Guest comment: Knutti Reto on an opening comment in an article ...

A reader quoted the two following two sentences from our recent review article about climate sensitivity:”The Earth’s climate is changing rapidly as a result of anthropogenic carbon emissions, and damaging impacts are expected to increase with warming. To prevent these and limit long-term global surface warming to, for example, 2°C, a level of stabilization or of peak atmospheric CO2 concentrations needs to be set.” That statement could be misinterpreted as a political statement of the authors of what has to be done about climate change, or a political statement driving the study.


First, we were not referring to the UNFCCC article, nor were we talking about "dangerous interference" but "damaging", and that was intentional. We only say there will be damaging impacts (which does not exclude that there may also be positive impacts) with warming, and they will get stronger with more warming. Few people would disagree. Also, based on evidence from the past, from models and from basic physics, more CO2 means more warming. Therefore, if the damages should be prevented or reduced, some upper bound on warming and therefore CO2 concentrations would be needed. What should be prevented, and where such a CO2 level might be, is a different question. We also explicitly mention “peak” as an alternative to stabilization, meaning CO2 may vary, and a peaking/overshoot scenario is an option. But there has to be some upper bound on CO2, otherwise warming would continue forever.

The crucial point however lies somewhere else. The sentences quoted are the two very first sentences of the summary paragraph of a review article in Nature Geoscience. Nature allows about two sentences at the beginning of a paper to set the stage, explain and motivate the problem, provide the context and relevance, and summarize all existing knowledge before jumping into results. At the same time these two sentences should not be technical, avoid jargon, and should be attractive to read, they should "draw readers into the piece" (in the words of the editor of our review), much like in a newspaper article where the first sentences need to catch the attention of the reader. So the challenge is to explain the link between CO2 and temperature, the link between climate change and impacts, and motivate the search for climate stabilization levels and therefore climate sensitivity as the most important number linking feedbacks and physical processes to stabilization targets. Everything in about two sentences, not more. The result, written in fact with substantial input from the editor, was the statement above, in our view a simple, short, yet reasonably balanced and attractive way of saying all that.

Seen in that context, we argue that the two sentences may look a bit different. We did not attempt to make a political statement; we just needed to explain in a few simple words why climate sensitivity is probably the most important quantity and source of uncertainty when talking about the overall magnitude and therefore impacts of climate change. Obviously if one had more space, such a statement could be formulated more clearly. But in any case it's important to realize that the first few sentences of such an article are setting the stage and sketching the problem, they are not a political statement that emerges as the conclusion of our work. As is often the case, placing individual sentences out of context may distort their interpretation.

Knutti Reto and Gabi Hegerl

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