Exploring the Effects of Clouds on Global Warming

[billiards]

Andre, this is mainly concerning your work, especially with your claim that CO2 is not a gas that causes global warming. From what I have gathered (and I hope I don't make a fool of myself by saying this) but you have argued that CO2 levels have risen over periods of cooling in the past, correct?

I was wondering about the effects of clouds, I have done some basic calculations about the overall effect of clouds on global warming and it I concluded that clouds would cause a net cooling (I might elaborate if probed but it's probably been done more vigorously by someon more qualified so it would be nice if someone could verify this for me). Anyway, it seems that clouds would also cause a drop in photosynthesis and thus an increase in CO2, right?

So is it not possible that in the past cooling due to an increase in cloud cover (possibly caused by a variety of mechanisms eg increased volcanism, forest fires etc..) would have been met with an increase in CO2. Would this not explain the apparent disparity in the records? Therefore we should be cautious when looking at the past records and saying that CO2 is not a significant greenhouse gas, it may simply have not appeared to be significant because of the clouds!

 

[Andre]

Good thinking.

About rising CO2 during cooling, that can be seen http://home.wanadoo.nl/bijkerk/epica5.GIF in the Antarctica ice core of Dome Concordia by EPICA, around 14,000 and 11,000 years ago. That is, if we assume that isotopes represent global temperatures and the bubbles in the ice being adequate storage containers for CO2.

About the role of clouds and albedo I’d recommend this thread.

About the relationship temperature precipitation and CO2 in the Pleistocene ice ages we do have some problems which are usually not recognized.

Forgetting about the oceans, during glacial periods world wide conditions appear to be cold-arid-steppe predominant-low CO2 and during interglacials: warm-moist-forests predominant-high CO2.

As with energy calculations always look at the checksum: Glacial: little carbon in the atmosphere, little carbon in the vegetation (steppe). Interglacial: much carbon in the atmosphere, much carbon in the vegetation (forests). Hence, much more carbon around during interglacials than during glacial period. But if the moist climates generate forest growth, it tends to take up the atmospheric CO2 instead of increasing it. Hence this reasoning about precipitation creating forests, creating high CO2 has the overall problem: what is the source of all that carbon?

The carbon cycle http://earthobservatory.nasa.gov/Library/CarbonCycle/Images/carbon_cycle_diagram.jpg shows that there is a lot of exchange going on of carbon and the major basin (source / sink) is the deep ocean. This cycle tends to straighten out unbalances and if atmospheric CO2 is predominantly high or low for longer periods, this is most like due to capacity changes of the sources / sinks, right?

Temperature could be an important reason for capacity changes but a few degrees in sea surface temperature changes (perhaps the difference between glacials and interglacials) is not affecting the deep sea where the Artic deep sea is slightly warmer than the deep sea of moderate latitudes.

So what else could change the capacity of the deep ocean CO2 sink?

 

[billiards]

I suppose the Carboniferous is renowned for its high carbon in the organic sediments. I always pictured it as a balance where the more carbon in the sediments the less carbon in the atmosphere, i guess this could be wrong?

Similarly if there were more carbon in the ocean there would have to be less in the atmosphere. Carbon is typically released by volcanism, which I assume is more-or-less constant (although I know this is not at all true). I have no idea how clouds would affect the oceans carbon absorbing capacity, but I presume that they would reduce the organic absorbtion of carbon by reducing photosynthesis. If the oceans remained unaffected, I would expect to see an increase in atmospheric CO2.

So is that what you are getting at? Would clouds change the capacity of the deep ocean CO2 sink? If so how would that work?

 

[billiards]

I believe that increased rainfall would result in increased weathering, this would have an overall negative affect on the CO2 levels so the complexity of my understanding thickens.

More clouds results in less photosynthesis => more CO2
More clouds results in more rainfall => less CO2

What are the relative importance of these mechanisms? If the photosynthesis mechanism holds then I guess my earlier mechanism (see OP) still holds. Although it must be noted that the whole story is of course infinitely more complicated, for example the rising of the Tibetan plateau is thought to have triggered the Monsoon which releases a stupendous amount of rainfall, but if we see this as an alternative cloud causing mechanism (as opposed to the increased volcanism/ forest fires I mentiones earlier) it could still fit.

 

[Andre]

So is that what you are getting at? Would clouds change the capacity of the deep ocean CO2 sink? If so how would that work?

I don't think that the clouds change the capacity, they may change the rate of CO2 exchange though indeed via algea photosynthesis. The main controls of a CO2 source/sink capacity as I said before is temperature and size. The rate of exchanges between sources and sinks balances the actual contends of a basin. That's why the ice age atmospheric CO2 yoyo is so interesting.

 

[billiards]

Shouldn't CO2 rise during an ice age because the hydrological cycle slows down?

 

[Andre]

and the biological cycle as well, the prevailing habitats change from forests to steppes. Large areas get ice covered and are no longer acting as CO2 sink.

Alternately at the transition from glacial stadials to interglacial, forests regrow, glaciated areas become carbon sinks again. All takes up carbon, hence the atmospheric CO2 should decrease.

But it's the other was around. A possible explanation is the ocean temperature. Colder oceans take up more CO2 during glacials and vice versa. However there may be other explanations.

 

[William Astley]

CO2 Changes During Ice Ages, Cause?

What is causing the drop in CO2, as the Earth cools, from the interglacial warm period to the glacial coldest period?

See this review article in Nature "Glacial/interglacial variations in atmospheric carbon dioxide" by Sigman and Boyle (2000) for an explanation of why there is a 100 ppm drop in atmospheric carbon dioxide (280 ppm to 180 ppm) as the glacial cycle progressed.

http://scholar.google.com/url?sa=U&q=http://www.atmos.ucla.edu/~gruber/teaching/papers_to_read/sigman_nat_00.pdf

The 100ppm drop in CO2 is not, primarily due to colder oceans. The following is an explanation of why colder oceans alone can not account for a 100 ppm drop in CO2. (See Nature paper for details).

As there is a vast amount of fresh water in the glacial period, in the new ice sheets, the ocean becomes Salter (3%). Salter water can hold less carbon dioxide (6.5 ppm less for a 3% increase in salt content). Colder water can hold more carbon dioxide, however, the deep ocean is already an average of 4C and will freeze (salty or not) at around -1.8C. The estimated maximum drop deep in deep ocean temperature is 2.5 C. The surface subtropical oceans were estimated to have cooled by about 5C. (Note vast areas of the high latitude oceans were covered by ice, during the coldest period and could hence no longer absorb carbon dioxide.)

The reduction in carbon dioxide, due to colder oceans, is estimated to be max. 30 ppm. Now as vast areas of land which are currently forested, were covered by the glacial period ice sheets, the temperate forest is no longer using carbon dioxide which adds carbon dioxide to the atmosphere. In addition, during the glacial period large sections of tropical rain forest changes to savannah (About a third of the tropical forest changes to savannah. The planet is drier when it is colder), as savannah is less productive that tropical forests that change also adds carbon dioxide to the atmosphere. The Nature article estimates the temperate forest change and the increase in savannah, adds 15 ppm of carbon dioxide to the atmosphere.

The net for this calculation is therefore = - 30 ppm + 6.5 ppm + 15 ppm = -8.5 ppm.

As there is 100 ppm to explain the above are not the solution. The above article explains that increased biological products in the ocean.
During the glacial period there are periodic (200yr, 500 yr, and 1500 yr cycle)rapid cooling events (RCCE "Rickeys") during which there is an increase in dust (800 times above current in the Northern Hemisphere, Greenland Ice sheet cores, and about 15 times in the Southern Hemisphere, Antarctic sheet cores). The iron and phosphate in the dust causes an increase in the biologic production in regions of the Earth's ocean which are currently almost lifeless due to a lack of nutrients. The increase in biologic production removes the C02.