Climate scientists are almost unanimous in the understanding that the bulk of th

Question

Climate scientists are almost unanimous in the understanding that the bulk of the warming observed in the 20th and 21st centuries can be attributed to human activities. This scientific conclusion can be summarized as follows: Humans industrial activity has led to emissions of large amounts of carbon dioxide (CO2) and other greenhouse gasses (such as methane, nitrous oxide, and halocarbons). The resulting increased atmospheric concentrations of these greenhouse gasses has caused a strengthening in the Earth’s greenhouse effect. In class we have called this “the perturbed greenhouse effect”. This strengthening of the greenhouse effect in the Earth’s atmosphere has warmed the planet’s surface. This warming is what we observed when we look at instrumental records of past temperature and see that the Earth has warmed. Despite this strong scientific consensus, the belief exists in parts of popular culture that the global warming that occurred in the 20th and early 21st centuries is natural variability. In this reflection, we ask you to consider the question of how we might distinguish natural climate variability from anthropogenic (human-caused) warming. Put more simply, “how do we know it’s us?”. We don’t expect you to know what all the pieces of the scientific argument are (we will cover some of them later in the term), but we would like you to discuss what types of observations or evidence we would need to settle the question. You might think of this problem another way -- if you had as many resources as you needed, what observation, analysis and simulations would you want in order to settle the question “what caused the global warming of the last 100 years?”

Explanation / Answer

The temperature of the earth is increasing at approximately twice the rate it was 50 years ago. This rapid increase cannot be explained by natural cycles alone. The major effect of this is the greenhouse gases released by human beings.

Greenhouse effect is the maintenance of the solar radiation in the Earth's lower atmosphere by greenhouse gases. These gases (mainly carbon dioxide, methane and nitrous oxide) act as a thermal cover for the planet, warming the surface to a life-supporting average temperature of 15 degrees Celcius. The recent rapid increase in greenhouse gases caused the thickening of the atmosphere. This thicker atmosphere entraps more solar radiation and raises the global temperature.

The Intergovernmental Panel on Climate Change, or IPCC reported that there are numerous greenhouse gases accountable for warming, and humans emit them in different ways. Most of these gases come from the combustion of fossil fuels in vehicles, factories and electricity production. The most important greenhouse gas is carbon dioxide. Other suppliers include methane emitted from landfills and agriculture (particularly from the digestive systems of grazing animals), nitrous oxide from fertilizers, gases used for refrigeration and industrial processes, and deforestation.

Different greenhouse gases have very diverse heat-trapping capabilities. Some of them can even entrap more heat than carbon dioxide. A molecule of methane causes greater than 20 times the warming of a molecule of carbon dioxide. Nitrous oxide is 300 times stronger than carbon dioxide. Other gases, such as chlorofluorocarbons are around thousands of times greater than carbon dioxide.

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