Climate change is happening, and human activity is responsible for at least half of it. The report is more certain about this assertion than just six years ago. Why has this certainty increased?
We now have more observed data than we did six years ago; in particular, from the oceans, such as sea levels and sea ice, but also permafrost on land surfaces. Science has also made advances in the last seven years and we now have a better understanding of the climate system. The new data confirms previous data and shows that our climate changes relatively quickly.

In the past 10 years, though, it has not become any warmer; temperatures have stayed almost constant. This doesn’t fit very well into the picture of climate change.
The atmospheric temperature is indeed not changing as much as in the decades before. Even so, it is increasing slightly and the last 10 years made up the warmest decade since we started taking measurements. However, atmospheric temperature is not the only indicator of climate change. Many other measured parameters are changing: sea levels continues to rise, there is less sea ice – in 2012 at a record low – and the oceans are getting warmer.

How do you explain the static atmospheric temperature in a climate that is getting warmer over the long term?
Global warming does not imply that each year must be warmer than the previous one; temperature fluctuates strongly from year to year. The primary driver for this is the El Niño effect in the Pacific Ocean: 1998 was an exceptional El Niño year with temperature outliers moving upwards. Over the past 10 years, El Niño has not had such a pronounced effect. In addition, the amount of solar radiation changes in a cyclical manner and is currently decreasing. Based on solar activity, it might be expected that the temperature would drop somewhat. Finally, we suspect that the ocean depths are absorbing more thermal energy. However, we do not yet have sufficient measurement data to be able to say that with certainty.

In the past months, the scientific community has had intensive discussions as to whether CO₂ is in fact not warming up the climate as much as we had previously assumed.
It is undisputed that CO₂ leads to global warming through the greenhouse gas effect. It is, though, not simple to quantify this influence. The previous IPCC report stated that a doubling of the CO₂ concentration in the atmosphere would lead to an increase in temperature of between 2 and 4.5 degrees. New studies, among them one in which I was involved, point out that this influence could be somewhat smaller. Thus, the range reported in the new report is wider: from 1.5 to 4.5 degrees. This, however, does not change the situation.

The new report states that at least half of global warming is caused by human activity. What about the other half?
A high probability exists that a large portion of the other half is also caused by humans. The report states that there is a 95 per cent likelihood that at least half of it is due to human activity,  but our best estimate is that practically all global warming is man-made. This might seem confusing, but consider an example from sport: I am certain that in the long jump I can get beyond three metres, but my best estimate for the next jump is six metres.

You have now spent four days at the Intergovernmental Panel on Climate Change conference in Stockholm. What was the nature of the meeting?
The assessment report was written by scientists and was available as a draft long before the conference. However, because the IPCC is a UN body, the report must be officially approved by policymakers. At the conference, much attention was paid in particular to the phrasing, and a roughly 30-page summary for policy-makers was discussed. It had to be unanimously accepted by the UN member states.

Do policymakers have any influence on the science?
Facts are facts and governments can't change them. But they can have a say as to which points are included in the summary and determine the exact phrasing. As a result, statements often become more understandable and clearer, but sometimes, in the spirit of compromise, so many amendments and footnotes are added that they become nearly unreadable. Throughout Thursday night we debated individual statements, and for several hours several governments attempted to water them down. In the end, though, all the scientific facts have remained unchanged.

The IPCC report from 2007 contained a much-discussed incorrect statement about retreating glaciers in the Himalayas. Retreating glaciers were overstated in the report. Are such errors also possible in the new report?
The IPCC has increased its quality assurance even more. As was already the case with previous versions, the new report has been reviewed for accuracy by external experts. Both scientists and government representatives were able to state their objections, which the authors had to respond to in writing. This process was monitored by what are known as review editors, a type of scientific referee. In this way, the number of errors should be reduced. However, it will never be possible to write thousands of pages without making a single error.

The new report no longer uses the same climatic scenarios as the last one. Why?
The previous reports worked with technology-based scenarios created by experts in the fields of energy, technology and economies. The scenarios differed primarily in the degree to which society will rely on fossil fuels in future and how much emphasis will be placed on alternative forms of energy. In making their calculations, climate physicists based their work on these scenarios. The experts for impacts and mitigation of climate change, in turn, relied on these results. This system had the disadvantage that the technology-based scenarios were often more out of date than the results available to the climate physicists and experts. Critics then questioned not only the technical feasibility of the scenarios, but also the ability of the report to make definitive statements as a whole.

How did you address this problem?
Now we define the scenarios using the atmospheric concentration of greenhouse gases. An optimistic scenario corresponds to the 2-degree climate target and a pessimistic scenario corresponds to more than 4 degrees, with two scenarios in between. This change has the advantage that we can now calculate the impact on the climate at the same time economists can estimate possible technologies and costs for them. But even though the scenarios are now defined differently, the climate forecasts have not changed significantly.

From your personal viewpoint, what was the most important finding in the report?

Emitted CO₂ remains and accumulates in the atmosphere for several centuries. The report states that the total amount of CO₂ is the primary cause of long-term global warming, and that greenhouse gas emissions lasting over centuries represent a continuing obligation from past and present generations for the future. For me, this is the primary statement. Society must think about how it plans to deal with this issue. When economists and policymakers start to address long-term questions, they generally mean a timeframe of the next 20, 30 or perhaps at most 50 years. Here we are dealing with long-term problems that go far beyond these timeframes. The total CO₂ budget that humans are permitted to emit is limited. We have already used up two-thirds of it if we are to achieve the 2-degree target. The earlier we drastically reduce our emissions, the more freedom of action we give to later generations. The longer we wait before taking action, the more difficult it will become to meet the 2-degree target.