Geophysikalisches Kolloquium: The road to two degrees warming

The road to two degrees warming

Prof. Reto Knutti, Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland


Many nations have set an upper limit of two degrees Celsius warming above preindustrial as their goal to stabilize climate change and prevent dangerous climate impacts. This requires a reduction in greenhouse gas emissions, but by how much? At what level do we need to stabilize atmospheric carbon dioxide? The key quantities that determine the long term warming are climate sensitivity (the response of temperature to radiative forcing) and the response of the carbon cycle (i.e. how much carbon remains in the atmosphere, and how much is taken up by the ocean and biosphere). The first part of the talk will provide a synthesis of recent estimates of climate sensitivity and show that climate sensitivity is unlikely to be small. High values of climate sensitivity are inherently difficult to rule out. The second part of the presentation will show that the warming per unit of emitted carbon is surprisingly constant. The implication is that the warming is determined by the total emitted carbon, almost independent of the emission pathway. Most of the warming, once it is realized, is shown to be irreversible for at least a thousand years. Decisions made today therefore affect the future of our planet many centuries into the future. Recent simulations with a comprehensive coupled climate model under strong mitigation scenarios suggest that about half of the climate change by the year 2100 can be avoided by mitigation, relative to a ‘business as usual’ scenario. The policy implications of the two degree target for the negotiations in Copenhagen is discussed at the end, and it is argued that adaptation and strong mitigation are both needed to limit the impacts of climate change.




15:15 Uhr


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Reto Knutti, ETH Zürich


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