In February 2007, the Intergovernmental Panel on Climate Change (IPCC) stated in their Fourth Assessment Report that human actions are “very likely” (i.e. with 90% or greater probability) the cause of global warming, indicated by an increase of 0.75 degrees in average global temperatures over the last 100 years. This statement is the result of very tough discussions on a worldwide scale between thousands of climate researchers whether human activity is the main cause of global warming. The results of this discussion were presented to the public in many publications, for instance in Martin Durkin´s documentary “The Great Global Warming Swindle”, presented in March 2007 at UK´s Channel 4. The main message of this production was that man-made global warming is “a lie” and “the biggest scam of modern times.” Martin Durkin and coworkers argue that the scientific consensus on climate change is the product of “a multibillion-dollar worldwide industry, created by fanatically anti-industrial environmentalists, supported by scientists peddling scare stories to chase funding and propped up by complicit politicians and the media”. The documentary showcases scientists, politicians, economists, writers, and others who are sceptical of the scientific consensus on anthropogenic (or man-made) global warming. Some of the scientists, opposing the main stream of greenhouse gas theories, simply argue that it has not yet been ascertained whether humans are the primary cause of global warming or if there are other natural variations responsible for this phenomenon like increased solar activity, cosmic rays or variations in natural climatic cycles. There is also a series of scientists questioning the temperature records used in the databases as temperature differences attributed to the greenhouse effect are reasonable small (fractions of a ºC). The so-called “urban heat island” effect leads to a local warming in more populated areas, showing slightly higher temperatures due to to increased heat generated by cities, rather than a global temperature rise. Anyhow, this argument was confuted by the IPPC, indicating that the effect of the urban heat island on the global temperature trend is no more than 0.05 °C (0.09 °F) degrees through 1990.

Other facts presented by the film were shown to be incorrect or misinterpreted. The film asserts for instance that records of atmospheric CO2 levels since 1940 show a continuing increase, but during this period, global temperature decreased until 1975, and has increased since then. Anyhow, it is well recognized that this cooling was driven mostly by aerosols (i.e. pollution) in the atmosphere. There is nothing contradictory about this cooling when all sources of radiation changes are considered. A second argument that easily can be invalidated is the impact of the so-called “solar variation theory” on global warming. According to the authors, solar activity (and involving cosmic rays as well as heat from the sun aiding cloud formation) is currently at an extremely high level and directly linked to changes in global temperature. The film argues that solar activity is far more influential on global warming than any other anthropogenic or natural activity on Earth. What the film does not mention is that solar activity has declined over the last 30 years – at the same time as the major spike in global temperature.

But there are some statements related to the influence of the oceanic mass and water vapour on climate change which are more difficult to confute. Water vapour makes up about 98% of the greenhouse gases by volume and provides something between 40 to 80 percent of the natural greenhouse effect. It probably has the largest impact on the planet’s temperature and climatic conditions, much larger than CO2. Water particles in the form of clouds act to reflect incoming solar heat, but the film argues that the effects of clouds cannot be accurately simulated by scientists attempting to predict future weather patterns and their effects on global warming. This argument probably is correct and it is well recognized that water vapor is responsible for the natural warming up of the surface temperature to approximately 30-35ºC. Anthropogenic greenhouse effect, according to the film’s argumentation, is only about 2% of the total “natural” greenhouse effect, which corresponds to a 0.6-0.7 ºC increase in temperature. This estimate is somewhat smaller compared to the results of much more sophisticated simulations (0.9 – 2.7ºC), but there is undoubtedly an anthropogenic effect of CO2 increasing the average surface temperature.

One example of the complexity of climatic simulations is the prediction of future storm events considering global warming effects. According to a recently published study, published online by research meteorologist Tom Knutson in the journal Nature Geoscience and resumed in the New York Times (May 18, 2008), global warming isn’t to blame for the recent jump in hurricanes in the Atlantic. The study predicts that by the end of the century the number of hurricanes in the Atlantic will fall by 18 percent. In the past, Knutson has raised concerns about the effects of climate change on storms. His new paper has the potential to heat up a simmering debate among meteorologists about current and future effects of global warming in the Atlantic. And Knutson is not alone with this view. Another group of experts, those who study hurricanes and who are more often skeptical about global warming, also say there is no link between global warming and hurricane frequency. They attribute the recent increase to a natural multi-decade cycle. According to the prediction, the number of hurricanes touching land in the US and its neighbors will drop by about 30 percent because of wind factors. However, the biggest storms, those with winds of more than 110 mph, would only decrease in frequency by 8 percent. The biggest decrease is forecasted for storms with winds between 39 and 73 mph (normal tropical storms), who would decrease by 27 percent.

It’s not all good news from Knutson’s study, however. His computer model also forecasts that “hurricanes and tropical storms will be wetter and fiercer. Rainfall within 30 miles of a hurricane should jump by 37 percent and wind strength should increase by about 2 percent”, Knutson’s study says.

There are already critical reactions on this new publication. MIT hurricane meteorologist Kerry Emanuel claims that the computer model used by Knutson is not adequate enough to look at storms and according to Kevin Trenberth, a climate scientist from the National Center for Atmospheric Research in Boulder, Colo., Knutson’s computer model is poor at assessing tropical weather and ”fail to replicate storms with any kind of fidelity.” It also does not considering well enough the intensity, duration and size of the storm events, as not only the number of hurricanes is important to evaluate.

Positive feedback comes from NOAA hurricane meteorologist Chris Landsea , who wasn’t part of this study, praised Knutson’s work as ”very consistent with what’s being said all along.” ”I think global warming is a big concern, but when it comes to hurricanes the evidence for changes is pretty darn tiny,” Landsea said.