Guest essay by Eric Worrall
We laughed when Alexandria Occasio Cortez claimed that the world would end in 12 years. According to a new climate study, AOC is an optimist; we might only have seven years. But the authors appear to make some fairly pessimistic assumptions to achieve that prognosis.
Climate change: Threshold for dangerous warming will likely be crossed between 2027-2042
Scientists introduce a new way to predict global warming, reducing uncertainties considerably
Date: December 21, 2020
Source: McGill University
The threshold for dangerous global warming will likely be crossed between 2027 and 2042 — a much narrower window than the Intergovernmental Panel on Climate Change’s estimate of between now and 2052. In a study published in Climate Dynamics, researchers from McGill University introduce a new and more precise way to project the Earth’s temperature. Based on historical data, it considerably reduces uncertainties compared to previous approaches.
“Climate skeptics have argued that global warming projections are unreliable because they depend on faulty supercomputer models. While these criticisms are unwarranted, they underscore the need for independent and different approaches to predicting future warming,” says co-author Bruno Tremblay, a professor in the Department of Atmospheric and Oceanic Sciences at McGill University.
Until now, wide ranges in overall temperature projections have made it difficult to pinpoint outcomes in different mitigation scenarios. For instance, if atmospheric CO2 concentrations are doubled, the General Circulation Models (GCMs) used by the Intergovernmental Panel on Climate Change (IPCC), predict a very likely global average temperature increase between 1.9 and 4.5C — a vast range covering moderate climate changes on the lower end, and catastrophic ones on the other.
A new approach
“Our new approach to projecting the Earth’s temperature is based on historical climate data, rather than the theoretical relationships that are imperfectly captured by the GCMs. Our approach allows climate sensitivity and its uncertainty to be estimated from direct observations with few assumptions,” says co-author Raphael Hebert, a former graduate researcher at McGill University, now working at the Alfred-Wegener-Institut in Potsdam, Germany.
…Read more: https://www.sciencedaily.com/releases/2020/12/201221160425.htm
The abstract of the study;
An observation-based scaling model for climate sensitivity estimates and global projections to 2100
Climate Dynamics (2020)
We directly exploit the stochasticity of the internal variability, and the linearity of the forced response to make global temperature projections based on historical data and a Green’s function, or Climate Response Function (CRF). To make the problem tractable, we take advantage of the temporal scaling symmetry to define a scaling CRF characterized by the scaling exponent H, which controls the long-range memory of the climate, i.e. how fast the system tends toward a steady-state, and an inner scale 𝜏≈2τ≈2 years below which the higher-frequency response is smoothed out. An aerosol scaling factor and a non-linear volcanic damping exponent were introduced to account for the large uncertainty in these forcings. We estimate the model and forcing parameters by Bayesian inference which allows us to analytically calculate the transient climate response and the equilibrium climate sensitivity as: 1.7+0.3−0.21.7−0.2+0.3 K and 2.4+1.3−0.62.4−0.6+1.3 K respectively (likely range). Projections to 2100 according to the RCP 2.6, 4.5 and 8.5 scenarios yield warmings with respect to 1880–1910 of: 1.5+0.4−0.2𝐾1.5−0.2+0.4K, 2.3+0.7−0.52.3−0.5+0.7 K and 4.2+1.3−0.94.2−0.9+1.3 K. These projection estimates are lower than the ones based on a Coupled Model Intercomparison Project phase 5 multi-model ensemble; more importantly, their uncertainties are smaller and only depend on historical temperature and forcing series. The key uncertainty is due to aerosol forcings; we find a modern (2005) forcing value of [−1.0,−0.3]Wm−2[−1.0,−0.3]Wm−2 (90 % confidence interval) with median at −0.7Wm−2−0.7Wm−2. Projecting to 2100, we find that to keep the warming below 1.5 K, future emissions must undergo cuts similar to RCP 2.6 for which the probability to remain under 1.5 K is 48 %. RCP 4.5 and RCP 8.5-like futures overshoot with very high probability.Read more: https://link.springer.com/article/10.1007/s00382-020-05521-x
From the study;
… For the high-emission scenario RCP 8.5, all methods show a high probability of a warming exceeding 2 K before 2100. According to the SCRF, the risk of overshooting 1.5 K is negligible before 2024 (or 2028), but extremely likely after 2036 (or 2047), similarly to the CMIP5 MME which reaches the 95% probability of overshooting 1.5 K in 2038. The 2 K threshold is also extremely likely to be crossed about 15 years later in 2055 for both the SCRF and CMIP5 MME (or 2068). …Read more: Same link as above
… Our analysis supports better constrained TCR and ECS likely range than the IPCC AR5. When using 𝐹𝐴𝑒𝑟𝑅𝐶𝑃FAerRCP (or 𝐹𝐴𝑒𝑟𝑄𝑎FAerQa), the range shrinks from [1.0, 2.5] K to [1.4, 2.0] K for the TCR (or [1.2, 1.5] K) and from [1.5,4.5]𝐾[1.5,4.5]K to [1.8,3.7]𝐾[1.8,3.7]K for the ECS (or [1.5, 2.7] K); the median estimates also decrease from 1.8 K to 1.7 K (or 1.4 K) for the TCR and from 3.0 K to 2.4 K (or 1.8 K) for the ECS. This agrees with other recent observation-based studies (Otto et al. 2013; Skeie et al. 2014, and Johansson et al. 2015) which also support a downward revision of the ECS upper 17% bound by at least half a degree. In addition, the ECS500500 was found to be significantly smaller, 2.2+0.6−0.5𝐾2.2−0.5+0.6K (or 1.7+0.4−0.2𝐾1.7−0.2+0.4K), than the ECS. This implies that if the ECS is on the higher end of the CI, then a large fraction of the warming would be experienced hundreds of years after a potential stabilization of anthropogenic forcing. An important and rather conservative claim supported by this evidence is therefore that the upper 5% ECS bound and median of AR5 can be safely revised downward to 4.0 K and 2.5 K. The lower 5% bound of 1.5 K, on the other hand, remains reliable. …Read more: Same link as above
Interestingly the TCR (Transient Climate Response) estimated by the authors appears to be fairly close to the ECS (Equilibrium Climate Sensitivity) estimated by Lord Monckton in his corrected feedback study. The authors of this study justify this difference between TCR and ECS by raising the possibility that the climate will take a long time to stabilise (“hundreds of years”) even if anthropogenic CO2 emissions were halted today, so a lot hinges on how much warming is still in the pipeline.
How can we get an estimate to constrain the ECS, without waiting for centuries for the climate to finish responding to anthropogenic CO2 emissions? One option is to consider past geological periods such as the Jurassic (1950ppm CO2, +3C global warming). If 1950ppm CO2 can only raise global temperature by 3C, even when the paleo-climate had millions of years to respond to elevated CO2 levels, this implies that ECS is small, and there is very little if any additional warming waiting in the pipeline. A low ECS implies that any anthropogenic global warming likely to occur as a result of our burning of fossil fuel is a total non event – we will run out of fossil fuel long before we reach 1950ppm. But this viewpoint does not appear to be popular with most government funded climate scientists.