What, Again? Greenhouse Gases Accumulate in the Atmosphere

Summary: This post tabulates important findings from the six Assessment Reports (ARs) that the United Nations Intergovernmental Panel on Climate Change (IPCC) has released since 1990.  As shown here, climate scientists have recognized, as of the date of each AR, that a) man-made GHGs continue to be emitted and accumulate further in the atmosphere, b) predicted emissions risk accumulating climatically dangerous levels of GHGs, and c) principles are proposed to minimize further emissions so as to keep accumulated GHG levels to as low a level as possible.

 Here attention is restricted to three topics, a historical review of accumulated atmospheric GHGs, projections of future emission rates and accumulated totals, and ways to stabilize accumulated atmospheric GHGs. Coming posts will consider other aspects of climate, presented over the three decades that ARs have been issued, resulting from the projected accumulated GHG levels.

Some may feel this and the coming posts sound like broken records; they 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 here 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 GHG emissions, due to human activity, of the principal greenhouse gases (GHGs) 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 to the end of this century.  They also discuss general principles (but not specific policies) for limiting future emission rates.

The general results and projections presented in ARs 1-6 are broadly consistent with each other across the AR series.  They record the profound increase, due to human activity, in accumulated atmospheric GHGs across the years, emphasizing the results for the most significant GHG, carbon dioxide (CO₂).  Accumulated CO₂ is shown in the graphic below, adapted here by adding vertical lines for the years in which the various ARs were released, as well as the additional red line corresponding to the date of the Paris Climate

Direct measurements of atmospheric carbon dioxide (smoothed over the 12 months of a year) taken atop the Mauna Loa volcano in Hawaii beginning in 1958.  The base level prior to the Ind. Rev. is 280 ppm.  Ppm, parts of carbon dioxide in 1 million parts of air. The vertical lines represent the dates of issue of the respective Assessment Report (AR), and the date of the Paris Agreement.

 

Agreement of 2015.  The principal goal of the Paris Agreement is to reduce emissions sufficiently to keep the increase in the global average temperature to below 2.0°C (3.8°F), and preferably 1.5°C (2.7°F) by 2100.  It is seen in the graphic that there is a consistent increase in CO₂ emission rate beginning with the first measurements in 1958 and continuing throughout the three decades following AR1. 

The topics selected for this post are tabulated below in the Details section.  The authors of AR1 recognized already in 1990 that atmospheric accumulation of man-made GHGs was increasing.  As the graphic above and the entries for the successive ARs in the table show, the growth in accumulated levels continued unabated up to the present (AR6).  Furthermore, projected future trends for various emission scenarios were generally comparable throughout the series.  Finally, the need to reduce annual emissions was recognized from the beginning and reiterated, with increasing urgency, throughout the series.

 Whereas the need to reduce emissions begins in AR1 and extends up to the present in AR6, the strength of climate science underpinning those conclusions has increased dramatically.  For example this first of the three volumes 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.  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. 

 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 given below in the Details section the column “Actions to Stabilize Atmospheric GHGs” summarizes the increasing urgency of acting to reduce annual emission rates essentially to zero as the AR series progresses.  For AR1 and AR2 ambitious reduction goals were already stated, but planetary manifestations of the effects of global warming were not yet readily distinguishable.  By the time of AR6, 2021, extreme weather and climate events have become the subjects of frequent headlines, distributed across heat waves and droughts, famine, uncontrolled wildfires across the globe, intense precipitation events and flooding, and melting of glaciers and ice sheets leading to sea level rise.  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.

As noted in the Actions column of the table, early action could have been taken at moderate levels of effort and expense, to avert future, if not yet apparent, hazards.  Such opportunities were not seized.  By 2021 hazardous events are now current, requiring immediate action.  Necessarily these current actions must be far more aggressive, pervasive and expensive.  They also require fundamental and comprehensive changes in social and cultural approaches to the problem.

We must encourage our political, corporate and civic leaders to accept these challenges and overcome them 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.

 

© 2021 Henry Auer