This was always an embarrassment for global warmists, because the troposphere has never warmed much, but in the last few years its cooled. The tropical troposphere has now not warmed at all. See http://www.climateaudit.org/?p=3048 for the graph of temperature according to three satellite series since 1978.
The Radiosonde (weather balloon) data series is an independent measurement of the tropical troposphere temperature. It goes back to 1958 and is presumably extremely reliable, because all they are doing is sending thermometers up in balloons. You can see the time series at: http://www.climateaudit.org/wp-content/uploads/2008/06/hadat42.gif The graph is flat, and the most recent data point is the coldest.
To quote an excellent Op-Ed from an Australian climate scientist http://www.theaustralian.news.com.au/story/0,25197,24036736-7583,00.html "Recently the alarmists have suggested we ignore the radiosonde thermometers, but instead take the radiosonde wind measurements, apply a theory about wind shear, and run the results through their computers to estimate the temperatures. They then say that the results show that we cannot rule out the presence of a hot spot. If you believe that you'd believe anything."
Moreover, the global temperature series (not the tropics, the global series) have been flat for so long (a decade) that the IPCC's prediction from 2001 has now been falsified by all five global temperature series at the 95% confidence level, see http://rankexploits.com/musings/2008/accounting-for-enso-cochrane-orcutt/ 2001-2008 is not a long temperature series, but the statistical analysis thus draws wide error bars, and the IPCC's prediction of 2 degrees/century falls well outside the error bars.
This shows that the IPCC's GCM's (Global Circulation Models) are wrong. Not that it can be too surprising that the GCM's are worthless since p 596 of the IPCC 4th report cautiously admitted they didn't know whether their GCM's had more data points or free parameters! Yet the GCM's are absolutely central to any argument for expecting warming by more than a few tenths of a degree by 2100, and to the amazingly porous argument the IPCC report gives to demonstrate man caused the alleged observed warming.
Then you've got what seems like much better science that cosmic rays affect the climate, cf box at http://www.sciencebits.com/RealClimateSlurs for a quick summary (click "brief summary" link there) and http://arxiv.org/abs/0804.1938 for a recent review article. I particularly recommend leafing through the figures in this latter document to see the incredibly striking impact of cosmic rays on the earth's climate. You will see figures, on which proxies for cosmic rays zig and zag up and down at the exact same time as proxies for climate, both over thousands of years and over the last 500 Million Years. The connection between cosmic rays and climate does not appear to be in any remaining doubt. And the connection between solar variation, through the solar magnetic field, and cosmic rays is also not in doubt. You will see figures where sunspots and cosmic rays zig and zag together repeatedly. The gradual increase in solar magnetic activity over the last few centuries (until its recent decline in the last few years) is apparent-- gradually increasing at the same time as the planet is claimed to have been warming. The mechanism by which cosmic rays affect the climate is not completely nailed down, but there is a strong emerging understanding including laboratory experiments. Solar fluctuations, which are not large enough to affect the climate much directly, none the less seem to affect the climate indirectly through the effect of the solar magnetic field on cosmic rays, and the effect of cosmic rays on cloud nucleation and/or other atmospheric effects.
The IPCC 4th report says "attribution of anthropogenic climate change is understood to mean demonstration that a detected change... is not consistent with alternative, physically plausible explanations."[p668] But the report contains several alternative possibilities that are said to be "not understood" or whose magnitude is said to be "largely unknown". For example, two are mentioned just in the last paragraph of 1.4.3. (p108): unknown large feedbacks from changes in solar irradiance, and the effects of galactic cosmic rays. Actually, as I point out in the above few paragraphs, cosmic rays seem to explain climate fluctuations extremely well. The IPCC devotes considerable space to the strawman that solar activity could directly affect the earth's temperature, but ignores the actual indirect means by which solar variation seems to affect temperature. Global Warmists routinely attack the strawman of direct solar effect any time the subject is raised.
Also, Mars, Jupiter, Triton, Neptune, and Pluto have recently been observed warming, suggesting some cause external to the earth, but none of them are mentioned anywhere in the 987 pages of the 4th Report. Another physically plausible explanation for recent warming (if indeed warming has actually occurred) as remarked by Lindzen would be thermal transfer from the deep oceans. The oceans and atmosphere are turbulent fluids prone to exchange heat in unpredictable ways over a wide range of time scales simply because chaotic systems do that kind of thing, which the computer models of the IPCC are completely inadequate to simulate.
Its also worth noting that intuitive physics (and pencil and paper calculation) says that greenhouse gas warming scales logarithmically. The theoretical reason for the effect is that CO2 molecules (for example) absorb and reflect certain wavelengths. But they only do it in certain wavelengths. Once you've got some molecules of CO2 in the air, the effect of each next molecule is less than the one before, because those wavelengths are already getting scattered, and mostly heat is already only getting out in other wavelengths. So even if you believed everything else, one's expectation would be that we've already seen the substantial majority of all the warming we will ever see, if we quintuple the CO2 from here. To believe otherwise, you have to rely in detail on the GCM's prediction of positive feedbacks, that they are not competent to calculate, to predict warming in the future that is several times greater than anything we've seen before.
You've also got the fact that 70% of the surface thermometers they're using are in locations like on roofs, next to air conditioners, where the systematic biases are a sizeable factor greater than the measured effect, and the only reason we know this is because a TV weatherman in Chico, CA organized people on the internet to go photograph the weather stations, cf http://www.surfacestations.org These pictures must be seen to be believed. If you just want the most ludicrous pictures, go to the weatherman's blog http://wattsupwiththat.com/category/weather_stations/. Keep scrolling down through previous pages.
The naively computed expected error from this, is .56*2 + .13*5 > 1.75 degrees C, since 56% of the stations are level 4 expected to have more than 2 degrees C error (according to NASA's own standards and estimates) and 13% of the stations are level 5 expected to have more than 5 degrees C error (assuming all the systematic error is positive, and almost all the errors are). This compares to a claimed signal of around .75 degrees C (warming in the last century), which makes one wonder if the planet hasn't actually cooled in the last 100 years. It doesn't seem as if anybody really knows.
By the way, on top of these very local siting problems, which have a scale of inches (for lightbulbs and electronic equipment placed inside some of the enclosures with the thermometers- I kid you not, look at the pictures) to feet (air conditioner exhausts etc) there is also the "Urban Heat Island Effect", the problem that as cities expand, pour concrete etc., the urban environment heats up significantly (which has a length scale of miles-- thus its a distinct effect). Almost all the thermometers are, of course, now located in metropolitan regions (although many of them were located in fields 100 yrs ago.) IPCC claims this isn't important or is corrected for (by contrast to the local siting problems, which they completely ignore) but they are almost certainly wrong-- for example because its possible to extract an excellent picture of worldwide urban growth from the temperature data. The Heat Island Effect seems to account for about half the purported temperature rise by itself.
By the way, NASA, which maintains the surface record, (Hansen is the chief archivist) doesn't appear merely satisfied with reporting these ludicrous figures. They also have adjusted the temperature record on numerous occasions, including retroactive changes, virtually every change making it more conducive to the Greenhouse Gas Global Warming Orthodoxy. For an amusing blink comparator between the current GISS time series, and the time series they were putting out a few years ago http://wattsupwiththat.com/2008/11/14/the-evolution-of-the-giss-temperature-product/.
In 1999 there was some reason to believe the earth was warming. The hockey stick graph was published, and was the main evidence, and the surface based temperature series showed the earth had been warming for more than 20 years. But since then, the hockey stick was invalidated, and the earth has stayed flat or cooled. The surface based record has been called into serious question. The Cosmic ray explanation has gotten better and better. In short, the scientific case for global warmism has simply fallen apart, at the same time as they claim greater and greater confidence and alarm. Alarmism is now based only on momentum from earlier days, and is strongly contradicted by the data.
Another interesting question is, how this mass delusion/fraud has happened. Richard Lindzen (MIT Professor of Climatology, National Academy Member) has an interesting paper on the subject: Climate Science: Is it currently designed to answer questions? http://arxiv.org/abs/0809.3762.
Its also a delusion that there is any sort of consensus among scientists that Global Warming is a threat. There are a lot of faculty members, who if you ask them "Is Global Warming real?" will say they have heard there is a consensus, but I expect that almost any decent scientist who genuinely tries to understand the subject will realize the actual situation, which is why more than 31,000 American scientists signed a mail-in petition. http://www.oism.org/pproject/ .