Now let’s look at a different time scale to see how things have changed over the last

10,0 years. This 10,000-year period is known as the Holocene; the climate was generally stable, with little fluctuation in temperature, and it spawned the dawn of civilization and agriculture. It includes the time known as the Medieval Optimum or Medieval Warm Period (800 to 1300 c. e.), when the Vikings settled Greenland, but also the Little Ice Age (1300 to 1800 c. e.)4 in Europe (27). Both of these events were probably caused at least partly by changes in solar intensity. A paucity of sunspots reflecting reduced solar output—known as the Maunder Minimum— occurred between 1650 and 1715 during the coldest part of the Little Ice Age (18). There were only small changes in concentrations of greenhouse gases in the last

10,0 years until roughly the last 100 years, when all of them have increased very rapidly (Figure 1.2). The inset shows changes since the industrial period began in 1750. Temperature and greenhouse gas measurements are also more precise dur­ing this latter period.

Carbon dioxide is the greenhouse gas that is of most concern, since it is in the highest concentration and is increasing most rapidly by burning fossil fuels and deforestation, so we will focus our attention on it now. While Figure 1.2 shows a dramatic increase in CO2 (top graph), methane (middle graph), and nitrous oxide (bottom graph) in the last 50 years, it does not give any indication of the global average temperatures. It does show a scale that indicates radiative forcing. This is a “measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the earth-atmosphere system and is an index of the impor­tance of the factor as a potential climate change mechanism. Positive forcing tends to warm the atmosphere while negative forcing tends to cool it.”(1) Radiative forc­ing is given in units of energy rate (power) per area (watts per square meter, or W/ m2) at the tropopause.5 (See Appendix A for more information on radiative forc­ing.) What this means is that CO2 is adding a positive radiative forcing (1.66 W/ m2) to the atmosphere that should contribute to global warming. So does it, or are the skeptics right when they say that global temperature is not actually increasing?