Before we evaluate that, let’s look at the modern measurements of CO 2 in the atmosphere. In the 1950s, David Keeling developed techniques to accurately mea­sure the concentration of CO2 in the atmosphere near his home in California and established a baseline concentration of 310 ppm. Later he established a laboratory on top of Mauna Loa, a volcano in Hawaii, and began taking daily measurements of CO2. One dramatic discovery was that the earth breathed in CO2 during the summer as leaves grew and consumed CO2 during the process of photosynthesis. During the fall and winter the leaves fell from the trees and released CO2 as they decayed. He measured semi-annual variations in the atmospheric concentration of CO2 reflecting the breathing earth, causing the zig-zag in Figure 1.3. Of more profound significance, he showed that the concentration of CO. in the atmo­sphere was steadily rising and not in a linear fashion but exponentially (29, 30). The concentration of atmospheric CO2 over time became known as the Keeling Curve (Figure 1.3).

One of the people profoundly affected by the Keeling Curve was Al Gore, who first heard about it when he was a student at Harvard from Professor Roger Revelle, Keeling’s scientific mentor. Al Gore subsequently made it famous to a

more general audience in his books Earth in the Balance and An Inconvenient Truth. The Keeling Curve was the prod that stimulated the scientific concern about global warming.

Now let’s look at whether temperatures really are increasing as you would expect from the increase in CO2 shown in the Keeling Curve. In a report on indi­cators of global climate change (31), the US National Oceanic and Atmospheric Administration National Climatic Data Center (NOAA/NCDC) presents data comparing atmospheric CO2 levels with global average temperatures since 1880. I have updated the figure with data through 2012, using historical CO2 data from Etheridge et al. (32) (Figure 1.4). The global temperatures have a lot of variabil­ity, but the trend to increased warming since about 1980 is indisputable, and the trend follows the increase in CO2 quite well. Skeptics such as George Will say that global warming has not occurred over the last decade (since 1998). Presumably he thinks that temperatures will increase every year if there really is global warming.

However, it is not very scientific to just say there appears to be a trend. A series of data can and should be analyzed by a mathematical process called linear regres­sion to determine if there is actually a change that is statistically valid over a cer­tain time range, and the range has to be large enough to avoid too much influence from any particular year. One cannot just cherry pick a particular year and say that there has been no warming since then, which is what George Will does. There are factors that can strongly affect the weather and temperatures in any individual

year. Volcanic eruptions eject aerosols into the atmosphere that cause cooling for a few years, depending on the size of the volcano, and the El Nino/Southern Oscillation (ENSO) causes warming in years of a strong El Nino. In contrast, years of a strong La Nina lead to global cooling.

A more detailed look at the temperature and CO2 records since 1980 will make these ideas clear. I have plotted the temperature anomalies (change in tempera­ture from the 20th century average) and the atmospheric concentration of CO2 in Figure 1.5. This graph also shows times of major ENSO events (El Nino), which cause transient warming, and the El Chichon and Mount Pinatubo volcano erup­tions in 1982 and 1991, which led to several years of cooling due to aerosols thrown into the atmosphere. 1998 was a year of a particularly strong ENSO, which is why it was an abnormally warm year. In scientific parlance, it is an outlier year. It is clear that the supposed lack of warming since 1998 is wrong, since it was slightly warmer in both 2005 and 2010. While the last decade has been the hottest ever, the five-year running average temperature has essentially been flat, probably due to natural climate variations from El Nino and La Nina events and aerosols (33) and an unusual warming of the deep ocean, which has absorbed much of the excess heat in the last decade (34).

The line in the graph is the linear regression line for temperature anomalies (the equation is in the upper right of Figure 1.5), which contains two interesting pieces of information. One is that the slope of the line gives the annual increase in temperature over this entire time period, which is 0.0153°C per year, or 0.153°C

(0.28°F) per decade, as it is usually stated. The other is the statistical correlation (given by the value of R2 = 0.79), which indicates a relatively strong correlation between the actual temperatures and an annual linear increase shown by the slope of the line. The closer the value of R2 is to 1, the more likely a straight line best describes the data.

The slope of the line can be compared to slopes of similar temperature changes over longer time periods to see if the rate of increase is changing. According to the IPCC 2007 (24), the slope over a 150-year period (ending in 2005) was 0.045°C per decade, over 100 years was 0.074°C per decade, and over 50 years was 0.128°C per decade. This means that the rate of increase in average annual temperature has been going up more in recent decades than previously (remember it has been going up by 0.153°C per decade since 1980). In other words, the actual global temperature is increasing more than linearly over the last hundred years (for the mathematically inclined, it can be best fit by a second order polynomial, not a straight line).

The annual atmospheric CO2 concentration is also plotted in Figure 1.5, which shows that CO2 is increasing similarly to temperature. In fact, CO2 is also going up more rapidly than linearly. Considering the data in Figures 1.4 and 1.5 together, it is hard not to conclude that rising CO2 levels are leading to higher global tem­peratures, especially since physics says that higher CO2 should cause warmer temperatures.

A simple but profound question to answer about the connection between CO2 and global warming is how much warming would be expected from a rapid dou­bling of CO2. The value is known as the climate sensitivity, and it is critical for evaluating what to expect as we continue to pour more CO2 into the atmosphere. Probably the best value for the climate sensitivity comes from paleoclimate stud­ies done by James Hansen. He compared the conditions during the last ice age with the recent Holocene period averaged over a millennium when the earth was in energy balance. The climate forcing from atmospheric CO2 and reflection from ice (albedo) determine the temperature difference between the two eras. This results in a calculation of 0.75°C for each W/m2 of climate forcing. Doubling CO2 from the long-term interglacial average of 280 ppm leads to a forcing of 4 W/m2, so that would mean a temperature rise of 3°C for doubled CO2. Hansen used this value to calculate the paleoclimate temperature, as in Figure 1.1, and could accu­rately model a 400,000-year range of temperatures based solely on atmospheric greenhouse gases and albedo from ice sheets. Models also suggest that climate sensitivity is 3°C (21, 35).

But, you say, how about Singer’s argument that it is actually solar irradiance, not CO2, that is causing any global warming that might be occurring and that leads to a roughly 1,500-year cycle of warming? Can the observed global warm­ing be explained by solar irradiance? The largest change in solar irradiance in the short run is the 11-year solar cycle. Satellite measurements of solar irra­diance at the top of the atmosphere (Figure 1.6) show that there is a regular 11-year (actually 10-12 year) cycle of changes in solar irradiance but no con­stant increase that would explain the continued warming of the earth (31).6 The average temperature, which jitters up and down from chaotic weather, shows lit­tle effect from the changes in solar output. In fact, the solar output was decreas­ing from 2001 to 2010, while the earth experienced the hottest decade in the last 100,000 years.

The IPCC also concludes that any radiative forcing from solar irradiance is just 0.12 watts per square meter, while the radiative forcing from CO2 is more than ten times greater (1.6 W/m2).7 Furthermore, the IPCC discusses the potential contri­bution of cosmic ray intensity to cloud cover and concludes that “the cosmic ray time series does not appear to correspond to global total cloud cover after 1991 or to global low-level cloud cover after 1994” (24). Thus, Singer’s hypothesis— that any global warming that might occur is from solar irradiance rather than CO2—does not hold up. The IPCC concludes that “most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (>90% chance) due to the observed increase in anthropogenic greenhouse gas concentra — tions.”(1) Each of the four IPCC reports have led to stronger and stronger state­ments about the likelihood that humans are causing the increased global warming as the data keep rolling in to support the statements.

Another point needs to be made about the global temperatures. Singer and other global warming skeptics raise a red herring that temperatures are not accu­rate because of “urban islands” where temperatures are hotter, thus skewing the temperature data. However, this has been taken into account in the land-based temperature records used in the official global temperatures and it makes only a very tiny correction of 0.01° C (36). Also, the “urban island effect” would not affect ocean temperatures, which are increasing very similarly to land temperatures (24).