The more I write about global warming, the more I realize I share some things in common with the doubters and deniers who populate the blogosphere and the conservative movement. Like them, I am dubious about the process used by the U.N. Intergovernmental Panel on Climate Change (IPCC) to write its reports. Like them, I am skeptical of the so-called consensus on climate science as reflected in the IPCC reports. Like them, I disagree with people who say "the science is settled." But that's where the agreement ends.

The science isn't settled -- it's unsettling, and getting more so every year as the scientific community learns more about the catastrophic consequences of uncontrolled greenhouse gas emissions.

The big difference I have with the doubters is they believe the IPCC reports seriously overstate the impact of human emissions on the climate, whereas the actual observed climate data clearly show the reports dramatically understate the impact.

But I do think the scientific community, the progressive community, environmentalists and media are making a serious mistake by using the word "consensus" to describe the shared understanding scientists have about the ever-worsening impacts that human-caused greenhouse gas emissions are having on this planet. When scientists and others say there is a consensus, many if not most people probably hear "consensus of opinion," which can -- and often is -- dismissed out of hand. I've met lots of people like CNBC anchor Joe Kernen, who simply can't believe that "as old as the planet is" that "puny, gnawing little humans" could possibly change the climate in "70 years."

Well, Joe, it is more like 250 years, but yes, most of the damage to date was done in the last 70 years, and yes, as counterintuitive as it may seem, puny little humans are doing it, and it's going to get much, much worse unless we act soon. Consensus of opinion is irrelevant to science because reality is often counterintuitive -- just try studying quantum mechanics.

Fortunately Kernen wasn't around when scientists were warning that puny little humans were destroying the Earth's protective ozone layer. Otherwise we might never have banned chlorofluorocarbons in time.

Consensus of opinion is also dismissed as groupthink. In a December article ignorantly titled "The Science of Gore's Nobel: What If Everyone Believes in Global Warmism Only Because Everyone Believes in Global Warmism?" Holman W. Jenkins Jr. of the Wall Street Journal editorial board wrote:

What if the heads being counted to certify an alleged "consensus" arrived at their positions by counting heads?

It may seem strange that scientists would participate in such a phenomenon. It shouldn't. Scientists are human; they do not wait for proof. Many devote their professional lives to seeking evidence for hypotheses, especially well-funded hypotheses, they've chosen to believe.

Less surprising is the readiness of many prominent journalists to embrace the role of enforcer of an orthodoxy simply because it is the orthodoxy. For them, a consensus apparently suffices as proof of itself.

How sad that the WSJ and CNBC have so little conception of what science really is, especially since scientific advances drive so much of the economy. If that's what Jenkins thinks science is, one would assume he is equally skeptical of flossing, antibiotics and even boarding an airplane.

(Note to WSJ: One reason science works is that a lot of scientists devote their whole lives to overturning whatever is the current hypothesis -- if it can be overturned. That's how you become famous and remembered by history, like Copernicus, Galileo, Newton, Darwin and Einstein.)

In fact, science doesn't work by consensus of opinion. Science is in many respects the exact opposite of decision by consensus. General opinion at one point might have been that the sun goes around the Earth, or that time was an absolute quantity, but scientific theory supported by observations overturned that flawed worldview.

One of the most serious results of the overuse of the term "consensus" in the public discussion of global warming is that it creates a simple strategy for doubters to confuse the public, the press and politicians: Simply come up with as long a list as you can of scientists who dispute the theory. After all, such disagreement is prima facie proof that no consensus of opinion exists.

So we end up with the absurd but pointless spectacle of the leading denier in the U.S. Senate, James Inhofe, R-Okla., who recently put out a list of more than 400 names of supposedly "prominent scientists" who supposedly "recently voiced significant objections to major aspects of the so-called 'consensus' on man-made global warming."

As it turned out, the list is both padded and laughable, containing the opinions of TV weathermen, economists, a bunch of non-prominent scientists who aren't climate experts, and, perhaps surprisingly, even a number of people who actually believe in the consensus.

But in any case, nothing could be more irrelevant to climate science than the opinion of people on the list such as Weather Channel founder John Coleman or famed inventor Ray Kurzweil (who actually does "think global warming is real"). Or, for that matter, my opinion -- even though I researched a Ph.D. thesis at the Scripps Institution of Oceanography on physical oceanography in the Greenland Sea.

What matters is scientific findings -- data, not opinions. The IPCC relies on the peer-reviewed scientific literature for its conclusions, which must meet the rigorous requirements of the scientific method and which are inevitably scrutinized by others seeking to disprove that work. That is why I cite and link to as much research as is possible, hundreds of studies in the case of this article. Opinions are irrelevant.

A good example of how scientific evidence drives our understanding concerns how we know that humans are the dominant cause of global warming. This is, of course, the deniers' favorite topic. Since it is increasingly obvious that the climate is changing and the planet is warming, the remaining deniers have coalesced to defend their Alamo -- that human emissions aren't the cause of recent climate change and therefore that reducing those emissions is pointless.

Last year, longtime Nation columnist Alexander Cockburn wrote, "There is still zero empirical evidence that anthropogenic production of CO2 is making any measurable contribution to the world's present warming trend. The greenhouse fearmongers rely entirely on unverified, crudely oversimplified computer models to finger mankind's sinful contribution."

In fact, the evidence is amazingly strong. Moreover, if the relatively complex climate models are oversimplified in any respect, it is by omitting amplifying feedbacks and other factors that suggest human-caused climate change will be worse than is widely realized.

The IPCC concluded last year: "Greenhouse gas forcing has very likely (>90 percent) caused most of the observed global warming over the last 50 years. This conclusion takes into account ... the possibility that the response to solar forcing could be underestimated by climate models."

Scientists have come to understand that "forcings" (natural and human-made) explain most of the changes in our climate and temperature both in recent decades and over the past millions of years. The primary human-made forcings are the heat-trapping greenhouse gases we generate, particularly carbon dioxide from burning coal, oil and natural gas. The natural forcings include fluctuations in the intensity of sunlight (which can increase or decrease warming), and major volcanoes that inject huge volumes of gases and aerosol particles into the stratosphere (which tend to block sunlight and cause cooling).

A 2002 study by the U.S. National Academy of Sciences warned, "Abrupt climate changes were especially common when the climate system was being forced to change most rapidly." The rapidly growing greenhouse warming we ourselves are causing today thus increases the chances for "large, abrupt and unwelcome regional or global climatic events."

Over and over again, scientists have demonstrated that observed changes in the climate in recent decades can only be explained by taking into account the observed combination of human and natural forcings. Natural forcings alone just don't explain what is happening to this planet.

For instance, in April 2005, one of the nation's top climate scientists, NASA's James Hansen, led a team of scientists that made "precise measurements of increasing ocean heat content over the past 10 years," which revealed that the Earth is absorbing far more heat than it is emitting to space, confirming what earlier computer models had shown about warming. Hansen called this energy imbalance the "smoking gun" of climate change, and said, "There can no longer be genuine doubt that human-made gases are the dominant cause of observed warming."

Another 2005 study, led by the Scripps Institution of Oceanography, compared actual ocean temperature data from the surface down to hundreds of meters (in the Atlantic, Pacific and Indian oceans) with climate models and concluded:

A warming signal has penetrated into the world's oceans over the past 40 years. The signal is complex, with a vertical structure that varies widely by ocean; it cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically [human-caused] forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences.

Such studies are also done for many other observations: land-based temperature rise, atmospheric temperature rise, sea level rise, arctic ice melt, inland glacier melt, Greeland and Antarctic ice sheet melt, expansion of the tropics (desertification) and changes in precipitation. Studies compare every testable prediction from climate change theory and models (and suggested by paleoclimate research) to actual observations.

How many studies? Well, the IPCC's definitive treatment of the subject, "Understanding and Attributing Climate Change," has 11 full pages of references, some 500 peer-reviewed studies. This is not a consensus of opinion. It is what scientific research and actual observations reveal.

Ignoring all the evidence, doubters and deniers keep asserting that the cause of global warming isn't human emissions, but is instead natural forcings, primarily the sun. Last year, brief presidential candidate Fred Thompson commented on claims that planets like Mars were supposedly also warming -- an idea debunked by RealClimate. Thompson said sarcastically:

I wonder what all those planets, dwarf planets and moons in our SOLAR system have in common. Hmmmm. SOLAR system. Hmmmm. Solar? I wonder. Nah, I guess we shouldn't even be talking about this. The science is absolutely decided. There's a consensus. Ask Galileo.

The view that the sun is the source of observed global warming seems credible mainly to people who are open to believing that the entire scientific community has somehow, over a period of several decades, failed to adequately study, analyze and understand the most visible influence on the Earth's temperature. Such people typically cannot be influenced by the results of actual research and observations. Those who can should visit Skeptical Science, which discusses deniers' favorite arguments. In one discussion, the site explains that the "study most quoted by skeptics actually concluded the sun can't be causing global warming." Doh!

And that brings us to a recent study by the Proceedings of the Royal Society, which examined "all the trends in the Sun that could have had an influence on the Earth's climate," such as sunlight intensity and cosmic rays. The study found that in the past 20 years, all of those trends "have been in the opposite direction to that required to explain the observed rise in global mean temperatures."

Those trying to prove the sun is the sole cause of warming have a double challenge. First they would have to show us a mechanism that demonstrates how the sun explains recent warming, even though the data shows solar activity has been declining recently. (In the past, increased warming was associated with an increase in solar activity). They would also have to find an additional mechanism that is counteracting the well-understood warming caused by rising emissions of heat-trapping greenhouse gases. The doubters have done neither.

But then the doubters aren't interested in things like data and observations and peer-reviewed research. If they were, why would they keep pointing out that, historically, global temperature rise precedes a rise in carbon dioxide emissions by a few hundred years -- as if that were a reason to cast doubt on the impact of human emissions of greenhouse gases? Rep. Joe Barton said to Al Gore:

I have an article from Science magazine that explains a rise in CO2 concentrations actually lagged temperature by 200 to 1000 years. CO2 levels went up after the temperature rose. Temperature appears to drive CO2, not vice versa. You're not just off a little. You're totally wrong.

Yes, historically, glacial periods appear to end with an initial warming started by changes in the Earth's orbit around the sun. This in turn leads to increases in carbon dioxide (and methane), which then accelerate the warming, which increases the emissions, which increases the warming. That amplifying feedback in the global carbon cycle is what drives the global temperature to change so fast.

But while this fact seems to make doubters less worried about the impact of human emissions, it makes most scientists more worried. As famed climatologist Wallace Broecker wrote in Nature in 1995:

The paleoclimate record shouts out to us that, far from being self-stabilizing, the Earth's climate system is an ornery beast which overreacts even to small nudges.

That is, you need a trigger to start the process of rapid climate change. Historically, that has been orbital changes, or sometimes, massive natural releases of greenhouse gases.

Now humans have interrupted and overwhelmed the natural process of climate change. Thanks to humans, carbon dioxide levels are higher than they have been for millions of years. Even more worrisome, carbon dioxide emissions are rising 200 times faster than at any time in the last 650,000 years.

If the "Earth's climate system is an ornery beast which overreacts to even small nudges," what will happen to people foolish enough to keep punching it in the face?

That brings us to another problem with the word "consensus." It can mean "unanimity" or "the judgment arrived at by most of those concerned." Many, if not most, people hear the second meaning: "consensus" as majority opinion.

The scientific consensus most people are familiar with is the IPCC's "Summary for Policymakers" reports. But those aren't a majority opinion. Government representatives participate in a line-by-line review and revision of these summaries. So China, Saudi Arabia and that hotbed of denialism -- the Bush administration -- get to veto anything they don't like. The deniers call this "politicized science," suggesting the process turns the IPCC summaries into some sort of unscientific exaggeration. In fact, the reverse is true. The net result is unanimous agreement on a conservative or watered-down document. You could argue that rather than majority rules, this is "minority rules."

Last April, in an article titled "Conservative Climate," Scientific American noted that objections by Saudi Arabia and China led the IPCC to remove a sentence stating that the impact of human greenhouse gas emissions on the Earth's recent warming is five times greater than that of the sun. In fact, lead author Piers Forster of the University of Leeds in England said, "The difference is really a factor of 10."

How decent of the IPCC not to smash the last hope of deniers like Fred Thompson, whose irrational sun worshiping allows them to ignore the overwhelming evidence that human emissions are the dominant cause of climate change.

How else does the IPCC lowball future impacts? The 2007 report projects sea level rise this century of 7 to 23 inches. Yet the IPCC itself stated that "models [of sea level rise] used to date do not include uncertainties in climate-carbon cycle feedbacks nor do they include the full effect of changes in ice sheet flow."

That is, since no existing climate models fully account for the kinds of feedbacks we are now witnessing in Greenland and Antarctica, such as dynamic acceleration of ice sheet disintegration or greenhouse gases released by melting tundra, the IPCC is forced to ignore those realities. The result is that compared to the "consensus" of the IPCC, the ice sheets appear to be shrinking "100 years ahead of schedule," as Penn State climatologist Richard Alley put it in March 2006

According to both the 2001 and 2007 IPCC reports, neither Greenland nor Antarctica should lose significant mass by 2100. They both already are. Here again, the conservative nature of the IPCC process puts it at odds with observed empirical realities that are the basis of all science.

It's no surprise then that three scientific studies released in the past year -- too late for inclusion by the IPCC -- argue that based on historical data and recent observations, sea level rise this century will be much higher than the IPCC reports, up to 5 feet or more. Even scarier, the rate of sea level rise in 2100 might be greater than 6 inches a decade!

And it's no surprise at all that sea-level rise from 1993 and 2006 -- 1.3 inches per decade as measured by satellites -- has been higher than the IPCC climate models predicted.

The deniers are simply wrong when they claim that the IPCC has overestimated either current or future warming impacts. As many other recent observations reveal, the IPCC has been underestimating those impacts.

Since 2000, carbon dioxide emissions have grown faster than any IPCC model had projected.

The temperature rise from 1990 to 2005 -- 0.33°C -- was "near the top end of the range" of IPCC climate model predictions.

"The recent [Arctic] sea-ice retreat is larger than in any of the (19) IPCC [climate] models" -- and that was a Norwegian expert in 2005. Since then, the Arctic retreat has stunned scientists by accelerating, losing an area equal to Texas and California just last summer.

"The unexpectedly rapid expansion of the tropical belt constitutes yet another signal that climate change is occurring sooner than expected," noted one climate researcher in December.

This last point, though little remarked on in the media, should be as worrisome as the unexpectedly rapid melting of the ice sheets. As a recent study led by NOAA noted, "A poleward expansion of the tropics is likely to bring even drier conditions to" the U.S. Southwest, Mexico, Australia and parts of Africa and South America. Also: "An increase in the width of the tropics could bring an increase in the area affected by tropical storms." And finally: "An expansion of tropical pathogens and their insect vectors is almost certainly sure to follow the expansion of tropical zones."

Why are recent observations on the high side of model projections? First, as noted, most climate models used by the IPCC omit key amplifying feedbacks in the carbon cycle. Second, it was widely thought that increased human carbon dioxide emissions would be partly offset by more trees and other vegetation. But increases in droughts and wildfires -- both predicted by global warming theory -- seem to have negated that. Third, the ocean -- one of the largest sinks for carbon dioxide -- seems to be saturating decades earlier than the models had projected.

The result, as a number of studies have shown, is that the sensitivity of the world's climate to human emissions of greenhouse gases is no doubt much higher than the sensitivity used in most IPCC models. NASA's Hansen argued in a paper last year that the climate ultimately has twice the sensitivity used in IPCC models.

The bottom line is that recent observations and research make clear the planet almost certainly faces a greater and more imminent threat than is laid out in the IPCC reports. That's why climate scientists are so desperate. That's why they keep begging for immediate action. And that's why the "consensus on global warming" is a phrase that should be forever retired from the climate debate.