It is not just some pundits and politicians who disagree with the theory that anthropogenic greenhouse gases are causing global warming. Some prominent scientists have also raised questions. One of the foremost is S. Fred Singer, who is most famous in the popular press for his (and co-author Dennis T. Avery’s) 2008 New York Times best seller Unstoppable Global Warming: Every 1,500 Years (16). Singer developed weather instruments for satellites, he was the first director of the National Weather Bureau’s Satellite Service Center, and he was a professor of environmental sciences at the University of Virginia. In 1990 he started a non­profit advocacy research institute, the Science & Environmental Policy Project, which identifies global warming as one of its main issues. It claims that “com­puter models forecast rapidly rising global temperatures, while data from weather satellites and balloon instruments show only slight warming.” He also began the Nongovernmental International Panel on Climate Control to rebut the conclu­sions of the IPCC reports (17). The criticisms of global warming by Fred Singer represent most of the arguments made by scientists who believe the evidence does not support the theory that man-made greenhouse gases cause global warming. Let’s look at what he says about the science.

First, he argues that climate on earth has undergone very large changes over geological time when humans weren’t around, which is clearly true. Much of ancient climate was driven by a very different arrangement of continents on earth.

About 225 million years ago (mya) the landmasses on earth were united in the supercontinent Pangaea, which began breaking up into two smaller superconti­nents—Laurasia and Gondwanaland. Laurasia consisted of what would become North America, Europe, and Asia; the southern supercontinent Gondwanaland consisted of what would eventually become the southern continents of South America, Africa, Australia, Antarctica, New Zealand, Madagascar, and India. The forces of plate tectonics began to separate Gondwanaland and move the continen­tal pieces about 170 mya. Eventually, Antarctica-Australia, Africa, Madagascar, and India separated from one another, creating the Indian Ocean. A little later, South America separated from Africa and created the South Atlantic Ocean. A seminal event for earth’s modern climate occurred about 30 to 40 mya, when Antarctica separated from Australia and migrated to the South Pole, breaking the Andean link with South America. Another major event was the rise of the Isthmus of Panama between North and South America about 3 mya. This discon­nected the Atlantic and Pacific Oceans and led to the Atlantic Ocean currents that take warm water from the Gulf of Mexico to the Arctic. The altered atmospheric circulation and precipitation patterns in the far north began modern climate pat­terns, and glaciers developed in Antarctica (18).

Two million years ago, a period of alternating Ice Ages and interglacial periods began, caused by “cycles in the earth’s relation to the sun,” according to Singer and Avery. They claim that weather patterns have undergone alterations on a roughly

1.500- year cycle over the last million years. This is based primarily on a paper in Science by Gerard Bond (19) that measured debris dropped from glacial ice into the north Atlantic seabed over the last 12,000 years and compared the variations with fluctuations in solar output. Additional evidence of cycles was obtained from Greenland ice cores analyzed by Dansgaard and Oeschger (20). The ice cores show the major ice age and interglacial climate swings but also show an approximately

2.500- year temperature cycle on top of the major swings. This smaller cycle was later changed to be about 1,500 years (plus or minus 500 years), so that is the basis of their book’s title and their principal rationale for saying that warming from greenhouse gases is irrelevant because the earth naturally undergoes fluctuations on an approximately 1,500-year cycle.

So what could cause a 1,500-year cycle? Singer and Avery try to explain it by the effect of solar activity on extragalactic cosmic rays. When solar activity is weak, more cosmic rays are able to bombard the atmosphere, ionizing air molecules and creating cloud nuclei, which would cool the earth. When the sun is more active, the extra ultraviolet rays (UV) create more ozone, which absorbs more near UV from the sun, warming the atmosphere. One problem with this rather complex theory is that there is no 1,500-year solar cycle. Surprisingly, they ignore the most prominent change in solar activity—namely the 11-year sunspot cycle— which can be readily measured and does affect temperature slightly. The majority of Singer and Avery’s book does not focus on the supposed 1,500-year cycle over the last million years but instead focuses on the last thousand or so years, specifi­cally on the Medieval Warm Period from 900 to 1300 c. e. and a two-stage Little Ice Age from 1300 to 1850. If there really is a 1,500-year cycle, then there should be another warming period beginning in about the year 2400, but we should still be in a cold phase now. But, of course, they give a 500-year fudge factor, so maybe we actually are in a new warming cycle based on their theory—or not. And, impor­tantly, measurements of solar output should be predictive of the climate. Thus, their theory can be tested, and later we will look at evidence as to whether it explains the current state of global temperatures.

Singer and Avery argue that greenhouse gases are not the cause of global warm­ing. They say that the only evidence for greenhouse gas warming is “(1) the fact that the Earth is warming, (2) a theory that doesn’t explain the warming of the past 150 years very well, and (3) some unverified computer models" They go on to give a list of things the greenhouse gas theory supposedly does not explain:

• CO2 changes do not account for the highly variable climate in the last

2.0 years.

• Greenhouse gas theory does not explain recent temperature changes in the twentieth century.

• CO2 increases have not led to planetary overheating.

• The poles should warm the most, but they do not.

• We should discount the “official” temperatures because of urban heat islands.

• The earth’s surface has warmed more than the lower atmosphere up to

30.0 feet, yet the theory says the lower atmosphere should warm first.

• CO2 has been a lagging indicator of temperature by 400 to 800 years for the past 250,000 years.

• Greenhouse gas warming should increase water vapor, but there is no evidence that it is increasing.

Another prominent scientist who is skeptical that CO2 is causing global warm­ing is Richard Lindzen, a prominent professor of meteorology at the Massachusetts Institute of Technology. He proposes that the earth’s temperature is self-regulating through an effect he calls the “iris” effect, essentially an opening in the high cirrus clouds that lets the heat out. According to his theory, when surface temperatures rise, moist air rises in the tropics, but more of it rains out than at lower tempera­tures so there is less moisture to form the ice crystals that make the high cirrus clouds. In effect, he postulates a thermostat mechanism that stabilizes temperature by tropical convection of heat high into the atmosphere, where it is radiated away. He also says that cloud formation is little understood and as oceans warm, more clouds would form, which would reflect more of the incoming solar radiation away (21, 22). While it is certainly true that clouds are not modeled very well, that does not mean that the earth is not warming. And if his theory were really true, then it should have applied in the past to prevent global warming from other mechanisms. So a good test of his theory is whether, in fact, the earth is warming or not.

Even scientists who are convinced that greenhouse gases cause current global warming do not deny that ice ages and interglacial periods have happened

repeatedly in the past and recognize that human activity had nothing to do with them. The main point at issue is whether climate changes over the last 50 years or so are caused by anthropogenic greenhouse gases resulting from burning fossil fuels and from deforestation. Singer and Avery and other global warming skeptics say that we are just having normal climate change that occurs periodically and cyclically, while most scientists say that natural factors cannot explain the recent changes.