Summary. A denier of global warming asserts that the recent U. S. National Climate Assessment includes statements “that are (at best) misleading”. The denier inappropriately claims the Assessment’s use of a graphic image overlaying recent annual carbon dioxide emissions and annual values of global average temperature “shows correlation, and implies causation”. The denier cites the Medieval Warming Period and the Little Ice Age, having occurred in the absence of significant change in carbon dioxide level, as showing that temperature variations need not even be correlated with carbon dioxide.
This post shows this view to be overly simplistic. Here the scientific findings presented in the previous post, “The U. S. National Climate Assessment: Warming Is Due to Human Actions” are summarized. These demonstrate the scientific and logical rigor leading to the conclusion, based on all the relevant science (not merely a portion portrayed in a single graphic image), that manmade greenhouse gas emissions arising from burning fossil fuels are indeed responsible for global warming. This post also uncovers inaccuracies and inconsistencies surrounding the denier’s citation of the Medieval Warming Period and the Little Ice Age in support of his assertion that temperature and carbon dioxide level are not connected.
In fact it is shown here that observed global temperatures result from the combined effects of many climatic factors, of which the CO2 level is only one. Carbon dioxide is, however, the principal driver of contemporary global warming.
Introduction. Overwhelming evidence and the concurrence of the vast majority of climate scientists lead to the conclusion that manmade emissions of greenhouse gases are responsible for global warming and its harmful effects. Even so, there are still those who are skeptical of or deny outright the causative relationship between greenhouse gases and global warming.
One such person is Thomas Wysmuller, a former meteorologist. He responded to the post “The U. S. National Climate Assessment: Warming Is Due to Human Actions” (Human Actions) in an email to this writer seeking to discredit the U. S. National Climate Assessment (NCA).
The denier accuses the NCA of “misleading” information concerning the correlation of global average temperature and atmospheric carbon dioxide (CO2) concentration (see the first graphic in the Human Actions post). First, the denier’s email incorrectly states that superimposed data for temperature and CO2 from 1880 to the present “shows correlation and implies causation” (emphasis in original). Human Actions and the NCA provide a comprehensive set of stringent scientific evidence, of which the temperature-CO2 correlation is but a part, that substantiates the causative relationship between emissions of CO2 from burning fossil fuels, and other sources, and the increase in the long-term global average temperature that is most apparent in the second half of the 20th century (see Details at the end of this post for full summaries of the data from Human Actions and the NCA).
Second, the denier’s email presented an alternative graphic from the work of Loehle and McCulloch (see Details) depicting positive temperature deviations during about 500-1150 CE (i.e., AD), a period called the Medieval Warm Period (MWP ) and negative temperature deviations during about 1400-1900 CE, called the Little Ice Age (LIA); during both of these periods the atmospheric concentration of CO2, the principal greenhouse gas before the industrial revolution, remained relatively unchanged at about 280 parts per million (ppm; volumes CO2 per million volumes of air). The denier asserts “CO2 and atmospheric temperature are almost entirely in disconnect”. There are several problems with these data (see Details).
The Loehle-McCulloch graphic (see Details) leaves the impression that both the MWP and the LIA are well-defined worldwide phenomena that occur despite no significant change in atmospheric CO2levels. This denier would have us believe that the absence of a relationship between temperature and CO2during these intervals shows these properties can never be dependent on each other.
This is a simplistic and improper assumption. As shown in the Details, numerous scientific publications make clear that there are many factors affecting observed atmospheric temperature of which CO2level is only one. The denier also infers that the MWP and the LIA are worldwide phenomena. Articles summarized below in Details show that these effects were only regional, not global.
What is significant for contemporary warming is that the increased temperature can be accounted for, after including the effects of all the other drivers of temperature, onlyby including the excess CO2 introduced into the atmosphere becausehumanity is burning fossil fuels for energy. This factor was absent during geological time periods before the industrial revolution.
Conclusion
A denier has failed to disprove that contemporary global warming is due to excess emissions of greenhouse gases that arise from human activity, primarily the burning of fossil fuels.
This post summarizes recent scientific reports demonstrating that manmade greenhouse gas emissions are causing contemporary global warming. It also presents selected scientific articles that consider contributions from many energetic sources that may contribute to global warming. They correctly emphasize that atmospheric CO2 concentration is not the sole factor governing warming. A denier must keep all these contributions in mind.
Finally, a denier must be able to explain the additional consequences of global warming on the complete earth system, not only on the temperature of the air close to the surface of the earth. These include facts such as a) about 90% of the excess heat from global warming ends up in the ocean, b) the total heat content of the ocean continues to increase, and c) mountain glaciers and ice sheets are melting at more rapid rates than earlier, leading to inexorable sea level rise.
Details
The Human Actions post and the NCA set out scientifically objective and logically rigorous demonstrations that excess atmospheric CO2 originating from mankind’s burning of fossil fuels is directly responsible for the increased global average temperature in the last several decades.
First, HumanActions summarizes its conclusion as follows:
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 time course strongly similar to those for temperature and CO2, suggesting that burning fossil fuels is a major contributor to the excess accumulation of atmospheric CO2;
3. Francey and coworkers (see Human Actions) analyze a particular physical property of atmospheric CO2, showing unambiguously that the excess CO2appearing during the industrial revolution originates from fossil fuels; and
4. Climate model “hindcasts” show that only by including contributions to atmospheric CO2from human factors can the global average temperature record from 1970 to the present be satisfactorily reproduced.
Second, the NCA, in its NCA Highlights, concludes
“Multiple lines of independent evidence confirm that human activities [such as burning of coal, oil, and gas, and clearing of forests] are the primary cause of the global warming of the past 50 years. The[y] have increased the concentration of carbon dioxide in the atmosphere by more than 40% since the Industrial Revolution….Natural factors like the sun and volcanoes cannot have caused the warming observed over the past 50 years….If not for human activities, global climate would actually have cooled slightly over the past 50 years. The pattern of temperature change through the layers of the atmosphere, with warming near the surface and cooling higher up in the stratosphere, further confirms that it is the buildup of heat-trapping gases (also known as “greenhouse gases”) that has caused most of the Earth’s warming over the past half century.”
In conclusion, because of the objective veracity of the scientific underpinnings of global warming, no legitimate assertions questioning these conclusions can be credibly proposed.
Loehle and McCulloch improperly convey misleading information concerning the MWP and the LIA. The graphic displaying temperature deviations during the MWP and LIA, adapted from the original and cited by a denier, is shown below:
Source: email to this writer from T. Wysmuller, citing Loehle, C. and McCulloch, J. H. 2008. Correction to: A 2000-year global temperature reconstruction based on non-tree ring proxies. Energy & Environment 19: 23-100;
First, the adapted graphic incorrectly labels the left vertical axis as “Global Temperature Anomaly”. The use of the word “Global” is misleading and incorrect. In the original article the corresponding graphic does not include this word in the label for the vertical axis. Only this adapted version includes the word “Global”.
Second, labeling the temperature anomaly as “Global” is wrong. As shown below in the various sections describing the work of other researchers, it becomes clear that the MWP and the LIA are regional in scope, not “global”.
Third, Loehle and McCulloch inappropriately excluded from consideration all temperature data based on analysis of tree ring properties. In fact, tree ring proxies for temperature continue to be used by other climate scientists up to the present (see descriptions for articles by Kaufman, Villalba, and Mann, below). This omission potentially introduces an unexplained bias into the Loehle-McCulloch graphic.
Fourth, it is highly misleading for a denier to imply that CO2 concentration is the only factor governing average temperature. Climate scientists have long recognized many other potential energetic factors affecting temperature (see the research articles described below). What is important is that since the industrial revolution the excess CO2added to the atmosphere by burning fossil fuels is the dominant factor driving warming in the recent decades (see Human Actions).
Villalba (Climatic Change 1994, Vol. 26, pp 183-197) used tree ring and radiocarbon data to characterize temperature and precipitation in southern South America . At one location studied, a cold interval from 900 to 1070 CE was followed by warmer intervals from 1080 to 1250, and again a cold period from 1270-1660. At another location glacial advances were observed for the periods 1270–1380 and 1520–1670 C.E. In a third location glacial advances were found from the late 1600s to the early 1800s. These are distinct patterns that fail to suggest single worldwide events. Villalba states that strong El Niño-Southern Oscillation (ENSO) Pacific oceanic trends were likely responsible for these events.
Mann shows that the LIA is regional, not worldwide (Mann, M.E.,2002, “Little Ice Age”, in Vol. 1, The earth system: physical andVolume 1, The Earth system: physical and chemical dimensions of global environmental change, pp 504–509; M. C. MacCracken and J. S. Perry (editors) John Wiley & Sons, Ltd., Chichester). Using temperature proxies including tree ring data, Mann presented temperature curves over 1,000 years from several regions in the world, shown below:
Estimated relative temperature variations for various regions from 1000 to 2000 CE, smoothed over as much as 100 years to show long-term trends. The vertical axis shows ºC for panels (a) and (e), and relative variations for panels (b), (c), (d), (f), (g) and (h). The rectangular box from before 1400 to after 1900 is labeled LIA. Panel (e) for Central England represents the European Little Ice Age well. Fennoscandia (panel (f)), Scandinavia . Panels (b) and (f) are based on tree ring data.
Source, Mann 2002, “Little Ice Age”; http://www.meteo.psu.edu/holocene/public_html/shared/articles/littleiceage.pdf.
It is evident that the period called the LIA is not a single global event, but is manifested in distinct ways in different regions around the world. In particular, according to Mann, the variability includes among its causes changes in atmospheric circulation patterns, especially the North Atlantic Oscillation. This is “the dominant mode of atmospheric circulation variation in the North Atlantic and neighboring regions, [having] a particularly strong influence on winter temperatures in Europe ”. The North Atlantic Oscillation cooled eastern North America and Europe while the western U. S. Middle East were warmer than usual, confirming the regional nature of temperature trends. Volcanic eruptions episodically led to cooling as well.
Kaufman et al. (Science 2009, Vol. 325, pp. 1236-1239) studied proxy temperatures in the Arctic regions of the Western and Eastern Hemispheres over the last 2000 years. A long-term cooling, more or less linear from 0 to 1900 CE is attributed to weakening solar irradiation from 0 to 2000 CE. Kaufman et al. find no evidence in the Arctic for either a MWP or a LIA. Arctic temperatures rise sharply after 1900 CE, deviating from the long preceding downward trend. This indicates that another climatic factor not previously present causes the strong positive deviation after 1900.
Mann et al. (Science 2009, Vol. 326, p. 1256) conclude that the MWP (termed the Medieval Climate Anomaly in their article) is partly due to La Niña cooling in the tropical Pacific, and that the LIA, most evident from 1400 to 1700 CE, is partly due to El Niño conditions and to the North Atlantic-Arctic Oscillation.
Miller et al. show that intense volcanic activity led to the onset of the LIA (Miller, G. H., et al., Geophys. Res. Lett., 39, L02708 (2012)). Based on geological findings based in Northern Canada and Iceland
Marcott et al. studied worldwide and regional trends in temperature over the past 11,000 years (Science 2013, Vol. 339, pp. 1198-1201). They found that temperature trends differed widely among (a) the Arctic and northern temperate, (b) the tropical, and (c) the southern temperate and Antarctic regions. This shows that single “events”, such as a worldwide warming period or a worldwide glacial period, did not prevail during this interval.
Data for possible sources for temperature variation were gathered. Solar irradiance was warming for both polar regions but less so for the tropics, effects that weakened as time progressed toward the present. Atmospheric sulfate aerosols from volcanic activity (exerting a cooling effect) were strongest in the earliest years but continued throughout the full interval. Greenhouse gases increased from about 7,000 to about 1,000 years ago; their warming effect opposes the observed cooling trend and so cannot explain it. The Atlantic Ocean meridional overturning circulation, which carries heat from the tropics to the North Atlantic, contributed cooling to the Northern Atlantic while lessening cooling in the South Atlantic, in agreement with observed land temperatures. This study clearly points out the many potential contributions to global cooling and heating prior to the industrial revolution, and shows that most effects, when significant, are regional rather than global.
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