Influential skeptics continue to challenge the scientific consensus that CO2 emissions are responsible for climate change. Have they got a point?
In January this year, 16 scientists wrote in the Wall Street Journal (WSJ) that they saw no scientific arguments supporting the need for urgent action to combat climate change.
They included prominent climate skeptics like MIT Atmospheric Science professor Richard Lindzen as well as the scientists, and former ExxonMobil employees, Roger Cohen and William Happer.
Even in Germany, where climate skeptics have less political influence than countries like the USA, a book called "Die kalte Sonne" (The Cold Sun) has been making waves since its publication earlier this year.
The authors Fritz Vahrenholt und Sebastian Lüning, employees of Germany's second-biggest energy company RWE, maintain that less than half the world's warming to date is human-made. They say solar activity, sunspots and magnetic fields, which change in cycles, are responsible.
As the sun is about to go into a cold cycle, they say, this will counteract global warming and we need not fear the worst. Calls for urgent action are no more than "panic-mongering". "I feel duped on climate change", Vahrenholt told the media. The German boulevard paper "Bild" ran the headline "The CO2 lie".
Is humankind responsible?
While statements like these have frequently made the headlines during the last few months, leading research institutes continue to produce further evidence that global warming is tied to humans.
In their latest calculations, Germany's Max-Planck Institute for Meteorology and the German Climate Computing Centre (DKRZ) warned that the internationally agreed two-degree target for global temperature rise can only be reached with an immediate and drastic reduction in carbon dioxide emissions (CO2) emissions.
In February, five French institutes published new forecasts backing this.
Where does the data come from?
The conclusions of climate scientists are based on continual monitoring on land, in the water, in the atmosphere and from space satellites.
The data is fed into complex computer models to simulate possible future scenarios. The models are calibrated with information on climate changes in the past.
Geological information and ice cores enable researchers to determine what the temperature was at a particular time, as well as the corresponding atmospheric conditions.
Predicting how the climate will change in the future is an extremely difficult business. Factors such as forest cover, ocean currents or cloud formation all influence the climate. Mechanisms known as feedback effects can intensify or reduce the greenhouse effect. For instance, the melting of ice-covered areas in the Arctic mean less solar energy is reflected back into space. Instead, the heat is absorbed by the darker-colored ocean.
As this complexity makes it impossible to predict exactly how strong global warming will actually be at a particular time, the forecasts of climate scientists always include a broad spectrum of possible temperature rises.
No more warming?
Data from some 36,000 weather stations around the world show that land and sea temperatures have risen by 0.8 degrees Celsius on a global average since 1900.
The decade from 2000 to 2009 is regarded as the warmest since the beginning of comprehensive temperature monitoring.
Until very recently, 1998, 2005 and 2010 shared the record for the hottest temperature ever measured.
Critics, like the signatories to the letter to the WSJ and the authors of "The Cold Sun" interpret the high point of 1998 as evidence that the earth has stopped heating.
Peter Lemke, head of climate science at the Alfred-Wegener-Institute for polar and marine studies and one of the authors of the last IPCC report, warns against this method of interpretation. "If you want to analyze a trend in the climate, you need to take the data from at least 30 years", says Lemke.
Global warming is not a linear process. Regional variations and natural phenomena like El Nino or the North Atlantic Oscillation sometimes create the impression that temperatures are cooling. However, on a long-term graph, there is clearly a warming trend.
A new study by the British Climatic Research Unit integrating missing measurements from the Arctic also indicates that 2005 and 2008 were in fact both warmer than 1998.
CO2 or natural climate variation?
Climate scientists see a direct connection between the rise in global temperature and the increase in CO2 in the air over the last 200 years.
"78% of this increase is linked to the burning of fossil fuels such as coal, oil and gas. The remaining 22% were caused by changes in land use", says Lemke.
"If you take the development as a whole, it is impossible to miss the obvious connection between the greenhouse gas CO2 and the temperature of the planet", says Professor Mojib Latif from the Helmholtz-Centre for ocean research.
Nevertheless, climate skeptics question this connection.
"We show that at least half the warming of the last forty years was caused by the influence of the sun and cyclical oscillations in the world's oceans", Vahrenholt and Lüning write in their book.
Leading climate scientists reject this. Lemke says the sun is definitely an important factor in influencing climate, but he stresses that solar activity alone cannot account for the current variations in climate.
"The IPCC report shows quite clearly that natural sun cycles only account for around five percent of the temperature change", says Lemke.
"The climatic changes cannot be attributed either to the eleven-year sun cycles or to any longer-term pattern," says Wolfgang Schmidt from the Kiepenheuer-Institute for Solar Physics in Freiburg.
During the last few decades, the global temperature has risen while solar activity experienced a slight cooling trend.
Healthy skepticism or hidden agenda?
The influence of climate skeptics varies in different parts of the world.
In the USA, the climate debate has long been politically divisive. Climate skepticism is widespread in the Republican Party. Yet while the opposition tries to block climate-protection legislation by the Obama administration, the US military is already calculating the security implications of climate change. The US Navy is preparing for less Arctic sea ice making the region more easily accessible.
There are various reasons why people are skeptical about climate change, not least economic interests.
"The big money in climate change involves firms, industries, and individuals who worry that their economic interests will be harmed by policies to slow climate change", wrote Yale University Economics Professor William D. Nordhaus, in a recent response to skeptics for the New York Review of Books.
Nordhaus compares the attacks on the science of global warming to the resistance offered by cigarette companies to scientific findings on the dangers of smoking. The amount of money at stake for energy companies confronted with the implications of climate change is far greater, says the Yale economist.
"Climate impact skeptics" is the term used by Professor Otmar Edenhofer, Chief Economist at the Potsdam Institute for Climate Impact Research in Potsdam, to describe those people who do not deny that the climate is changing, but doubt whether it could really have the catastrophic consequences so many scientists and politicians fear.
They stress the positive aspects of climate change, such as the possibility of farming land previously covered by permafrost or of accessing oil and gas reserves in the Arctic. They play down the danger of desertification or increasing droughts and floods. At the same time, a new study by the Potsdam Institute indicates a "clear link" between the increase in extreme rainfall and heat waves and "human-made global warming".
Edenhofer warns of the danger of neglecting the "worst-case-scenario" when it comes to preparing for the effects of climate change. Risk experts from the world's largest reinsurance company Munich Re also regard climate change as a major strategic challenge for coming years.
Many regions of the world are already feeling the effects of climate change, especially in the poorer countries of the developing world. However, the political will to take appropriate action has not been keeping pace with the broad scientific consensus. The skeptics still have a wide sphere of influence.
Leading scientists say there is only a short window of a few decades to make the turnaround that is necessary to reduce emissions and halt climate change.
The question is whether the earth and humankind can really afford to wait until the last sceptic has been convinced.