The Evidence of Global Warming

Scientific Evidence

Increasing Temperatures & Greenhouse Gases

Through the study of ancient ice cores from Antarctica it is possible to compare atmospheric concentrations of one of the key greenhouse gases, carbon dioxide (CO₂), with temperature variations over the past 400 thousand years of the earth's history (figure below).

The two trends are clearly related to each other, with fluctuations in one plot almost exactly mirrored in the other for more than 400,000 years. 

But suddenly in the 1800s, as the Industrial Revolution takes off, atmospheric CO₂ concentrations begin an unprecedented upward climb, rising rapidly from 280 ppmv (parts per million by volume) in the early 1800s to a current level of about 400 ppmv, well above the highest concentrations previously attained in the course of the preceding 400,000 years. 

Notice how CO₂ concentration (the upper line) seems to rise vertically at the end of the time series, as "now," the present time, is approached.  The increase appears vertical because of the large time scale, but it actually occurs over the past 150 years, which corresponds to the age of fossil fuels (the modern industrial age). This is the primary evidence of the long-term relationship between CO₂ concentration in the atmosphere, and the earth’s temperature.

On a shorter and recent time scale, direct measurements of CO₂ concentrations in the atmosphere show the sharp upward rise of the past decades. This is shown in the figure below. (The red squiggles reflect the day to day variations, but month by month the average value of CO₂ concentration increases, and the year by year trend is clear.)

The main point of contention regarding the reality of Global Climate Change and the claim that it is a consequence of increased man-made emissions of greenhouse gases—primarily CO₂—lies in the scientific evidence (examples of which are presented above) that the earth is warming, and the conclusions reached by several independent groups of scientists that there is a cause and effect connection between observed temperature changes and CO₂ concentrations measured over an extended period of time.

We turn now to the next issue: The gathering of data on global warming, from land and sea and polar ice. The analysis of the data is an enormous task involving a large number of scientific establishments. How is this task coordinated, monitored, and evaluated?

The Intergovernmental Panel on Climate Change (IPCC)

The data presented above, clearly showing a relationship between CO₂ and the earth’s temperature, does not of itself prove a causal relationship. 

Upon noting these indications of global warming, and recognizing the potential for dramatic changes in the climate due to continued unchecked accumulation of greenhouse gases in the atmosphere, the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) established the Intergovernmental Panel on Climate Change (IPCC) in 1988. 

The purpose of the IPCC is to objectively review existing and developing peer-reviewed scientific literature to form an objective evaluation about the risk of human-induced climate change. 

There are two key points here. The data and analyses considered must have appeared in peer reviewed scientific journals, and hence have already been subjected to the criticism of knowledgeable scientists. And the reviewers must be objective, free of commercial and political bias. Objectivity is not always easy to evaluate and ensure. 

Subjecting a proposed scientific paper to multiple reviewers makes it easier to eliminate bias from the peer review process.

What is the Peer Review Process?

The peer review process varies from one scientific publication to another, but in general it goes like this. A manuscript is submitted to a journal for publication. The editor of the journal selects several (typically three or four) individuals who are acknowledged experts in the field of study. 

The reviewers might include, depending on the topic of the manuscript, chemists, climatologists, statisticians, and/or others whose expertise is essential to evaluating the reliability of the scientific study and its conclusions. 

The reviewers respond to the editor, point out any errors or weaknesses in the work, and recommend publication if the noted errors/deficiencies are corrected. A reviewer may recommend outright rejection of the manuscript as unsuitable for publication. 

The set of reviews is transmitted by the editor to the author(s) of the manuscript, and they may make recommended changes, or they may argue that the reviews include unfair or incorrect criticism. 

The editor makes a judgment as to whether the revised manuscript (or the author’s rebuttals) has dealt with the reviewers’ comments. (Journal editors are usually highly regarded scientists in the fields covered by the journal. 

Most journals have a panel of editors with expertise in various aspects of the particular field(s) covered by the journal.) The editor may send the revised manuscript back to the reviewers for another assessment. 

In some cases, the editor may ask an additional reviewer to give an opinion on the author’s rebuttal of a particular point in the reviews.

After years of investigation and in consultation with thousands of scientists, the IPCC was able to write, in its Second Assessment Report in 1995, that climate has changed over the past century and that the twentieth century had a mean temperature “at least as warm as any other century since 1400 A.D.” 

Their report concluded that the dramatic increase in carbon dioxide concentration in the atmosphere over the past 150 years (noted above) is largely due to anthropogenic (human-caused) effects and further concluded that “the balance of evidence suggests a discernible human influence on global climate.” 

The predictions of mathematical models presented in the report indicated future temperature rises in the range of 1.8 to 6.3 degrees Fahrenheit in the global mean surface temperature during the next century, with sea-level rises predicted to be in the range of 6 inches to 3 feet by 2100.

You can see that these predictions vary widely. In part, this is due to varying assumptions made about what corrective steps might be taken by various governments in the future. 

The variations also reflect the real uncertainties in our current understanding of the physics of climate change. Detractors of the global warming scenario use this as an argument to disregard the results of warming models. They fail to note that even the low end of predicted changes is of significance, and warrants action. 

Disagreement in the science community about how much warming may occur in the future does not imply that significant warming will not occur.

The conclusions of the IPCC gained broad support in the world scientific community and, in the summer of 1997, a letter signed by 2,600 scientists called for the United States to take a leadership role in reducing greenhouse gas emissions to diminish the likelihood of intense, continuous global warming. 

Improved Models, Growing Confidence

The Third Assessment Report of the IPCC was released in 2001, incorporating new research undertaken in the five years since the Second Assessment Report. Increased confidence in evolving modeling techniques lent added weight to the linkage between rising temperatures and continued greenhouse gas accumulations.

Based on the large amount of evidence reviewed by scientific experts on the Panel, the IPCC's 2001 report stated emphatically that "concentrations of atmospheric greenhouse gases has continued to increase as a result of human activities." 

More recent revised models of the future predict by 2100 even higher anticipated temperature increases than did the First Assessment Report (under different likely scenarios), while estimates for sea-level rise, though still significant, decreased slightly with refinements to the models used.

The most recent report of the IPPC (2014) can be found on the following website: http://ipcc.ch/   where you can examine various details of the report.

To summarize to this point, it is apparent that the vast majority of members of the scientific community, specifically those people trained and experienced at obtaining and analyzing experimental observations relevant to climate and its impact, agree that global warming is occurring, and it is caused by worldwide man-made releases of greenhouse gases into the atmosphere. 

Let’s turn then to the next topic: the impact of Global Warming.