The present rapidly accelerating global warming is now mostly of human origin. We are approaching a doubling of the atmosphere's carbon dioxide levels above pre-industrial levels, which is very far outside the variation over many thousands of years in the past. The consequence is warming by the “greenhouse effect” which is fairly easy to calculate and in agreement with what we see. The effects of variation in the sun's output are smaller.

The best reference material especially for any remaining skeptics is the set of UN reports, available here. If you take the issue seriously, you should read at least the summary for policymakers in the Synthesis report. There are of course many publications on the subject, of varying quality. Here is a good summary of the global warming “argument” from the American Physical Society (APS News Online, Feb. 2006). It would seem to be more reasonable to look at the writings of those who have seriously studied the technical issues with at least a little scientific background rather than Ann Coulter, Rush Limbaugh etc. The UN reports are clearly enough written that there is no need to go to “secondary” reports, even Al Gore's. Nine years ago I listened with respect but skepticism to the arguments of a scientist doubting a human source for the already acknowledged global warming, in a talk at Brookhaven National Lab. That was a long time ago. Much has happened since, especially the development of computer simulations with newer fast machines that give answers that agree well with reality when tested by “predicting” current situations using data from years ago. Occasionally there is a serious article disputing elements of the UN reports, such as in the March 2008 Physics Today (Scafetta and West “opinion”) *. It partially connects the fluctuations in surface temperature mentioned below to solar output variation. The final conclusion is rather weak, that if the sun cooled off a lot in the next few decades we would all be saved. In the near future, I am willing to say that the global mean temperature in 2012 might possibly be close to what it was in 2002, because the sun is currently decreasing in output due to normal cyclical variation. The sharp upward trend over the next few decades though is unmistakable, and supported by simple cause-and-effect reasoning about the effect of carbon dioxide.

Since the above was written, a new and interesting study was published and referred to in several news articles.  Here is a report from www.telegraph.co.uk; the original paper in Nature is not free on the web.
Science News has an informative article on the same subject.  It concludes that natural variations in climate i.e. solar variability and ocean current cycles may result in no rise of global temperature over the next decade. Note my earlier remark above!  But I quote this article:   "However, the effect of rising fossil fuel emissions will mean that warming will accelerate again after 2015 when natural trends in the oceans veer back towards warming, according to the computer model."  I emphasize again that the overall rise of  human-caused CO2 emissions is measured rather directly by air sampling, looking at ice cores etc, The physical effects of this "blanket" are easy to understand and are directly calculable, perhaps without a computer.  The uncertainties come when we try to predict the future, but the main uncertainty is in what we will do to mitigate it!  If you are interested in more depth I suggest reading Topic 2 in the UN "synthesis report"  referenced above. It appears to me that a minor flaw in the UN report was in treating the yearly variations as "random"; they are cause-and-effect at a deeper level with more information available, such as the fit to solar variation in the Scarfetta & West analysis. An analogy is the behavior of a jar full of air; in classical (not quantum) physics all the air molecules collide with each other & with the jar in a strictly predictable cause and effect manner, but due to a lack of such detailed information the behavior of the gas is best described as random motion.

If you are really interested in contrarian discussion, the relatively slow response time of Physics Today has yielded several reader responses to Scafetta and West (March issue mentioned above) and a response by these authors.  The reader answers are mostly quite negative.  The letters can be viewed (free) at http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_10/10_1.shtml  (Unfortunately the original March article is still available only to subscribers.)

There is another recent article from Science News pointing out there was never a "consensus" on the (widely quoted by warming skeptics) "prediction" of a cooling climate decades ago: See this reference
(which is too long for this page width!) from the October 25, 2008 Science News.

There are at least two reasons for uninformed skepticism. One is confusion about statistical fluctuations. If you look at the famous “hockey stick” diagram publicized by Al Gore, you will see a lot of “fuzz” i.e. the variations in climate from year to year are frequently larger than the overall trend, which is sharply rising mean temperatures over the last few decades. Although  we can't predict the weather five days from now very well, we can better predict the weather averaged over many days several months from now; farmers have found such predictions useful in improving their income. Similarly, climate averages over decades are more accurately predictable than next year's average weather.  Our current very cold winter does not disprove global warming!  For example, here is a quote from a recent blog (I won't identify to avoid embarrassment about this stupid remark): “guess ( ) missed the news flash where NASA data showed global temperatures dropped .75 C in 2007. Obviously CO2 emissions are pretty much irrelevant to global temperatures.” **   Another way to put it is that the effect is statistical or chance; as seen throughout the UN reports the results are given as odds or probabilities. A wise gambler doesn't bet against the house, or believe that a “winning streak” will continue in violation of the odds. Here your children's or grandchildren's welfare or even lives are at stake!  Let us hope we get lucky and a lot of volcanoes erupt cooling the climate, or the sun's output decreases beyond currently known oscillations.  But don't bet on it. The other reason is simple greed. If you look more closely you find that many “dissident” reports have been funded by oil companies etc. with an obvious “axe to grind”. Please show me the big profits to scientists that would result from raising the alarm about global warming! Where's the “big money”?

It seems to me that some of the skeptics become fascinated, even obsessed, by the statistical analysis of climate variation ranging from the centuries-old Fourier analysis to the currently fashionable wavelet analysis and thus lose sight of the giant "hockey stick" right in front of their eyes.  Yes, it's obvious that solar variability (mainly the sunspot cycle) affects climate, but it's also hard to argue with a CO2 effect that to a great extent can be calculated directly and which is demonstrated in past geological events.

Now for even worse news: The UN reports are already out of date. In particular, the models used were not able to accurately take into account the nonlinear effects of ice melting, and relied on data from a few years ago. When arctic or antarctic ice melts, the reflection of sunlight by white ice is reduced so more heat goes into the atmosphere instead thus accelerating global warming, melting still more ice, etc. etc. This “feedback effect” can cause a catastrophically rapid change. This in fact seems to be happening, there has been a great thinning of ice in many parts of the Arctic. (The Antarctic is less affected because of the configuration of the continents and ocean currents.) The Arctic may soon be ice-free most of the year allowing commercial shipping through the historically sought “Northwest Passage”. The melting of floating ice will not raise the sea level although it will make the planet warmer by not reflecting heat, but the melting of ice on land in Greenland and possibly the Antarctic could raise sea level by many feet, submerging nearly all of Bangladesh and much of Florida, and many other low areas and countries. This disaster in Florida would not be like Katrina because it would be very slow, but it would still be an economic catastrophe. But it would be small compared to the displacement of many millions in Bangladesh and elsewhere! This is how wars start. In fact the melting of Greenland ice is now indeed alarming. As mentioned, Antarctica is less affected. But a state-sized area of antarctic sea ice broke up and melted recently; this has “uncorked” several glaciers which used to feed it, leading to a much more rapid flow of continental ice into the sea which will raise sea levels.

The effects on humanity are also outlined in dire terms in the UN reports. The flooding of low lying areas can lead to mass migration on a scale not seen since WW2 or ever, perhaps. The scale of the problem is of course due to the explosive population growth in the last century which has stressed the world's ecosystems to the limit or beyond. (The reason we have not already had the mass starvation forecast in the “Club of Rome” report many years ago is the “green revolution” in agricultural productivity due to the geneticist Dr. Norman Borlaug and many other relatively unsung heroes. We may not be able to rely on more such “technological miracles” in the future.) It is true that some of the effects of warming are beneficial. My relatives in Saskatchewan are likely to profit in their wheat farming from a longer growing season. At the moment more people die of cold than of heat worldwide, maybe this will reverse. All depends on where you live; here in the Southwest we will likely face extreme drought. It's happened before! The climate models also predict more extreme and violent weather. A supercomputer isn't needed to understand the rudiments of this effect: With a warmer climate more moisture is held in the atmosphere, so that more energy is released when it condenses. This energy is very large, note how long and how much heat is needed to boil water (the opposite of condensing). This is what fuels storms and hurricanes. I won't claim this caused Katrina, we can only say “maybe so” because the variation in such extreme events is so large anyway.

More than the probably negative average of positive and negative effects, the instability from the predicted changes and migrations may be the greatest threat, from historical precedent leading to bloody and prolonged wars.

The UN reports are also “conservative” in that they were subject to much political pressure to put a more favorable light on these matters for obvious economic/political reasons. Thus the projections are generally at the “almost certain” level rather than the “most likely” level as you will see from reading the reports. If you want to plan rationally, you should use the “most likely” level, as a farmer would use planning his crops using the best available long range weather forecasts. Unfortunately, especially with the newer alarming results I mentioned above, the rational forecast says we have very little time. We will suffer much with the consequences of what has already been put into the air, all we can do with a really serious effort over the next twenty years is lessen the bad effects. These efforts will have to be on many fronts; the quickest effect will be from energy conservation and increased efficiency. A major change can be obtained in the near future if we have the will. But it will take a lot more than this, we have to pursue several ways of obtaining large amounts of energy from sources which don't emit carbon dioxide or are at least “carbon neutral” i.e. only emitting what was originally absorbed from the atmosphere to form wood or other biofuel. These web pages are mostly about new large scale energy sources which I consider interesting and possibly practical. The list is by no means comprehensive, it is just some items that have come to my attention. I am very interested in hearing about new possibilities!

• * I'm sorry I can't provide a live reference or a local copy of the Physics Today article; if you click here you'll see an abstract and an option to purchase. Perhaps copyright law allows me to email you a copy on request.

• ** Also, I don't know where the blogger got this number, it disagrees with the data shown by the warming skeptics in the Physics Today article.