The U. S. National Climate Assessment: Warming Is Due to Human Actions

Summary.  The U. S. government released its Third National Climate Assessment in May, 2014.  It shows that emissions from human use of fossil fuels since the industrial revolution began are reflected in increased carbon dioxide concentrations in the atmosphere, and that this is directly responsible for the increase in the long-term global average temperature.  This post summarizes these findings, affirming their validity beyond any credible scientific doubt.

A future post will consider the Assessment’s climate findings.  Warming has already had adverse effects across the U. S., including heat, drought, wildfires, changes in availability of water, floods, ocean storm surges, extreme weather and climate events and socioeconomic effects.  Worsening of these trends is foreseen during this century.

Introduction.  The Third National Climate Assessment (NCA) was issued May 6, 2014 as mandated under the Global Change Research Act of 1990 to “assist the nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change”.  Its tasks, however, do not include formulation of policies to address global warming.

Significance of the NCA.  The NCA was prepared by hundreds of scientists and experts, overseen by federal officials from thirteen departments and agencies (see Details at the end of this post).  Its procedures assure that the results reflect the highest scientific standards presented in an objective, unbiased manner.  Its importance lies also in its emphasis on observations and projections of climate in the U. S. by region and by the sector of ecological or economic activity.

This post and one to follow are based on a Fact Sheet distributed by email by Bess Evans in the White House Office of Science and Technology Policy, and on the NCA Highlights (Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Highlights ofClimate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 148 pp.).  Here the scientific findings leading to the conclusion that global change arises from human activities are summarized.  A second post will present selected findings of the NCA drawn from the regions and segments mentioned above.  

The Earth is warming as a result of “human activities, predominantly the burning of fossil fuels” (NCA).  Carbon dioxide (CO₂) has been recognized as a greenhouse gas since at least the 19^th century.  Although it has been present in the atmosphere for geological time frames, the recent abrupt increase due to human use of fossil fuels has raised its concentration, and that of other man-made GHGs, higher than before these human activities began.  The incremental CO₂ raises average temperatures around the globe.  This has led to several effects identified in the NCA and shown in the following table:

Increasing Values or Amounts	Decreasing Values or Amounts
Primary Effects
Temperature Over Land
Sea Surface Temperature
Temperature Over Oceans
Air Temperature Near Earth’s Surface (Troposphere)
Ocean Heat Content
Consequent Effects
Water Vapor (Moisture Content of Air)	Sea Ice
	Snow Cover
	Glaciers and Ice Sheets

 

Strong Correlation between CO₂ and Global Temperature.  The NCA presents data showing a close correlation between rising atmospheric CO₂ concentration and global average temperature during the industrial era, shown in the graphic below.

Correlation between global temperature and CO₂ concentration from 1880 to 2012.  The thick black line gives the yearly CO₂ concentration in parts per million (volumes of CO₂ present in 1 million volumes of air) with values along the right vertical axis.  The ends of the blue and red lines show the temperature with values along the left vertical axis.  The horizontal line shows the average temperature across this time span.  The ends of the blue lines show the temperature values for years in which the temperature was below the average, and the ends of the red lines show the temperature values for years in which the temperature was above the average.

Source: NCA; www.nca2014.globalchange.gov.

 

The ends of the blue and red lines, ending at the average temperature value for each year, and the solid black line for annual CO₂ concentrations are highly correlated, suggesting a relationship between them. 

 

Emissions of CO₂ have risen sharply from zero since the beginning of the industrial revolution.  The sources of increasing atmospheric CO₂ from 1850 to the present are shown in the following graphic.

Annual rates of emission of CO₂ (evaluated on the basis of the carbon portion of the carbon dioxide molecule) from the three main fossil fuels, plus the production of cement from limestone.  The amounts shown represent about 80% of all human-derived carbon, with most of the remainder arising from human-induced deforestation.

Source: NCA (data from Boden et al. 2012); www.nca2014.globalchange.gov.

 

 

It is important to note that the time course for the annual rate of emission of C from burning fossil fuels in this graphic follows closely the time course for the annual values of atmospheric CO₂ concentration and annual temperature rise shown in the first graphic.   These data strongly suggest that the increased atmospheric concentration of CO₂ during industrial times originates from the burning of fossil fuels, and that the excess CO₂ causes the global temperature rise because of the greenhouse effect.

 

Burning fossil fuels is directly responsible for the increase in the concentration of atmospheric CO₂.  This conclusion is based on use of a distinctive molecular signature for CO₂, a physical property (the isotopic ratio of ¹³C to ¹²C in CO₂) which differs for CO₂ originating geologically in the atmosphere and CO₂ originating from burning fossil fuels.  The time course for this property, measured for the past 1,000 years, also follows the trends shown in the previous graphics.  Significantly, pronounced changes arise only during the industrial period, retracing the time courses shown above for temperature, atmospheric CO₂ and fossil fuel emissions.  The trend of values deviates from those for the first 800 years and curves strongly in the direction characterizing CO₂ originating from fossil fuels as the industrial revolution progresses (R. J.Francey et al., Tellus (1999), 51B, 170–193).   The researchers conclude “The overall [change in the isotopic ratio] during the last 1-2 centuries is attributed to anthropogenic [i.e., man-made] emissions”.  This result removes any credible basis for scientific doubt that the excess CO₂ in the atmosphere accumulating during the industrial period originates from burning fossil fuels.

 

Man-made CO₂ is responsible for global warming.  Climate models that simulate past global average temperature have been run for the cases that a) excess CO₂ originating from burning fossil fuels is omitted, and b) it is included.  The results are shown below.

 

Observed annual average global temperature changes from 1890 to 2010 (black line), and “hindcasts” from climate models shown as light green (models using natural factors only) and gray (models using both natural and human factors).   The change defined as 0ºF is a reference obtained from a temperature average over many of the years in the center of this period.

Source: NCA (data adapted from Huber and Knutti 2012); www.nca2014.globalchange.gov.

 

 

It is important to note that both models reproduce the long-term trend of observed temperature from 1890 to about 1970.  This validates the use of the models for “hindcasting” temperature from 1970 to the present.  The graphic shows, however, that after about 1970 only the climate models that include human factors, i.e., mankind’s use of fossil fuels and deforestation, reproduce the observed temperature data.  This shows unequivocally that the differential increase in global average temperature over the last several decades arises from the incremental addition of CO₂ to the atmosphere that comes from burning fossil fuels, and from deforestation. 

 

Summary.  The NCA Highlights have shown:

1.     Since 1880 the atmospheric concentration of CO₂ and the long-term average global temperature are highly correlated;

2.     Over the same time period emission of carbon (dioxide) into the atmosphere from burning fossil fuels and producing cement follows a strongly similar time course, suggesting that burning fossil fuels is a major contributor to the excess accumulation of atmospheric CO₂;

3.     In a report not included in the NCA Highlights Francey and coworkers analyze a particular physical property of atmospheric CO₂, showing unambiguously that the excess CO₂ appearing during the industrial revolution originates from fossil fuels; and

4.     Climate model “hindcasts” show that only by including contributions to atmospheric CO₂ from human factors can the global average temperature record from 1970 to the present be satisfactorily reproduced.

 

This sequence of results, rigorously and accurately collected by climate scientists over the past several decades, conclusively establishes the validity of the NCA Highlight statement:

“The majority of the warming at the global scale over the past 50 years can only be explained by the effects of human influences, especially the emissions from burning fossil fuels (coal, oil, and natural gas) and from deforestation.”

 

Conclusion

 

The NCA has been prepared by hundreds of scientists and other experts from the academic world, nonprofit organizations, government research facilities, private sector consultants, private corporations and foreign research facilities (see Details below).  Their work was overseen by federal experts drawn from thirteen U. S. federal agencies dealing with aspects of global change.  Their report was subjected to external review, which garnered several thousand comments.  These were accommodated in preparing the final document.  There can be no reasonable basis for doubting or dismissing the impartiality, cogency and veracity of the NCA’s findings.

 

The NCA Highlights state unequivocally that most of the warming of our planet observed over the last 50 years is due to the excess contributions of greenhouse gases to the atmosphere from human activities.

 

Most of the NCA details the effects of the warming of the planet on the U. S.  This will be examined in a coming post.

 

Details

 

Significance of the NCA.  The NCA was prepared under the supervision of a federal advisory committee whose members are drawn from every department and agency, thirteen in all, substantively involved in an aspect of global change science or policy.  The Assessment itself has been assembled by over 300 scientists and experts drawn from academic research settings, federal agencies and research facilities, state agencies, nongovernmental organizations, private consulting organizations, corporations and foreign research organizations.  The Assessment is based on citations to a large number of original research articles published in peer-reviewed journals, and other reports of comparable scientific validity.  A draft version was reviewed by others in federal agencies, the academic community, the public, and the National Academy of Sciences.  The final Assessment took 4,000 comments submitted by such groups into consideration.  Entry portals to the full document, in sections, are available here . 

 

The details of this process provide high assurance that the Assessment communicates the scientific basis of the various aspects of global change treated, without projecting particular points of view not supported by scientific findings.  While remedial approaches are discussed in broad terms, no particular policy position is espoused.

 

The importance of this Third NCA is that it is written almost entirely by American experts and emphasizes aspects of global change manifested in the United States.  (This is in distinction, for example, from the recent Fifth Assessment Report of the United Nations-sponsored Intergovernmental Panel on Climate Change, which addresses the issue at a global level.) 

 

The Third NCA is noteworthy for addressing first, separate results for eight geographic regions of the United States, and second, separate results for twelve climate-based and socioeconomic segments affected by global change.  This more refined approach is enabled, compared to earlier Assessments, because of the accumulation of more detailed, sophisticated information, as well as refinement and enhancements in computational treatment of climate change in the U. S. at the regional level and across various ecological and socioeconomic segments.  These improvements were not available earlier.  A second post will consider some of these aspects of the NCA.

 

© 2014 Henry Auer