November 2001
Vol. 31, No. 11, pp 56–58.

Table of Contents

Jay Lehr
Richard S. Bennett

It’s the Sun that makes it hot

The Sun supplies the energy to warm the Earth. The atmosphere, which is mostly transparent to the incoming sunlight, absorbs outgoing reflected or internal thermal radiation to keep the Earth warmer than it would be otherwise.

This absorptive property of the atmosphere is commonly called the greenhouse effect. Gases in the atmosphere that absorb infrared radiation, thereby preventing some of the outgoing energy from returning to space, are called greenhouse gases.

Not all gases in the atmosphere absorb outgoing IR radiation. Nitrogen and oxygen, which make up most of the Earth’s atmosphere, have no blocking effect. The gases that absorb the IR radiation and create the greenhouse effect are mainly water vapor, carbon dioxide, methane, and nitrous oxide. Water vapor and water in clouds absorb nearly 90% of the IR radiation, whereas CO2, CH4, and the other minor greenhouse gases together absorb little more than 10% of the radiation (1).

Therefore, most of the greenhouse effect is natural and caused by the different forms of water in the atmosphere. However, human activities over the past 100 years, such as burning wood, coal, oil, and natural gas, have increased CO2 alone by more than 50%, according to the George C. Marshall Institute, which has supported global climate change research for many years (1). Many studies project that the amount of CO2 in the atmosphere from human activities will double in the next 100 years.

The average global temperature has increased ~0.6 °C during the past 120 years. Much of the observed temperature rise occurred before 1940, whereas most (>80%) of the additional CO2 entered the atmosphere after 1940. Increased greenhouse gases cannot explain a temperature rise that occurred before the major increases in these gases existed in the atmosphere. Furthermore, from 1940 to 1970, CO2 built up rapidly in the atmosphere, and according to computer climate projections, the Earth’s temperature should also have risen rapidly. Instead, the temperature dropped.

Climate records of the past 100 years provide no support for the idea that human activities, such as burning coal and oil for energy, caused the early 20th-century global warming. Natural factors must have caused most of that warming.

Lack of correlation
When scientists analyzed ice cores from Greenland and Antarctica to determine the relationship between atmospheric CO2 levels and temperatures dating back 250,000 years, they found that sometimes the concentration of CO2 was high when the temperature was low, and sometimes the CO2 was low when the temperature was high (2). Moreover, a careful analysis showed that some of the atmospheric CO2 changes did not precede the temperature changes, as the greenhouse warming theory would predict. Instead, changes in atmospheric CO2 followed the temperature changes. The atmospheric CO2 changes were not the cause of the temperature changes but were likely driven by changes in vegetation in response to natural variations in air and sea-surface temperatures.

During the past 10,000 years, the climate has remained relatively warm and stable, allowing humans to advance and thrive. But even during this generally warm period, the temperature has fluctuated significantly. During the Holocene Climate Optimum, ~6500 years ago, the climate was warmer than it is today. Evidence shows that ~1000 years ago, during the Medieval Climate Optimum, regions of the Earth were again substantially warmer than they are today. By the 14th century, a cold period called the Little Ice Age had begun. The warming that began in the late 19th and early 20th centuries seems to be a natural recovery from the Little Ice Age (3).

Closer to the present, some researchers believe that the 1980s decade was the hottest in 100 years, and that some years in the 1990s may have been even hotter. Actually, more local U.S. temperature records were set in the 1930s than during any other decade in the past century. However, the recent trend is for surface temperatures to change no more than 0.1–0.2 °C every one or two decades. Such a change is well within the range of the climate’s natural variations, and climatic change mechanisms are not all understood. It is safe to conclude that the natural climatic variability adds confusion to the effort to diagnose human-induced climate change. Apparent long-term trends can be artificially amplified or damped by the contaminating effects of undiagnosed natural variations (4).

Climate and the Sun
One natural factor attributable to climate change may be that the brightness of the Sun varies over decades or centuries. Brightness variations are in step with the sunspot cycle, a series of changes in the Sun’s magnetism that have a period of ~11 years. Climate models suggest that changes of ~0.5% in the Sun’s brightness would produce global average temperature changes of ~0.5 °C over a century or so (5).

In defining the tremendous impact the Sun has on climate, we must understand the actual movement of the Earth around the Sun. Three variables—orbit shape, tilt, and wobble—profoundly affect weather patterns. The Earth’s orbit is not a circle, but an ellipse, in which one end is farther from the Sun than the other. In a 100,000-year cycle, the tug of other planets on the Earth causes its orbit to change shape. It shifts from a short, broad ellipse that keeps the Earth closer to the Sun to a long, flat ellipse that allows it to move farther from the Sun and back again.

While the Earth travels in its orbit, it also spins around an axis that tilts lower then higher over a 41,000-year cycle. Close to the poles, the contrast between winter and summer temperatures is greatest when the angle of the tilt is large. The Earth also wobbles because it is spinning on its axis, tilting back and forth. Thus a temperature drop in the Northern Hemisphere occurs when the planet tilts away from the Sun; the Southern Hemisphere cools as the tilt changes, and the cooling trend moves north again over a 22,000-year cycle. This means that ~11,000 years from now, the northern midwinter will fall in July instead of January, and the glaciers may return in full force. It also means that summer temperatures peak in the tropics twice as often as the concentrated heat of the Sun passes back and forth across the equator.

The Sun drives the climate. Even if the Sun’s energy output did not vary as it does, the amount of sunlight reaching different areas of the Earth would still change because of the way the Earth moves around the Sun. Climate drives the ebb and flow of glaciers and vegetation: Ice sheets spread and shrink within the 100,000-year cycle of orbital change. Glaciers dominate the land for 60,000–90,000 years during the cold phase of the cycle, and they all but disappear during the warm phase.

The Milankovitch cycle
A Yugoslavian astrophysicist, Milutin Milankovitch, discovered the connection between Earth’s cycles of rotation and climate—thus it is called the Milankovitch cycle (6). Short, warm gaps, or interglacials, break up glacial ages and occur when summer temperatures rise in the north.

The last interglacial ended ~122,000 years ago. The interglacial in which we are living, the Holocene epoch, began ~10,000 years ago and is approximately half over. The warming trend that melted the glaciers started much earlier, but it peaked ~8000 years ago.

Because many forces influence climate, temperatures do not rise and fall uniformly. Small cycles occur simultaneously with larger ones, producing cold or warm periods. Cycles in the energy output of the Sun and shifting ocean currents contribute to these swings in the Earth’s temperature. The changes in the Sun’s radiation are particularly important and have been known for a decade.

The almost perfect correlation between the Sun’s magnetic activity and the Earth’s temperature is too close to be readily dismissed as a coincidence (7). The magnetic activity is caused by strong magnetic fields that erupt on the Sun’s surface visible to us through high-power telescopes as sunspots, in addition to bursts of energetic particles and radiation. The changes in the surface magnetic fields do not by themselves transfer enough energy to the Earth and its atmosphere to have a direct impact on climate. However, satellite observations of the Sun have shown that when its surface magnetic activity goes up, its energy output increases; when the surface magnetic activity diminishes, its energy output decreases. It is safe to conclude that energy output from the Sun is the major factor in changes in global temperature.

Temperature measurements
Until satellite measurements began in 1979, there was no accurate method to measure temperatures from all parts of the globe. Readings were taken mainly at land-based points; ocean, rainforest, and mountain surface temperatures were simply not available. Weather stations were established for the most part in cities and, starting in the 1920s, at airports.

Unfortunately, as cities grew and expanded and air conditioning and paved areas became more common, “heat islands” appeared. Urban temperature readings rose, especially in cities like Phoenix, where the heat exhausted from cooling commercial, industrial, and residential buildings attained such high levels that airport temperatures reached historic highs. Rural areas, however, did not experience the same temperature increases.

Weather balloons provided the first advance beyond fixed-temperature measuring stations. But balloons had limitations: They were quite expensive, their height or direction of flight couldn’t be controlled, and storms or crash landings at sea often destroyed their instruments.

In 1979, the Tyros satellite changed all that. For the first time, temperature readings could be taken around the globe, and could be adjusted for altitude—from a few feet above sea level over the oceans to the top of the Himalayas. Critics have charged that satellite readings are not accurate, but weather balloon observations correspond so closely with them that this argument is no longer used. Another criticism was that the satellite readings did not allow for orbital decay, but this has been corrected.

Satellite readings have been available for >21 years. Although that length of time is much too short to establish any trend in climate change, the readings do show that during the 21 years, the Earth temperature change was +0.03 °C/decade, less than the 0.05 °C/decade increase of the previous 100 years. Climatologists generally agree that the temperature of the Earth has increased by 0.6 °C since 1880 and that most of this increase occurred before 1940.

Computer simulations of the Earth’s climate are known to contain huge uncertainties. We know from physics that the rate of energy added to the climatic system by the doubling of atmospheric CO2 is about 4 W/m2—a small amount compared to solar radiation of 342 W/m2 at the top of the troposphere. But 4 W/m2 is also small compared to the uncertainties in the climate change calculations. For example, measurement of the amount of energy flowing from the equator to the poles is uncertain by ±25–30 W/m2. The amount of sunlight absorbed by the atmosphere or reflected by the surface is also uncertain by as much as ±25 W/m2. Some computer models include adjustments to the energy flows of as much as 100 W/m2. Imprecise treatment of clouds may introduce another 25 W/m2 of uncertainty into the basic computations (8).

These uncertainties in modeling climate processes are many times larger than the 4 W/m2 input of energy resulting from a doubling of CO2 concentration in the atmosphere. It is difficult to see how the climate impact of the 4 W/m2 can be accurately calculated in the face of such huge uncertainties. As a consequence, current forecasts based on the computer simulations of climate may not even be meaningful. A comparison of nearly all of the most sophisticated climatic models with actual measurements of current climate conditions found the models in error by ~100% for cloud cover, 50% during precipitation, and 30% in temperature change. In addition, even the best models give temperature change results differing by a factor of 2 or more (9).

The NAS report
The National Academy of Sciences (NAS) Report on Global Climate Change, issued in June 2001, has added considerably to the debate (10). The report, which the news media trumpeted as having confirmed global warming, resolved very little. What it reported as fact—that global temperatures as measured by land-based thermometers have risen in the past 20 years—was already known and not a point of contention.

What the report did not conclude was that global warming was the result of human activity. It actually concluded in a passage not quoted in the press that “A causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established” (10, p 1 of the summary).

The NAS report also points out that current warming could be completely natural. The scientist-authors state that because of inadequacies in the global warming computer simulation models, they cannot tell for sure whether this warming is anything more than natural climate variation:

    The fact that the magnitude of observed warming is large compared to natural variability as simulated in climate models . . . does not constitute proof of a linkage [to increases in greenhouse gases] because the model simulations could be deficient (10, p 2 of the summary).

The study emphasizes that land-based measurements of global temperatures during the past 20 years are inconsistent with satellite readings, which have shown an increase of only 0.04 °C/decade. The report states that “satellite measurements beginning in 1979 show little warming of air temperature in the troposphere.” This is significant because the global warming hypothesis predicts that satellite measurements should show warming in the upper atmosphere prior to surface warming. The NAS authors note that they have no explanation for this apparent contradiction with theory, stating, “The finding that surface and troposphere temperature trends have been as different as observed over intervals as long as a decade or two is difficult to reconcile with our current understanding of the processes that control the vertical distribution of temperature in the atmosphere” (10, p 17).

This evidence appears to directly contradict the global warming hypothesis and suggests that surface warming is not caused by human activities. Of the three kinds of measurements of global temperatures (satellite, land-based, and weather balloon), only the land-based data show any significant warming.

Even assuming the worst-case scenario, delaying substantial cuts in CO2 emissions for the next 25 years would produce an additional global temperature rise of no more than a few tenths of a degree by 2100 (11). This means we have at least 25 years in which to sharpen our understanding of climate and seek valid predictions, without contributing to serious climate change. An incremental warming of a few tenths of a degree, spread over decades, constitutes no hazard while we seek important additional information to build a foundation for national and worldwide energy policy. Policies made in haste or based on poor information are likely to adversely affect the economies and citizens of the United States and the world. Rather, we should foster efficient use of our resources to meet the world’s needs with minimal environmental impact.

The predictions of severe warming over the coming century are based on developing climatic models that have so far failed to simulate history accurately, let alone predict the future. The satellite record of temperatures in the lower at mosphere (troposphere) since 1979 shows an increase of 0.03 °C/decade, whereas models predicted the fastest warming in the troposphere. The models also utterly fail to accurately simulate the transition into and out of the ice ages for the past few hundred thousand years.

Continued investment in climate science is necessary to unlock the mysteries of what really makes our climate tick. Understanding and predicting our weather offer many benefits. We should also plan for the inevitable natural changes in climate. We need to invest in agricultural adaptability and water management. We should also recognize that many scientific studies document that high CO2 concentrations improve agricultural yields. It is time to sit back calmly and ignore the volatile rhetoric of politicians and environmental zealots, while we continue to gather better data, create better models, and act slowly so as to do no harm.


  1. A Guide To Global Warming; George C. Marshall Institute: Washington, DC, 2000.
  2. Fischer, H.; Wahlen, M. Science 1999, 283, 1712–1714.
  3. Lamb, H. H. Climate, History and the Modern World, 2nd ed.; Methuen: New York, 1995.
  4. Mahlman, J. D. Science 1997, 278, 1416–1417.
  5. Soon, W. H.; Posmentier, E. S.; Baliunas, S. Astrophys. J. 1996, 472, 891–902.
  6. Imbrie, J.; Imbrie, K. P. Ice Ages: Solving the Mystery; Enslow Publishers: Short Hills, NJ, 1986.
  7. Raymond, D. Science 1993, 259, 926.
  8. Cess, R. D.; Zhang, M. H.; Minnis, P.; Corsetti, L.; Dutton, E. G. Science 1995, 267, 496–499.
  9. Barnett, T. P. J. Climate 1999, 12, 511–515.
  10. National Academy of Sciences. Report on Global Climate Change; National Academy Press: Washington, DC, 2001.
  11. Wigley, T.M.L.; Richels, R.; Edmonds, J. A. Nature 1996, 379, 240–243.

Jay Lehr ( is the science director of the Heartland Institute (Chicago) and editor of McGraw-Hill’s new Standard Handbook of Environmental Science, Health & Technology.

Richard S. Bennett ( is president of the Society of Environmental Truth in Corpus Christi, TX.

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