Scientific causes of climate change: The greenhouse effect
Let us begin by examining the evidence on global temperatures. Figure 9.1 graphs global temperatures over the last 140 years, courtesy of the National Aeronautic and
Figure 9.1 Global temperatures, 1880 to present. Source: NASA Goddard Institute for Space Studies (Wikimedia Commons)
Space Administration [NASA (2006)].This figure suggests that there has been a noticeable uptick in global temperatures over time. The black dots indicate actual global mean temperatures in each year, while the solid line shows the general implied trend, while eliminating (“smoothing”) the year-to-year fluctuations.1 These data reflect two things. First, there is indeed short-term variation in global temperatures from year to year. Second, global temperatures are trending noticeably upwards. Notice in particular that since 1980 or so, global temperatures have risen by about 0.5 degree Celsius and they are on track to rise by nearly 1 degree Celsius by 2020. This is nearly 2 degrees Fahrenheit!
This recent trend coincides with a steady build-up in the atmosphere of certain types of gases that scientists call greenhouse gases (GHGs). These GHGs include carbon dioxide (C02), methane, nitrous oxides (NOx), and a few other gases. This correlation is no accident. Scientists believe that rising GHGs are responsible for rising global temperatures.The mechanism for this connection, shown in Figure 9.2, is known as the greenhouse effect.
To explain: if you have ever been in a greenhouse when the sun is shining, you may have noticed how warm it is.This is because the sunlight enters the greenhouse through the transparent roof, which traps some of the sun’s energy, heating the interior of the greenhouse. Similarly, the atmosphere of the earth traps the sun’s energy, heating the earth’s surface. Without this greenhouse effect, the earth’s surface would be frigid, like the surface of the moon.
Here’s the thing: the higher the concentration of GHGs in the atmosphere, the greater is the greenhouse effect; that is, the more heat gets trapped in the atmosphere. This is why global temperatures are rising: GHGs are accumulating in the atmosphere. Figure 9.3 shows the time trend for C02, the most common greenhouse gas.2 This figure shows that from 1958 to 2020, atmospheric C02 has increased by nearly 100 parts per million, or over 31%. Where are these GHGs coming from? Though the overall picture is complicated, scientists almost entirely agree that a key cause is the burning of fossil fuels.
Figure 9.2 The greenhouse effect. Source: Shutterstock
Figure 9.3 Atmospheric C02,1958 to 2020. Source: Keeling et al. (2001) Scripps C02 Program.