What’s causing it?
It is one thing for the average surface, air, and ocean temperatures of the Earth to be rising at unprecedented rates, it is another thing to determine what is causing it. Knowing the cause of climate change helps scientists and policy makers to address the consequences. This entire site is about the causes of climate change and the subsequent sections go into more detail on sociological perspectives of such causes. Below I examine the direct sources of the cause, subsequent chapters examine the underlying social forces of the direct human causes. Why and how have humans caused it and why have political and social systems not done more to stop it?
Global warming and climate change are caused by the levels of carbon dioxide and other gases increasing in the atmosphere. Essentially, these gases trap more of the sun’s energy in the Earth’s atmosphere and this increases the average air and surface temperatures. Carbon dioxide (CO2) and the other emissions are often referred to as “greenhouse gases” (GHGs). That’s because, like the material of a greenhouse (glass or plastic sheeting), GHGs trap more heat “inside” instead of allowing it to escape back into space. The energy emanating from the sun (in the form of short-wave radiation) has enough energy for much of it to penetrate the Earth’s atmosphere and then it is partially absorbed by the mass of the Earth and, now with less energy (long-wave radiation), it is partially reflected back toward space. However, now with a higher concentration of GHGs in the atmosphere, less of the energy reflected off the Earth escapes the atmosphere and returns to space. More of it is reflected back again toward Earth — warming it.
Below is a short video from the The National Academies of Sciences, Engineering, and Medicine explaining the process.
Many refer to this specifically as “anthropogenic” climate change. Anthropogenic means that it is human-caused. But how do we know that these GHGs came from human-generated sources and not naturally occurring sources, like volcanos? As the journal Scientific American reports:
“Some greenhouse gases (most of the halocarbons, for example) have no natural source. For other gases, two important observations demonstrate human influence. First, the geographic differences in concentrations reveal that sources occur predominantly over land in the more heavily populated Northern Hemisphere. Second, analysis of isotopes, which can distinguish among sources of emissions, demonstrates that the majority of the increase in carbon dioxide comes from [the] combustion of fossil fuels (coal, oil and natural gas). Methane and nitrous oxide increases derive from agricultural practices and the burning of fossil fuels.” See more details on this below.
But couldn’t the sun just be producing a higher level of energy? It could. But if that were the case, scientists expect the upper levels of the atmosphere (the stratosphere) to record an increase in energy “received” as that is where any increased energy from the sun would arrive first. Instead, they find “that while the troposphere (the lower region of the atmosphere) has warmed, the stratosphere, just above it, has cooled. If solar changes provided the dominant forcing, warming would be expected in both atmospheric layers. The observed contrast, however, is just that expected from the combination of greenhouse gas increases and stratospheric ozone decreases. This collective evidence, when subjected to careful statistical analyses, provides much of the basis for the increased confidence [moving from likely to very likely (90 percent probable)] that human influences are behind the observed global warming.”
The figure below (from NASA) shows that there is variability in the solar radiation from the sun over time, but especially since 1960 the relationship between the sun and the Earth’s average temperature has decoupled or not followed a similar pattern. The temperature of the Earth has risen dramatically and the solar radiation has generally declined since 1950. Find more debate about the influence of the sun on global warming trends here.
Additionally, scientists have develop models that predict warming as certain variables are altered.
As the US National Academy of Science states (on p.5 in this source), “Calculations using climate models have been used to simulate what would have happened to global temperatures if only natural factors were influencing the climate system. These simulations yield little surface warming, or even a slight cooling, over the 20th century and into the 21st. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes.”
Next, we will look in more detail at the anthropogenic emissions and their sources from human activity.