A Study of Anthropogenic Causes of Global Warming

Introduction

It appears crucial in the debate on whether there is significant anthropogenic climate change to establish a number of facts. First, analysis of temperature data for a necessarily lengthy period in the past has to be performed. Second, this data should provide, if any, the direction in which the global temperature is moving, and of course its intensity and pace. If these findings are verified and pass into plausibility (not necessarily certainty), a number of theories to explain them would naturally arise from the scientific community, amongst which the human factor hypothesis. This would lay the basis for determining the impact on the lives and livelihoods of individuals, and also provide ground for further research on mitigation and adaptation.

I. The scientific background

1.1 Atmospheric events

The Earth's atmosphere is composed mainly of nitrogen gas, oxygen and smaller amounts of other gases like water vapor, carbon dioxide and methane - these are generally called "greenhouse gases". Most radiation from the sun passes right through these gases, because it has a short wavelength, it heats up the planet's surface and is reemitted as long wavelength radiation, such as infrared. But, although the short-wave radiation is allowed to pass, the long-wave type is absorbed and causes the molecules to vibrate, thus creating heat as a release of energy. The heat is radiated through the atmosphere and produces the so-called "greenhouse effect". A lack of greenhouse gases would hinder the creation and accumulation of heat on the planet, but the opposite is also true: an abundance of these gases can cause the Earth's surface to overheat. These events can be observed in Fig 1.1, which also presents the amounts of energy inflows and outflows created by irradiation and reflection. It appears that the greenhouse gases are absorbing or returning to Earth some part of the outgoing longwave radiation, which can lead to an increase in temperature.

Thermodynamics states that, for a body at constant temperature, the amount of energy absorbed and the amount emitted must be the same. In Earth's case, it releases 239 w/m2, which would imply a global temperature of about -18 OC if it were to absorb the same amount from the Sun. Fig 1.1 clearly shows that this is not the situation on the surface, since the energy absorbed amounts to around 349 w/m2, causing the temperature to rise. Most of the radiation is emitted at the level where greenhouse gases can be found, and in fact temperatures there are at around the expected norm.

The previously discussed issue is largely held as irrefutable by the scientific community, so we shall leave it as an insightful introduction into the events concerning Earth's atmosphere, and move on to more debated subjects.

1.2 Temperature changes

There has been and continues to exist a growing and well documented concern among climatologists and other scientists that the average global temperature has been changing during the past few centuries, and will continue to do so. A number of bodies that study the existence of this phenomenon are the IPCC , the NASA Goddard Institute and many others, but a lot of their research, in order to be presented in a more accessible format to the wide, non-technical public, is adapted and publicized by newspapers, magazines, politicians and other outsiders. On Monday, June 24th 1974, Time Magazine produced an issue with an intriguing article: "Another Ice Age?"; the title, in different forms, also appeared in other publications such as the New York Times and Newsweek. However, after a closer review of the article and its sources, one finds there is no mention of an impending ice age by the scientists credited with the findings.

There have naturally been two major schools of thought that maintained to accurately infer from the data that the temperature has either been increasing or decreasing. A phenomenon known as "global dimming" explains that, due to pollutants in the atmosphere, known as aerosols, sunlight may become reflected and overcome any warming effect from increased levels of carbon dioxide. Researchers went even further and proposed that a cooling process might also take place. Conversely, "global warming" proposes that the build-up of carbon dioxide would be so great that the ensuing heating process would overcome any cooling produced by reflective aerosols.

As early as the 1980s, the contest between the two hypotheses was largely settled by temperature readings on the ground. Fig 1.2 shows an image of how the temperature on Earth has evolved throughout the years 1880 to 2000. We can easily spot an increase in the temperature and it appears that the largest rise has been in the period between 1970 and 2000. Also, Fig 1.3 presents this year's surface temperature anomaly across the globe, as compared to the period 1951-1980. On the top-right corner of the image there is the average global temperature increase, while the grey areas denote a lack of consistent or reliable data. The image clearly indicates that a rise in temperatures has occurred as compared to the base period, and while it is not uniformly distributed, it does aggregate to an average of 0.80 OC.

While this debate appears largely settled, we now arrive at another source of dispute among scientists. An interesting discussion, one which I will leave for the next section, would be to determine what climate-relevant events have occurred in the studied period that could have influenced temperature increases to such a high extent.

II. Causes of global warming

2.1 Proponents of human causes

In a 2004 issue of the journal "Science", a paper by Naomi Oreskes briefly related the state of researcher's consensus on the problem of global warming, its causes and effects. Setting aside the media hype, the author states that the Inter-governmental Panel on Climate Change, whose purpose is to evaluate the state of climate science in order to inform policy makers, clearly expressed a consensus after analysing the research: "Human activities are modifying the concentration of atmospheric constituents ... that absorb or scatter radiant energy ... [M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations" . The author also posits that "In recent years, all major scientific bodies in the United States whose members' expertise bears directly on the matter have issued similar statements." Since it is widely within the scientific arena that the human race is mainly responsible