Thongchai Thailand


Posted on: October 9, 2020




Figure 1 and Figure 2 above demonstrate the relevant features of a climate science anomaly known as ETCW or Early Twentieth Century Warming.

Figure 1 is a graphical rendition of 20th century temperatures and decadal warming rates (in a moving 10-year window). What we see here is that the rate of warming is strong in both the early third and the late third of the time span, particularly in the latter half of the calendar year from July to December, but with a flat period of no warming and perhaps some cooling in the middle of the 20th century. The issue here is that the warming similarity between the late third and the early third is anomalous in the CO2 greenhouse warming context because of very low CO2 in the early third and high CO2 in the late third of this study period. This comparison is well known in climate science and is referred to as the ETCW (Early Twentieth Century Warming) anomaly. As seen in the bibliography below, a significant area of research in climate science is the study of this anomaly to discover its hidden causes. The no-warming middle period is of course well known as the so called 1970s cooling where cooling is seen from 1945 to 1975.

The anomaly at issue here is easier to see in the corresponding ECS climate sensitivity values shown in Figure 2. Here ECS is computed for four different temperature reconstructions, Hadcrut4, GISS, Berkeley, and the RCP8.5 business as usual temperatures derived from the Charney high end climate sensitivity of ECS=4.5. The relevant pattern in Figure 2 for the ETCW is that very high values of ECS are needed in the early twentieth century to explain the warm temperatures at low atmospheric CO2 concentration. The ETCW issue is best understood in terms of these anomalous ECS values.

As seen in the bibliography below, the ETCW remains an anomaly and a mystery although there are several papers out that claim to have resolved this issue. One such paper is reviewed at this site in a related post: LINK: where Gabriele Hegerl and co-authors write that the ETCW can be explained by the CO2 emissions from a number of volcanic eruptions in that period. The paper is contentious in climate science circles because what she is saying is that ETCW is not anthropogenic global warming but natural warming; implying that AGW went on a hiatus back then and natural forcings in terms of volcanic emissions did the job.

This explanation by Hegerl etal suffers from circular reasoning because the volcanism theory was constructed after she had seen the data. She then tested that theory with the same data. Testing a theory with the data used to construct that theory is a form of circular reasoning.

An alternative explanation is found in the works of Colin Morice and others that have identified large uncertainties in older temperature reconstructions such that the older it is the greater the uncertainty: LINK:

Uncertainty in Global Mean Temperature | Thongchai Thailand


An odd curiosity of the ETCW issue is that the much championed and heralded Callendar 1938 paper published by the Royal Society in England is presented as a foundational work in climate science. It is the world’s first paper on anthropogenic global warming and climate change. It is claimed to show that the theory in climate science of the role of the greenhouse effect of CO2 in climate is well established in the history of science as seen in the Callendar 1938 paper.

What we find in the Callendar 1938 paper is that Guy Callendar studied the fossil fuel emissions and the corresponding rise in atmospheric CO2, and temperature for the 38-year period 1900 to 1938. He used the Tyndal papers on infrared absorption and re-radiation by CO2 (the greenhouse effect), to compute the equilibrium climate sensitivity as ECS=2 (later confirmed by Manabe and Wetherald 1965). Callendar concluded that the 1900-1938 warming was human caused by way of the coal burning of the industrial revolution and the greenhouse effect of atmospheric CO2 concentrations thus caused. This paper is the origin and the birth of AGW greenhouse effect climate science.


{The Callendar paper is discussed in some detail in a related post on this site LINK: }.

A recent paper in 2020 by Karsten Haustein challenges the ETCW anomaly idea. Karsten is at Oxford University where he is a member of the Climate Systems and Policy group. He specializes in the climate cooling impact of dust and aerosols and says that the ETCW is not an anomaly because he can explain it in the context of AGW in terms of the climate impact of dust and aerosols. Incidentally, Karsten is also an expert in the attribution of extreme weather events to AGW climate change (more on that in a related post). Details of this work by Karsten are provided by ARS Technica: LINK:

Karsten’s analysis of ETCW is summarized in the two images below.

Here are the external forcings: greenhouse gases, aerosol pollution, solar activity, and short-lived effects from volcanic eruptions. Each is compared to estimates used in the last IPCC report ("AR5") to show the difference when accounting for spatial patterns.
Modeled temperature trends (green line) compared to global temperature datasets. The datasets appear smoother here because year-to-year El Niño/La Niña wiggles were removed, but the match is the same with them added in.

He says that the analysis above shows that a correct inclusion of localized land based dust and aerosol forcing along with AGW GHG forcing of CO2 matches temperature reconstructions from 1880 to 2020 and that therefore there is no such thing as an ETCW anomaly. The analysis is similar to that of Gabriele Hegerl except that she used volcanoes to warm the early twentieth century while Karsten used volcanoes to cool the late twentieth century. Both contain the confirmation bias and circular reasoning of searching for things to explain the anomaly and interpreting the available data in that context. The theory to resolve the ETCW anomaly was constructed from the data and then tested with the same data. This pattern of empirical research, though common, is a form of circular reasoning.

Gabriele and Karsten notwithstanding, what we find in the bibliography below is that the ETCW is an unresolved issue in climate science and is the likely motivation for climate scientists such as James Hansen, Peter Cox, and NASA to move their AGW timeline to begin in 1950 or later well after the ETCW period. Of note in the bibliography below is the paper by Tom Knutson in which he acknowledges that the ETCW is a serious issue in the theory of AGW. Tom Knutson of NOAA is a climate scientist with a degree of scientific discipline that appears not to be a common feature of climate scientists in general. Photo of Dr. Thomas Knutson below.

Tom Knutson Homepage – Geophysical Fluid Dynamics Laboratory


  1. Delworth, Thomas L., and Thomas R. Knutson. “Simulation of early 20th century global warming.” Science 287.5461 (2000): 2246-2250.  The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model’s climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.
  2. Brönnimann, Stefan. “Early twentieth-century warming.” Nature Geoscience 2.11 (2009): 735-736.  The most pronounced warming in the historical global climate record prior to the recent warming occurred over the first half of the 20th century and is known as the Early Twentieth Century Warming (ETCW). Understanding this period and the subsequent slowdown of warming is key to disentangling the relationship between decadal variability and the response to human influences in the present and future climate. This review discusses the observed changes during the ETCW and hypotheses for the underlying causes and mechanisms. Attribution studies estimate that about a half (40–54%; p > .8) of the global warming from 1901 to 1950 was forced by a combination of increasing greenhouse gases and natural forcing, offset to some extent by aerosols. Natural variability also made a large contribution, particularly to regional anomalies like the Arctic warming in the 1920s and 1930s. The ETCW period also encompassed exceptional events, several of which are touched upon: Indian monsoon failures during the turn of the century, the “Dust Bowl” droughts and extreme heat waves in North America in the 1930s, the World War II period drought in Australia between 1937 and 1945; and the European droughts and heat waves of the late 1940s and early 1950s. Understanding the mechanisms involved in these events, and their links to large scale forcing is an important test for our understanding of modern climate change and for predicting impacts of future change. This article is categorized under: • Paleoclimates and Current Trends > Modern Climate Change
  3. Cowan, Tim, et al. “Factors contributing to record-breaking heat waves over the Great Plains during the 1930s Dust Bowl.” Journal of Climate 30.7 (2017): 2437-2461.  Record-breaking summer heat waves were experienced across the contiguous United States during the decade-long “Dust Bowl” drought in the 1930s. Using high-quality daily temperature observations, the Dust Bowl heat wave characteristics are assessed with metrics that describe variations in heat wave activity and intensity. Despite the sparser station coverage in the early record, there is robust evidence for the emergence of exceptional heat waves across the central Great Plains, the most extreme of which were preconditioned by anomalously dry springs. This is consistent with the entire twentieth-century record: summer heat waves over the Great Plains develop on average ~15–20 days earlier after anomalously dry springs, compared to summers following wet springs. Heat waves following dry springs are also significantly longer and hotter, indicative of the importance of land surface feedbacks in heat wave intensification. A distinctive anomalous continental-wide circulation pattern accompanied exceptional heat waves in the Great Plains, including those of the Dust Bowl decade. An anomalous broad surface pressure ridge straddling an upper-level blocking anticyclone over the western United States forced substantial subsidence and adiabatic warming over the Great Plains, and triggered anomalous southward warm advection over southern regions. This prolonged and amplified the heat waves over the central United States, which in turn gradually spread westward following heat wave emergence. The results imply that exceptional heat waves are preconditioned, triggered, and strengthened across the Great Plains through a combination of spring drought, upper-level continental-wide anticyclonic flow, and warm advection from the north.
  4. Wegmann, Martin, Stefan Brönnimann, and Gilbert P. Compo. “Tropospheric circulation during the early twentieth century Arctic warming.” Climate dynamics 48.7-8 (2017): 2405-2418.  The early twentieth century Arctic warming (ETCAW) between 1920 and 1940 is an exceptional feature of climate variability in the last century. Its warming rate was only recently matched by recent warming in the region. Unlike recent warming largely attributable to anthropogenic radiative forcing, atmospheric warming during the ETCAW was strongest in the mid-troposphere and is believed to be triggered by an exceptional case of natural climate variability. Nevertheless, ultimate mechanisms and causes for the ETCAW are still under discussion. Here we use state of the art multi-member global circulation models, reanalysis and reconstruction datasets to investigate the internal atmospheric dynamics of the ETCAW. We investigate the role of boreal winter mid-tropospheric heat transport and circulation in providing the energy for the large scale warming. Analyzing sensible heat flux components and regional differences, climate models are not able to reproduce the heat flux evolution found in reanalysis and reconstruction datasets. These datasets show an increase of stationary eddy heat flux and a decrease of transient eddy heat flux during the ETCAW. Moreover, tropospheric circulation analysis reveals the important role of both the Atlantic and the Pacific sectors in the convergence of southerly air masses into the Arctic during the warming event. Subsequently, it is suggested that the internal dynamics of the atmosphere played a major role in the formation in the ETCAW.
  5. Stolpe, Martin B., Iselin Medhaug, and Reto Knutti. “Contribution of Atlantic and Pacific multidecadal variability to twentieth-century temperature changes.” Journal of Climate 30.16 (2017): 6279-6295.  Recent studies have suggested that significant parts of the observed warming in the early and the late twentieth century were caused by multidecadal internal variability centered in the Atlantic and Pacific Oceans. Here, a novel approach is used that searches for segments of unforced preindustrial control simulations from global climate models that best match the observed Atlantic and Pacific multidecadal variability (AMV and PMV, respectively). In this way, estimates of the influence of AMV and PMV on global temperature that are consistent both spatially and across variables are made. Combined Atlantic and Pacific internal variability impacts the global surface temperatures by up to 0.15°C from peak-to-peak on multidecadal time scales. Internal variability contributed to the warming between the 1920s and 1940s, the subsequent cooling period, and the warming since then. However, variations in the rate of warming still remain after removing the influence of internal variability associated with AMV and PMV on the global temperatures. During most of the twentieth century, AMV dominates over PMV for the multidecadal internal variability imprint on global and Northern Hemisphere temperatures. Less than 10% of the observed global warming during the second half of the twentieth century is caused by internal variability in these two ocean basins, reinforcing the attribution of most of the observed warming to anthropogenic forcings.
  6. Tokinaga, Hiroki, Shang-Ping Xie, and Hitoshi Mukougawa. “Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability.” Proceedings of the National Academy of Sciences 114.24 (2017): 6227-6232.  With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean–atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.
  7. Hegerl, Gabriele C., et al. “The early 20th century warming: anomalies, causes, and consequences.” Wiley Interdisciplinary Reviews: Climate Change 9.4 (2018): e522.  The most pronounced warming in the historical global climate record prior to the recent warming occurred over the first half of the 20th century and is known as the Early Twentieth Century Warming (ETCW). Understanding this period and the subsequent slowdown of warming is key to disentangling the relationship between decadal variability and the response to human influences in the present and future climate. This review discusses the observed changes during the ETCW and hypotheses for the underlying causes and mechanisms. Attribution studies estimate that about a half (40–54%; p > .8) of the global warming from 1901 to 1950 was forced by a combination of increasing greenhouse gases and natural forcing, offset to some extent by aerosols. Natural variability also made a large contribution, particularly to regional anomalies like the Arctic warming in the 1920s and 1930s. The ETCW period also encompassed exceptional events, several of which are touched upon: Indian monsoon failures during the turn of the century, the “Dust Bowl” droughts and extreme heat waves in North America in the 1930s, the World War II period drought in Australia between 1937 and 1945; and the European droughts and heat waves of the late 1940s and early 1950s. Understanding the mechanisms involved in these events, and their links to large scale forcing is an important test for our understanding of modern climate change and for predicting impacts of future change.
  8. Butler, James H., et al. “A record of atmospheric halocarbons during the twentieth century from polar firn air.” Nature 399.6738 (1999): 749-755.  Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today’s value.
  9. Tett, Simon FB, et al. “Estimation of natural and anthropogenic contributions to twentieth century temperature change.” Journal of Geophysical Research: Atmospheres 107.D16 (2002): ACL-10.  Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well‐mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well‐mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well‐mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well‐mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near‐surface temperature from well‐mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near‐surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.
  10. Thompson, David WJ, et al. “Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change.” Nature Geoscience 4.11 (2011): 741-749.  Anthropogenic emissions of carbon dioxide and other greenhouse gases have driven and will continue to drive widespread climate change at the Earth’s surface. But surface climate change is not limited to the effects of increasing atmospheric greenhouse gas concentrations. Anthropogenic emissions of ozone-depleting gases also lead to marked changes in surface climate, through the radiative and dynamical effects of the Antarctic ozone hole. The influence of the Antarctic ozone hole on surface climate is most pronounced during the austral summer season and strongly resembles the most prominent pattern of large-scale Southern Hemisphere climate variability, the Southern Annular Mode. The influence of the ozone hole on the Southern Annular Mode has led to a range of significant summertime surface climate changes not only over Antarctica and the Southern Ocean, but also over New Zealand, Patagonia and southern regions of Australia. Surface climate change as far equatorward as the subtropical Southern Hemisphere may have also been affected by the ozone hole. Over the next few decades, recovery of the ozone hole and increases in greenhouse gases are expected to have significant but opposing effects on the Southern Annular Mode and its attendant climate impacts during summer.
  11. Compo, Gilbert P., et al. “The twentieth century reanalysis project.” Quarterly Journal of the Royal Meteorological Society 137.654 (2011): 1-28. The Twentieth Century Reanalysis (20CR) project is an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea‐surface temperature and sea‐ice distributions as boundary conditions. It is chiefly motivated by a need to provide an observational dataset with quantified uncertainties for validations of climate model simulations of the twentieth century on all time‐scales, with emphasis on the statistics of daily weather. It uses an Ensemble Kalman Filter data assimilation method with background ‘first guess’ fields supplied by an ensemble of forecasts from a global numerical weather prediction model. This directly yields a global analysis every 6 hours as the most likely state of the atmosphere, and also an uncertainty estimate of that analysis.The 20CR dataset provides the first estimates of global tropospheric variability, and of the dataset’s time‐varying quality, from 1871 to the present at 6‐hourly temporal and 2° spatial resolutions. Comparisons with independent radiosonde data indicate that the reanalyses are generally of high quality. The quality in the extratropical Northern Hemisphere throughout the century is similar to that of current three‐day operational NWP forecasts. Intercomparisons over the second half‐century of these surface‐based reanalyses with other reanalyses that also make use of upper‐air and satellite data are equally encouraging. It is anticipated that the 20CR dataset will be a valuable resource to the climate research community for both model validations and diagnostic studies. Some surprising results are already evident. For instance, the long‐term trends of indices representing the North Atlantic Oscillation, the tropical Pacific Walker Circulation, and the Pacific–North American pattern are weak or non‐existent over the full period of record. The long‐term trends of zonally averaged precipitation minus evaporation also differ in character from those in climate model simulations of the twentieth century.
  12. Smith, Karen L., Lorenzo M. Polvani, and Daniel R. Marsh. “Mitigation of 21st century Antarctic sea ice loss by stratospheric ozone recovery.” Geophysical Research Letters 39.20 (2012).  We investigate the effect of stratospheric ozone recovery on Antarctic sea ice in the next half‐century, by comparing two ensembles of integrations of the Whole Atmosphere Community Climate Model, from 2001 to 2065. One ensemble is performed by specifying all forcings as per the Representative Concentration Pathway 4.5; the second ensemble is identical in all respects, except for the surface concentrations of ozone depleting substances, which are held fixed at year 2000 levels, thus preventing stratospheric ozone recovery. Sea ice extent declines in both ensembles, as a consequence of increasing greenhouse gas concentrations. However, we find that sea ice loss is ∼33% greater for the ensemble in which stratospheric ozone recovery does not take place, and that this effect is statistically significant. Our results, which confirm a previous study dealing with ozone depletion, suggest that ozone recovery will substantially mitigate Antarctic sea ice loss in the coming decades.
  13. Egorova, Tatiana, et al. “Contributions of natural and anthropogenic forcing agents to the early 20th century warming.” Frontiers in Earth Science 6 (2018): 206.  The warming observed in the early 20th century (1910–1940) is one of the most intriguing and least understood climate anomalies of the 20th century. To investigate the contributions of natural and anthropogenic factors to changes in the surface temperature, we performed seven model experiments using the chemistry-climate model with interactive ocean SOCOL3-MPIOM. Contributions of energetic particle precipitation, heavily (shortwave UV) and weakly (longwave UV, visible, and infrared) absorbed solar irradiances, well-mixed greenhouse gases (WMGHGs), tropospheric ozone precursors, and volcanic eruptions were considered separately. Model results suggest only about 0.3 K of global and annual mean warming during the considered 1910–1940 period, which is smaller than the trend obtained from observations by about 25%. We found that half of the simulated global warming is caused by the increase of WMGHGs (CO2, CH4, and N2O), while the increase of the weakly absorbed solar irradiance is responsible for approximately one third of the total warming. Because the behavior of WMGHGs is well constrained, only higher solar forcing or the inclusion of new forcing mechanisms can help to reach better agreement with observations. The other forcing agents considered (heavily absorbed UV, energetic particles, volcanic eruptions, and tropospheric ozone precursors) contribute less than 20% to the annual and global mean warming; however, they can be important on regional/seasonal scales.
  14. Polvani, Lorenzo M., et al. “Significant weakening of Brewer‐Dobson circulation trends over the 21st century as a consequence of the Montreal Protocol.” Geophysical Research Letters 45.1 (2018): 401-409.  It is well established that increasing greenhouse gases, notably CO2, will cause an acceleration of the stratospheric Brewer‐Dobson circulation (BDC) by the end of this century. We here present compelling new evidence that ozone depleting substances are also key drivers of BDC trends. We do so by analyzing and contrasting small ensembles of “single‐forcing” integrations with a stratosphere resolving atmospheric model with interactive chemistry, coupled to fully interactive ocean, land, and sea ice components. First, confirming previous work, we show that increasing concentrations of ozone depleting substances have contributed a large fraction of the BDC trends in the late twentieth century. Second, we show that the phasing out of ozone depleting substances in coming decades—as a consequence of the Montreal Protocol—will cause a considerable reduction in BDC trends until the ozone hole is completely healed, toward the end of the 21st century.
  15. Polvani, Lorenzo M., and Katinka Bellomo. “The Key Role of Ozone-Depleting Substances in Weakening the Walker Circulation in the Second Half of the Twentieth Century.” Journal of Climate 32.5 (2019): 1411-1418.  It is widely appreciated that ozone-depleting substances (ODS), which have led to the formation of the Antarctic ozone hole, are also powerful greenhouse gases. In this study, we explore the consequence of the surface warming caused by ODS in the second half of the twentieth century over the Indo-Pacific Ocean, using the Whole Atmosphere Chemistry Climate Model (version 4). By contrasting two ensembles of chemistry–climate model integrations (with and without ODS forcing) over the period 1955–2005, we show that the additional greenhouse effect of ODS is crucial to producing a statistically significant weakening of the Walker circulation in our model over that period. When ODS concentrations are held fixed at 1955 levels, the forcing of the other well-mixed greenhouse gases alone leads to a strengthening—rather than weakening—of the Walker circulation because their warming effect is not sufficiently strong. Without increasing ODS, a surface warming delay in the eastern tropical Pacific Ocean leads to an increase in the sea surface temperature gradient between the eastern and western Pacific, with an associated strengthening of the Walker circulation. When increasing ODS are added, the considerably larger total radiative forcing produces a much faster warming in the eastern Pacific, causing the sign of the trend to reverse and the Walker circulation to weaken. Our modeling result suggests that ODS may have been key players in the observed weakening of the Walker circulation over the second half of the twentieth century.
  16. Abalos, Marta, et al. “New Insights on the Impact of Ozone‐Depleting Substances on the Brewer‐Dobson Circulation.” Journal of Geophysical Research: Atmospheres 124.5 (2019): 2435-2451.  It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role on ODS‐driven age of air trends.
  17. Polvani, L. M., et al. “Substantial twentieth-century Arctic warming caused by ozone-depleting substances.” Nature Climate Change (2020): 1-4.  The rapid warming of the Arctic, perhaps the most striking evidence of climate change, is believed to have arisen from increases in atmospheric concentrations of GHG since the Industrial Revolution. While the dominant role of carbon dioxide is undisputed, another important set of anthropogenic GHGs was also being emitted over the second half of the twentieth century: ozone depleting substances (ODS). These compounds, in addition to causing the ozone hole over Antarctica, have long been recognized as powerful GHG. However, their contribution to Arctic warming has not been quantified. We do so here by analysing ensembles of climate model integrations specifically designed for this purpose, spanning the period 1955–2005 when atmospheric concentrations of ODS increased rapidly. We show that, when ODS are kept fixed, forced Arctic surface warming and forced sea-ice loss are only half as large as when ODS are allowed to increase. We also demonstrate that the large impact of ODS on the Arctic occurs primarily via direct radiative warming, not via ozone depletion. Our findings reveal a substantial contribution of ODS to recent Arctic warming, and highlight the importance of the Montreal Protocol as a major climate change-mitigation treaty.

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