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Climate Change Explanation

2014 November 25. A question I asked on the Wikipedia Reference Desk. I have removed the responses that don't directly attempt to answer my questions.

Q: I've been hearing about Climate Change for years, but I've never come across a good, solid explanation of the phenomena. I just saw a story in IEEE Spectrum that "Climate scientists have definitively shown that the buildup of carbon dioxide in the atmosphere poses a looming danger.", but when I follow the link I only find more of the same kind of stuff I have been seeing.
What I would like to see would include, at the minimum,
- an explanation of how much radiation is reflected/absorbed depending on the level of CO2 in the atmosphere,
- a historical record of global temperatures going back 100,000 years along with an estimate of how accurate those temperatures are, and
- a summary of CO2 sources and sinks and their relative size.
Oh, and without all the political claptrap that seems to the largest part of any story about climate change. Is that too much to ask? (talk) 15:37, 25 November 2014 (UTC)

So, to directly address the original questions: Grab the 2007 IPCC Assessment Report (2007 is the most current complete report, though a new report Climate Change AR5 will be forthcoming in the next few weeks). Among many thousands of pages of background material, it contains, in direct answer to the OP's specific asks:
  • "a historical record of global temperatures going back 100,000 years along with an estimate of how accurate those temperatures are":
  • "an explanation of how much radiation is reflected/absorbed depending on the level of CO2 in the atmosphere":
    • Box 1, Figure 5.1, schematic illustration of multiple interactions between ice sheets, solid earth and the climate system which can drive internal variability and affect the coupled ice sheet–climate response to external forcings on time scales of months to millions of years
    • Chapter 07: Clouds and Aerosols.
    • Chapter 08: Anthropogenic and Natural Radiative Forcing.
    • Figure 8.1, Calculation Methodology cartoon; and in fact, this entire chapter.
  • "a summary of CO2 sources and sinks and their relative size.":
    • Chapter 8.2.1, "Introduction" to atmospheric chemistry. Anthropogenic and natural inputs are considered.
Nimur (talk) 19:59, 25 November 2014 (UTC)
1) Carbon dioxide (CO2) enhances the greenhouse effect, causing more radiative energy to be trapped near the surface of the earth. Ultimately, nearly all energy in the atmosphere derives from the sun, and the sun's input isn't changing significantly, but the CO2 holds that energy closer to the surface. The analogy that is often used is something like putting on a winter coat. The coat itself doesn't generate any heat, but it holds in the heat allowing the person inside to stay warmer. For convenience, scientists often use a simplified description of CO2 impacts on radiation, wherein they equate the radiative impact of CO2 changes with an equivalent amount of excess sunlight at the top of the atmosphere. That is to say, if the sun increased by X then that's about the same as CO2 increasing by Y. This is an over simplification for a number of reasons, but it is a convenient framework for thinking about the problem of greenhouse gases, so it is widely used. For CO2, the equivalent top of atmosphere radiative forcing is
\Delta F \approx 5.35 \times \ln {C \over C_0}~\mathrm{W}~\mathrm{m}^{-2} \,
where C is the current CO2 concentration and C0 is the preindustrial concentration (280 ppm). The logarithm comes about because the wings of the CO2 absorption band are approximately exponential. So, to give some concrete numbers. Today's CO2 level is about 400 ppm. So that is roughly the same as \Delta F \approx 5.35 \times \ln {400 \over 280} = 1.9 ~\mathrm{W}~\mathrm{m}^{-2} \, . Averaged over the whole Earth, the sunlight at the top of the atmosphere is about 340 W/m2, so the impact of CO2 thus far is equivalent to about a 0.55% increase in sunlight. Now, 0.55% doesn't sound like a lot, but the average temperature of the Earth is presently about 288 Kelvin (15 Celsius / 60 Fahrenheit), and without the sun we'd be near 0 Kelvin (-273 C / -460 F). 0.55% of 288 K is still about 1.5 degrees C (2.7 degrees F) of warming. That's a very oversimplified way of looking at it, but still gets about the right magnitude of effect. A more sophisticated way is to introduce the notion of a climate sensitivity, which is just a fancy way of saying how much will the temperature change for a given amount of radiative forcing. Unfortunately, the climate sensitivity still has large uncertainties, but recent estimates suggest the equilibrium climate sensitivity is roughly 2 to 4.5 degrees C per doubled CO2 (= 0.5 to 1.2 degrees C per W/m2 radiative forcing equivalent). Using the present 400 ppm of CO2, that would lead to an estimate of 1.0 to 2.3 degrees C (1.7 to 4.1 degrees F) at equilibrium, i.e. allowing that CO2 levels stayed approximately constant for long enough for the oceans to reach a steady temperature. Observed warming since 1850 is about 1.0 C (1.8 F), at the low end of the predicted range, but the oceans are still absorbing heat and even if CO2 levels stopped increasing we would have a long time to go before temperatures stabilized. Dragons flight (talk) 20:12, 25 November 2014 (UTC)
And if you're not willing to read long documents (I'll be honest--I'm not), here are a few links to get the OP started.
Carbon cycle has a good chart showing the major carbon sinks/sources, along with how much they release/absorb per year. It doesn't have percentages, but you can compute them yourself.
Temperature of the Earth, with multiple sources for the past 100,000 years: [3] --Bowlhover (talk) 21:21, 25 November 2014 (UTC)
3) Carbon in the Earth system can roughly be divided into four groups: atmosphere, biosphere on land (including both living and recently deceased plants/animals), oceans, and near-surface geosphere (carbon bound in rocks). Each of these pools include very large quantities of carbon and except for the geosphere fluxes between pools are also large. Environmental carbon is usually expressed as gigatons of carbon (GtC) equal to 1012 kg of carbon, and only the carbon content is counted regardless of if it is bound to oxygen (as CO2) or some more complex organic form. Using the numbers in carbon cycle, the four pools of carbon contain roughly the following at present: atmosphere 720 GtC, biosphere on land 2,000 GtC, oceans 38,400 GtC, geosphere 75,000,000 GtC. As one can see the atmospheric piece is actually the smallest, while the geosphere pool is huge. A small fraction of the geosphere pool consists of exploitable fossil fuels, about 4000 to 6000 GtC. The fluxes between the pools are also large, except for those involving the geosphere. In addition, most of the exchanges are roughly symmetrical. For example, nearly the same amount of carbon moves from ocean to air as from air to ocean each year, about 90 GtC/yr, with an estimated net flux of only about 2 GtC/yr from atmosphere to ocean. Similarly from atmosphere to biosphere and from biosphere to atmosphere, the flux each way is about 120 GtC/yr, with a net flux towards the biosphere estimated at about 3 GtC/yr. With exchanges this large (e.g. 120 GtC/yr), the atmosphere and biosphere take only about 15 years to equilibrate, so for long-term purposes they can be imagined as a single pool. The ocean carbon pool is much larger, so it can take hundreds of years to equilibrate. The natural flux from the geosphere to the atmosphere and ocean is small, ~0.5 GtC/yr, and mostly due to a combination of weathering of rocks and volcanic activity. The net flux into the geosphere is even smaller at present, ~0.1 GtC/yr. By contrast, the carbon we are adding to the atmosphere by intentionally extracting and burning fossil fuels is about 8 GtC/yr. As we add this carbon to the atmosphere, it shifts into the biosphere and ocean. This redistribution is important to us as it presently offsets about half of what we emit. If we could magically stop burning fossil fuels tomorrow, then the atmospheric levels would decline for a long time as carbon dioxide continued to move out of the atmosphere and into the ocean and land. However, because the flux into the geosphere is so low, it will take many thousands of years, to move carbon out of the land/air/ocean system and return it to the ground.
Related to this, the ability of the ocean to take up carbon is also rather complicated. Though it contains 38,000 GtC, only about 1% of that exists as CO2. Most of the remainder is either bicarbonate (HCO3-) or carbonate ions (CO4-2). The balance between the ocean and the atmosphere happens when the partial pressure of dissolved CO2 in the surface water is equal to the partial pressure of CO2 in the overlying air, which sets the boundary condition; however, the transformation from CO2 to carbonate and bicarbonate is also influenced by the pH of the ocean and the abundance of cations (e.g. Ca+2). The influences of these factors are expressed through the Revelle factor, which expresses the change in total ocean carbon as a function of changing CO2 levels. In rough terms, a 100% increase in atmospheric CO2 equilibrates with about an 8% increase in ocean total carbon content. The consequence of this is that even though the ocean is a huge carbon pool, it can only capture about 50% of our fossil fuel emissions. The land captures another 35%, leaving ~15% of the emissions to linger in the atmosphere for thousands of years. Dragons flight (talk) 05:37, 27 November 2014 (UTC)
The fundamentals first. The laughable attempts to pass off CO2 sensitivity estimates based on the temperature record since 1850 should be regarded as stabs in the dark, as the error bounds associated with a temeperature reconstruction such as HADCRUT4 are large and ever increasing, once we go back beyond 30 odd years ago. Greglocock (talk) 21:44, 25 November 2014 (UTC)
Svante Arrhenius computed an approximate climate sensitivity from first principles around 1900. Since we are fairly far from climate equilibrium, it's indeed non-trivial to derive climate sensitivity directly from the recent temperature and CO2 record. But then I don't think that is a major method used. --Stephan Schulz (talk) 16:16, 27 November 2014 (UTC)
it is a difficult subject. In absolute terms the changes are quite small, temperature is relative to absolute zero at -273°C so a change of 2°C is less than a single percent - and yet it means a huge change for us. It is difficult to get within a factor of 2 about the probable change, a lot of work has been done and the estimates and error bounds are the best that can be done at the moment. At the end of the day it comes down to whether you think all those scientists are actually doing their best to come up with a good estimate or whether you think they are practically all deluded or involved in a giant conspiracy. As to acting on what they say the question is rather like going to the doctor and being told you have cancer. Of course a lot of people will just deny anything is wrong with them, and sometimes nothing bad does happen as the outcome isn't definite, but is it a rational way of dealing with bad news? Dmcq (talk) 23:36, 25 November 2014 (UTC)
The effects of global warming on the human world is hard to predict, but the OP is asking for more definite information. The sizes of carbon sources/sinks and how much radiation CO2 absorbs are both measurable, the latter to a very high precision. --Bowlhover (talk) 00:24, 26 November 2014 (UTC)
I was explaining that what they were asking for was too much to ask for which is what they asked. Dmcq (talk) 08:44, 26 November 2014 (UTC)
  • CO2 as a forcing is fairly easy to find in IPCC. I believe the total increase of CO2 from 1750 to present (50% increase) is equivalent to 1.5 W*m2 (the total from the sun is on average around 1500 W*m2). Annual, seasonal, and daily fluctuations of CO2 are pretty large. Solar activity variation is on this order of magnitude. The larger concern isn't what's happened so far as it's not particularly significant (or even attributable to CO2), but the lifetime and accumulation of CO2 makes the 50 and 100 year projection significant. Also, it's not clear what happens to other GHG's such as water vapor so while the forcing of CO2 might be known, how it affects other forcings (i.e. how sensitive the climate is to CO2) is still being researched.
  • There's nothing that will compare to the recent record. Even since 1850, natural variations have swamped the global warming signature. For example, glaciers that are in retreat since 1850 had periods of growth such as from 1950 to 1970. To see global wrming effects directly requires pulling a very small signal from data that naturally varies. It's also not understood why most of the surface warming is arctic with little tropical warming and antarctic cooling. It was only recently discovered that a lot of measured sea level rise in Greenland and even the landfall of Sandy was due to a teeter-totter effect of melting glaciers (land rises under the glacier, sinks in another place).
  • Carbon cycle [4] shows some of the numbers. Human contributions are mostly fossil fuel combustion and cement. It is a relatively small contribution and some of the sinks have actually adapted (i.e. oceans have absorbed about half of all human emissions which changes the pH of the oceans, also a warming ocean will start returning CO to the atmosphere). One of the difficulties in assessing the ultimate effect of global warming is that the contributions year over year are very small. Much smaller than natural variation which is why climate change must be studied over decades. It's also why weather variations cannot be attributed to climate change. --DHeyward (talk) 07:33, 26 November 2014 (UTC)
There is no "good, solid explanation" but we clearly see "good and solid" evidence that something is seriously going wrong(like rapidly melting glaciers). Additionally it is absurdly strange that every producer has to prove his products can cause no harm and that they are foolprove reliable but in the climatechange debate this seems turned around and made near impossible that way that even clear evidence is not enough to prove massive pollution is causing seriouse harm. --Kharon (talk) 16:04, 26 November 2014 (UTC)
Since we are coming out of an Ice Age wouldn't it be a bit odd if the glaciers weren't melting? Greglocock (talk) 19:39, 26 November 2014 (UTC)
The ice age cycle is approximately 100,000 years long. You would indeed not expect any noticeable changes after only 100 years, which is 0.1% of the cycle, but drastic changes have been observed in the past few decades. Look at the data and convince yourself that the ice age temperature changes are orders of magnitude too small to explain the recent warming. Also notice in that graph that we're already near the peak of the cycle, where natural warming rates are expected to slow down or reverse. --Bowlhover (talk) 20:17, 26 November 2014 (UTC)
You are assuming some sort of gradualism argument. Since the climate is chaotic there is no reason to assume that all changes should be slow. Greglocock (talk)
Here is a graph of temperature change over the last 450,000 years [5]. This is part of a series of graphs of global temperature over different time scales. Each page links to the next longer and shorter time scales. The next shorter time scale is 12,000 years, which has estimates by several different methodologies plotted on the same graph, and can give you an idea of the inherent error in the estimations. Given that glaciation cycles last about 100,000 years, I find it useful to look at the 5 million year time frame (the next longer time scale). At some point continental drift comes into play, so time scales longer than a few million years are probably not useful in separating the anthropogenic component from the natural short-term baseline. It is still instructive to look at the 65 million year plot to see how much cooler it's gotten since the dinosaurs went extinct. Ice age gives a pretty good overview of natural temperature cycles, and includes the 450,000 and 5 million year plots I've mentioned (click on them to see the bigger version). I hope you're still reading in spite of the political diatribes.--Wikimedes (talk) 07:48, 27 November 2014 (UTC)

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