An answerer just wrote that “it cooled in the mid 20th century in spite of increased CO2 concentrations.”
That it cooled during the mid 20th century is an observation. Rain, buoyancy, and hurricanes are also observations. But the role of science is not to sit back a marvel at all of these natural “mysteries”. It is to find out how they happen, right?
Do climatologists have any explanation as to why the mid 20th century was cool? Was the mid 20th century really cooler than the beginning of the 20th century? Do you think that the answerer’s statement is disingenuous if there is no attempt to explain why it cooled?
Update:@ jim z, That aerosols scatter and absorb shortwave radiation, thereby altering the radiative balance of the Earth-atmosphere system, is an observation, not an “amateurish theory” or a “pet theory”. You may wish to read more of the scientific literature if you’re interested in learning more about this topic. I’m skeptical hat you’re actually interested in the science though.
Also, a scientific theory is not some hair brain idea, which is how you seem to be using the word. Theories are constructed in order to explain, predict, and to master natural phenomena. A scientific theory can be thought of as a model of reality.
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There was a very small cooling from about 1940-1975, probably due primarily to increasing amounts of aerosols in the atmosphere. In particular after WWII, people were burning a lot of fossil fuels without a lot of pollution controls. Human SO2 emissions skyrocketted during that same period of 1940-1975. See pages 12-14 here:
http://www.pnl.gov/main/publications/external/tech...
These rapidly increasing anthropogenic SO2 emissions are precisely what led some scientists to conclude that if they continued the rapid rate of increase, it could lead to significant global cooling. Of course this scenario didn't happen. Various countries passed various Clean Air Acts from the '50s to the '70s, and as the link above shows, global anthropogenic SO2 emissions decreased after 1975. Not coincidentally, that's when the current rapid warming trend began. Aerosols have a very short atmospheric lifetime (they get washed out rapidly by rain), so if they're not continually being emitted, their cooling effects don't last very long. That's also why volcanoes only have cooling effects for 1-2 years after major eruptions.
Human SO2 emissions weren't the only cause of the warming pause. Volcanic activity also played a role, for example, and the PDO was in its cool phase.
http://www.globalwarmingart.com/wiki/File:Climate_...
http://jisao.washington.edu/pdo/
It's notable that despite all these cooling effects (including the natural PDO cycle), the planet only cooled about 0.1°C over this period, compared to a 0.55°C warming over the ensuing 30 years.
http://data.giss.nasa.gov/gistemp/graphs/Fig.A2.lr...
As for Meadow's comment, the Asian Brown Cloud is suspected of having masked quite a bit of warming.
"aerosols in [atmospheric brown clouds] have masked 20 - 80 per cent of GHG forcing in the past century."
http://www.unep.org/pdf/ABCSummaryFinal.pdf
This cooling, approximately 0.1 C, was probably a natural fluctuation. It fits the calculated "natural only" climate curve, according to which there should have been a further 0.1 C cooling since then, rather than the observed roughly 1.0 C warming.
See "Global Warming", John Houghton, Prof of atmospheric physics, Oxford, for a very detailed discussion.
Notice that the observed overall warming since 1950 takes place DESPITE a slight natural tendency to cool. So we cannot rely on the slow natural cooling to rescue us, and when this natural cooling reverses it will add to our problems.
Aerosols is the reason given for the mid century cooling.
The problem is, aerosols are still poorly understood. Different types of aerosols behave differently, with different parts of the atmosphere.
Anthropogenic (human produced) aerosols make up about 10 percent of the amount of aerosols in our atmosphere. Tiny particles of black carbon, or soot, are a major component of smoke produced by many kinds of burning. Coal-burning power plants generate lots of black carbon and often loft it high into the atmosphere as emissions from tall smokestacks. Internal combustion engines in cars, trucks, and construction vehicles also emit plenty of black carbon. Diesel engines are especially prolific producers of this type of aerosol. Black carbon is a strong light absorber. Aerosols that also absorb sunlight (especially black carbon) effectively increase albedo warming the atmosphere in their vicinity when the reradiate the absorbed energy in the infrared portion of the spectrum. Note that such absorption and heating may occur near Earth's surface or high above it in the stratosphere, and that the location of that heating can make a big difference in terms of the overall effect on climate.
Pollutant gases such as sulfur oxides and nitrogen oxides, which are emitted by fossil fuel combustion and a variety of industrial processes, can generate secondary emissions of aerosols as a result of chemical reactions in the atmosphere. These aerosols generally scatter sunlight and form a chemical reaction that prolongs cloud formation, which then acts as an albedo to reflect sunlight.
The problem is when some human aerosols cause warming, while others cause cooling, it is hard to determine the overall effect. Even the IPCC states the level of understanding of aerosols as low. So anyone who states with near certainty that the mid century cooling was a result of aerosols is misleading.
AGW advocates concede the mid-century decline (they kind of have to since, at the time, some of them were concerned that the decline would continue) but blame it on, of all things, man (they kind of have to since those who were concerned about the decline at the time blamed it on man).
So - do AGW advocates have an explanation? Sure. Is there some intuitive logic to it? Sure. Is it proven? Do we or they, or does anyone, KNOW why the temperatures declined from the 1940s to the late 1970s? No. We know that they did, despite the continued rise in atmospheric CO2 concentration.
Does that disprove the theory that the late-20th-century warming was caused by man? No. But it does mean that there are other, more substantial climate drivers over a multi-decade span than CO2, and that these are not fully understood, or close to it, which means it's at the very least premature to start calling for carbon taxes.
My point is that you don't know why it cooled, Dana's amateurish theory of human aerosols not withstanding. I doubt if Dana could acknowledge a natural variation in climate if his life depended on it. Blaming humans for the cool spell we had in the 1970s on humans borders on the most ridiculous claim I have heard in a long time. Just because you don't know, doesn't mean that you can insert your pet theory in there and claim it as fact.
The role of science isn't to assign blame just because the answer fits your world view. A role of science is to acknowledge its limitations. Those who claim to know things they don't are not practicing good science.
As someone who is interested in paleoanthropology, I am well aware how many times scientists exaggerate their new discoveries and theories. It happens more than not. Just because they claim something, doesn't make it so.
There are plenty of theories why it cooled. In my opinion, human emissions of CO2 or human emissions of anything else can't be credibly blamed for that cooling yet they are blamed for the subsequent warming.
They'll probably blame it on dimming caused by pollution or some other BS. Which will leave the question of why that's not happening now with China burning ever more coal.
Yes.