Global Climate Change

April 22, 2017

Summary of the Issue

The existence, degree, causes and implications of climate change or global warming have been issues of much debate in recent decades.  The deliberations are unlikely to be settled in the near term due to the complexity of the matter, scientific uncertainties and political polarization. This paper reviews existing research to identify underlying evidence regarding the issues, particularly the sources of global warming.  Climate change is caused by both natural phenomena and human activities, with the latter likely exerting more impact from about the mid-19th Century-onward due to increases in industrialization and the burning of fossil fuels.



Greenhouse gases (GHGs) in the form of carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and hydrochlorofluorocarbons (HCFCs) prevent long-wave radiation from leaving the Earth’s atmosphere and reflect it back to Earth which serves to increase surface temperatures (Guoguang, 2010).   Natural sources of CO2 consist of volcanoes, decomposition, animal/plant respiration and the ocean (What’s Your Impact, n.d.).  CO2 created by man occurs predominantly through burning of fossil fuels, deforestation, mineral conversion processes, and cement production among others.  In the U.S. during 2014, manmade GHGs were from electricity production (30%), transport (26%), industry (21%), commercial/residential (12%), agriculture (9%) (Environmental Protection Agency, 2014).


In 1969, the Union of Concerned Scientists (UCS) was formed by experts and students at the Massachusetts Institute of Technology.  The premise was discontent with government actions seen as a threat to the sustainable existence of mankind and the environment through the misuse of science and technology, largely to facilitate the military and industrial complex (MIC). The UCS focused on arms control, addressing nuclear hazards, alternatives to non-renewable energy, and the threat of climate change (Union of Concerned Scientists, 1969). In 1961, President Dwight D. Eisenhower had expressed similar concerns with the growing post-World War II MIC in his farewell speech (This Day in History, n.d.).  In 1988, the Intergovernmental Panel on Climate Change (IPCC) was formed to scientifically evaluate causes, ramifications, and solutions regarding global warming.   The IPCC generated four assessment reports and various other technical papers from 1990 to 2007 on the topic.  The 2007 four-volume Fourth Assessment Report (AR4) consolidates findings in a Synthesis Report from three Working Groups including more than 500 authors and 2000 expert reviewers with critique by representatives from more than 100 countries.  The respective Working Group I, II and III reports are entitled “The Physical Science Basis,” “Impacts, Adaption and Vulnerability,” and Mitigation of Climate Change.”  The AR4 finds that global climate is caused predominantly by humans (Intergovernmental Panel on Climate Change, 2007, Forward).


Evidence and Consequences of Climate Change

The Synthesis Report documents a 0.74 degree C increase in global daily average surface temperatures for the period of 1906-2005 with the 11 highest temperatures occurring in the last 12 years since 1850.  Other characteristics of this trend are that it is occurring world-wide and it is greater on land and in northern locales.  Further, it is accompanied by sea levels that have risen an average of 1.8 mm/yr and 3.1 mm/yr since 1961 and 1993, respectively, due to increases in melting rates of artic sea ice and mountain glaciers/snow pack.  Precipitation has increased substantially over various parts of the world while declining in other locations.  It is likely that areas of drought have increased since the 1970s, land-based heat waves have increased, heavy rain events have increased over most areas, and very likely that cold days/nights have decreased while hot days/nights have increased over the last 50 years.  There is high confidence that global warming has increased runoff, increased temperatures of rivers/streams, altered polar ecosystems, and caused changes in salinity, circulation and oxygen levels which in turn have altered levels of fish, algae and plankton.  The Synthesis Report finds with medium confidence, due to other factors such as adaptation and lack of data in some locales, that climate change is linked to more impacts from pests and fire to forests, the ability to plant crops earlier, and altered vectors of infectious disease (Intergovernmental Panel on Climate Change, 2007, page 2-4).


Linkages to Human Activities

The Synthesis Report finds that worldwide GHG emissions have increased since the pre-industrial age and have grown by 70% over the period of 1970-2004.  More specifically, CO2 releases increased by 80% during these years.  Analysis of ice cores demonstrates that CO2, CH4, and N20 levels have risen substantially since 1750 due to human activities and are far above the thresholds from pre-industrial periods.  The causal actions are primarily the burning of fossil fuels and, to a lesser extent, land use changes in the form of agriculture.  The Synthesis Report states with very high confidence that human influences as a whole have contributed to global warming.  More specifically, human activities very likely had a causal effect on sea level rises from the middle of the 20th Century onward, likely were a factor in wind pattern alterations, likely had positive correlations to higher hot and cold temperature extremes, and more likely than not heightened the threat of extreme heat events, increased areas covered by drought since the 1970s, and expanded the number of heavy rain events.  Anthropogenic climate change also has likely had a significant impact in altering physical geography and biological systems.  However, the level of impacts by region are not consistent due to natural variations, other causal factors, and limitations of research (Intergovernmental Panel on Climate Change, 2007, pages 5-6).


The IPCC’s 2013 Fifth Assessment Report (AR5) builds upon AR4 with new research findings. Highlights of the AR5 Summary for Policymakers state that that the surface temperatures during the previous three decades have been increasingly warmer, more so than any other comparable period since 1750.  The researchers find with medium confidence that the Northern Hemisphere in these three recent decades likely never experienced temperatures this high over the previous 1400 years.  AR5 finds that more than one-half of global warming occurring since 1950 is extremely likely to have been caused by human activities.  It is likely that anthropogenic radiative forcings, or the change in atmospheric energy, in every continental area of the world have contributed to climate change except for Antarctica.  Radiative forcing due to human actions has increased more quickly since 1970 than any other comparable period since 1750 due in part to growth in GHGs.  Of note is that the warming trend was less during 1998-2012 than 1951-2012 due to lower radiative forcing as a result of volcanic activity, and solar activity was on the descending portion of its cycle (Intergovernmental Panel on Climate Change, Summary for Policymakers, 2013).


Non-human Factors

            According to Guoguang (2010), natural causes of global warming consist primarily of solar radiation disparities, volcanic occurrences, and mild fluctuations of climate system mechanisms.  Changes in the Earth’s orbital characteristics alter positioning with the sun which slightly modifies the amount of radiation reaching the Earth’s surface.  The result can be substantive changes in seasonal temperatures that vary by region.  Solar activity, particularly sun spot variations, impacts the amount of radiation reaching the Earth’s surface but is not considered by most scientists to be a primary factor in climate change. Volcanic dust shrouds can prevail for decades and limit the amount of sun reaching the Earth’s surface, thereby lowering temperatures (Guoguang, 2010).


The Nongovernmental International Panel on Climate Change (NIPCC) was formed in about 2003 in part by the Science & Environmental Policy Project and is associated with a worldwide cadre of scientists to counter concerns that the IPCC has political motivations that compromise objective reasoning.  NIPCC is affiliated with the Heartland Institute, a non-profit conservative/libertarian think-tank, and the Center for the Study of Carbon Dioxide and Global Change, a non-profit entity focused on studying the impacts of increases in CO2.  NIPCC has released several studies and publications (Nongovernmental International Panel on Climate Change, n.d.).  One of these, Why Scientists Disagree About Global Warming, refutes most findings of the IPCC.  Some of them will be stated here.  The extent of global warming since the middle of the 20th Century is within the bounds of prior variabilities and temperature peaks that were caused completely by natural forcings and reactions.  Evidence shows that past increases in temperatures occurred prior to rising CO2, not vice versa.  It is possible that solar forcings account for most of any global warming.  NIPCC asserts through analyses that many of the conclusions reached by IPPCC very well could be circumstantial and are not conclusive.  In particular, melting of polar ice caps and Arctic sea ice is not abnormal, sea-level increases are not intensifying and vary within past bounds by region.  Rainfall levels vary by region and indications of an overall trend towards drought due to human activity are nebulous.  There is not any substantial evidence of increases in extreme weather events due to global warming (Idso, Carter, & Singer, 2015).


According to Huang, Wang, Luo, Zhao & Wen (2012), the Medieval Warm Period (MWP) of about 950 to 1250 and the Little Ice Age (LIA) of about 1300 to 1870 are respective pre-industrial periods substantively warmer and colder than overall averages that had little influence from human activity.  Total solar irradiance (TSI), ultraviolet radiation and galactic cosmic rays (GCRs), and atomic nuclei originating outside the solar system, are the three primary solar impacts on the Earth’s climate.  Since about the mid 1980’s, it is clear that the negligible change in TSI does not explain the change in surface temperatures.  Ultraviolet radiation increases when solar activity intensifies which is characterized by higher concentrations of ozone (O3).   O3 absorbs the ultraviolet radiation, increasing temperatures in the stratosphere, which has substantive impacts on the troposphere.  Nevertheless, the causal effects of ultraviolet radiation on climate are not well understood.  Solar activity is the dominant influence during these times.  Research has shown that GCRs are relatively weak compared to solar activity.  However, the GCRs appeared to have more impact during the LIA period.  GCRs also have a close association with the Earth’s climate due in part to correlations with movement of the Andean glacier in South America.  Weakening of solar irradiance increases GCR which leads to more low cloud coverage resulting in lower surface temperatures.  However, the GCR-climate relationship is not well understood (Huang, Wang, Luo, Zhao & Wen, 2012).


Huang, Wang, Luo, Zhao & Wen (2012) conclude that both anthropogenic and natural mechanisms are causal factors in climate change with the former more predominant onward from the middle of the 20th Century and facilitated by the aforementioned GCR influence on solar radiation received by the Earth. Triacca, Attanasio & Pasini (2013) identify various research using dynamical climate models, neural network models, and bivariate Granger causality studies.  The authors use other applicable variables, namely TSI, cosmic ray intensity (CRI), and optical thickness of stratospheric aerosol, and teleconnection climate variability, to identify causal connections with global temperature.  The authors corroborate other studies by finding that human activities are the predominant cause of climate temperatures since 1950 (Triacca, Attanasio & Pasini, 2013).



Human activities appear to have played an increasing role in climate change since about 1850 due to the modernization of society that has included industrialization and associated increases in fossil fuel usage to potentially supplant natural phenomena as the primary causal factor.  However, the research is far from conclusive due to data gaps that increase retrospectively, disagreement by scientific experts, and the complications of interpreting cross-impacts amongst the various factors.  Nevertheless, the evidence is clear that human activities cause the release of GHGs consisting of CO2, N2O, CH4 and HCFCs which are known contributors to global warming.


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Huang, Wang, Luo, Zhao & Wen, (2012). Debates on the Causes of Global Warming. Advances

in Climate Change Research. 3:1, 38-44. Reviewed via


Idso, C. Carter, R. Singer, S. (2015). Why Scientists Disagree About Global Warming. Nongovernmental International Panel on Climate Change. The Heartland Institute. Reviewed via



IPCC, 2007: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp.


IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Reviewed at

Nongovernmental International Panel on Climate Change. (n.d.). About the NIPCC. Reviewed at

This Day in History. (n.d.). Eisenhower warns of military-industrial complex. Reviewed at

Triacca, U. Attanasio, A. Pasini, A. (2013). Anthropogenic global warming hypothesis: testing its robustness by Granger causality analysis. Environmetrics. 24: 260-268. Reviewed via

Union of Concerned Scientists. (1969). Founding Document: 1968 MIT Faculty Statement. Reviewed at What’s Your Impact. (n.d.). Greenhouse Gas Emissions – Main Sources. Reviewed via

Information on this web site is based primarily on research and analysis conducted by R. Arkell, AICP.  Apart from the citations, this information including conclusions, interpretations, and opinions does not necessarily represent the interests, views, or position of any other person, organization or agency. 

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