In everyday terms, climate sensitivity refers to the amount of warming of global surface temperatures with a doubling of atmospheric carbon dioxide - a potent planet-warming greenhouse gas emitted by human industrial activity. In practice, establishing Earth's actual climate sensitivity has proven very problematic. A large part of this likely stems from the emerging realisation that climate sensitivity is not a fixed value, but varies with the background state of the planet (Armour [2012], Meraner [2013]).
The recent (2013) AR5 Intergovernmental Panel on Climate Change (IPCC) Report published a summary of peer-reviewed research on climate sensitivity and found that the likely values (greater than 66% probability) ranged from 1.5°C-4.5°C (for a doubling of atmospheric CO2). This range was lower than the previous (AR4) IPCC assessment because it included climate model/observationally-based research which implied a lower climate sensitivity (see Otto [2013] for example).
However, a research paper just published, Sherwood (2014), suggests that climate sensitivity of relevance today is in excess of 3°C - near the upper range of estimates from the latest IPCC report. Climate models exhibit a large range of climate sensitivities and the main reason for this is down to the way that each model handles cloud feedback. In brief: an increase in cloud cover in response to global warming would act as a negative (counteracting) feedback - reflecting more sunlight back out to space and thereby cooling the Earth, whereas a decline in cloud cover would act as a positive (reinforcing) feedback - as more sunlight reaches the Earth's surface and this leads to greater warming.
The authors of Sherwood (2014) looked at the way that the various climate models handled the cloud feedback and found models with a low climate sensitivity were inconsistent with observations. It turns out that these models were incorrectly simulating water vapour being drawn up to higher levels of the atmosphere to form clouds in a warmer world. In reality (based on observations) warming of the lower atmosphere pulls water vapour away from those higher cloud-forming levels of the atmosphere and the amount of cloud formation there actually decreases. The diminished cloud cover leads to greater warming (a positive feedback).
Of course, this is just one study on one aspect of climate sensitivity and is certainly not the last word on this topic. But Sherwood (2014) is consistent with, and builds upon, another peer-reviewed paper, Fasullo & Trenberth (2012), which found only those climate models with high climate sensitivity correctly simulated drying in key cloud-forming regions of the atmosphere.
Professor Steve Sherwood talks about his research paper in the video below (thanks to Peter Sinclair at Climate Denial Crock of the Week). If correct, this research indicates that business-as-usual fossil emissions may lead to globally-averaged surface temperatures rising in excess of 4°C by 2100 - a formidable threat to global civilization.
Posted by Rob Painting on Wednesday, 8 January, 2014
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