Andrew A. Lacis, the NASA climatologist whose 2005 critique of the United Nations climate panel was embraced by bloggers seeking to cast doubt on human-driven climate change, has sent in two more commentaries on the state of climate science.
Human-induced warming of the climate system is established fact.
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 except 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.
Greenhouse Tutorial
In the context of global climate, absorbed solar radiation (about 240 W/m2, with 30 percent of the incident radiation being reflected back to space) is the energy source that keeps the Earth’s surface warm. The Planck radiation law determines that a temperature of 255 K (about 0° F) is needed to have energy balance with the absorbed solar radiation. If the Sun were suddenly turned on, the Earth would begin warming, and would keep warming until it reached a 255 K temperature, at which point it would be radiating 240 W/m2 of thermal energy out to space, in equilibrium with the solar energy input.
The global-mean surface temperature of the Earth is observed to be 288 K (60° F). Why is this so much warmer than the 255 K effective temperature of the thermal radiation emitted to space? The reason is that the Earth has an atmosphere that contains gases that absorb thermal radiation. These gases are distributed throughout the atmosphere, and they also must maintain energy balance on a local scale, meaning that the same amount of radiation absorbed (e.g., from the ground), must be re-emitted (in both upward and downward directions) so as to maintain constant temperature. This radiative process of localized absorption and emission of thermal radiation establishes a temperature gradient within the atmosphere, and in so doing, results in heating the ground surface to a higher temperature than would be the case with no atmosphere. This is the greenhouse effect, and it keeps the surface temperature of the Earth 33 K (60° F) warmer than it would otherwise be for the same 240 W/m2 of solar heating.
It is helpful to analyze the Earth’s energy balance in terms of global-mean and decadal-average quantities, so as not to be distracted by having to worry about local energy balance variations due to regional, clear-sky, cloudy-sky, diurnal, seasonal, and interannual fluctuations. A good climate GCM can be adapted to perform this task.
We know from direct measurement that there are atmospheric constituents that absorb thermal radiation. The most important are: water vapor, cloud particles, and CO2, with smaller contributions coming from methane, N2O, CFCs, ozone, and aerosols. We also know quite accurately the spectral absorption characteristics for the absorbing gases, and how cloud and aerosol particles interact with thermal radiation. This basic knowledge comes from a combination of laboratory measurements and theoretical analyses. This input data is tabulated in the HITRAN database for all significant atmospheric gases, and is available for use in radiative transfer calculations.
It is important to know the relative contribution of each absorbing gas to the total (33 K) greenhouse effect. Precise attribution is difficult because there is significant overlapping absorption with varying degrees of saturation, and there is the need to take into account the vertical structure and time-spatial distribution of absorbers, requiring a good climate GCM with good radiative transfer. The problem is akin to defining the actual weight that is borne by individual support columns of a bridge. We know that collectively the columns must support the entire weight of the bridge, but actual measurements removing one column at a time would be extremely problematic since the weight of the bridge would get redistributed among the remaining columns.
With good mathematical models that accurately represent the radiative interactions of atmospheric interest, we can conduct virtual experiments to determine the radiative contribution of each individual gas within the context of current atmospheric structure. We have performed such experiments for the principal greenhouse gases, clouds, and aerosols using the [Goddard Institute] climate model by systematically inserting, or taking out, each atmospheric constituent one at a time, and recording the corresponding radiative flux change.
These experiments show that water vapor accounts for about 50 percent of the total 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. It is significant that CO2 and the minor GHGs do not condense or precipitate at current atmospheric temperatures. This provides a stable temperature structure for the fast feedback processes to operate and maintain the quasi-equilibrium amounts of water vapor and clouds. Hence, the strength of the terrestrial greenhouse effect is effectively sustained and controlled by the atmospheric temperature structure provided by CO2 and the other non-condensing greenhouse gases.
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.
Dr. Andrew A. Lacis
National Aeronautics and Space Administration
NASA Goddard Institute for Space Studies
B.A., Physics, 1963, University of Iowa
M.S., Astronomy, 1964, University of Iowa
Ph.D., Physics, 1970, University of Iowa
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