Guest post by Francis Massen – francis.massen [at] education.lu [see also previous posts by Francis].
Abstract:
AGHG induced climate change theory often points to decreasing diurnal temperature range [DTR] as a clear sign of global warming. The European continent does not follow that pattern: since 1970 DTR increases throughout Europe, as shown in the data of an individual station and two research papers using the European Climate Assessment dataset.
1. The global DTR trend from IPCC’s FAR.
The global trend of DTR (Daily Temperature Range = difference between daily maximum and daily minimum temperatures) has been used in IPCC’s FAR as a clear sign of global warming by human emitted greenhouse accumulation: anthropogenic GHG emissions are said to partially block and reflect outgoing heat radiation (OLR = outgoing Longwave Radiation) and thus will increase the surface temperature: this heating will be most noticeable during the night in the absence of incoming sunlight. The result should be a larger increase of the minimum daily temperature than the corresponding increase in the daily maximum temperature. As a consequence, DTR should decrease, as shown in fig.1
Fig.1 Global DTR anomaly since 1950 (IPPC, FAR 2007 [1])
Regional trends may well be different from this global decrease; this is indeed the case for Europe, as tacitly acknowledged by FAR in fig. 3.11
Fig.2 Positive DTR trend shown in FAR for France and Western Germany (turquoise arrow and circle added) [2]
Numerous papers have been written on DTR trends, and how DTR may be influenced by UHI, cloud cover, greenhouse gases, moisture, precipitation, solar radiation etc. Roger Pielke Sr. is vocal in his warnings on the unreflected use of DTR as a proxy for global warming: measuring the nightly minimum temperature is especially fraught with problems related to the stability of the boundary layer. As a consequence Roger recommends using daily maximum temperature (if the urge in relying on surface temperatures is irresistible). This warning is repeated in his discussion of a paper by McNider et al. [3], were these authors conclude that “minimum temperatures in a stable boundary layer are not very robust measures of the heat content of the deep atmosphere”
2. The European situation.
Do European observations document a continuous decrease in DTR? The answer is a resounding NO!
Let me first start by my own measurements done at meteoLCD, Diekirch, Luxembourg [4]. The trend is clearly positive, as shown by figure 3.
Fig. 3: Trends of average daily minimum, maximum, and DTR from 1998 to 2010 at Diekirch, Luxembourg [4].
The positive trend of 0.03 °C/y is not caused by the heat-wave year 2003 with an exceptional high DTR of 10.03: if this year is skipped, linear regression gives an even higher DTR slope of 0.04°C/y, significant at the 95% level.
This positive DTR trend has been documented in two papers, which both use the European Climate Assessment Dataset (ECA) of more than 2000 stations, usually distant by 75 km or less.
Klok E.J. et al [5] use 333 stations from this dataset covering most of Europe to calculate DTR trends.
Fig. 4. ECA stations (August 2007); figure from [5]
They find an average annual trend of +0.09 °C/decade; the trend is negative for Winter (-0.11 °C/d), practically zero for autumn (-0.01°C/d) and strongly positive for spring and summer (0.15 and 0.14 °C/d).
Knut Makowski from ETH, Zürich has his doctoral thesis in 2009 titled “The daily temperature variation and the solar surface radiation” [6]. He writes in the abstract: “It has been widely accepted that DTR decreased on a global scale during the second half of the twentieth century. In this thesis it is shown, however, that the long-term trend of annual DTR has reversed from a decrease to an increase during the 1970s in Western Europe and during the 1980s in Eastern Europe”. For the Benelux nations (Belgium, the Netherlands, Luxembourg) he shows a figure which suggests a DTR increase of 0.2°C during the 1998-2005 period, i.e. a mean annual trend of about +0.03°C/y, quite similar to that found at meteoLCD by linear regression.
Fig.5. DTR increase for the BeNeLux nations: +0.2°C from 1998 to 2005
(figure adapted from [6])
Makowski explains that increasing water vapour content would diminish incoming shortwave radiation from the sun and increase downwelling longwave radiation from the greenhouse gas: this means cooler days and warmer nights, i.e. decreasing DTR. This intuitive explanation should be taken with a grain of salt. Using the ESRL website [7] to calculate specific humidity at 300mb gives for the box [2.5-7.5 East latitude, 45-55 North longitude] covering a good part of Western Europe indeed a decreasing trend in specific humidity at the 300mb level, but also shows several short-time reversals, the last starting at 1998.
Fig.6. Specific humidity at 300mb decreases in a box centered on Western Europe; but this overall 40 year trend may reverse during shorter periods, as can be seen for 1976-1983 and 1998-2010.
3. Conclusion
The urge to find a simple (simplistic?) parameter documenting a global climate change may lead to hasty and faulty conclusions. If a whole continent follows a pattern opposite to that chosen as a proof of a warming world, should one not be more careful using “global” parameters? Roger Pielke Sr, with admirable tenacity keeps telling us that regional climate is what matters and that playing with global averages may be interesting for the junior academic, but useless for solid political decisions. The DTR case is another validation of this position.
REFERENCES:
1. Climate Change 2007: Working Group I: The Physical Science Basis
Fig.3.2
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-2-2.html
2 Climate Change 2007: Working Group I: The Physical Science Basis
Fig.3.11
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3-2-2-7.html
4 F. Massen, meteolcd: http://meteo.lcd.lu/trends/meteolcd_trends.html
5. Klok, E.J., Klein Tank A.M.G. Updated and extended European dataset of daily climate observations. International Journal of Climatology, Vol.29, issue 8, pp. 1882-1191, 2009
6. Makowski, K.: The daily temperature amplitude and surface solar radiation. PhD dissertation. Diss. ETH 18319, 2009
http://www.iac.ethz.ch/people/mknut/diss
7. Earth System Research Laboratory: http://www.esrl.noaa.gov/psd/cgi-bin/data/timeseries/timeseries1.pl
Climate Science: Roger Pielke Sr. 