On the February 27 edition of Fox News' The O'Reilly Factor, in response to a viewer email about whether the current global warming “scare” is “natural” or “man-made,” host Bill O'Reilly claimed: “It's all guesswork.” The viewer asked: “Bill, you stated in the dinosaur piece that global warming is cyclical. Does that mean the current scare is natural, not man-made?” O'Reilly responded: “Who knows, Scott? It's all guesswork, and I'll leave the definitive word to the deity.” In fact, the United Nations Intergovernmental Panel on Climate Change (IPCC) has concluded that the Earth is warming and human activity is very likely responsible for most of that warming.

On the previous day's O'Reilly Factor, O'Reilly stated: “As you may know, global warming is cyclical, and right now is the focus of a ferocious debate -- almost as ferocious as a T. rex.” He later asked Utah Museum of Natural History paleontologist Terry Gates: “The bottom line is, you guys don't know why the dinosaurs disappeared. Now, I've heard that the climate change ... could have led to the dinosaurs disappearing, because you found a lot of bones under ice, you know, frozen there. And that it used to be tropical, and then it was frigid. Am I reading it wrong?” Gates responded: “It was very tropical. It was much, much warmer than it was today. There was very high CO2 levels. There were no permanent ice poles. But climate change may have impacted it once again and made one of the factors that contributed to the dinosaur extinction.”

Contrary to O'Reilly's claim that the question of whether global warming is natural or man-made is “all guesswork,” in its most recent report, the "Synthesis Report" of the IPCC's “Fourth Assessment Report,” the IPCC stated:

There is very high confidence that the net effect of human activities since 1750 has been one of warming.

Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG (greenhouse gases) concentrations. It is likely there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica). [italics in original]

The authors of the IPCC report stated that the phrase “very high confidence” translates to an “at least 9 out of 10” chance of being correct, and “very likely” translates to greater than 90 percent probability.

The IPCC used climate simulations to represent comparisons between global mean surface temperature anomalies from observations and Atmosphere-Ocean General Circulation Models (AOGCM) simulations driven with (chart “a” below) both anthropogenic (man-made) and natural forcings and (chart “b”) natural forcings only. According to the IPCC, the figure demonstrating the effects of anthropogenic and natural forcings was obtained from 58 simulations produced by 14 models. The figure demonstrating the effects of natural forcings only was obtained from 19 simulations produced by five models. The IPCC stated: “The fact that climate models are only able to reproduce observed global mean temperature changes over the 20th century when they include anthropogenic forcings, and that they fail to do so when they exclude anthropogenic forcings, is evidence for the influence of humans on global climate.” From the IPCC:

Figure 9.5 shows that simulations that incorporate anthropogenic forcings, including increasing greenhouse gas concentrations and the effects of aerosols, and that also incorporate natural external forcings provide a consistent explanation of the observed temperature record, whereas simulations that include only natural forcings do not simulate the warming observed over the last three decades. A variety of different forcings is used in these simulations. For example, some anthropogenically forced simulations include both the direct and indirect effects of sulphate aerosols whereas others include just the direct effect, and the aerosol forcing that is calculated within models differs due to differences in the representation of physics. Similarly, the effects of tropospheric and stratospheric ozone changes are included in some simulations but not others, and a few simulations include the effects of carbonaceous aerosols and land use changes, while the naturally forced simulations include different representations of changing solar and volcanic forcing. Despite this additional uncertainty, there is a clear separation in Figure 9.5 between the simulations with anthropogenic forcings and those without.

[...]

The fact that climate models are only able to reproduce observed global mean temperature changes over the 20th century when they include anthropogenic forcings, and that they fail to do so when they exclude anthropogenic forcings, is evidence for the influence of humans on global climate. Further evidence is provided by spatial patterns of temperature change.

In the IPCC's charts below, the black lines represent observed global mean temperature anomalies, while the mean temperatures produced by climate models are represented by the thick red line (anthropogenic and natural forcings) and the thick blue line (natural forcings only).

An IPCC press release accompanying the “Synthesis Report” stated: “The science related to climate change is vast and complex, and the IPCC has worked with scientists around the world to collect, assess and process the body of solid and up-to-date scientific literature.” The press release later stated: “Like all other IPCC reports, this text has undergone a multi-stage review process.”

The first section of the IPCC's Fourth Assessment Report, "The Physical Science Basis," released in February 2007, responded to the question: “How Reliable Are the Models Used to Make Projections of Future Climate Change?” in its Frequently Asked Questions section. The report stated:

There is considerable confidence that climate models provide credible quantitative estimates of future climate change, particularly at continental scales and above. This confidence comes from the foundation of the models in accepted physical principles and from their ability to reproduce observed features of current climate and past climate changes. Confidence in model estimates is higher for some climate variables (e.g., temperature) than for others (e.g., precipitation). Over several decades of development, models have consistently provided a robust and unambiguous picture of significant climate warming in response to increasing greenhouse gases.

It explained: “One source of confidence in models comes from the fact that model fundamentals are based on established physical laws, such as conservation of mass, energy and momentum, along with a wealth of observations.” It added: “A second source of confidence comes from the ability of models to simulate important aspects of the current climate. Models are routinely and extensively assessed by comparing their simulations with observations of the atmosphere, ocean, cryosphere and land surface. Unprecedented levels of evaluation have taken place over the last decade in the form of organised multi-model 'intercomparisons.' ” The report also stated:

A third source of confidence comes from the ability of models to reproduce features of past climates and climate changes. Models have been used to simulate ancient climates, such as the warm mid-Holocene of 6,000 years ago or the last glacial maximum of 21,000 years ago (see Chapter 6). They can reproduce many features (allowing for uncertainties in reconstructing past climates) such as the magnitude and broad-scale pattern of oceanic cooling during the last ice age. Models can also simulate many observed aspects of climate change over the instrumental record.

The report noted that “models still show significant errors” and that "[s]ignificant uncertainties, in particular, are associated with the representation of clouds, and in the resulting cloud responses to climate change. Consequently, models continue to display a substantial range of global temperature change in response to specified greenhouse gas forcing." However, it added that "[d]espite such uncertainties ... models are unanimous in their prediction of substantial climate warming under greenhouse gas increases, and this warming is of a magnitude consistent with independent estimates derived from other sources, such as from observed climate changes and past climate reconstructions."

The IPCC says its Assessment Reports are written by hundreds of authors and reviewed by more than 2,500 scientific experts. Government and expert reviewers also contribute to the content of the reports. According to the IPCC's Synthesis Report:

IPCC reports are written by teams of authors, nominated by governments and international organizations. They come from universities, research centres, business and environmental associations from all over the world. More than 800 contributing authors and more than 450 lead authors were involved in the writing of the AR4 (Fourth Assessment Report).

Review is an essential part of the IPCC process, to ensure an objective and complete assessment of current information. More than 2.500 scientific expert reviewers were involved in the two-stage scientific and technical review process of the AR4.

For the first review, the drafts are circulated to specialists with significant expertise and publications in the field. A wide circulation process ensures contributions from independent experts in all regions of the world and all relevant disciplines. Revised drafts are distributed for the second review to governments and to all authors and expert reviewers. Governments and expert reviewers can provide comments on the accuracy and completeness of the scientific/technical/socio-economic content and the overall balance of the drafts. Differing views for which there is significant scientific or technical support are clearly reflected in the final documents.

From the February 27 edition of Fox News' The O'Reilly Factor:

O'REILLY: And finally tonight, the mail. Thousands of letters to choose from. And we appreciate all of you who corresponded with us.

Scott Kelly, Brandywine, Maryland: “Bill, you stated in the dinosaur piece that global warming is cyclical. Does that mean the current scare is natural, not man-made?”

Who knows, Scott? It's all guesswork, and I'll leave the definitive word to the deity. What I do know is the cleaner the planet is, the better.

From the February 26 edition of Fox News' The O'Reilly Factor:

O'REILLY: “Factor Follow-up” segment tonight: Did global warming wipe out the dinosaurs 65 million years ago? And if not, what did? As you may know, global warming is cyclical, and right now is the focus of a ferocious debate -- almost as ferocious as a T. rex.

To get a better view on the subject, I spoke with Dr. Terry Gates, a well-respected paleontologist.

[begin video clip]

O'REILLY: So doctor, let's put it in perspective right off the bat. Sixty-five million years ago, the last dinosaurs were running around. Then they disappeared. When did people show up?

TERRY GATES (Ph.D., paleontologist): People showed up about 100,000 years ago. And that's what we know today as Homo sapiens, so modern man, 100,000 years ago.

O'REILLY: All right. So you're telling me that Raquel Welch was not chased around by a dinosaur? Is that what you are saying, Doctor?

GATES: I'm sorry to say, but yes. That is true. She was not chased around by a dinosaur. They were separated by --

O'REILLY: And Barney Rubble and Fred Flintstone -- no dinosaur interaction?

GATES: I think that Hanna-Barbera got it wrong.

FRED FLINSTONE: Yabba-Dabba-Doo!

[...]

O'REILLY: The bottom line is, you guys don't know why the dinosaurs disappeared. Now, I've heard that the climate change --

GATES: Yes.

O'REILLY: -- could have led to the dinosaurs disappearing, because you found a lot of bones under ice, you know, frozen there. And that it used to be tropical, and then it was frigid. Am I reading it wrong?

GATES: It was very tropical. It was much, much warmer than it was today. There was very high CO2 levels. There were no permanent ice poles. But climate change may have impacted it, once again, and may have been one of the factors that contributed to the dinosaur extinction.

O'REILLY: I also hear a theory about dinosaurs and bacteria. Some new bacteria came in.

GATES: It may be true for a very small group of dinosaurs. Bacteria are evolving into new forms all the time. They're one of the most quickly evolving life forms on Earth. And you don't get massive, whole species-wide deaths because of bacteria today. And we're not talking about one species going extinct. We're talking about an entire group of dinosaurs.