New paper finds "greenhouse cooling" of the stratosphere over past 52 years

A new paper published in Atmospheric Chemistry and Physics finds the stratosphere of the Northern Hemisphere cooled over the past 52 years due to the increase of greenhouse gases. The paper suggests that stratospheric cooling is a "more suitable" signal of anthropogenic global warming than trying to find a mid-troposphere hot spot (which was previously considered to be the definitive "fingerprint" of man-made global warming, but still has not been found despite millions of weather balloon and satellite observations over the past 60 years):

According to the authors, 

A major open question that still remains to be answered is whether the stratosphere can be considered as a more suitable region than the troposphere to detect anthropogenic climate change signals and what can be learned from the long-term stratospheric temperature trends. Indeed, the signal-to-noise
ratio in the stratosphere is, radiatively speaking, more sensitive
to anthropogenic GHG forcing and less disturbed by
the natural variability of water vapour and clouds when compared
to the troposphere. This is because (a) the dependence
of the equilibrium temperature of the stratosphere on CO2 is
larger than that on tropospheric temperature, (b) the equilibrium
temperature of the stratosphere depends less upon
tropospheric water vapour variability and (c) the influence
of cloudiness upon equilibrium temperature is more pronounced
in the troposphere than in the stratosphere where
the influence decreases with height (Manabe and Weatherald,
1967). Furthermore, anthropogenic aerosols are mainly
spread within the lower troposphere (He et al., 2008), and
presumably have little effect on stratospheric temperatures. 

Another open question is whether the lower stratosphere
has been cooling in the time since a reasonable global network
became available, i.e. after the International Geophysical
Year (IGY) of 1957–1958. Such a long-lasting cooling
from the 1960s until today would need to be explained.
To what extent are the cooling trends in the lower stratosphere
related to human-induced climate change? Has the
cooling been accelerating, for instance at high latitudes in
winter/spring due to ozone depletion? Has it been interrupted
by major volcanic eruptions and El Niño events (Zerefos et
al., 1992) or large climatological anomalies. 

This study addresses those questions and presents a new
look at observed temperature trends over the Northern Hemisphere
from the troposphere up to the lower stratosphere in a
search for an early warning signal of global warming, i.e. a
cooling in the lower stratosphere relative to the warming in
the lower atmosphere.

However, even the most ardent fans of anthropogenic global warming theory don't agree on why an increase of "heat trapping" greenhouse gases would have the opposite effects of causing the stratosphere to cool and the troposphere to warm. 

Further, many warmists claim any source of warming including solar activity, cloud changes, ocean oscillations, etc. would cause a mid-troposphere "hot spot" and overlying cooling of the stratosphere, and would not necessarily be a signal or "fingerprint" of anthropogenic global warming. 

The authors also find from 1958-1979 the lower troposphere either slightly cooled or remained unchanged, followed by significant warming 1980-2010:

From 1958 until 1979, a non-significant trend (0.06 ± 0.06 °C decade−1 for NCEP) and slightly cooling trends (−0.12 ± 0.06 °C decade−1 for RICH) are found in the lower troposphere. The second period from 1980 to the end of the records shows significant warming (0.25 ± 0.05 °C decade−1 for both NCEP and RICH). Above the tropopause a significant cooling trend is clearly seen in the lower stratosphere both in the pre-1980 period (−0.58 ± 0.17 °C decade−1 for NCEP, −0.30 ± 0.16 °C decade−1 for RICH and −0.48 ± 0.20 °C decade−1 for FU-Berlin) and the post-1980 period (−0.79 ± 0.18 °C decade−1 for NCEP, −0.66 ± 0.16 °C decade−1 for RICH and −0.82 ± 0.19 °C decade−1 for FU-Berlin).

Thus, although it appears the stratosphere may be cooling, and this could be due to increased greenhouse gases increasing radiative surface area and thus emissivity to space, there is still no evidence of a mid-troposphere "hot spot" predicted by climate models. The slight cooling to no change of lower tropospheric temperatures from 1958-1979 found by this paper also don't support AGW theory since CO2 levels rose ~7% during that period. 

Atmos. Chem. Phys., 14, 7705-7720, 2014 www.atmos-chem-phys.net/14/7705/2014/ doi:10.5194/acp-14-7705-2014 Evidence for an earlier greenhouse cooling effect in the stratosphere before 1980 over the Northern Hemisphere

C. S. Zerefos^1,2, K. Tourpali³, P. Zanis⁴, K. Eleftheratos⁵, C. Repapis^1,10, A. Goodman⁶, D. Wuebbles⁷, I. S. A. Isaksen⁸, and J. Luterbacher^9
1Research Centre for Atmospheric Physics and Climatology, Academy of Athens, Athens, Greece
²Navarino Environmental Observatory (N.E.O.), Messinia, Greece
³Laboratory of Atmospheric Physics, Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece
⁴Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, Thessaloniki, Greece
⁵Laboratory of Climatology & Atmospheric Environment, University of Athens, Athens, Greece
⁶Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
⁷Department of Atmospheric Sciences, University of Illinois, Urbana, IL, USA
⁸Department of Geosciences, University of Oslo, Oslo, Norway
⁹Climatology, Climate Dynamics and Climate Change, Department of Geography, Justus-Liebig University of Giessen, Giessen, Germany
¹⁰Mariolopoulos-Kanaginis Foundation for the Environmental Sciences, Athens, Greece
Abstract. This study provides a new look at the observed and calculated long-term temperature changes from the lower troposphere to the lower stratosphere since 1958 over the Northern Hemisphere. The data sets include the NCEP/NCAR reanalysis, the Free University of Berlin (FU-Berlin) and the RICH radiosonde data sets as well as historical simulations with the CESM1-WACCM global model participating in CMIP5. The analysis is mainly based on monthly layer mean temperatures derived from geopotential height thicknesses in order to take advantage of the use of the independent FU-Berlin stratospheric data set of geopotential height data since 1957. This approach was followed to extend the records for the investigation of the stratospheric temperature trends to the earliest possible time. After removing the natural variability [it is impossible fully distinguish natural variability from anthropogenic] with an autoregressive multiple regression model our analysis shows that the period 1958–2011 can be divided into two distinct sub-periods of long-term temperature variability and trends: before and after 1980. By calculating trends for the summer time to reduce interannual variability, the two periods are as follows. From 1958 until 1979, a non-significant trend (0.06 ± 0.06 °C decade⁻¹ for NCEP) and slightly cooling trends (−0.12 ± 0.06 °C decade⁻¹ for RICH) are found in the lower troposphere. The second period from 1980 to the end of the records shows significant warming (0.25 ± 0.05 °C decade⁻¹for both NCEP and RICH). Above the tropopause a significant cooling trend is clearly seen in the lower stratosphere both in the pre-1980 period (−0.58 ± 0.17 °C decade⁻¹ for NCEP, −0.30 ± 0.16 °C decade⁻¹ for RICH and −0.48 ± 0.20 °C decade⁻¹ for FU-Berlin) and the post-1980 period (−0.79 ± 0.18 °C decade⁻¹ for NCEP, −0.66 ± 0.16 °C decade⁻¹ for RICH and −0.82 ± 0.19 °C decade⁻¹ for FU-Berlin). The cooling in the lower stratosphere persists throughout the year from the tropics up to 60° N. At polar latitudes competing dynamical and radiative processes reduce the statistical significance of these trends. Model results are in line with reanalysis and the observations, indicating a persistent cooling (−0.33 °C decade⁻¹) in the lower stratosphere during summer before and after 1980; a feature that is also seen throughout the year. However, the lower stratosphere CESM1-WACCM modelled trends are generally lower than reanalysis and the observations. The contrasting effects of ozone depletion at polar latitudes in winter/spring and the anticipated strengthening of the Brewer–Dobson circulation from man-made global warming at polar latitudes are discussed. Our results provide additional evidence for an early greenhouse cooling signal in the lower stratosphere before 1980, which appears well in advance relative to the tropospheric [assumed] greenhouse warming signal. The suitability of early warning signals in the stratosphere relative to the troposphere is supported by the fact that the stratosphere is less sensitive to changes due to cloudiness, humidity and man-made aerosols. Our analysis also indicates that the relative contribution of the lower stratosphere versus the upper troposphere low-frequency variability is important for understanding the added value of the long-term tropopause variability related to human-induced global warming.