Human-induced warming of the climate system is established fact.

I want to share this article because I know you are more likely to read if I post it here, than if I post the link [ dot Earth Feb 17th, Andrew C. Revkin].
How do we know this to be true? What does it take to get something established as fact? I will try to explain this quandary here the same way that I explain it to myself.
We have come to understand that nothing happens in this world expect as allowed by the laws of physics. What this means is that for every physical action there is going to be a well-defined cause, and a well-defined effect. Quantum mechanical weirdness that operates at atomic scale does not invalidate this physical description of the macroscopic range that is of interest.

Human experience has demonstrated that it is through measurement and physics that we understand the world that we live in. The term “physics” includes also the mathematical description of these laws which permits mathematical models to be constructed to conduct virtual experiments of real-world situations.

In this way, by utilizing global-mean decadal-average quantities, we have come to understand that water vapor accounts for 50 percent of the (33 K, 60 deg F) greenhouse effect. Longwave absorption by clouds contributes 25 percent, and CO2 accounts for 20 percent. The remaining 5 percent of the greenhouse effect is split between methane, N2O, CFCs, ozone, and aerosols. Significantly, CO2 and the minor GHGs do not condense or precipitate at current atmospheric temperatures. This provides a stable reference temperature structure for the fast feedback processes to operate and maintain the amounts of atmospheric water vapor and clouds at their quasi-equilibrium concentrations. Hence the strength of the terrestrial greenhouse is sustained and effectively controlled by the atmospheric temperature floor that is provided by CO2 and the other non-condensing greenhouse gases. (More detail is contained in my Greenhouse Tutorial which is a related supporting commentary.)


The bottom line is that CO2 is absolutely, positively, and without question, the single most important greenhouse gas in the atmosphere. It acts very much like a control knob that determines the overall strength of the Earth’s greenhouse effect. Failure to control atmospheric CO2 is a bad way to run a business, and a surefire ticket to climatic disaster.


My earlier criticism had been that the IPCC AR4 report was equivocating in not stating clearly and forcefully enough that human-induced warming of the climate system is established fact, and not something to be labeled as “very likely” at the 90 percent probability level. It would seem that the veracity of the human-induced warming would hinge on establishing the pedigree of the observed increase in atmospheric CO2. On this point, the IPCC report is crystal clear. Pages 137-140 of IPCC AR4 describe high-precision in situ measurements of atmospheric CO2 at Mauna Loa, documenting the steady increase in CO2 along with its characteristic seasonal fluctuation. These measurements, supplemented by analyses of air bubbles trapped in ice core samples, show unequivocally that atmospheric CO2 has increased from a pre-industrial level of 277 ppm in 1750 to present day concentrations that are approaching 390 ppm.

The IPCC report also shows the corresponding decrease in atmospheric oxygen, thus providing irrefutable verification that the increase in atmospheric CO2 is linked directly to fossil fuel oxidation. In Chapter 7, the IPCC report states it clearly: “the increases in atmospheric carbon dioxide and other greenhouse gases during the industrial era are caused by human activities”. Undoubtedly, volcanic eruptions have contributed some atmospheric CO2, but this can only be miniscule as neither the 1991 Pinatubo eruption (largest of the century), nor the 1986 Lake Nyos CO2 eruption that killed thousands, so much as registered a blip in the Mauna Loa CO2 record.


In view of all this, the IPCC AR4 Chapter 9 Executive Summary states that: “It is likely (66 percent probability) that there has been a substantial anthropogenic contribution to surface temperature increases in every continent except Antarctica since the middle of the 20th century.” How can this be considered anything other than inaccurate and misleading?


To understand climate change, it is necessary to know the radiative forcings that drive the climate system away from its reference equilibrium state. These radiative forcings have been analyzed and evaluated by Hansen et al. (2005, 2007). They include changes in solar irradiance, greenhouse gases, tropospheric aerosols, and volcanic aerosols. Of these forcings, the only non-human-induced forcing that produces warming of the surface temperature is the estimated long-term increase by 0.3 W/m2 of solar irradiance since 1750. Volcanic eruptions are episodic, and can produce strong but temporary cooling. All of the other forcings are directly tied to human activity. When it comes to radiative forcing of global climate change, it is abundantly clear that whether we like it or not, or whether we care to admit it, it is humans who are driving the bus.