Climate Sensitivity to CO2 … determines the degree of warming Planet Earth incurs as the atmospheric concentration of CO2 rises. People and especially governmental policy makers need to understand the consequences that will occur due to past and future emissions. The images of fires and smoke along the USA’s West Coast now repeat yearly, with new extremes often breaking old records for heat and fire. Studies are mounting and one headline reads: Earth Is Barreling Toward a ‘Hothouse’ State Not Seen in 50 Million Years The image depicts the situation and suggests abrupt changes in global climate can follow relatively small changes in atmospheric CO2 concentration. This suggestion is not one that is politically acceptable because of its dire consequences. The truth hinges on the topic of this post.
This is a technical post on the topic of Climate Sensitivity. A team of scientists believe they have narrowed the range of potential values from “the previously accepted range of 1.5 to 4.5ºC first established in 1979 (roughly 3 to 9ºF), to a narrower range of 2.6 to 3.9ºC.” NASA Blog: Making Sense of Climate Sensitivity In their discussion of climate sensitivity, the authors settled on a 150 years as the time frame for the faster consequences to be measurably complete. I note that they appear to have ignored the exponential amplification of effects that I posted about in discussing methane and other amplifications. Still, it is an excellent article for presenting the topic well in broad strokes.
Humanity, and more imminently human society, is raising the level of greenhouse gases, primarily of CO2. The higher the climate sensitivity to CO2, the more temperatures will rise as CO2 increases. Look to the image at the right. We already saw in earlier IPCC reports what to expect from various increases in the global average temperature. After many subjective choices made in the setting of parameters considered this statement at the top of the NASA article (IMO) underscores the primary result that they politically needed to arrive at: “Essentially, by narrowing the range of estimates, the researchers found that climate sensitivity isn’t so low that it should be ignored, but it’s also not so high that there is no hope for the planet’s recovery.”
I’ve long considered the IPCC level discussions as striving to generate concern without despair. Ongoing emissions demonstrate the IPCC has not raised concerns enough to get human society to change. Despair not caused by warnings is instead arriving in the experiences of burnt homes, of shrinking islands, lost crops and of softened land that is no longer permafrost. Full equilibrium for any given change in CO2 concentration takes centuries to millennia to be reached, but much of it is done within far shorter time frames. Calculating the effect to expect is made harder because humans continue adding more insulating greenhouse gases, adding to their concentrations in the atmosphere. Despite decades of past international warnings, net CO2 in the atmosphere continues to increase.
Looking more deeply
Debate over climate sensitivity is about how much the temperature will shift. The IPCC was tasked with the magnitude of the shift this century, which requires identifying and tracking all the factors that shift most quickly. Different research teams and then the IPCC came up with different values, based upon their selection of factors that shift, their accuracy of tracking them and the quality of the algorithms in their models.
Separately, we know the ultimate shift will be larger, taking millennia for the planet’s equilibrium to be restored. Dr David Wasdell proposed a way of determining that upper limit by looking at the paleoclimate record. His analysis of the empirical geological relationship between CO2 and temperature when given time to reach equilibrium is summarized in this next graph: “Whole Earth Sensitivity.” In his paper Dr. Wasdell then includes examples of major modeling results from the 1979 Charney Report up to the modeling by Dr James Hansen. Those two open source papers plus this expanded warning by James Hansen as to sea levels give non-researchers a peek into the complexity of variables embedded in the summary statements as to what values each assign to climate sensitivity.
Scientists now agree that the hardest variable to track and model is cloud feedback. The latitude, altitude and density of clouds (condensed water vapor) is extremely transient data to measure and compute into models. See below for reference links to more papers on clouds and climate sensitivity.
Details of the Latest Study
I did not see the time frame defined in NASA’s posted article, but when I followed the article to this PDF “An assessment of Earth’s climate sensitivity using multiple lines of evidence” I saw the team chose to use 150 years. There is this acknowledgement:
“Bringing all the evidence to bear in a consistent way requires using a specific measure of ECS, so that all lines of evidence are linked to the same underlying quantity. We denote this quantity S (see section2.1). The implications for S of the three strands of evidence are examined separately in sections 3-5, and anticipated dependencies between them are discussed in section 6.”In Section 2.1 they define the time frame as 150 years, adding a reason:“Our reference scenario does not formally exclude any feedback process, but the 150-year time frame minimizes slow feedbacks (especially ice sheet changes).”
“the effective sensitivity S that we will use—a linear approximation to the equilibrium warming based on the first 150 years after an abrupt CO2 quadrupling—is a practical option for measuring sensitivity, based on climate system behavior over the most relevant time frame while still approximating the traditional ECS. Moreover, the quantitative difference between this and the traditional equilibrium measure based on a CO2 doubling (with fixed ice sheets) appears to be small, albeit uncertain. This uncertainty is skewed, in the sense that long-term ECS could be substantially higher than S but is very unlikely to be substantially lower. Further work is needed to better understand and constrain this uncertainty.”
“IPCC AR5 concluded that climate sensitivity is likely (≥ 66% probability) in the range 1.5-4.5 K [ºC]. The probability of S being in this range is 93% in our Baseline calculation, and is no less than 82% in all other “plausible” calculations considered as indicators of reasonable structural uncertainty (seesection7.3). Although consistent with IPCC’s “likely” statement, this indicates considerably more confidence than the minimum implied by the statement. We also find asymmetric probabilities outside this range, with negligible probability below 1.5 K [ºC] but up to an 18% chance of being above 4.5 K [ºC] (7% in the Baseline calculation).”
Why this is Alarming
Are Humans Responding?
An April 17, 2020 PNAS Report considers arguments that humans are surely not going to stay on the path of emissions indicated by RCP8.5. The key analysis (image to the right) shows that cumulative emissions since 2005 have most closely tracked against RCP8.5. It highlights that while human emissions might be less than that IPCC scenario, the net emissions between 2005 and 2020 are within 1% of that the cataclysmic path. Continuation on this path suggests a 3.3 to 5.4ºC increase in Earth’s temperature. The report points out that feedback effects such as thawing permafrost, and burning of forests are already sufficient to match the emission reductions humans have managed. These are positive (amplifying) feedback loops that humans need get ahead of and reverse – if there is to be any opportunity to restore Holocene like climate conditions.