Anthropogenic Climate Change: Evidence, Causes, and Denial

Introduction

Global warming — or, more accurately, climate change — is the phenomenon, repeatedly confirmed through scientific study, that the planet is experiencing warming and shifting climates at a pace far more rapid than has been observed in any prior era. The acceleration of climate change has been correlated with the onset of the industrial age and, in particular, with the mass adoption of fossil fuels. The burning of hydrocarbons to generate energy triggers a chemical reaction that releases carbon into the atmosphere, where it traps solar radiation and drives the warming process. This paper outlines that process and the evidence supporting the fact of anthropogenic global warming.

Observations of Climate Change

The first step in understanding climate change is understanding how scientists determine that it is occurring. Weather is an observable phenomenon, and over long periods thousands of individual data points can be gathered globally. When processed, these data points reveal trends in variables such as temperature and extreme weather events. Weather information has been collected since the late nineteenth century in some areas, and since the early twentieth century across a much broader range of locations. By the mid-twentieth century, even extreme locations such as the South Pole had weather stations installed, allowing a minimum of sixty years of data from tens of thousands of sites around the world to be gathered.

Weather stations do not only collect data about storms and temperature; they also monitor atmospheric composition. Because of this data set, we know that atmospheric concentrations of gases such as carbon dioxide, tropospheric ozone, methane, and nitrous oxide have all increased significantly since measurements began (Seinfeld & Pandis, 2006). The earliest studies of climate change — when it was still commonly called global warming — focused on CO₂ but paid little attention to other gases, the broader chemistry of the atmosphere, or climate feedbacks (Seinfeld & Pandis, 2006). The major greenhouse gases — nitrous oxide, CO₂, and methane — are all known to alter atmospheric chemistry by trapping solar radiation in the Earth's atmosphere. Under normal, pre-industrial conditions, a certain percentage of the solar radiation that strikes Earth would be reflected back into space. With higher concentrations of greenhouse gases, less radiation escapes.

The observed changes in global temperature and atmospheric composition are, in isolation, short-range. The planet is approximately five billion years old, and a few decades do not capture any long-range phenomenon accurately. Concluding that recent climate changes are out of step with past history requires an understanding of historical temperature and atmospheric composition. Ice cores represent one of the best sources of such information. Some ice on this planet has been frozen for millions of years — long enough to determine whether recent changes in weather and atmospheric composition are abnormal. Evidence from the Vostok ice core in Antarctica, for example, reveals the climate and atmospheric history of the past 420,000 years. This examination found that atmospheric and climate properties "oscillated within stable bounds" and that the upper bound of those oscillations falls well below today's observations. Thus, current concentrations of carbon dioxide and methane in the atmosphere are unprecedented for at least that length of time (Petit et al., 1999).

Ice cores from Kilimanjaro provide evidence of climate change over the Holocene era specifically. These cores suggest that during the Holocene there were three abrupt periods of climate change, and they indicate that current climate change is stronger than those past events — strong enough to eliminate the Kilimanjaro ice cores entirely between 2015 and 2020 (Thompson et al., 2003). Taken together, samples from different parts of the world demonstrate that current climate and atmospheric conditions fall outside the natural range observed throughout recent history, and that these anomalous conditions have emerged only recently, as distinct from other Holocene climate events. Moreover, the intensity of current climate change exceeds that of other events in the Holocene record. This conclusion has been confirmed by additional studies. One analysis of Northern Hemisphere climates over the past 1,000 years found that current conditions are outside the normal range for that entire period, and that projections for the twenty-first century far exceed natural climate variability (Crowley, 2000).

Ice core science actually predates the modern study of climate change. In a 1984 climate journal, the technique of analyzing ice core composition was used to identify different climate cycles in the North Atlantic linked to massive iceberg releases from Greenland and the Canadian Arctic. Even then, scientists were able to rule out alternative explanations such as discontinuities in the cores or ice sheet instability. Today, using hundreds of ice cores from around the world, the findings are even more robust, and observed variations can only be explained by broad-based changes in the atmosphere (Dansgaard et al., 1984). When climate scientists began examining ice cores to test hypotheses about climate change, in other words, they were employing a technique established in the field long before the climate change debate existed.

A preponderance of additional studies has reached the same conclusions. The climate changes observed across all parts of the world over sixty to one hundred and twenty years of accurate record-keeping exceed natural variation — both within short time frames and over longer periods extending beyond the Holocene. This confirms what all anecdotal evidence already suggests: the climate is changing, and the evidence is incontrovertible.

Attribution of Climate Change

That climate change can be observed is only part of the argument. For policymakers and the general public alike, denying that climate change is occurring is untenable: failure to plan for a verifiable fact leaves one dangerously vulnerable to negative outcomes. There is, however, a meaningful difference between preparing for natural climate change and responding to anthropogenic climate change — that is, change driven by human activity. The second critical question is therefore: are humans causing it, and if so, how?

From the earliest studies of global warming, researchers have sought to establish the timeline of climate change and then identify what differs between this period and earlier ones. The hypothesis quickly focused on greenhouse gases, and the underlying science was explored accordingly. It was found that greenhouse gases are present in the atmosphere at concentrations higher than at any point in recorded history, lending strong support to the hypothesis. Researchers have also been able to test the individual contributions of each greenhouse gas to the overall greenhouse effect. Rodhe (1990) found that carbon dioxide makes the greatest contribution to climate change. Methane is actually a more potent greenhouse agent molecule for molecule, but far greater quantities of carbon dioxide are entering the atmosphere, making CO₂ the larger overall contributor.

Further research has demonstrated that multiple human activities contribute to greenhouse gas emissions. Agriculture is one such activity. Fertilizers have been found to release greenhouse gases at rates higher than non-cultivated land in the same area, indicating that the problem extends well beyond transportation. Searchinger et al. (2008) illustrated why this matters: greenhouse gas emissions actually increased when land was converted to biofuel production. The theory had been that biofuels would reduce emissions by displacing petroleum consumption, but the fertilizers required to produce biofuels generated more emissions than the displacement saved.

Both the burning of fossil fuels and the conversion of fertilizers have increased substantially since the turn of the twentieth century. A considerable body of evidence links these specific activities to changes in atmospheric composition that are, in turn, driving climate change by raising global atmospheric temperatures. The problem has only accelerated in recent years.

Conclusion

The only conclusion that can be reached, on the basis of the evidence presented by both sides, is that human activity — specifically the burning of fossil fuels and modern agricultural practices — is responsible for releasing greenhouse gases into the atmosphere at concentrations never previously recorded. The saturation of greenhouse gases now exceeds historical norms as far back as researchers can measure, which is at least 420,000 years. These gases trap solar radiation in the Earth's atmosphere, warming it over time. That atmospheric warming is triggering feedback loops that will drive further warming regardless of future human behavior. Even if all burning of fossil fuels ceased tomorrow, the planet will continue to warm for another 1,000 years.

All of this has been subjected to rigorous scientific study. Hundreds of studies have demonstrated the reality of anthropogenic climate change. There are no comparable studies demonstrating that humans are not driving climate change. There are, however, studies demonstrating that climate change denial is rooted not in evidence but in public relations campaigns funded by those with vested economic interests in maintaining the status quo. These campaigns need only generate a small degree of public doubt, since most people recognize that meaningful action against climate change would carry short-term costs to their quality of life. Critical analysis of the available evidence, however, makes it clear that humanity is responsible for changing the climate.

The conflation of denial with legitimate scientific opposition is regrettable. There is no meaningful comparison between scientific consensus — built from hundreds of independent studies using rigorous methods — and the assertion that one simply disagrees. In the domain of empirical fact, unsupported opinion carries very little weight and cannot stand as credible opposition to established evidence. To present climate change denial as the scientific equivalent of climate science is not only inaccurate but misleading, and it does a disservice to public understanding of one of the most consequential challenges humanity faces.

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