Das Wetter und der Klimawandel

Beiträge mit Schlagwort ‘global warming’

Scientific Award for Michael Mann

From 22. – 27. April 2012 General Assembly of European Geosciences Union (EGU) takes place in Vienna. During this meeting climate scientist Michael E. Mann received the Oeschger-Medal for his research! Hans Oeschger (1927) was a Swiss scientist, famous because of his ice core research. Together with Willi Dansgaard he discovered a series of abrupt climate changes (Dansgaard-Oeschger Events) during the last glacial period by analysis of Greenland ice cores. Also he measured first the glacial-interglacial change of atmospheric CO2 encased in ice core´s tiny air bubbles.

Prof. Michael E. Mann holding a tree grate with tree rings Source: Homepage Michael E. Mann

Michael E. Mann came to be known for his “hockeystick”-curve. This curve was the result of a reconstruction of average temperatures last millenium-round on northern hemisphere, published by Mann and his colleagues in 1999 (M.E.Mann,R.S.Bradley, M.K.Hughes: „Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations“, Geophysical Research Letters, 1999) It was really a pioneering feat!

The team of scientists used proxy data from tree rings, corals and ice cores on the one hand and instrumental temprature data on the other hand such as it was. The „hockeystick” demonstrated at first how extraordinary (human-made) global warming during 20th century really was! For that reason all climate skeptics dislike this curve up to the present day.

The Hockeystick-Curve. The graph resembles a hockeystick in shape: with a long part of curve declining slightly as the shaft and with a short part of curve upturning suddenly as the blade.   Source: Mann, Bradley und Hughes 1999

Assembling and evaluation of all data for “hockeystick” happened in a very sophisticated way. The proxy-data, mainly tree rings extending to 1980 were validated by instrumental data back to 1854, occasional to 18th century yet. Long proxy record and shorter instrumental record overlapped a good way. Using only instrumental data from time intervall 1902 to 1980 for calibration of proxy data, Mann and his collegues were able to use leftover intervall from 1854 to 1901 (and the rare earlier instrumental data from 18th century) to verify this calibration. Calibration and validation of proxy data  worked accurately, meaning that „hockeystick“ is correct in this regard.

CO2 is a greenhouse gas but it also acts as a fertilizer, especially for trees in high elevations. More CO2 let tree ring width and tree ring density (of summerly late wood) mimic an additional rise in temperature that does not really exist. Thus the tree ring data were readjusted.

Above: Records of two tree ring data series (ITRDB Millenial, North American Treeline), which diverged from 19th century on due to influence of CO2. ITRDB Millenial includes trees in high elevations much sensitive to dunging effect of CO2. Below: Variations in atmospheric CO2 and residual between the two data series, used for readjustment. Source: Mann, Bradley und Hughes, 1999 

Two examples how carefully Mann and his team had gone about it! All that and then some you can read in original paper, downloadable on Mann´s homepage (see sources below). It´s worth it! Michael E. Mann was also a lead author of 3rd. IPCC-Report about global warming in 2001. Again and again some climate sceptics tried to discredit Mann scientifically and also personally. Fortunately they failed yet.  I hope it stays that way!

Jens Christian Heuer

Sources: European Geosciences Union (EGU) , Homepage Michael E. Mann

Global Warming, Arctic Amplification and Extreme Weather

A brand-new paper from Jennifer A. Francis and Stephen J. Vavrus „Evidence linking Arctic amplification to extreme weather in mid-latitudes“, (GEOPHYSICAL RESEARCH LETTERS, VOL. 39, 2012)
sheds new light on the issue of extreme weather events due to global warming!
The two scientists found evidence that enhanced Arctic warming relative to mid latitudes (Arctic amplification, AA) leads to prolonged extreme weather events, like droughts, heat waves (such as in Russia, summer 2010!), heavy rain and cold spells (remember persistent chilly conditions in recent winters!).

Arctic Amplification means enhanced arctic warming relative to mid latitudes due to a sort of inverse Ice-Albedo-Feedback. Decreasing bright, highly reflective sea ice is replaced by dark open water, absorbing sunlight strongly. Open water also releases a lot of moisture and latent heat in artctic atmosphere.
Due to the fact that jetstream is driven by gradient in air-temperature (and gradient in air pressure arising thereby, respectively) between polar regions and mid latitudes, jetstream slows down and becomes wavier.

Seasonal anomalies in 1000-500 hPa thicknesses (m) north of 40°N during 2000–2010 relative to 1970–1999: (a) autumn (OND), (b) winter (JFM), (c) spring (AMJ), and (d) summer (JAS). White asterisks indicate significance with chance level p < 0.05. 1000-500 hPa thickness reveals aerial vertical thermal expansion by means of distance between 1000 hPa- and 500 hPa isobaric surface. Source: Francis and Vavrus, 2012

As a result high and low pressure systems moved by jetstream slow down, too.
These weather systems arise from turbulence in jetstream like the vortexes of a raging river do.

A wavy jetstream results in cold spells via wave troughs (in reverse warm spells via wave crests, respectively), whereas high air moisture makes possible  a lot of snow.

Finally a nice animation of the jetstream of northern hemisphere from NASA/Goddard Space Flight Center (Scientific Visualization Studio):

Jens Christian Heuer