The climate models that the Climate Alarmists rely on to support their "existential threat" claims produce projections of "alarming" global warming due to a particular parameter estimate. That parameter is "climate sensitivity", which is the temperature change produced by a doubling of atmospheric CO2. The value the modelers use is chosen to make the models fit recent past data. It is high enough to justify considerable concern about the future climate. However, to the extent that the model is not complete, such a parameter estimate can be expected to be biased and measures of its uncertainty to be meaningless. In fact, these models' forecasted warming has been much higher than the actual warming.
LS provide evidence that these climate models' estimates of climate sensitivity are inconsistent with the actual data. They argue that the latter imply a climate sensitivity of about one third of the models' estimates.
Here are some excerpts from the LS paper.
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It is commonly accepted that increasing CO2 in the atmosphere should lead to some warming (e.g. Arrhenius, 1896; Callendar, 1938). This, per se, is not particularly worrisome. As has been recognized since antiquity, the dose makes the poison. The no on that any warming, however small, is evidence of coming disaster defies reason. Remember, in natural systems, fl uctua ons are the norm. For example, your body temperature always fluctuates a little. Skyscrapers always sway a li le. This is a characteris c of all stable systems.
With respect to CO2, the dose is determined by what we call climate sensitivity. By convention, this is the eventual total increase in global mean temperature associated with a doubling of CO2. The reason we refer to a doubling is that the impact of each doubling is the same: i.e. a well-established equation based on empirical data shows that we get the same warming from an increase from 400 parts per million (ppm) to 800 ppm as we would from 200 ppm to 400 ppm (Pierrehumbert, 2011). That is to say, the impact of each added unit of CO2 is less than the impact of its predecessor. In addition, reasonably straight forward calculations suggest that, all other indirect factors (e.g. clouds) being held constant, a doubling of CO2 should produce about one degree Celsius (1°C) of direct warming—a value that is not generally held to be alarming (Wilson and Gea-Banacloche, 2012). The radiative forcing effect of CO2 is measured in units of Wa s per square meter. Each doubling of CO2 is expected to provide about 3.7 Watts per square meter (Pierrehumbert, 2011). This can be compared to the natural flows of radiant energy in and out of the climate system, es mated to be 235 to 245 Watts per square meter (Trenberth et al., 2009).
Of course, CO2 is not the only anthropogenic greenhouse gas, and according to the United Nations Intergovernmental Panel on Climate Change (IPCC, 2013) the increase in anthropogenic greenhouse forcing since the beginning of the industrial era (which happens to coincide with the end of the Little Ice Age) is already almost what one expects from a doubling of CO2, and we have seen a welcome warming of about 1°C. A er all, the Little Ice Age was hardly considered optimal. The IPCC does not claim all of this small warming is due to increased greenhouse gases, but even if it were, it does not, on the face of it, suggest a high sensitivity. However, most models employed by the UN’s Intergovernmental Panel on Climate Change display higher sensitivities (currently ranging from 1.5° – 4.5°C). Moreover, the UN argues that higher values portend profound dangers (a dubious claim in its own right).
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Before turning to systematic approaches, we may ask whether there is, in fact, any evidence that at least suggests unusual warming. As we see in Figure 6 , warming since 1978 is considerably less than almost all models use d by the UN projected. 
The UN acknowledges that anthropogenic contributions were only significant since the 1960s . Yet, as we see in Figure 7, warming from 1920 until 1940 was indistinguishable from the recent warming.
The contention of dangerous warming has always depended on special pleading rather than unusual behavior of the temperature record. Arguments about changes of tenths of a degree have been relatively pointless since the data are far from reliable for such small changes. Similarly, the use of demonstrably inadequate models to determine sensitivity is also inappropriate
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The situation with respect to climate sensitivity is that we basically see no reason to expect high sensitivity. The original basis for considering that high sensitivity is possible (namely, the hypothetical water vapor feedback of Manabe and Wetherald, 1975) is clearly contradicted by the measurements of TOA radiative fluxes which show that the total long-wave feedback, including cirrus cloud variations, may even be negative. Analysis of the temperature data leads to the conclusion that if anthropogenic contributions are the cause of warming since the end of the Little Ice Age, and if aerosols are limited to a contribution of 1 Watt per square meter, then climate sensitivity in excess 1.5°C is precluded.
Have we then proven that dangerous warming is truly impossible? Not quite. Although current estimates of short-wave feedbacks don’t even suggest positive feedback factors in excess of about 0.3 (with the possibility of negative values remaining), we can’t preclude that something may someday be discovered that raises this to a value that is significantly larger. Our simple calculation that suggested that sensitivities in excess of 1.5°C were precluded depends upon the assumption that models are correct in producing negligible natural internal variability. It is, however, remotely conceivable that there was in reality (as opposed to in models) natural internal variability that was exactly what was needed to cancel the effect of high sensitivity, but that this internal variability would eventually be overwhelmed, and allow the high sensitivity to reveal itself.
This remote possibility is far from “settled science,” and the thought that multi -trillion dollar policies would be implemented to putatively prevent this, seems far from rational. This is especially so when one considers that for about 95 percent of the me since complex life systems appeared (about 600 million years ago), levels of CO2 were much higher than they are anticipated to become (as much as 10-20 mes today’s levels) without evidence of a relationship to global mean temperature.
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