Ponderer and I have posted “dueling” commentaries on global warming. A reader sent a paper to join the discussion. It is longer than commentaries usually posted on this site, but it has so much information that deserves consideration that I’ve decided to post it in its entirety with a few minor edits.
The so-called ‘greenhouse effect’ comes about by short wave radiation impinging on the earth from the sun. Some fraction of this short wave radiation is reflected back into space with little effect. Another fraction is absorbed by the earth. Essentially blackbody long wave radiation is emitted from the earth’s surface as a result. Carbon dioxide (and a few other gases that we will get to) absorbs and reemits this longer wave radiation. It emits the longer wave radiation in all directions, so some fraction comes back to be reabsorbed by the earth’s surface (either soil or water). On balance under these conditions there is more heat (in the form of both long and short wave radiation) entering the system than leaving it, so overall heating occurs.
This greenhouse effect is logarithmic in that a doubling of CO2 increases the earth’s temperature by a constant amount (all other things being equal, and we will see that not all things are equal). This so called forcing number (the number of degrees of warming due to a doubling of CO2) has been given a great deal of study. My best reading of the literature is that the forcing is around 1.5 degree Celsius. The current level of CO2 in the atmosphere is about 400 ppm. So, all other things being equal, a doubling to 800 ppm should heat the earth by about 1.5 degrees Celsius. CO2 levels in 1750 (approximately the start of the industrial revolution) were about 280 ppm.
Complicating matters is that land and sea both absorb CO2 from the atmosphere and release it to the atmosphere under varying conditions. Certainly plants take CO2 into the soil and plankton takes it into the sea. Also warmer seas release CO2 into the atmosphere. All of the pluses and minuses of these processes are not well understood (e.g. it is only recently known that fungi take a great deal of CO2 from the atmosphere). Interestingly, much of the CO2 residence time data comes from residual carbon-14 from the nuclear testing era.
We know that the earth was as warm as or warmer than today through much of earth’s history. By looking at ice core data (where air bubbles are trapped in the ice), we can see both CO2 levels back many millions of years, but also see from the oxygen-18 content, a proxy for temperature. As the earth’s temperature has increased and decreased, the atmospheric CO2 concentration has lagged these changes. At higher temperatures, there are always higher levels of CO2.
In historical times, we know that the mid-Roman period was a great deal warmer than today (ocean levels, agricultural practices in northern climates, etc.). We also know that from about 1000-1300 AD, temperatures were a great deal warmer than today (the famous Viking settlements in Greenland example). More recently, from about 1600-1850 the earth was unusually cold. This period is called the “little ice age”. We had the Thames frozen in London, and the canals of Holland frozen for skating. Since about 1850 to today, the earth has been slowly warming and the sea levels have been slowly rising due to ice melt and water expansion (at warmer temperatures). In the past century, we know that the 1930’s were as warm as or warmer than today, and that from around 1960 to 1980 the earth cooled (thus, the cries that we were entering another ice age that were heard in the 1970’s). From 1980 to about 1997, the earth warmed a small amount (less than half a degree Celsius), and since then the temperature has been relatively flat. Overall, from the end of the little ice age to today, the earth has warmed between 1.0-1.5 degrees Celsius.
Unfortunately, the only reliable global temperature measure that we have is from the satellite era (about 1979 to today). That time period happens to correspond to a period of warming followed by a period of relatively constant temperatures. Different proxies have been used to try to reconstruct historical temperatures to varying degrees of success. These proxy methods all suffer from problems of linearity (does the proxy change measure the temperature change or some other phenomenon) and lack of global coverage. The best measure we have is the central England temperature measurement that appears to measure temperature using thermometers at one location from about 1750 to today. Those temperatures match our understanding of the little ice age and the 20th century changes.
Why has this 1.0-1.5 degree of warming occurred, and should we be worried? The models in vogue to forecast global temperatures assume that a doubling of CO2 will increase the earth’s temperature from about 5-7 degrees Celsius. If that is true, we should be somewhat worried. The models get this much larger forcing (than the 1.5 degrees of CO2 forcing) by assuming there are positive feedbacks in the system that amplifies the CO2 forcing. The largest of these assumed positive feedbacks comes from water vapor. Water vapor in the atmosphere certainly can act as a greenhouse gas, and increasing temperatures could increase the water vapor level and thus amplify temperature increases. However, increased water vapor levels also increase cloud formation. Increasing clouds would reduce the amount of short wave radiation reaching the earth’s surface, and would tend to lower the earth’s temperature.
The climate models do not account for cloud formation. Experimental data is sparse, and there are reports of both positive and negative temperature feedback from water vapor. There are other feedback mechanisms possible (e.g. methane), but the water vapor feedback is probably the largest.
The bottom line is that climate alarmists believe there is a strong positive feedback that amplifies the CO2 forcing, and that the cumulative forcing is dangerous. I would characterize myself as a climate realist. I believe there is a possible warming effect due to human effects, but that this warming (if it exists at all) is so small that we should not be overly concerned. Certainly, if there are low cost methods to reduce human CO2 emissions, we should do them. But, we should not penalize our fragile economy (or the developing world’s more fragile economies) if the costs are greater.
The “Guest Commenter” sent a follow-up email to add the following: One aspect that I did not deal with directly, but that is very important, is the other phenomena that can change the global climate. Obviously, since in historical times we know that the earth was as warm or warmer than today for long periods of time (roughly 100 BC to 300 AD, and roughly 1000 AD to 1300 AD) and much colder than today (roughly 1600 to 1850). In fact, the current warming trend dates back to about 1850, and over half of the current warming occurred prior to 1950. Clearly these temperature changes were not caused by SUV driving. Science tells us that solar variations affect earth’s climate, but the mechanisms are not well understood. Sadly, the IPCC charter is limited to looking at human causes of climate change, so they spend no time and money on these other mechanisms.
For more on this site’s on-going discussion about global warming see the previous commentaries listed at the link. My guess is that there is more to come.