Fig. (a) shows global temperature anomalies (1880- 2013) as functions of radiative forcing using CO2 forcing as a linear surrogate. The line has a slope of 2.33 C per CO2 doubling. Fig. (b) shows the residuals from the straight line in (a), which are estimates of the natural variability.
In an audacious new paper appearing in Eos Transactions (1st December, pp. 8-9) author S. Lovejoy of the Dept. Of Physics, McGill University, provides a sound basis for "climate closure" in terms of the primary cause of global warming. The paper is timely, given the ubiquitous narrative by the anthropogenic denialists that the Earth has been subject to warming - but not from humans - rather via a "giant fluctuation of solar, nonlinear dynamics that is internal to the atmosphere" or some other natural origin. Of course, this is poppycock, the active straws grabbed by desperate people - mainly driven by economics- who have no other out. After all, most people in the world with more than air between the ears already concur some type of climate havoc is occurring. The only issue is to pin the cause on the right source.
This is where Lovejoy's paper enters. While acknowledging the effects of climate forcings are "difficult to quantify" he points out that ever since 1880 the forcings have been directly linked to economics. In other words:
"To a good approximation, if you double the world economy (e.g. by increase in GDP %), double the carbon dioxide (CO2), double the methane and aerosol outputs, double the land use changes (i.e. from rural to urban) you get double the warming."
As he notes, this justifies using the global CO2 forcing since 1880 "as a linear surrogate for all the anthropogenic forcings". With reference to the graphs shown, Figure (a) (he calls it 1(a)) shows the global annual temperature plotted not as a function of the date but as a function of the CO2 forcing. As he points out:
"Even without fancy statistics or special knowledge, it is easy to see that the temperature (plotted in green) increases very nearly linearly with some additional variations" - which represent the "natural variability" Then the gradient (black) is found to be 2.33 C per CO2 doubling which then is the actual historical increase in temperature arising from the observed increase in CO2. This is the "effective climate sensitivity".
Lovejoy further has put a check on his assumptions, noting that the figure "sits comfortably" with the IPCC range of 1.5 C - 4.5 C per doubling for the "equilibrium climate sensitivity"
This is important because by avoiding many statistical details, Lovejoy has succeeded in presenting a paper that ought to be accessible just to a person who's been exposed to some basic algebra with only a minimum of physics background.
Then in Figure (b) we see the differences (residues) between the actual temperature and the specifically anthropogenic part. These residuals are the natural variability. That this is reasonable is confirmed, as Lovejoy notes, because the average amplitude of the residues (+ 0.109 C) is "virtually the same" as the errors in 1-year global climate model hindcasts (+ 0.105 C and + 0.106 C, from Smith et al, Science, 317, 796, 2007 and Laeple et al, Geophys. Research Lett., 35, L10710, 2008), respectively.
In effect, as Lovejoy observes:
"Knowing only the slope of Figure 1(a) and the global annual CO2, we could predict the global annual temperature for next year to this accuracy. Clearly this residue must be close to the true natural variability."
The range of the straight line in Fig. (a) is thus an estimate of the total anthropogenic warming since 1880. Or about 1 C. Lovejoy next asks:
"What is the probability the denialists are right and that this is simply a giant natural fluctuation?"
To answer this Lovejoy compared industrial variations to preindustrial ones. Then, applying a Gaussian distribution to the data, he arrives at the result that "the chance of a 1 C fluctuation over 125 years being natural is in the range of 1 in 100,000 to 1 in 3,000, 000. "
He further notes that "for long periods the standard deviation of temperature differences is twice the 0.1C value. Hence, a 1 C fluctuation is about five standard deviations or a 1 in 3 million chance".
This appears whopping improbable but he reminds us that nonlinear geophysics "tells us the extremes should be stronger than the usual Gaussian (bell) curve allows". In other words, such global fluctuations would be about "100 times more likely than the bell curve would predict".
Factoring this in, we still arrive at a lower bound probability of at most 1 in 1,000 for natural causes to predominate over anthropogenic ones. Most of us would take those odds - say in a gambling venue - any time we get them
What impressed me about the work and conclusions is how much it conforms with the earlier research into C14:C12 isotope deviations compiled by P.E. Damon ('The Solar Output and Its Variation', The University of Colorado Press, Boulder, 1977). (When the Sun is more active, the heliosphere will be stronger, shielding the Earth from more intense cosmic rays. The effect of this is to reduce the C14 produced in the Earth’s upper atmosphere. Conversely, when the Sun is less active – as it’s been from 2000- 2008 then the shield is weaker and more intense cosmic rays penetrate to our upper atmosphere yielding more C14 produced. )
Damon's results are shown in the accompanying graph below. To conform with solar activity the plot is such that increasing radiocarbon (C14) is downward and indicated with (+). The deviations in parts per thousand are shown relative to an arbitrary 19th century reference level (1890).
As John Eddy observes concerning this output (Eddy, The New Solar Physics, p. 17):
“The gradual fall from left to right (increasing C14/C12 ratio) is…probably not a solar effect but the result of the known, slow decrease in the strength of the Earth’s magnetic moment. exposing the Earth to ever-increased cosmic ray fluxes and increased radiocarbon production.
The sharp upward spike at the modern end of the curve, representing a marked drop in relative radiocarbon, is generally attributed to anthropogenic causes—the mark of increased population and the Industrial Age."
Assuming the validity of the arbitrary norm (zero line or abscissa) for 1890, then it is clear that the magnitude of the Middle Ages warming period (relative C14 strength of -18), for example, is less than about ½ the relative effect attributed mainly to anthropogenic sources in the modern era (-40). Even if one fourth the latter magnitude is assigned to solar activity (based on solar variability component detected over 1861-1990 amounting to 0.1- 0.5 W/m^2 vs. 2.0 to 2.8 W/m^2 for heating component arising from greenhouse gas emissions, cf. Martin I. Hoffert et al, in Nature, Vol. 401, p. 764) the anthropogenic effect is at least 3/2 times that for the last (exclusively solar) warming period..
Is there still a role for natural variability? Of course, and as Lovejoy notes "without it the warming would have become unrealistically strong".
The problem is that now, there are few or no know agents of "natural variability" that can put a cap on warming. The only man-made one we know would work for sure, is a "nuclear winter" effect induced by a massive exchange of thermonuclear warheads. But I am sure nobody wants that!