Climate Sensitivity: Quantifying It is Likely the Best Way to Validate Climate Change Models but Estimates Still Vary Considerably

 

     Indeed, most estimates of climate sensitivity still vary about as much as they did in the late 1970s. There have been many scientific papers with estimates through the years and that variation has persisted. Not surprisingly, those climate scientists who say climate change is a crisis tend to come up with high climate sensitivity estimates and those who say it is not a crisis tend to come up with lower estimates.  

     Climate sensitivity is simply a measure of how much the Earth's surface will warm for a doubling in the atmospheric CO2 concentration. It is “the average change in global mean surface temperature in response to a radiative forcing, which drives a difference between Earth's incoming and outgoing energy.” As I have always said, if climate sensitivity is lower, we can go slower. If it is higher, we may be in more trouble than we know, or as Wikipedia explains: “If climate sensitivity turns out to be on the high side of scientific estimates, the Paris Agreement goal of limiting global warming to below 2 °C (3.6 °F) will be difficult to achieve.

     Climate sensitivity is predicted as two different measurements, one short-term and one longer-term. The reason for this is that it takes time for the global climate system to reach a “steady state” after an increase in CO2 atmospheric concentration. Transient climate response is the short-term measurement of the increase in global average temperature expected when the atmospheric CO2 concentration has doubled. Equilibrium climate sensitivity (ECS) refers to “the higher long-term increase in global average temperature expected to occur after the effects of a doubled CO2 concentration have had time to reach a steady state.” Climate sensitivity is calculated in three ways: 1) direct observations of temperature and levels of greenhouse gases, 2) indirectly estimated temperature and other measurements from the Earth's distant past, a kind of paleoclimatic reconstruction, and 3) computer modeling of climate. Only number one, direct observation, is free of potential bias and other sources of error. The imbalance of incoming and outgoing radiation to and from the Earth is known as radiative forcing and is expressed in units of Watts per square meter (W/m²). Thus, climate sensitivity is a measure of how much temperature change a specific amount of radiative forcing will cause. Radiative forcing is influenced by many factors and for this reason, climate sensitivity is also influenced by many factors. It should be noted that ECS, also known as “fast feedback” climate sensitivity, makes the assumption that ice sheets, vegetation, and long-lived GHGs are fixed. Ice sheets in particular are thought to exhibit “slow feedback.” That view has been challenged in a new paper by James Hansen. There is another metric, Earth system sensitivity (ESS) that refers to a more general and inclusive mechanism that includes the amplifying feedbacks of GHGs and ice sheets. Climate sensitivity is not specific to CO2 even though accurate prediction of climate sensitivity to CO2 is the main goal of the metric. Feedbacks and other possible influences must be considered as well. Comparison to paleoclimatic warming and cooling events and direct measure of the influences of CO2-spewing volcanic eruptions are some of the evidence considered in determining climate sensitivity.

     Earth system sensitivity is not always exactly the same. For instance, it can change in time relative to the number of aerosol particles in the atmosphere since those particles have a net cooling effect. One can mathematically cancel out factors that change the Earth’s overall sensitivity to arrive at climate sensitivity. Thus, climate sensitivity is considered to be a true measurement of how our climate system reacts to temperature with all other influences accounted for and subtracted out. Even so, the quantitative effects of different climate variables such as aerosol particles and clouds on climate sensitivity are still hotly debated. For aerosols, this is important because aerosols are still being emitted into the atmosphere, mainly from coal and wood combustion and wildfires. Despite aspirations and plans to phase out coal globally, 2022 and 2023 have been record coal production years. While pollution abatement at coal plants has led to lower aerosol emissions, that number is still very significant. Climate scientists like Jim Hansen often point out that as coal burning is phased out, the cooling effect of the aerosols will also phase out. One could also say that burning wood, coal, and to a lesser extent oil is a form of geoengineering. Thus, we already have been monkeying around with our climate by artificially cooling it. Even so, experiments that had planned to release sulfate aerosol particles into the stratosphere to counter global warming have been canceled on grounds of fear of unknown consequences. The same is generally true of plans to fertilize the ocean so that it can take up more carbon.

 

Image credit: NASA; "The relentless rise of carbon dioxide"

 

 

     The target number we hear about, usually 1.5 degrees C or 2.0 degrees C, refers to the global mean surface temperature (GMST). That is a statistical average of many daily readings throughout the globe. This number is not in dispute. One thing its measure through time has shown quite conclusively is that warming is accelerated along the poles, most particularly in the Arctic. This Arctic Acceleration as it is dubbed is certainly a cause for concern. The graph below from NASA depicts it.

 

 

     Indeed, one of the biggest uncertainties about climate change is climate sensitivity. The range of sensitivity put forth by climate scientists has not budged much, if at all since the 1970s, from about 1.5 deg C to 4.5 deg C. Others have pegged the range from 1.8 deg C to 5.6 deg C and others yet have sought to put it between 2.1 deg C and 3.6 deg C. Two NASA climate scientists, John Christy and Roy Spencer, published a recent paper about their new one-dimensional climate model that shows climate sensitivity near the lowest end of the range at 1.9 degrees C. Other NASA scientists have recently ranged climate sensitivity between 2.6 degrees C and 3.9 degrees C. Christy and Spencer claim their model is better because it accounts for heat storage in deeper layers of land which other models do not incorporate. I do not know how valid this point is. There are many studies of climate sensitivity that have come up with values across the range. Other climate scientists that are climate skeptics like Richard Lindzen have also argued that climate sensitivity is at the low end of the range.

     Below the light blue section shows the current IPCC range of 2-4.5 degrees C for climate sensitivity and estimates from different lines of evidence. 

     

Source: How sensitive is our climate? Skeptical Science. How sensitive is our climate? (skepticalscience.com)

     Climate scientists who are climate advocates have tended to find values closer to the higher end of the range. James Hansen is the lead author of a May 2023 paper from Columbia University - Global warming in the pipeline – which argues predictably for a more alarmist conclusion based on global climate models (GCMs). The IPCC estimates ECS at 3.0 degrees C. The Hansen paper concludes that ECS is “near 4°C or higher.” In 2008, he accepted 3 degrees C as fast feedback ECS but pegged slow feedback ESS at 6 degrees C. Thus, we have two groups of climate scientists (Christy & Spencer and Hansen et al), both groups likely biased in their own ways, predicting climate sensitivity where one prediction is more than double the other(1.9 vs. 4.0 or more). Climate sensitivity is an assumption that is readily plugged into GCMs, and results will be drastically different for each group. Hansen et al predict that we will exceed 1.5 degrees C for GMST by 2030 and 2.0 degrees C by 2050. I should point out that Hansen’s predictions in the 1980s of the global response to CO2 forcings turned out to be significantly overestimated. Uncertainties in aerosol forcing quantification and climate response times in particular make climate sensitivity difficult to pin down except to confine it to a range, which has been done in the past. Hansen’s paper utilizes oxygen isotope analysis from the Cenozoic Era, which began about 66 million years ago, to try and pin down temperature responses to CO2 forcings and compare them to today. During the Cenozoic, atmospheric CO2 levels rode from 300 ppm to 400 ppm, which resulted in a nearly ice-free planet. However, any paleoclimatic studies, while quite useful, are difficult to get quantified effects. The continents were at different positions on the earth due to plate tectonics and ice sheet changes likely occurred at different rates as a result. In any case, Hansen et al argue that ESS may be a better measure because greenhouse gas feedbacks that amplify effects and ice sheets do not have the previously assumed slow response, but faster response times. They use the Cenozoic paleoclimatic analogy for this argument. The argument is essentially that if those feedbacks have faster response times than previously predicted as they suggest, then climate sensitivity is higher. They also note that the aerosol cooling effect will likely wear off as less coal and wood are burned so that too will speed up global warming in time. The aerosol effects from wildfires will likely continue. They argue that along with slow and fast feedbacks, there are ultrafast feedbacks that may not be properly accounted. While I am not sure about aerosol effects, the numbers from the graph below from Hansen’s paper seem to give aerosols a pretty strong cooling effect. He calls it a Faustian Bargain that we are enjoying with the cooling effects of aerosols. 

 

Source: Hansen et al  2023.

 

References:

Climate Sensitivity. Wikipedia. Climate sensitivity - Wikipedia

Anthropogenic aerosol drives uncertainty in future climate mitigation efforts. E. J. L. Larson and R. W. Portmann. Scientific Reports. 2019; 9: 16538. November 12, 2019. Anthropogenic aerosol drives uncertainty in future climate mitigation efforts - PMC (nih.gov)

Climate model provides data-driven answer to major goal of climate research. Dr. Roy W. Spencer, University of Alabama in Huntsville. Phys.org. September 29, 2023. Climate model provides data-driven answer to major goal of climate research (phys.org)

Global warming in the pipeline. James E. Hansen, Makiko Sato, Leon Simons, Larissa S. Nazarenko, Isabelle Sangha, Karina von Schuckmann, Norman G. Loeb, Matthew B. Osman, Qinjian Jin, Pushker Kharecha, George Tselioudis, Eunbi Jeong, Andrew Lacis, Reto Ruedy, Gary Russell, Junji Cao, and Jing Li. Columbia University. May 19, 2023. PipelinePaper.2023.05.19.pdf (columbia.edu)

Making Sense of ‘Climate Sensitivity': New Study Narrows the Range of Uncertainty in Future Climate Projections. Alan Buis, NASA's Jet Propulsion Laboratory. September 8, 2020. Making Sense of ‘Climate Sensitivity' – Climate Change: Vital Signs of the Planet (nasa.gov)

How sensitive is our climate? Skeptical Science. How sensitive is our climate? (skepticalscience.com)