Climate Sensitivity

Pro: Climate Sensitivity is High Enough to Be Dangerous

From The Climate Brief

The sensitivity of the Earth’s climate to increases in atmospheric CO2 concentration is a question that sits at the heart of climate science.

Essentially, it dictates how much global temperatures will rise in response to human-caused CO2 emissions, but it is a question that does not yet have a clear answer.

For many years, estimates have put climate sensitivity somewhere between 1.5C and 4.5C of warming for a doubling of pre-industrial CO2 levels. This range has remained stubbornly wide, despite many individual studies claiming to narrow it. However, recent work combining multiple lines of evidence may have helped modestly narrow this range.
Source: https://www.carbonbrief.org/explainer-how-scientists-estimate-climate-sensitivity

The sensitivity of the Earth’s climate to increases in atmospheric CO2 concentration is a question that sits at the heart of climate science. Essentially, it dictates how much global temperatures will rise in response to human-caused CO2 emissions, but it is a question that does not yet have a clear answer. Graph by Zeke Hausfather and animation by Rosamund Pearce for Carbon Brief.

Climate models provide a wide range of climate sensitivity estimates. The CMIP5 models featured in the most recent IPCC report have ECS values ranging from 2.1C to 4.7C per doubling, with an average sensitivity of 3.1C.

However, not all models are created equal. Some perform better than others at matching historical temperatures and other climate variables. One idea, called “emergent constraints”, aims to narrow down (“constrain”) model sensitivity estimates using only the best-performing models.

This has been done in a number of different ways. For example, low-altitude cloud cover is strongly related to climate sensitivity. Models with clouds that more closely match observations show an ECS of between 3C and 4.8C – on the high end of the model range – according to a 2014 paper in Nature by Prof Steve Sherwood and colleagues.

Similarly, a second high-profile Nature paper used satellite observations of how much energy is emitted by Earth to space as their “emergent constraint”. They found that models best matching observations showed an ECS of 3C to 4.2C.

It is possible to argue that some of these studies or constraints are more compelling or physically realistic than others. This is an area of very active research. For now, it is probably premature to suggest that emergent constraints decisively show sensitivity to be lower or higher than previously thought.

These studies do have one consistent finding, however, which is that a climate sensitivity of less than 2C is very unlikely.
Source: https://www.carbonbrief.org/explainer-how-scientists-estimate-climate-sensitivity

Con: Climate Sensitivity is Likely Low Enough to be of Little Concern

Climate sensitivity estimates from new research beginning in 2009. Note that in blue, two studies using actual real-world data to infer climate sensitivity values between 0.8 and 1.3°C, far lower than model-based estimates. Diagram by Anthony Watts.

Predictions of substantial global warming assume high climate sensitivity to a doubling of carbon dioxide emissions in the atmosphere. For decades, scientists have debated the effect of climate sensitivity, due to the uncertain nature of climate feedback in various models and estimates vary widely.

Estimates in peer reviewed studies range from 0.8°C warming to almost 6.0°C warming by 2100. Such a large range of uncertainty means climate model temperature projections remain dubious, at best.

The best evidence indicates climate sensitivity is at the low end of the range, unlikely to exceed 1.5°C in the 21st century.

Factors that determine climate sensitivity. After increasing carbon dioxide (CO) levels, there is an initial warming. This warming could be amplified or reduced by the net effect of feedbacks (weather processes that change the characteristic of the planet). Diagram by Femkemilene from WikiMedia Commons.

Short Summary:

Declaring future predictions of global warming “settled science” requires a fairly precise calculation of future temperatures. However, since climate sensitivity was first identified more than 40 years ago, scientists and climate models have produced a very broad range of potential future temperature patterns. Calculations for a doubling of atmospheric carbon dioxide range from 0.8°C warming to 6.0°C future warming by 2100.

If climate scientists don’t understand the Earth’s atmosphere well enough to nail down a true climate sensitivity estimate for increased carbon dioxide, how can we trust climate model projections of future warming that rely on such an uncertain value? Climate sensitivity estimates from real-world atmospheric data suggest (see here and here) man-made global warming this century is unlikely to exceed 1.5°C total and its climatic effects might actually be beneficial for humans and the environment.

References:

Explained: Climate Sensitivity. MIT news. http://news.mit.edu/2010/explained-climate-sensitivity
Ad Hoc Study Group on Carbon Dioxide and Climate (1979). Carbon Dioxide and Climate: A Scientific Assessment. National Academy of Sciences. doi:10.17226/12181. ISBN 978-0-309-11910-8. https://web.archive.org/web/20110813231807/http://www.atmos.ucla.edu/~brianpm/download/charney_report.pdf
Earth’s Climate Sensitivity: Apparent Inconsistencies in Recent Assessments. Schwartz et al, 7 November 2014, Earth’s Future. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014EF000273
On the Observational Determination of Climate Sensitivity and Its Implications Richard S. Lindzen and Yong-Sang Choi Asia-Pacific J. Atmos. Sci., 47(4), 377-390, 2011 DOI:10.1007/s13143-011-0023-x http://www-eaps.mit.edu/faculty/lindzen/236-Lindzen-Choi-2011.pdf
On the Misdiagnosis of Surface Temperature Feedbacks from Variations in Earth’s Radiant Energy Balance Remote Sens. 2011, 3, 1603-1613; doi:10.3390/rs3081603 http://www.drroyspencer.com/wp-content/uploads/Spencer_Misdiagnos_11.pdf