Here’s a reminder of a climatic footnote to the 9/11 terrorist attacks on New York and Washington DC, ten years ago this weekend. With civilian aircraft grounded for three days, and without the contrails that usually criss-cross the skies of the USA, the difference between daytime and night-time temperatures at the surface increased. (See the Travis reference below.) Apparently like many other clouds (not all) the contrails reduce sunshine during the day and blanket the loss of heat at night. Take away those man-made clouds and the days become a little warmer and the nights a little cooler. In the jargon: the diurnal temperature range (DTR) increases.
An echo of that mini-climatic event comes with the news that the DTR in Europe increases when there’s a big reduction in cosmic rays arriving at the Earth. With the implication that the skies are less cloudy at such times, it’s strong evidence in favour of Henrik Svensmark’s hypothesis that cosmic rays help to make clouds. The report comes from Aleksandar Dragić and his colleagues at the Institute of Physics in Belgrade. I’m grateful to Bengt Andersson for drawing their paper to my attention. It was published on 31 August and the full text is available here http://www.astrophys-space-sci-trans.net/7/315/2011/astra-7-315-2011.pdf
It’s typical of the pathetic state of science reporting that I still seem to have the story to myself ten days later.
More than a year ago I began a succession of posts on whether or not observations in the real world support or falsify the Svensmark hypothesis. The most explanatory was the first – see http://calderup.wordpress.com/2010/05/03/do-clouds-disappear/
The focus was on the “natural experiments” in which big puffs of gas from the Sun block some of the cosmic rays coming from the Galaxy towards the Earth. The resulting falls in cosmic ray influx, called Forbush decreases, last for a few days. The game is to look for observable reductions in cloudiness in the aftermath of these events. The results are most clearly favourable to the Svensmark hypothesis for the Forbush decreases with the largest percentage reductions in cosmic rays. Scientists keen to falsify the hypothesis have only to mix in some of the weaker events for the untidiness of the world’s weather to “hide the decline”.
The Serbs avoid that blunder by picking out the strongest Forbush decreases. And by using the simple, reliable and long-provided weather-station measurements of temperature by night and day, they avoid technical, interpretive and data-availability problems that surround more direct observations of clouds and their detailed properties. The temperatures come from 184 stations scattered all across Europe (actually, so I notice, from Greenland to Siberia). A compilation by the Mount Washington Observatory that spans half a century, from 1954 to 1995, supplies the catalogue of Forbush decreases.
The prime results are seen here in Dragić et al.‘s Figure 5. The graphs show the increase in the diurnal temperature range averaged across the continent in the days following the onset of cosmic ray decreases (day 0 on the horizontal scales). The upper panel is the result for 22 Forbush events in the range 7−10%, with a peak at roughly +0.35 oC in the diurnal temperature range. The lower panel is for 13 events greater than 10%. The peak goes to +0.6 oC and the influence lasts longer. It’s very satisfactory for the Svensmark hypothesis that the effect increases like this, with greater reductions in the cosmic rays. The results become hard (impossible?) to explain by any mechanism except an influence of cosmic rays on cloud formation.
To be candid, these results are much better than I’d have expected for observations from a densely populated continent with complex weather patterns, where air pollution and effects of vegetation confuse the picture of available cloud condensation nuclei. Svensmark’s team has emphasised the observable effects over the oceans. Now the approach taken by the Belgrade team opens the door to similar investigations in other continents. Let a march around the world’s land masses begin!
USA: diurnal temperatures post-9/11
D.J. Travis, A. Carleton and R.G. Lauritsen, “Contrails reduce daily temperature range”, Nature 418, 601, 2002
Europe: diurnal temperatures after Forbush decreases
A. Dragić, I. Aničin, R. Banjanac, V. Udovičić, D. Joković´, D. Maletić and J. Puzović, “Forbush decreases – clouds relation in the neutron monitor era”, Astrophysics and Space Sciences Transactions, 7, 315–318, 2011.
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