C (3.6°F; acceptable) above pre-industrial values (AR6). Higher global temperatures are projected (Column 4) to lead to extreme weather and climate events, many of which we are already experiencing around the globe.

Whereas the need to reduce emissions is expressed in AR1 and extends up to the present in AR6, the strength of climate science underpinning those conclusions has increased dramatically over time.  The capabilities of gathering data and using more powerful computers to analyze them, and to develop more refined, detailed climate models have all increased dramatically.  (Incidentally the Nobel Prize in Physics, awarded October 5, 2021, recognized the development of early climate models.  These have served as the foundation for today’s highly refined models.)

Recent ARs reflect this enhancement.  For example the current first volume of AR6 was compiled by 234 climate scientists chosen from among all the nations of the IPCC.  They reviewed over 14,000 research articles published since AR5.  Drafts of the chapters in AR6 were reviewed by other scientists as well as by national governments.  We can feel assured that the final text represents scientific and political consensus views.

Conclusion

In the table shown in the Details section Column 4, “Projected Warming Patterns”, summarizes the increasing urgency of acting to reduce annual emission rates to near zero as the AR series progresses.  AR2 as long ago as 1995 foresaw some regions of the Earth having more severe floods or droughts while others would be less affected.  There would be more extremely hot days and fewer extremely cold days. The forecasts in AR4 in 2007 are made with higher confidence than in AR3.   AR6 in 2021 foresees continued warming with worse hot extremes and droughts in some areas.

Indeed, actual extreme weather and climate events have become the subjects of frequent current headlines, documenting heat waves and droughts, famine, uncontrolled wildfires, intense precipitation events and flooding, and melting of glaciers and ice sheets leading to sea level rise, documenting effects that were only predicted in the early ARs.  Also, by AR6 the science of attribution of extreme climate events has progressed dramatically, and permits ascribing the severity, if not the actual occurrence or not, of events to the effects of global warming.

Early action could have been taken at moderate levels of effort and expense to avert future, if not yet apparent, hazards such as described in Columns 3 and 4 of the table.  Such opportunities were not seized.  By 2021 such hazardous events are now current, requiring immediate action.  Necessarily these current actions must be far more aggressive, pervasive and costly in order to deal with a warming Earth approaching criticality.  They also require fundamental and comprehensive changes in social and cultural approaches to adapt to the consequences of warming.

Unequivocally we must encourage our political, corporate and civic leaders to embark on bold, comprehensive actions without further delay.

Details

This writer collated the entries in the following table from either the Summary for Policymakers, a “Headline” document or a press release, all issued by the IPCC in conjunction with each AR.  The entries are necessarily selective rather than comprehensive, and have been edited for brevity.

EVALUATIONS OF GLOBAL WARMING AND TEMPERATURE IN IPCC ASSESSMENT REPORTS

© 2021 Henry Auer

We Need to Begin Abatement of Greenhouse Gas Emissions Right Away

Summary.  The Intergovernmental Panel for Climate Change (IPCC) has issued four climate Assessment Reports between 1990 and 2007, warning about the perils of emitting greenhouse gases and the resulting global warming and its impacts on human society.  These phenomena originate largely from humanity’s burning of fossil fuels.  The dangers include higher global temperatures, regionally dependent worsening of heat waves and drought, or rainfall and flooding, and sea level rise with attendant worsening storm surges.

The Assessment Reports and a recent article by Stocker point out that embarking early on mitigation measures makes it easier to constrain the overall warming and its attendant harms to humanity, whereas delay removes the easier paths of mitigation from consideration and requires more stringent, more expensive mitigation to be undertaken  later to make up for the opportunity lost by not starting earlier.

It is concluded that an important way forward could be for only major emitting countries to agree among themselves to begin mitigation measures at an early date.  This would overcome the inability of the almost 200 countries of the United Nations to reach a climate agreement despite annual conferences convened for almost two decades.

Introduction

The Intergovernmental Panel for Climate Change (IPCC), established under the United Nations Framework Convention on Climate Change (UNFCCC), has been warning about the perils of global warming and its impacts on human society since 1990.  In that year the IPCC issued its First Assessment Report (AR), and has since issued its Second (1995), Third (2001) and Fourth (2007) ARs.  The Fifth AR is due in 2014. Their calls for mitigation of emissions are summarized here.

In addition, we consider a recent report by Stocker characterizing the urgency of potential timelines remaining for worldwide action on mitigating global warming.

IPCC Assessment Reports

The procedures used in preparing the ARs are set forth in the Details section at the end of this post.  They ensure the scientific integrity, objectivity, and broad acceptance of the results and recommendations among the large community of climate scientists around the world.  They further affirm the significance of the findings for policymakers in the member nations of the U. N.

A detailed tabulation of selected findings and recommendations contained in the four ARs issued from 1990 to 2007 is presented in the Details section below.  The table shows that, starting with the very first AR climate scientists recognized the great impacts that manmade emissions of carbon dioxide (CO₂) and other greenhouse gases (GHGs) are wreaking on the global climate.  These include an ever-increasing accumulation of CO₂ and other GHGs in the atmosphere, and the consequent effects of a) rising long-term average global temperature, b) rising sea levels, and c) increasing occurrence of, and increasingly harmful, extreme weather and climate events.  (Other considerations presented in the ARs, such as regional features of global warming projections, impacts of global warming on human society, and aspects of adaptation to global warming were not included in the table because they are less central to the topic of this post.)

The table makes clear that these broad conclusions have not changed since they were first documented in the 1^st AR of 1990.  What has changed, as explained in the Details section, is that the improvements in data acquisition, analysis and computational modeling have provided greater certainty for the forecasts originally made.  The passage of time also shows, as seen in the table, that predictions made in early ARs have actually come true.  Trends of worsening are turning out to be more severe in the 4^th AR than in earlier versions.

The ARs began recommending abatement of the emission of GHGs with the 1^st AR of 1990 (see table in Details).  It recommended beginning reduction in emissions right away as of that date, including moving to renewable energy and undertaking measures to improve energy efficiency.  The 2^nd AR of 1995 stated “…immediate stabilization of the concentration of carbon dioxide at its present [i.e., 1995] level could only be achieved through an immediate reduction in its emissions [rate] of 50-70% and further reductions thereafter.…[to achieve] a given stabilization value [of accumulated CO₂], higher emission[ rates] in early decades require lower emission[ rates, i.e. more stringent reductions] later on.”  By the time of the 4^th AR in 2007, policy recommendations noted that mitigation undertaken in the next 20-30 yrs (i.e. early) minimizes the harmful later impacts of GW. Delay in undertaking mitigation reduces the world’s ability to achieve lower GHG levels and increases the risk of severe GW impacts.  The costs of mitigation rise with the stringency of the target that would be necessitated by putting off the start of mitigation.

“The Closing Door of Climate Targets” is the title of a brief article by Thomas F. Stocker in a recent issue of Science (see Details section).  He develops a graphical representation of the amount of mitigation required to meet a range of target global average increases in temperature above the temperature that prevailed before the start of the industrial revolution (i.e., before humanity started burning fossil fuels).  These are shown below.

 

Options available for Global Mitigation Schemes to achieve selected temperature increase targets over the value that prevailed before the industrial revolution began. 

Source: Adapted from Stocker, Science, 2013, Vol. 339, pp. 280-282; http://www.sciencemag.org/content/339/6117/280.full.pdf. 

 

The graphic shows that the lower the target for the temperature increase is, the more stringent the mitigation effort must be.  As of 2013, for example (the beginning of the scenarios shown), in order to constrain the global temperature increase to about 1.5ºC (2.7ºF; lower horizontal dashed line) starting in 2013 the annual rate of reduction of emissions would have to be about 6%/yr.  If mitigation were to be postponed until 2021, the mitigation rate would have to be about 10%.   The constraint to a 1.5ºC temperature increase becomes unattainable shortly thereafter, in 2028 (red curve). 

In order to constrain the global temperature increase to 2ºC (3.6ºF; upper horizontal dashed line) beginning in 2013 the mitigation rate would have to be about 2.6%/yr, 3%/yr if begun in about 2019, or 10%/yr if delayed until about 2038; this target becomes unattainable in about 2044 (red curve).  Stocker concludes “…every year counts; if mitigation actions are delayed, much larger emissions reductions are later required to maintain a selected target”. The area of unachievability in the graphic results from Stocker’s model because “past cumulative emissions up to the time of sustained emissions reductions leave a legacy, or commitment, in the future, irrespective of any long-term mitigation efforts.  [The longer] the starting time of [a Global Mitigation Scheme] is delayed, the [more] low [limiting temperature] targets are progressively lost.  The door for these climate targets closes irreversibly.”

 

Analysis

The IPCC, starting in 1990, has concluded that our planet is warming as the result of manmade emissions of GHGs including CO₂.  In its 1^st AR and thereafter, it has urged the nations of the world to reduce emissions drastically in order to constrain anticipated increases in the long-term average global temperature to low increments.  If meaningful abatement steps were not undertaken, the ARs have warned, serious consequences to human welfare will 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 coming to pass with increasing regularity and ferocity in recent years; such extreme events are ascribed at least partly to global warming.  Recent ARs corroborated the more tenuous statements of 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.

Shortly after the 2^nd AR, the Kyoto Protocol for mitigating GHG emissions was finalized; it came into force in 2005 and expired at the end of 2012.  Major GHG emitting countries were not included under its terms, the U. S. because ratification of the Protocol failed, and all developing countries, including China and India, because they were excluded from coverage by its negotiated provisions. 

 

Thus nations currently making major contributions to global GHG emissions have continued their activities unimpeded by international agreements.  As a result the levels of accumulated atmospheric CO₂ and other GHGs continue to grow, and the long-term average global temperature continues to increase.  Developed countries, such as those included in the Organization for Economic Cooperation and Development (OECD), have been holding emission rates relatively constant in recent years, but developing countries, especially China and India, continue radically to expand their use of fossil fuels, and their emissions of GHGs, at dramatically high rates. This is shown in the following graphic:   

Historical emissions of CO₂ from 1990 to 2008, and projections under the Reference Scenario to 2035 for developed countries (OECD, black line) and developing countries (non-OECD, red line).

Source: U. S. Energy Information Administration; http://www.eia.gov/forecasts/ieo/emissions.cfm.

 

Countries such as these are making no serious effort at reducing their annual GHG emission rates in meaningful ways.

 

The four ARs of the IPCC and Stocker’s recent model for abating CO₂ emissions all point to the need for international agreements to mitigate emissions of GHGs immediately.  In view of the increasing rate of annual emissions, both historical and projected, it is imperative to reach agreement on mitigation at the earliest possible date.  As this post has summarized, delay in implementing abatement measures only means there will need to be even more drastic measures taken at a later time, incurring disproportionately higher expense to put those more extreme measures into practice.

 

The 193 member nations of the UNFCCC have been unable to reach agreement on a successor to the Kyoto Protocol despite the urgency of the situation.  As an alternative, we conclude it is now time for the major emitting nations and regions of the world to gather as a small group and reach an agreement encompassing meaningful abatement measures among themselves as soon as possible (see this post).  The climate welfare of all the world’s people depends on mitigation steps taken by such a group at the earliest possible time.

 

Details

 

The IPCC has issued four Assessment Reports since 1990; the fifth is currently in preparation.  Since the IPCC is an instrument of the United Nations, large numbers of climate scientists from member countries all around the world are assembled into teams of researchers and authors to prepare each AR.  The overall efforts are carried out under the direction of the Chairman of the IPCC.  A different person has served as the chairman of the IPCC each time an AR has been issued.  The personnel on the teams likewise differ from one AR to the next.  In the recent ARs three Working Groups were assembled to write their respective portions, and summary documents were prepared based on the Working Group results.  Additional groups of scientists serve as reviewers of these documents.  These arrangements and others assure the member nations of the U.N., as well as the worldwide community of climate scientists, that the ARs are objective, represent the results of the best science available at the time writing, and offer validated proposals for acting on the scientific findings and conclusions presented.

 

Principal findings from each AR have been extracted into the table shown below.  The author turned variously to Executive Summaries or Summaries for Policymakers of the full ARs to obtain the information presented here. 

 

The ARs span a period of 17 years.  Over this period vast enhancements in climate science have been made and are reflected in the successive ARs.  These include an increase in the number of climate scientists at work, significant improvements in the instrumentation used to gather climate data, and major expansions in computing power.  Improved computational capabilities permit more intensive analysis of data as well as creation of more sophisticated climate models for use in projections of likely future climates that may be expected when assuming various emissions scenarios. 

 

Table of selected topics extracted from the four Assessment Reports of the IPCC

Assess-ment Report	First AR 1990	Second AR 1995	Third AR 2001	Fourth AR 2007
Source of new CO2	Human activities substantial-ly increase emissions of GHGs	Largely manmade	Primarily manmade; reached 368 ppm in 2000	Manmade CO2 and other GHGs have risen markedly since beginning of ind. revol.; now higher than in past thousands of yrs.  Annual emission rate increased ca. 80% from 1970 to 2004.
Project-ed future CO2 Increase (note end dates, and rates vs. total, in this row.)	Doubling of total CO2 by 2025-2050	Various scenarios for emission rate in 2100 range from no change to 6 times higher. Predict CO2 doubling by 2100 without action.	As of 2001 the 1990's very likely had highest ever recorded CO2, mainly fr. humans burning fossil fuels. Range of scenarios project total atmospheric CO2 of 540-970 ppm by 2100.	In absence of mitigation annual emission rates for manmade GHGs increase 25-90% from 2000 to 2030 due mostly to continued fossil fuel use.
Actions to Stabilize Atmos-pheric CO2	Immediate reduction in rate by 60%	To stabilize CO2 at 1995 level need immediate reduction in emission rate of 50-70%; reductions are technically possible and economically feasible.	Stabilizing CO2 at lower levels requires more stringent mitigation than for higher levels.  Substantial reductions could be made by 2010 and 2020 starting “now” (2001).	Mitigation next 20-30 yrs minimizes GW impacts. Delay reduces ability to achieve lower GHG levels and increases risk of severe GW impacts. Six emission scenarios considered.  Most stringent: Stabilized CO2 concentration=350-400 ppm (2005=379 ppm). Least stringent: Stabilized CO2 concentration= 660-790 ppm.
Past Temp Rise	0.3-0.6°C since 1890; warmest 5 yrs occurred in most recent decade (1980s).	0.3-0.6°C since late 19th cent.; discernible influence fr human activity; 20th century at least as warm as any since 1400.	Incr. by 0.6±0.2ºC in 20th cent.; 20th cent. likely warmest over No. Hemisph. in last 1000 yrs. "New and stronger evidence" that this is manmade.	GW is unequivocal, shown by temp. increase, melting glaciers, and global SLR.  1906-2005  temp trend is now 0.56-0.92ºC, higher than stated in 3rd AR for 1901-2000. Manmade incr. in GHGs since 1950 very likely responsible for GW.
Project-ed Further Rise in Global Average Temp.	0.2-0.5°C per decade; 1.5-4.5°C by 2025-2050	Median scenario for 2100 predicts 2°C (full range  1°C-3.5°C) rise; 30-50% loss of mtn. glaciers; more warm days & fewer cold days	Various climate models w. various emission scenarios project temp. 1.4-5.8°C above 1990 value by 2100 (greater than in 2nd AR); not seen in past 10,000 yrs.	As of 2007 various scenarios project likely temp. incr. of 1.1-6.4ºC by 2090-2099 over temp. during 1980-1999 reference. Temp. continues to increase for centuries even if GHGs stabilize, due to slow global response.
Project-ed Weather Patterns	More temp change near poles; pptn hard to predict	Warmer land temps esp. at northern latitudes; region-dependent stronger or weaker flood/drought events; rate of temp incr. more than seen in past 10,000 yrs	Project temp. 1.4-5.8°C above 1990 value by 2100 (greater than in 2nd AR); not seen in past 10,000 yrs. Global pptn. incr., depends on latitude. Loss of glaciers continues in 21st cent. Project very likely more extreme events and variability, more hot days, fewer cold days, more heavy pptn., more droughts.	Higher confidence than in 3rd AR of changes: Warming strongest over northern latitudes. Snow cover loss. Frequency of heat extremes and of heavy pptn. very likely higher. Changes in regional climate patterns.  Manmade GW and SLR will continue for centuries even if emissions stabilize due to slow planetary response times.
Past Sea Level Rise	10-20 cm	10-25 cm over past 100 yrs, most due to incr. temp.	Incr. 0.1-0.2 cm/yr during 20th cent., very likely consistent w. warmer planet	Global SLR since 1961 is 0.13-0.23 cm/yr, and since 1993 is 0.24-0.38 cm/yr, higher than in 3rd AR. Very likely at least partly due to manmade GW.
Project-ed Sea Level Rise	20 cm by 2030; 65-100 cm by 2100	50 cm (range 15 cm to 95 cm) by 2100; ca. 100 mln people subject to SLR	9-88 cm SLR fr. 1990 to 2100	Likely extreme high sea level. Various emission scenarios project 18-59 cm rise from 1980-1999 reference to 2090-2099. SLR continues for centuries even if GHGs stabilize due to slow global response.
Confid-ence/ Certainty	Moderate uncertainty; still much to under-stand about climate; uncertain lag times betw GHG rise and effects	Increased realism of models increases confidence of average climate projections; as of 1995 inadequate data to ascribe extreme events to human activities; potential for large unexpected climate changes	High confidence in projections variously  associated with likely and very likely probabilities	Higher confidence than in 3rd AR of GW patterns. Higher confidence in incr. extreme events and their adverse effects.
Recom-mended Actions	Begin emissions reductions now;  include energy efficiency & clean energy	Adaptation technologies available	Reduced emission rates yield more time for adaptation.  Stabilizing CO2 at lower levels requires more stringent mitigation than for higher levels.  Technology for mitigation has advanced since 2nd AR.  High political, economic and cultural barriers impede implementation. NOTE: Substantial reductions could be made by 2010 and 2020 starting “now” (2001). Much of the expense is recovered in energy savings.	Mitigation next 20-30 yrs minimizes GW impacts. Delay reduces ability to achieve lower GHG levels and increases risk of severe GW impacts. For lax to stringent reduction scenarios, technology exists or can be commercialized to achieve targets. Costs rise with stringency of target. E.g. emission tax of US$20-80/ton CO2-equivalent could stabilize atmosphere at 550 ppm CO2-equivalent by 2100, or lower with technology advances. Kyoto Protocol was effective first step. Adaptation complements mitigation to lower effects of GW.

Abbreviations:        AR, Assessment Report

                             BAU, Business-As-Usual (no mitigation policy)

                             betw, between

cent., century

cm, 0.39 in./cm

GHG, greenhouse gases

                             GW, global warming

                             incr, increased

                             ind. revol., industrial revolution

                             mln, million

                                   ppm, parts per million
                                   pptn, precipitation
                             SLR, sea level rise
                             temp, temperature

                             temperature: 1ºC = 1.8ºF

                             yr, year

Statements of likelihood: Likely: greater than 60% probability

                                       Very likely: greater than 90% probability

Sources: The Intergovernmental Panel for Climate Change (IPCC)

First AR: http://www.ipcc.ch/ipccreports/1992%20IPCC%20Supplement/IPCC_1990_ and_1992_Assessments/English/ipcc_90_92_assessments_far_overview.pdf

Second AR    http://www.ipcc.ch/pdf/climate-changes-1995/ipcc-2nd-assessment/2nd-assessment-en.pdf

Third AR       http://www.grida.no/climate/ipcc_tar/vol4/english/pdf/spm.pdf

Fourth AR     http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf

 

Stocker’s model for Global Mitigation Schemes.  Stocker in Science, 2013, Vol. 339, pp. 280-282 (DOI:10.1126/science.1232468) presents a simple yet convincing model showing the options for undertaking measures in a Global Mitigation Scheme.  Its features include modeling the increase in the accumulated CO₂ level by a fixed percent each year, which he sets conservatively at 1.8%/yr.  This leads to a model of an exponentially increasing rate of CO₂ accumulation.  (This model is identical to the notion of the effect of a compounded interest rate on a savings account over many years.)  Then, beginning at a time chosen by us, the modelers, or by policymakers, a fixed percent of reduction in the annual rate of emission is imposed; Stocker has chosen to illustrate this with reduction rates from a fraction of 1%/yr up to 10%/yr or more.  Again, this superimposes an exponential decrease in the rate of accumulation beginning at the chosen time (note that this reduction does not remove CO₂ from the atmosphere but only slows its accumulation rate).  Finally, he chooses an intermediate value, among those evaluated by climate scientists, for the extent of increase of the global average temperature caused by a given level of accumulated atmospheric CO₂ of 2ºC (3.6ºF) per trillion tons of atmospheric carbon.  The results of this model are shown above in the body of this post.

 © 2013 Henry Auer