Is global warming chiefly caused by humans or by natural phenomenon?
Background and context
Few reputable scientists at this point dispute the claim that humanity has played some role in global warming.
Nonetheless, the consensus on this issue is somewhat fragile, with scientists disagreeing on the degree to which global warming might also be caused simply by natural changes in the earth's climate. Some claim that humans are the chief cause of the warming that has occurred over roughly the past century, while others claim that our role has been relatively insignificant as compared to natural forces. Therefore, the debate roughly defined here centers on the question of whether human are the "chief" or "most significant" cause of the recent global warming trend seen over the past century and today. This debate is important in the way of determining policy responses. If, for instance, the human cause of global warming was concluded to be relatively minor, then some would argue that policies attempting to address these human causes (carbon emissions regulations...) are misplaced. Conversely, a conclusion that humans are the chief cause of global warming would provide valid support for efforts that attempt to stem these human-causes. Of course, conclusions that humans are, for instance, 30% of the cause would create a more challenging set of questions in the way of how to prioritize a human response. Still, the undeniable fact that the earth is getting warmer leads to a range of separate debates that need not consider the underlying cause.
Warming trend: Is there a clear warming trend that shows human finger-prints?
The correlation in the acceleration of greenhouse gas emissions and the acceleration of global warming outlines the causal relationship between humans and climate change. This is not to say that humans are necessarily wholly responsible for global warming, and this position acknowledges (typically) some natural causes of global warming. But, it is argued that humans have added dramatically to the acceleration of global warming far beyond what would have occurred naturally.
Cooling between 1940 and 1975 can be attributed to sulphate aerosols During this period, CO2 warming was overtaken by an increase in human particulates and aerosol pollution. But, as pollution regulations and technology improved, sulphate aerosol concentrations decreased, human-C02's warming effect re-emerged, and global warming continued apace. This "global dimming" effect does not negate the fact that the general trend is human-caused global warming.
If the earth was following natural trends, we should be experiencing a cooling trend now. In the natural cycle of 100,000 year long glacial periods and 10,000 year interglacial periods, we are and the very end of a 10,000 interglacial period. This is the peak where temperatures are at their peak in the natural cycles. At this peak, we are at something of a plateau. But, on this plateau, we are on a slow course to entering the next ice age. In fact, we were very gradually cooling over the length of the preindustrial Holocene at a slow pace of around a .5C reduction over 8000 years. While we should be on a cooling course, we are actually on a warming course. And, dramatically so. This should not be the case in the natural context, and is only explained by human activities.
Even if we were in a natural warming cycle, the rate of warming currently is many times faster than any rate of warming in the historical record.
The earth's climate has always changed through history due to natural cycles. Temperature and CO2 levels have varied widely throughout Earth's cyclical patterns of 100,000-year-long ice-ages and 10,000 year-long inter-glacial warming periods. The Earth is currently peaking in an inter-glacial warming period, before it will, inevitably, enter the next 100,000-year ice age. It is natural to expect warming during this period.
The temperature record shows no consistent global warming trend The earth actually cooled between 1940 and 1975, contrary to human-caused global warming theory. Why would the earth cool during this period if anthropogenic global warming were occurring? 1940 to 1975 was a major period of unfettered industrial development.
The earth warmed significantly between 1910 and 1940 when emissions were not very high: How can we account for the warming of the Earth early in the century, from 1910 to 1940? At that time, the global emission of carbon dioxide from cars, trucks, and factories was relatively low--too low to account for the half-degree warming. Here also, natural factors in climate change, rather than human activities, must have been primarily responsible.
Greenland's temperatures rose fifty percent faster in the 1920s than they rose around 2007.
The accelerating rate of global warming may be naturally caused. Some scientists note that global warming actually weakens the ability of the environment to absorb C02, thus making the planet even more susceptible to global warming. Given this "positive feedback loop", it could be expected that natural global warming would lead to faster and faster rates of warming (acceleration). Therefore, it is not appropriate to credit all acceleration in warming to human-causes; this may be largely a natural phenomenon. And, the historical record does show sudden temperature spikes in history that would include a dramatic acceleration of warming due to natural causes.
There is no proof that greenhouse gases are causing global warming.
Greenhouse gases: Are human-greenhouse gas emissions capable of causing global warming?
Human greenhouse gas emissions have a clear "greenhouse" warming effect: Greenhouse gases are named as such because they have a "greenhouse effect" in the atmosphere, allowing sunlight to enter the earth's atmosphere but trapping a percentage of the radiation that would otherwise be reflected back into space. The trapped radiation emits added heat into the Earth's atmosphere with the result being "global warming". The "greenhouse effect" is not disputed.
While temperature changes "lead" CO2 historically, CO2 still caused added temperature change - It is probable that the sun-earth interactions did lead to increased temperatures historically that caused melting of ice and other effects that caused increased CO2 emissions. BUT, these increased CO2 emissions then acted as "positive feedback loops" in increasing temperature. Nobody disputes that CO2 is a greenhouse gas that has a "greenhouse effect" on climate, so why would the fact that temperature lead CO2 in the historical record even matter? Is this supposed to indicate that CO2 is not a "greenhouse gas"? It certainly is, and is certainly acted as a "positive feedback" in the natural historical record.
Humans have increased significantly the number of greenhouse gases in the atmosphere, adding substantially to the "greenhouse effect" Levels of greenhouse gases in the atmosphere have increased substantially through the industrial revolution. Sources cite that humans have contributed roughly 10 to 30 percent of the greenhouse gases that are currently in the atmosphere since large-scale industrialization began around 150 years ago. This substantial increase due to humans is certainly capable of producing the global warming effects we are seeing today. One of the reasons that the Earth's atmosphere is so vulnerable to such a substantial increase in atmospheric greenhouse gases is that it is so thin.
Global temperature variations have not been caused historically by C02 - The historical record shows that temperature changes have actually preceded C02 changes. This is demonstrated on the Vostok graph and shows that C02 changes could not actually be primary cause of global warming, or else they would have preceded temperature changes. This evidence would appear to indicate that global warming actually causes changes in C02 levels. Notable, in this fact, is that increases in CO2 levels cannot be wholly attributed to human-activities, as much if it is the side-effect of natural warming.
Humans are a small source of C02 emissions into the atmosphere. According to some sources [SUCH AS?], humans produce only single digit percentages of all the C02 that is released into the atmosphere every year. Volcanoes produce more than humans each year. This makes it impossible that humans are the main cause of global warming, even if we take greenhouse gases to be the main driver of global warming.
Sun: Does the sun have little or no role in global warming?
Sun-spot variation has a "negligible impact" on global climate change:
"Don't Blame Sun for Global Warming, Study Says". National Geographic. September 13, 2006 - "But sunspot-driven changes to the sun's power are simply too small to account for the climatic changes observed in historical data from the 17th century to the present, research suggests. The difference in brightness between the high point of a sunspot cycle and its low point is less than 0.1 percent of the sun's total output. "If you run that back in time to the 17th century using sunspot records, you'll find that this amplitude variance is negligible for climate"'...according to Solar astronomer Peter Foukal of Heliophysics, Inc.
In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.
Foukal, Peter; et al. (2006-09-14). "Variations in solar luminosity and their effect on the Earth's climate.". Nature. Retrieved on 2007-04-16.
A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays. Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.
Some graphical analyses of sun variations and global temperatures reveal a very tight relationship through history. This graphical correlation is really the main argument here. The question is, does the sun's relationship to temperature correlate more strongly than the relationship of greenhouse gases and global temperature? This side of the argument makes the point that it does.
The sun is causing warming on mars just like on earth: - This is a major demonstration of the inter-planetary effect that the sun is having on climates. The problem of global warming is not reserved solely to the Earth at this time. In fact, the trends on Mars match closely what is occurring on Earth. The only common factor between Mars and the Earth is the Sun. This seems to demonstrate the the sun is a central force driving Earth's climate change.
On-the-ground: What do on-the-ground effects demonstrate?
The effects of global warming are clear, but they don't demonstrate human-causes: Global warming is occuring and this is indeed resulting in the melting of ice-bergs and so-forth. Yet, this does not necessarily demonstrate human causes. Natural warming in Earth's history has resulted in such melting and on-the-ground climate changes.
The acceleration of global warming could be natural, and therefore the accelerated on-the-ground effects could be natural as well. Global warming produces many on-the-ground effects that add, in turn, to additional global warming. These are known as "positive feedback loops" and include such things as the depleted ability of forests to act as carbon-sinks as well as the added heat absorption of the oceans due to the melting of ice (more dark, non-reflective surface area). These effects can accelerate global warming and thus accelerate the effects of global warming on the ground. This makes it difficult to assume that accelerating on-the-ground effects from global warming demonstrate human-causes; they could be part of natural causes.
Glaciers and ice-sheets melt and re-form annually. Images are often presented that attempt to show the recession of both, and are held up as demonstrations of human-caused global warming. But, what is essential is that these images are taken in the appropriate context. If an image from the past from the middle of the winter is compared with a present image taken during summer months, than this is a faulty comparison; the comparison needs to be made in the same months of the year between different years.
The atmosphere is not warming in the way human-caused global warming theories would predict. If greenhouse gases were causing the climate warming then scientists would expect the troposphere to be warming faster than the surface, but observations do not bear this out
Scientific community: Is there a consensus that global warming is chiefly human-caused?
Wikipedia:Global warming. Retrieved 11.07.07 - "The detailed causes of the recent warming remain an active field of research, but the scientific consensus identifies elevated levels of greenhouse gases due to human activity as the main influence."
2001 Intergovernmental Panel on Climate Change report: "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities." 
Joseph Romm. Book Hell and High Water. 2007. pp 12. - "How strong is the scientific consensus? Back in 2001, President George W. Bush asked the National Academy of Sciences for a report on climate change and on the conclusions of the IPCC assessments on climate change. The eleven member blue-ribbon panel, which included experts previously skeptical about global warming, concluded: Temperatures are rising because of human activities; the scientific community agrees that most of the rise in the last half-century is likely due to increased greenhouse gas concentrations in the atmosphere; and 'the stated degree of confidence in the IPCC assessment is higher today than it was 10, or even 5 years ago.'"
Claims that the vast majority of scientists believe global warming is mostly due to humans.
Many scientists are pressured to deny global warming is human-caused: New Scientist notes that in surveys a much larger fraction of U.S. scientists consistently state that they are pressured by their employers or by U.S. government bodies to deny that global warming results from human activities or risk losing funding.
Al Gore's "An inconvenient truth" - Out of 928 peer-reviewed articles, all of them concluded that humans are the cause of global warming.
NASA's Goddard Institute for Space Studies (GISS) made significant changes to their temperature records in 2007, downgrading the magnitude of recent rises.
The IPCC report should not be trusted.
The conclusions are politically motivated.
Fredrick Sites, former president of the National Academy of Sciences.
"I have never witnessed a more disturbing corruption of the peer review process than the events that led to this IPCC report."
"this report is not what it appears to be - it is not the version that was approved by the contributing scientists listed on the title page."
"None of the studies cited has shown clear evidence that we can attribute the observed [climate] changes to the specific cause of increases in the greenhouse gases."
Scientists have a financial interest in calling climate change a crisis. Money is often directed toward studies that could be considered interesting or dramatic. This creates a financial incentive for scientists to "find" dramatic global warming effects. Because the prevailing opinion is that global warming is human-caused, there is an interest in seeing articles published that support this view. Some even suggest that scientists are tweaking their computer models so that the results show a more dramatic relationship between human activities and temperature changes than actually exists.
Global warming has become an industry in itself. Many businesses are benefiting from the global warming crisis. Solar energy, for example, has a direct interest in seeing the propagation of the notion that global warming is human caused.
Media: Is the media fairly representing both sides of this debate or is it biased?
The media is representing the scientific consensus that humans are the primary cause of global warming in the modern era. This consensus should be relied-upon by journalists; what basis do they have to doubt it?
A new generation of environmental journalists has emerged whose jobs are dependent on the existence of the Global Warming crisis.
Precautionary principle: Should we error on the side of precaution?
The precautionary principle would hold, in the context of global warming, that it would be better for humans to take precautionary measures to cut human greenhouse gas emissions. The risk is too high that global warming is mostly caused by humans. As such, it is reasonable to take action to mitigate these likely human causes, and before it is too late to have any impact on the problem.
The costs of such a precautionary principle are very high.
In the developing world the promotion of solar panels has been harmful. They are up to three times more expensive then other sources of electricity and often do not produce enough electricity for the various uses of electricity that are demanded. For example, with the provision of standard solar panels alone, it may be possible to turn on only the lights but not to use the refrigerator at the same time.
Patrick Moore PhD, the Co-founder of Greenpeace, said in an interview in the film "The Great Global Warming Swindle", "The environmental movement has evolved into the strongest movement there is in preventing development in developing countries."
Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, said natural changes in the environment cannot account for the magnitude of global warming in the past four decades.
NASA climate scientist James Hanson says that greenhouses gases other than carbon dioxide, particularly methane and CFCs, are "probably the main cause of observed global warming".
Richard Lindzen--professor of meteorology at MIT, highly respected atmospheric physicist, and member of the National Academy of Sciences as well as the special NAS panel on global warming--said in a recent commentary, "I cannot stress this enough--we [cannot] confidently attribute past climate change to carbon dioxide."