What is the exact sensitivity of the climate system to changing greenhouse gas, dust and aerosol loading in the atmosphere?

The IPCC AR4 has shown that, although we understand better and better how the climate system works, there are still considerable uncertainties in evaluating its future evolution. Not only due to uncertainties in human emissions, but also because of flaws in our physical and biogeochemical understanding of the system. The unpredicted recent evolution of Arctic sea ice or of the Greenland ice sheet are magnificent examples of such flaws.

Although everyone legitimally wants to understand the impacts of climate change (which drives many other global changes such as water resources, biodiversity, agriculture, extreme events, etc…), or wants to develop strategies of mitigation and adaptation to global change, there is little hope in doing a good job in the two latter aspects of global change if one does not improve our physical understanding of the system.

Climate sensitivity is the clear prerequisite to all other physical aspects such as tipping points, feedbacks, thresholds, etc… Solving this question allows one to basically improve the response to all other questions, from the physics to the mitigation, going through the impacts.

10 comments

1. orlando says:

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   The quest for exactitude in the question seems utopian. What is needed for practical purposes is to reduce uncertainties, of course, but also to recognise when we have reached the point where we can judge that likely responses to these anthropogenic forcings represent significant risks of unacceptable changes in the Earth system. This requires advances in quantifying the projections of the manifold consequences of “climate change”, and their potnetial feedbacks.
2. Alastair says:

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   There is no such thing as an exact sensitvity to CO2. The senstivity in the tropics is different from the Arctic, as is shown by polar amplificaion. And the Antarctic has at least two sensitivities: One in the Peninsula and another at the South Pole. Moreover, on a temporal dimension, the sensitivity has not even been monotonic in the Northern Hemisphere on exit from the last glacial. Climate sensitivity is a chimera.
3. Retallack says:

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   Future climate change can be understood from a record of greenhouse crises in deep time which is only beginning to be appreciated. Numerous greenhouse forcings of various magnitude provoked a variety of climatic responses in the geological past, and these can be used to derive power laws for earth-system behavior.
4. Kalense says:

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   I think it is quite wrong to imagine that climate models operate in a dead world. The interaction between life on Earth and its atmosphere are profound, far more than just “climate change drives biodiversity change” as this question implies. The uncertainties in anthropogenic emissions are pretty irrelevant if you don’t account for the non-linear and complex interaction of vegetation and phytoplankton with the chemistry and physics of the atmosphere.

   Understanding sensitivities is one thing, but correctly modelling the main components of the system is surely the number one priority.
5. clim101 says:

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   I’m not sure it is actually possible to ever give a precise climate sensitivity due to inherent chaotic and non linear variability in the system. Knowledge of its global value will not necessarily aid us in determining more local tipping points either.
6. EMMICSU says:

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   Climate sensitivity is not only to be assessed for, but coped, managed and lived definetively. Modelling efforts should put some perception on how decisionmaking process shape sensitivity analysis as a way of humankind lifestyles of adapting.
7. Meine van Noordwijk says:

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   A specific subset is the role of controlling SO2 air pollution and the increase in warming rate. Did past air pollution mask warming and is there a tradeoff between warming and pollution control?
8. nazdiod says:

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   Faced with a considerable uncertainties in evaluating the future evolution of the Earth Climate, there is a well-known influence of human activities on the climate of cities. So it is possible to work on actual data and research projects aimed at recovering more healthy microclimate and energy saving.
9. africain says:

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   It is certainly useful to improve our physical understanding of the climate change, but it is urgent to begin some action for slowing down the climate change. Uncertainties work in the sense that the real effects are larger than the predictions.
   Some actions are simple, easy to begin with: avoid wasting precious ressources, recycle used material …
10. sandpiper says:

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    Placing a value on climate sensitivity isn’t easy, since the sensitivity can vary because of changes in feedbacks as temperatures rise. As the global mean temperature increases, more water vapor in the atmosphere can lead to even further warming.