The cold truth about climate change

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?

Next page: The other problem with "consensus"