If a student asks why Alaska warms from January to July, saying "that's the annual cycle" isn't a real answer. It's just a new name for the mystery. A real answer would involve physics: the northern hemisphere receives more energy in July because it points toward the Sun more directly in July than in January.
Similarly, many people blame the recent 50 year warming trend on various "climate cycles" but don't explain the physics causing Earth to gain energy. Tung and Zhou 2013 is the latest attempt to blame a large part of Earth's warming on the Atlantic Multidecadal Oscillation (AMO), which is a long-term fluctuation in N. Atlantic sea surface temperatures. Previous attempts include Bob Tisdale's claim and Zhou and Tung 2012 which WUWT advertised.
Tung and Zhou subtract any apparent "oscillations" with periods between 50 and 90 years without explaining the physics causing Earth to gain and lose energy, as depicted in their Figure 4. They seem to imply that these cycles are more compelling than the mainstream explanation which involves nonrepeating events. But the universe is full of cycles and nonrepeating events, all of which obey the laws of physics. Because mainstream science addresses the energy balance, I find it more compelling than math without a basis in physics.
Tung and Zhou repeat the analysis of Foster and Rahmstorf 2011 (PDF) in Figure 5 while also removing the AMO, and obtain an anthropogenic warming trend over the last 33 years of 0.07°C/decade, less than half of Foster and Rahmstorf's 0.17°C/decade.
I've discussed a short video based on Foster and Rahmstorf 2011, which subtracted some natural phenomena that cause Earth to gain and lose energy:
Tung and Zhou also subtracted the AMO index, which is defined as detrended N. Atlantic sea surface temperatures. It mostly adds energy to the surface by cooling the deep ocean (see below). Note that the AMO index is defined solely on temperatures, so anthropogenic global warming can increase the AMO index. This means subtracting it potentially ignores AGW.
Their analysis concludes that:
"The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07-0.08°C/decade..." [Tung and Zhou 2013]
First, that's unphysical. Anthropogenic warming is caused primarily by the radiative forcings of greenhouse gases, which didn't skyrocket until about 1950 when our population and energy use per person both skyrocketed. More comprehensive analyses also show total anthropogenic radiative forcings increasing dramatically after 1950. Basic physics show that the anthropogenic warming rate should be higher after 1950.
Second, their unphysical claim isn't really a conclusion; it's actually the assumption that (if true) would have justified removing the AMO to determine the anthropogenic warming trend. Their paper is a circular argument. Here's why:
"The removal of the AMO in the determination of the anthropogenic warming trend is justified if one accepts our previous argument that this multidecadal variability is mostly natural." [Tung and Zhou 2013]
No. Removing the AMO to determine anthropogenic warming would only be justified if detrending the AMO from 1856-2011 actually removed the trend due to anthropogenic warming. But that's unphysical: basic physics show that the anthropogenic warming rate should be higher after 1950. As a result, their approach overestimates anthropogenic warming before 1950, and underestimates it after 1950.
Warming the globe also warms the N. Atlantic. Tung and Zhou have subtracted N. Atlantic temperatures that contain an anthropogenic trend over the last 33 years from global temperatures, and seem surprised to find a lower anthropogenic trend over the last 33 years. I'm not.
Tung and Zhou implicitly assumed that the anthropogenic warming rate is constant before and after 1950, and (surprise!) that's what they found. This led them to circularly blame about half of global warming on regional warming. Where'd that heat come from, if not from CO2?
Since the words "energy" and "heat" don't appear in Tung and Zhou 2013, let's approximate the energy they're ignoring. Warming a crude model of the atmosphere and upper layer of the ocean by 1°C requires about 10,000,000 megaton atomic bombs of energy. Tung and Zhou 2013 magic away 0.1°C/decade over the last 33 years, so they've ignored about 3,000,000 megaton atomic bombs of energy.
Internal variability moves heat around the Earth's climate without altering its total heat content. I've already noted that the deep ocean can't be the source of surface warming because the Argo probes show that the deep ocean is also warming. However, that claim was based on global averages. If AMO internal variability were causing surface warming through changes in circulation, the deep N. Atlantic should be losing enough heat to account for the surface warming. But the N. Atlantic as a whole has warmed over the past 50 years (PDF). So where'd that heat come from, if not from CO2?
Posted by Dumb Scientist on Thursday, 21 March, 2013
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