2010-08-13 07:18:40Basic rebuttal for 33 - "It cooled mid-century" REVISED
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
80.42.222.121
It cooled mid-century

Although temperatures increased overall during the 20th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20th century trends offer a good framework through which to understand climate change and the role of numerous factors in determining the climate at any one time.

Early and late 20th century warming has been explained primarily by increasing solar activity and increasing CO2 concentrations, respectively, with other factors contributing in both periods. So what caused the cooling period that interrupted the overall trend in the middle of the century? The answer seems to lie in solar dimming, a cooling phenomenon caused by airborne pollutants.

The main culprit is likely to have been an increase in sulphate aerosols, which reflect incoming solar energy back into space and lead to cooling. This increase was the result of two sets of events.

  1. Industrial activities picked up following the Second World War. This, in the absence of pollution control measures, led to a rise in aerosols in the lower atmosphere (the troposphere).
  2. A number of volcanic eruptions released large amounts of aerosols in the upper atmosphere (the stratosphere).

Combined, these events led to aerosols overwhelming the warming trend at a time when solar activity showed little variation, leading to the observed cooling. Furthermore, it is possible to draw similar conclusions by looking at the daily temperature cycle. Because sunlight affects the maximum day-time temperature, aerosols should have a noticeable cooling impact on it. Minimum night-time temperatures, on the other hand, are more affected by greenhouse gases and therefore should not be affected by aerosols. Were these differences observed? The answer is yes: maximum day-time temperatures fell during this period but minimum night-time temperatures carried on rising.

The introduction of pollution control measures reduced the emission of sulphate aerosols. Gradually the cumulative effect of increasing greenhouse gases started to dominate in the 1970s and warming resumed.

As a final point, it should be noted that in 1945, the way in which sea temperatures were measured changed, leading to a substantial drop in apparent temperatures. Once the data are corrected, it is expected that the cooling trend in the middle of the century will be less pronounced.

 

2010-08-13 09:43:31Hey
Robert Way

robert_way19@hotmail...
142.162.181.139
Great Post there. One minor thing is whether or not the data is available for forcings because if it is then it would be useful to have a cleaner looking graph.

Robert Way
2010-08-13 16:59:14
Ari Jokimäki

arijmaki@yahoo...
91.154.102.68

There's an interesting paper that relates to this:

A large discontinuity in the mid-twentieth century in observed global-mean surface temperature - Thompson et al. (2008)

Abstract:

http://www.nature.com/nature/journal/v453/n7195/abs/nature06982.html

Full text:

http://www.atmos.colostate.edu/ao/ThompsonPapers/Thompson_etal_Nature2008.pdf

2010-08-14 04:15:30
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
80.42.222.121

Will look around for some data, Robert.

Thanks Ari. I thought about adding the issue of uncorrected instrumental bias and I'm still debating it - a simple sentence would do, do you think?

2010-08-14 20:01:08
Ari Jokimäki

arijmaki@yahoo...
91.154.102.68
Yes, I think single sentence is enough, if you want to mention it.
2010-08-15 04:24:56Little tweak
doug_bostrom

dbostrom@clearwire...
184.77.83.151
Might change "The introduction of Clean Air Acts..." to something more reflective of a general trend in individual national legislation and regulations as well as case-specific, such as "The growth of particulate emissions regulations in many countries..."
2010-08-15 16:17:57War! What is it good for?
gpwayne
Graham Wayne
graham@gpwayne...
81.152.239.255
Hi Bio - I have a question about the cool period. I'm familiar with the particulate arguments, but considering how much stuff got blown up during the war (and how much industrial activity there was making all the stuff to blow up other stuff etc), have any studies shown a correlation between wartime pollution and dimming?
2010-08-15 19:26:52
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
212.139.87.36
Good morning Graham - I haven't found anything on the Second World War itself, but I know that in some ice core records (Greenland if I remember correctly) sulphate aerosols didn't increase substantially during this period. The increase certainly seems to follow the increase in industrial activities, and the decline follows the introduction of control measures. From what I've read, I don't think that anyone has suggested that the war contributed much to the cooling. Might look into it more.
2010-08-17 03:09:16
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
80.42.219.219
Another update.
2010-08-17 03:36:24
Ari Jokimäki

arijmaki@yahoo...
91.154.102.68
I'll give this thumbs up.
2010-08-17 14:11:45Ping
doug_bostrom

dbostrom@clearwire...
184.77.83.151
I think it's ready to go
2010-08-17 17:17:58Another thumb
gpwayne
Graham Wayne
graham@gpwayne...
86.156.59.156
Looks good to me...
2010-08-18 13:22:00Night-time temperatures
James Wight

jameswight@southernphone.com...
58.105.164.221
Not a bad summary. One other thing you might want to mention is that during the mid-century cooling period, minimum night-time temperatures did continue to increase, so the greenhouse effect didn’t just disappear in that time.
2010-08-18 13:51:14
Michael Searcy

scentofpine@yahoo...
72.91.223.97
Sounds good.
2010-08-18 17:52:22
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
80.42.219.219
Hi James. Yes, I did think about including the pattern of warming of day-time and night-time but thought it wasn't necessary for this level of rebuttal. I'll think about it for a bit.
2010-08-19 19:08:38
Anne-Marie Blackburn
Anne-Marie Blackburn
bioluminescence@hotmail.co...
80.42.219.219
Updated the rebuttal to include a discussion on daily temperature cycle
2010-08-19 22:04:54good to go
John Russell

jr@johnrussell...
82.70.63.102

Gets my Vote. Well written!

Best wishes,

JR

2010-08-21 18:16:59Twitter title suggestion
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17
Mid-century cooling: no fire without smoke
2010-08-21 23:09:37Published
John Cook

john@skepticalscience...
121.222.17.49
Just gone live but didn't use Graham's suggested heading - sorry, Graham, I got confused by the fire/smoke metaphor :-(