So, let’s look at some of the evidence. To really understand the science behind climate change, it is necessary to look at actual data in graphical form. This may be intimidating to some readers—even many of the students in my class have trouble following graphs—but I encourage you to study the figures carefully, and I will lead you through them in the figure captions and the text. Much of the evidence cited here is obtained from the 2007 IPCC Fourth Assessment Report, the latest IPCC consensus report of over 2,000 scientists that references over

6.0 peer-reviewed scientific publications, as well as data from the US National Oceanic and Atmospheric Administration/National Climatic Data Center (NOAA/NCDC). A mini-scandal broke out in 2010 when hackers broke into computers of leading climate scientists and published e-mails and documents that purported to show that the scientists were manipulating their data to exaggerate the case for global warming. However, five different investigations exonerated the scientists of misconduct (23).

First, let’s look at the record of temperature and greenhouse gases over hun­dreds of thousands of years. How is that possible? When snowflakes fall, they form layers with air trapped in them. In areas such as Antarctica and Greenland, the snow compresses into ice that contains bubbles of air with the constituents of the atmosphere at the time the snow fell. Each year a new layer of ice forms, with a new record of the atmosphere. Cores taken from ice sheets go back for

125.0 years in Greenland and 800,000 years in Antarctica (10). The gases in the bubbles from the ice cores can be analyzed to get a yearly record of the atmo­spheric composition (18). Temperature can also be inferred from these ice cores by measuring the amount of deuterium, an isotope of hydrogen.3

The graphs in Figure 1.1 showhowthe main greenhouse gases (CO2, methane, and nitrous oxide) and temperature have varied over 650,000 years (24). While a temperature scale is not given, modern temperature is about 6°C higher than the average during ice ages (10). It is worth noting that if there is a 1,500-year cycle in temperature, it is a very small effect compared to the large temperature changes over much longer time periods. The shaded areas are interglacial periods—times when the earth is warm and glaciers have melted. There are several important points to be taken from this figure.

First, the greenhouse gases are all at high relative concentrations during the interglacial periods. Second, the concentration of greenhouse gases and the tem­perature rise much more rapidly at the beginning of interglacial periods than they fall as a prelude to a glacial period. Third, although it is not obvious from the graph because of the time scale, a more detailed analysis shows that the concen­tration of CO2 actually lags the temperature by several hundred years. However, recent evidence indicates that the rise of CO2 led the rise in Northern Hemisphere temperatures and the melting of the ice sheets at the end of the last ice age about

20.0 years ago (25). Fourth, there is no precedent during the entire preceding

650.0 years for the dramatic increases in the greenhouse gases in the present age. The concentration of CO2 in previous interglacial periods was about 280 ppm (parts per million), but currently (July 2013) it is at 397 ppm (26).

Clearly, humans did not cause these changes over hundreds of thousands of years, so what did? These cycles of ice ages and warming periods were triggered by cyclical changes in the earth’s tilt, coupled with changes in the shape of the elliptical orbit (eccentricity) of the earth around the sun (the Milankovitch cycle) (15, 18, 27). The earth is currently tilted at 23.4° relative to the plane of its orbit around the sun but this varies from 22.1° to 24.5° in a 41,000-year cycle. This tilt is the primary cause of the seasons in the Northern and Southern Hemispheres. The tilt also precesses like the wobble of a top in a cycle of 23,000 years. Because of the elliptical orbit of the earth around the sun, the earth is sometimes closer and sometimes further away from the sun, which also affects the seasons. The shape of the ellipse changes from more circular to more elliptical in a 100,000-year cycle. Combinations of these cycles affect how much sun the northern and southern latitudes get, and this determines whether snow builds up and forms ice sheets (ice ages) or melts and glaciers recede (18). Looking back at Figure 1.1, the inter­glacial warm periods occur at roughly 100,000-year intervals, indicating that the principal effect is the change in ellipticity of the earth’s orbit (28). Any 1,500-year cycle is a small blip on these large changes.

But why does CO2 follow the temperature changes? The precise details are not clear, but absorption of CO2 in the oceans is the most prominent factor. The upper layer of the ocean contains a similar amount of CO2 as the atmosphere, about 800 Gt (10). Cold ocean water absorbs more CO2, and warmer ocean water releases CO2. (This effect is exactly like the difference between opening a cold can of soda and a warm can of soda. The warm soda can will likely overflow when the can is opened because of the rapid release of CO2, while the cold soda does not release much CO2 and does not overflow.) This results in a positive feedback mechanism whereby warming begins by greater solar exposure due to changes in the earth’s orbit, releasing CO2 from the oceans, which causes more warming, which releases more CO2 in a positive feedback loop. It is entirely expected that there would be a lag of several hundred years because it takes a long time for the vast oceans to warm up and begin releasing more CO2 . But the melting at the end of the last ice age was preceded by an increase in CO2 that helped to warm the Northern Hemisphere and melt the ice sheets, so we cannot take comfort in thinking that rising CO2 has no effect on warming.

Another positive feedback is related to the reflection of sunlight from ice sheets, known as albedo. As ice sheets over continents melt, less sunlight is reflected by ice and more sunlight is absorbed by exposed soil, rocks, and vegetation, which causes more warming. These changes in albedo and the release of CO2 from oceans are the main reasons that the warming periods are much faster than the cooling periods, though still taking hundreds to thousands of years. The most striking thing about Figure 1.1 is the very rapid rise in the concentrations of greenhouse gases in the modern era, which is unprecedented in historical times.