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David Hafemeister replies:
We thank Professor Ribbing for his continued interest in our work. As we pointed out in our January 2010 response, our three-page paper contained some simple climate models, which were obtained from my text Physics of Societal Issues (Springer 2007). Let us take a moment to consider an even simpler model. In the building sciences we know that there will be a heat flow (dQ/dt) when there is a temperature difference between surfaces () over an area (A) with thermal resistance (R),
The necessary temperature difference needed to expel the internal heat power through the thermal resistance in the steady state is
Note that an increase in R from increased carbon dioxide and other gases (absorption and re-radiation, convection) requires an increased surface temperature to force the heat power through the atmosphere to space. This is the basic cause of the warming: more thermal resistance requires a greater temperature difference to dispatch a given heat source in the steady state. To this, one must add positive feedbacks (more water vapor, less ice to reflect, methane release, IR absorption in clouds), which are larger than the negative feedbacks (reflecting clouds). Aerosols and volcanic dust lower surface temperatures, but these particles leave the atmosphere after a few years.
My simple models have been successful for non-climate physicists to understand the basic physics, but they clearly are not sufficient to determine policy, compared to the work of the professional atmospheric and climate scientists . The IPCC global circulation model calculations agree with the time dependent temperature data over the past century ONLY if CO2 absorption is taken into account . The blue curves (natural forcing due to solar activity and volcanoes) remain relatively constant in temperature over the century. On the other hand, the red curves (both natural and anthropogenic forcing) separate from the blue curves in 1955, with an increased temperature of about 0.7 oC in 2000.
Some climate skeptics say the Earth is cooling: But many skeptics plot temperature starting in 1997, where they should begin the plot 50 to 100 years earlier. The IPCC stated in 2007 that “eleven of the past 12 years (1995 to 2006 – the exception being 1996 – rank among the 12 warmest years on record since 1850.” In the last few decades we have seen the area of the Artic Icecap reduced 7.5%/decade (from 8.5 Mkm2 in 1979 to 6.5 Mkm2 in 2009), the rapid rise in the melting of Greenland, twelve fewer days of frozen lakes, and a doubling of the ocean level rise rate. Our Alaska and your Lappland have increased in temperature by 1-2 oC. It is unclear how skeptics can say the Earth’s climate is cooling.
Other climate skeptics say the Earth is warming but argue that, rather than the warming being due to carbon dioxide, it is caused by a sun that is emitting more ultraviolet photons or increasing the flux of galactic cosmic rays that make clouds. This has been refuted by a variety of authors . The main argument is that the 11-year solar cycle has been quite constant, not showing a trend over the past 30 years. The amount of extra ultra-violet photons and cosmic rays has been minimal; the sun did not cause significant warming of the Earth at the end of the 20th century.
Some climate skeptics say that CO2 additions are irrelevant because its effect is saturated by the exponential absorption from a collimated beam of infrared photons. But this is not true for the atmosphere. Each micro-layer absorbs IR photons and re-radiates new IR photons, both up and down. As one goes higher in the atmosphere it is cooler, thus reradiating fewer IR photons upwards. It is this physics that produces considerable forcing from carbon dioxide; it is logarithmic in concentration and it is not a negative exponential with concentration. For today’s increase of 2 ppm/year for a decade, the thermal forcing increases linearly with time. For a doubling of CO2, the forcing is not doubled but increased by a factor of 1.69. Temperature increases (direct and feedbacks) are approximately proportional to the additional thermal forcing.
Some climate skeptics ignore the fact that carbon dioxide is now at 390 ppm, which is over 100 ppm above the pre-industrial revolution level of 280 ppm, and rising 2 ppm/year. This level will double somewhere in the next century, it is already the highest level in the past 650,000 years, which has always been less than 280 ppm. Carbon dioxide cycles in and out of the atmosphere over past ice age cycles, but at lower levels and a much slower rate. At the same time methane is now at 1800 ppb, compared to its pre-industrial value of about 600 ppb. It is the rate of change of carbon dioxide and methane that is worrisome since they are exploding the natural norms.
Ribbing holds out the hope that increased photosynthesis from raised CO2 levels will help (with water), but not in all ecosystems, and the additional sequestering of carbon is far, far smaller than global emissions of 30 billion tons of CO2 per year (now) to 50 billion tons in 2050. I wish there was an easy fix, but it will be difficult.
Cal Poly University, San Luis Obispo, CA
 J. Lean, G. Rottman, J. Harder and G. Kopp, Solar Physics 230, 27 (2005). J. Lean, “Living with a variable Sun,” Physics Today 58, 32-38 (June 2005), P. Duffy, B. Santer, and T. Wigley, “Solar variability does not explain late-20th century warming,” Physics Today 62, 48-49 (January 2009).
This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.