Global warming "skeptics" are a hanful of industry-sponsored advocates who have loudly proclaimed that global warming is a hoax, and that average global temperatures are not rising.  

"I don't believe there is any conclusive evidence of global warming. And I certainly don't believe that it can be attributed to human activity – and particularly not by activity by the United States. That is the political agenda behind the global warming scare. It is an anti-West, anti-U.S., anti-free enterprise movement." Rush Limbaugh

The oil, coal, gas, and mining industries stand to lose enormous sums if the public understands the truth about global warming. Just as the tobacco industry invested millions in keeping its deadly secret, so also have the oil, coal, gas, and mining industries attempted to hide and discredit the link between CO₂ emissions and a warming earth. (In fact, many of today’s global warming skeptics were tobacco-cancer deniers in previous years.) Fossil fuel interests and conservative advocacy groups have funded, promoted, and used as witnesses a handful of greenhouse skeptics, who have widely and loudly proclaimed that global warming is a myth. 

“Climate change is 'a myth', sea levels are not rising and Britain's chief scientist is 'an embarrassment' for believing catastrophe is inevitable.” These are the controversial views of a new London-based think-tank that will publish a report on the heels of the Report on Arctic Warming, attacking the view that man-made greenhouse gases will warm the planet.  The think tank has received cash donations from the US oil giant ExxonMobil, which has long lobbied against the climate change agenda. Exxon lists the donation as part of its 'climate change outreach' program’ “ The Observer.

"Greenhouse ‘skeptics’ subvert the scientific process, ceasing to act as objective scientists, rather presenting only one side, as if they were lawyers hired to defend a particular viewpoint.” says James Hanson of the Goddard Institute for Spaces Studies. The skeptics avoid the scientific arena of peer-reviewed literature in order to promote unsubstantiated and unsupported guesses about the nature of the global climate.

"The (NAS) report states that the temperature on Earth, again, let me state this, may or may not be, may or may not be, 1 degree warmer than it was 100 years ago. One degree change over 100 years. Think about that. These experts cannot predict the weather one day in advance. How can they predict and calculate and analyze the weather back 100 years ago, when they did not have any of the scientific equipment that was available to them, that is available to them today? Remember, 100 years ago they did not have any satellites; they did not even have telephone communications in most of the world. But across the face of this planet, that it was cooler then by a whole 1 degree? Can anyone listen to that with a straight face? Give me a break."      Rep. Dana Rohrabacher, R-C

Rather than advancing a theory of climate, they simply claim that warming isn’t happening, if it is, it will be minimal, if it isn’t minimal it will be beneficial, and if it isn’t beneficial, it will be entirely natural in origin.

Nearly every mechanism that the skeptics have proposed to explain the global climate has been discredited in scientific review. And nearly every day, new research lends further weight to the gradual accumulation of evidence that suggests that humans are affecting the global climate.

The skeptics have been wrong over and over again.  Many of the leading CO₂ skeptics got their start as ozone layer skeptics, and a few still maintain this position, even in the face of overwhelming scientific evidence linking ozone depletion to man-made chemicals in the atmosphere. Here’s Sallie Baliunas' confident assertion that "the ozone hole cannot occur in the Arctic". 

Only a few weeks after Baliunas testified before Congress that the science on ozone depletion was unsettled, the  Nobel Prize for Chemistry was awarded to Paul Crutzen, Sherwood Rowland, and Mario Molina for their work on stratospheric ozone. Rowland and Molina were explicitly cited for proposing the CFC-ozone depletion theory. Shortly thereafter, news reports  announced that "A large hole has opened up in the ozone layer over the Arctic, which scientists believe will severely damage the natural shield protecting the northern hemisphere from cancer-causing sunlight."

Conservatives constantly criticize environmental advocates for alarmism--by which they mean the exaggeration of various risks and potential outcomes. Let’s say that melting ice caps, mass extinctions, and rising sea levels aren’t alarming enough to justify being alarmed by the possibility of such a future. Misusing and misrepresenting science, I would argue, is far more disturbing than judging such outcomes to be alarming. Artificially manufacturing "skeptical science" to raise doubts about mainstream science, in order to promote political influence and protect industry profits, is even more disgraceful.

Alaska's lone congressman, Republican Rep. Don Young, went so far as dismissing the major new report on Arctic climate change. He called it “ammunition for fear mongers”. "I don't believe it is our fault. That's my opinion," Young said. "It's as sound as any scientist's."

The skeptics are yet another group of paranoid pseudoscientific cranks inventing a conspiracy to suppress science that contradicts their political agenda. It's the same technique used by the creationists, who dress up their arrogant superstitions in scientific lingo and call it Intelligent Design. 

The “skeptics” are advocates for a political ideology, lobbying for a policy agenda that promotes the interests of fossil fuel industries.  

Skepticism is a good thing. Science is a process of challenge and questioning. However, the “skeptics” are engaging in a misuse and politicization of science by refusing to acknowledge inconvenient data, and a narrow-minded self-serving interpretation of the research. This is not skepticism. This is an irresponsible, politically motivated hoax masquerading as science. 

As evidence has mounted against them, the skeptics have tried to offer some explanation for global warming other than human causes. 

Solar variation, volcano’s, increasing plant growth, cosmic rays, the “urban-heat island” effect, satellite discrepancies, among other discredited assumptions, are offered to explain the observed climate changes. These explanations are still clung to by many of the skeptics, long after they have been put into the trash heap of crackpot theories by the progress of scientific inquiry.

SATELLITE DISCREPANCIES EXPLAINED

Until recently, conflicting studies suggested that temperatures measured by satellites revealed a slight cooling trend, whereas the surface temperature record showed a warming trend. This confounded the global warming issue, giving ammunition to the skeptic's global warming denial.

Satellites began measurements in 1979 and the surface temperature record, which reveals global warming of about 1.1°F, dates back to about 1860. Satellites take temperatures through vertical slices of the atmosphere, not at the surface. Satellite measurements are also known to be influenced by ozone layer depletion, which has caused cooling of the upper atmosphere. For these and other reasons, satellite and surface data are not expected to be a perfect match. Nevertheless, the discrepancy between the two data sets was too large to be ignored.

Then scientists discovered they neglected some measurement and calibration problems with the satellites, including the fact that satellites were falling from their orbits, which produced an artificial cooling trend. Correcting the satellite data for these problems reveals a small warming trend. These corrections bring the satellite record into better agreement with surface measurements. Any remaining discrepancies do not invalidate the fact that surface temperatures are rising.
 

Solar variations alone cannot explain the current warming. 

 "The likeliest cause of current climate trends seems to be solar activity, perhaps in combination with galactic cosmic rays caused by supernovas...Dr. Thomas Sieger Derr "A Biblical Perspective on Environmental Stewardship"

There is no doubt that solar variability plays an important role in global climate change.  However, it has also been found that the amount of solar radiation that hits the earth is closely related to parameters that can be recorded from ground level. Since some of these provide records stretching back several hundred years, it is possible to reconstruct solar activity with quite high resolution over these time spans. 

These reconstructions of solar activity have allowed scientists to investigate the relative impact of solar activity and greenhouse gases on current climate change. Several studies have investigated the relationship between changes in solar activity, volcanoes and temperatures over the past 1000 years and have concluded that, although changes in solar activity can explain much of the temperature changes, there is a 'residual' unexplained temperature increase in the 20th century. For example, Mann et al concludes that "While the natural (solar and volcanic) forcings appear to be important factors governing the natural variations of temperatures in past centuries, only human greenhouse gas forcing alone ... can statistically explain the unusual warmth of the past few decades". A similar analysis by Crowley et al concludes that there is "a very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.”

The temperature changes in this century have been studied using sophisticated statistical methods. Such an analysis is presented by Thomas et al (1997), who show that changes in CO₂ over the last century have had around three times the

impact on temperature as have changes in solar irradiance, and that there is no evidence in the statistics of any major unidentified source of natural variation.

Another, more recent, analysis (Kaufmann  & Stern, 2002) found that temperature changes could only be explained by taking into account changes in solar activity, sulphate aerosols and greenhouse gases.

The detailed causes of the recent warming trend have been investigated by the UK Meteorological Office, and are presented here (see also Stott et al, 2000). They found that about half of the warming is caused by solar variability but that, in the second half of the century, these effects have been countered by sulphate emissions from dirty fuel and from volcanoes (which contributed to a global cooling observed in the 1960s and 1970s). The overall effect of all the natural causes (sun and volcanoes combined) has been quite small. They conclude that there is "very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.”

Similarly, two recent studies of ocean temperatures have found that the observed increase is best explained by the effect of greenhouse gases.

Dr. Judith Lean, of the Naval Research Laboratory's Space Science Division, and Dr. Jerry Mahlman, Director of NOAA's Geophysical Fluid Dynamics Laboratory presented evidence that changes in solar radiation output alone cannot explain the global warming that has taken place in the last century.  They also said that the recent warming trend could be explained by a combination of greenhouse gases increase, natural variability, and increased solar irradiance. 

In the short term, volcanoes exert a net cooling effect at the surface due to their emissions of sulphur dioxide, because particles in the stratosphere absorb the sun's energy before it can reach the surface.  Volcanoes do also emit CO₂, and massive eruptions in the past may have emitted enough CO₂ to cause long-term climate change. However, in the recent climatic record, volcanic emissions have been much lower. Gerlach (1991) estimated a total global release of 3-4 x 10¹² mol/yr from volcanoes. Man-made (anthropogenic) CO₂ emissions overwhelm this estimate by at least 150 times.

The Little Ice Age and the Medieval Warm Period were local, not global climate phenomenon.

The skeptics frequently point to historical periods of temperature extremes as evidence that the current warming of the climate is nothing unusual. In the northern hemisphere, there seems to have been a more pronounced warm period early in the millennium (the 'Medieval Warm Period'), and cool period in the latter part of the millennium ('Little Ice Age').

These do not appear to have been global events (see this review), and instead are likely to reflect periodic changes in the North Atlantic Oscillation, and changes in solar activity.

Recent research points to a possible human influence on these climate periods as well. Human influence of the Earth's climate did not begin just a few decades or centuries ago, but 8000 years ago, with the birth of agriculture. William Ruddiman, a climate scientist at the University of Virginia in Charlottesville, started to suspect that ancient human activities have affected the climate when he noticed a telltale discrepancy in levels of greenhouse gases revealed by ice cores. After ruling out possible natural causes for the greenhouse gas increases, Ruddiman now thinks that early farmers clearing forests in Europe, India and China account for the surge of carbon dioxide, while rice paddies and burgeoning herds of livestock produced the extra methane.

Intriguingly, Ruddiman thinks the anomalous cooling of the "little ice age" that gripped the world for several centuries from around 1300 was caused by a specific setback to agriculture: plague. He notes that pandemics of bubonic plague depopulated Eurasia during those same centuries. Fields and villages were abandoned and reclaimed by fast-growing forests that sucked carbon dioxide out of the atmosphere, resulting in the cooler temperatures felt worldwide. This completely reverses the widely held idea that it was the little ice age that caused the famine, depopulation and disease.

For more information on the Medieval Warm Period, see this article from the NOAA Paleoclimatology Program.

The skeptics are fond of asserting that increasing levels of CO₂ will be absorbed by increasing plant growth brought on by warmer weather.

Satellite observations suggest that plant growth has been increasing, although it is thought that this is mainly due to rising temperatures and increased rainfall, rather than a direct effect of increased CO₂ (see this article from the Earth Observatory). Whatever the cause, the resultant increases in plant mass captures some CO₂ from the atmosphere. The effect of this can be seen in assessments in carbon balance – the natural world is already absorbing more than it emits, and predictions of climate change are based on the assumption that this effect will continue.

Unfortunately, there are several problems with the idea that increased plant growth will absorb the excess CO₂:

• Firstly, there is simply too much CO₂. Data from Mauna Loa show that carbon dioxide levels have steadily increased throughout the century. Studies have shown that the ability of trees to soak up excess carbon is limited by the availability of other nutrients, as discussed in this article from National Geographic.
• Secondly, there's more to greenhouse gas than CO₂, as this link shows. Methane, a very important greenhouse gas, has doubled in concentration since the start of the industrial age.
• Thirdly, we are actively reducing biomass by cutting down forests and draining marshland – deforestation in the 1980s was responsible for around 1.6 to 1.8 GtC emissions, according a report from the Edinburgh Centre for Carbon Management. In the 1990s, changing land use meant that deforestation has been balanced by reforestation of disused farmland, and there has been a net absorption of 1.4 billion tons. However, as these new forests mature, it is predicted that the balance will shift once again to a net loss of CO₂ to the atmosphere. Fourthly, although forests contain more CO₂ than grassland, they are darker. This means that they absorb more heat. Scientists from the USA's Lawrence Livermore Laboratories report that deforestation likely played an important part in the global cooling that took part over the last millennium.
• Finally, and most importantly, climate change is likely to have important effects on the carbon cycle. For example, as the oceans warm they will release CO₂, and as the permafrost thaws it will start to decay, releasing large amounts of CO₂. Peat marshes, too, are likely to release increasing amounts of CO₂ as they warm (see this news report). Although the exact effects of these changes are hard to calculate, the current estimate is CO₂ that the net effect will be to increase global warming by a further 3ºC over the next 100 years – making the total increase up to 8ºC.

Long a staple of the skeptics unending denial of global warming, the “urban-heat island effect” claims that urban development has skewed thermometer readings of global temperature.

The effects of these 'urban heat islands' are calculated by comparing changes in urban stations with nearby rural stations, and the records from the urban stations are then reduced accordingly. A description of this adjustment is provided in this article from NASA's Earth Observatory.  The overall effect is that the rate at which the temperature of heat islands has increased is not much greater than the background rate.

A recent study suggests that even this small effect is an overestimate. A recent paper (Peterson; 2003) reporting that "Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures." 

Global warming is not an artifact of incomplete coverage

 At the beginning of the 20th century, thermometer measurements covered only 20 percent of the Earth's surface, compared to more than 87 percent in 1987. The skeptics have argued that the gradual increase in coverage during the 20th century introduced an artificial warming trend into the temperature record, which accounts for most or all of the 20th century's measured warming. A study from Lawrence Livermore found no evidence to support the hypothesis that incomplete observational data has caused us to overestimate the true warming trend.

Satellite temperature data confirm other evidence of global warming

Until recently, conflicting studies suggested that temperatures measured by satellites revealed a slight cooling trend, whereas the surface temperature record showed a warming trend. This was loudly promoted by the skeptics as evidence that global warming was not happening.

Satellites began measurements in 1979 and the surface temperature record, which reveals global warming of about 1.1°F, dates back to about 1860. Satellites take temperatures through vertical slices of the atmosphere, not at the surface. Satellite measurements are also known to be influenced by ozone layer depletion, which has caused cooling of the upper atmosphere. For these and other reasons, satellite and surface data are not expected to be a perfect match. Any remaining discrepancies do not invalidate the fact that surface temperatures are rising.

There are currently two reconstructions that are generally considered to be potentially accurate. The most well known is that of Christy and Spencer, first published in 1996. This caused a great deal of controversy, since it contradicted the predictions from climate models. Recently, Christy has revised his data, showing not a cooling, but a warming trend in the data. More recently, in 2003, Mears and Wentz have reanalysized the data using a different technique. Their results suggest that the atmosphere has warmed in line with model predictions.
 
Scientists discovered they neglected some measurement and calibration problems with the satellites, including the fact that satellites were falling from their orbits, which produced an artificial cooling trend. Correcting the satellite data for these problems reveals a warming trend. These bring the satellite record into better agreement with surface measurements.

In a new study published Dec. 15, 2004 in the Journal of Climate, Fu and Johanson used direct stratosphere temperature measurements to examine the contamination from the stratosphere in the satellite channel that measures troposphere temperatures, indicating the troposphere has been warming at about two-tenths of a degree Celsius per decade. That closely resembles measurements of warming at the surface; something climate models have suggested would result if the warmer surface temperatures were the result of greenhouse gases.

The findings are important because, for years, satellite data inconsistent with warming at the surface have fueled the debate about whether climate change is actually occurring.

The skeptics have long argued that a few degrees temperature increase won't matter much, and besides, warmer is better: longer growing seasons, lower heating bills. How many people actually notice the difference between 86 and 88.5°F? Considering that in some regions people experience large daily temperature ranges (20-30°F), climate skeptics try to convince the public that global warming by a few degrees is nothing to worry about.

This is a case of deliberately confusing weather and climate. The daily and seasonal temperature changes are highly variable, but the average global temperatures have been remarkably stable. The difference in global average temperature between the last ice age and the present is only 9°F. This puts the projected range of climate change-related global average temperature increases of 2.5-10.4°F in an entirely different light.

Humans generate an insignificant 4.5% of the world's greenhouse gases is when compared to the 95.5% generated by nature. False

It is indeed true that human emissions of CO₂ are a small percentage of the total carbon cycled through the different components of the Earth system: plants, soils, the oceans, and the air. But these human emissions are by no means insignificant. For the last 420,000 years, until the beginning of the industrial revolution, this cycle of carbon exchange was in a stable equilibrium. The continual release and uptake of carbon kept CO₂ concentration in the Earth's atmosphere relatively stable. Since 1750, the atmospheric concentration of carbon dioxide has increased by 31%, to a present level of 367 ppm. This increase in the CO₂ concentration of the atmosphere is mainly due to the burning of fossil fuels and large-scale deforestation and land-use change. These human activities have forced the carbon cycle out of the state of equilibrium and out of the historic range of variation.

WHAT THE SKEPTICS HAVE IN COMMON