Tag Archives: simulations

Claim: Ozone treaty is delaying first ice-free Arctic summer

From Tallbloke’s Talkshop

 May 24, 2023 by oldbrew

Sea ice optional? [image credit: BBC]

Why has it taken so long for greenhouse gas obsessives to come up with this, using ‘new climate model simulations’? The Arctic summer sea ice was supposed to be on its last legs at least fifteen years ago. Of course natural variation is ignored or discounted, as usual.
– – –
A 1987 global deal to protect the ozone layer is delaying the first ice-free Arctic summer by up to 15 years, new research shows.

The Montreal Protocol – the first treaty to be ratified by every United Nations country – regulates nearly 100 man-made chemicals called ozone-depleting substances (ODSs), says EurekAlert.

While the main aim was to preserve the ozone layer, ODSs are also potent greenhouse gases, so the deal has slowed global warming.

The new study shows the effects of this include delaying the first ice-free Arctic summer (currently projected to happen the middle of this century) by up to 15 years, depending on future emissions.

The researchers – from UC Santa Cruz, Columbia University and the University of Exeter – estimate that each 1,000 tonnes of ODS emissions prevented saves about seven square kilometres of Arctic sea ice.

“While ODSs aren’t as abundant as other greenhouse gases such as carbon dioxide, they can have a real impact on global warming,” said Dr Mark England, Royal Commission for the Exhibition of 1851 senior research fellow at the University of Exeter.

“ODSs have particularly powerful effects in the Arctic, and they played a major role in driving Arctic climate change in the second half of the 20th Century.

“While stopping these effects was not the primary goal of the Montreal Protocol, it has been a fantastic by-product.”

Dr England said opponents of the protocol predicted a range of negative consequences, most of which did not happen, and instead there are numerous documented instances of unintended climate benefits.

Professor Lorenzo Polvani, from Columbia University, said: “The first ice-free Arctic summer – meaning the Arctic Ocean practically free of sea ice – will be a major milestone in the process of climate change.

“Our findings clearly demonstrate that the Montreal Protocol has been a very powerful climate protection treaty, and has done much more than healing the ozone hole over the South Pole.

“Its effects are being felt all over the world, especially in the Arctic.”

ODS decline

The study, which used new climate model simulations, shows that protection of the ozone layer itself played no part in slowing the loss of Arctic sea ice – all the benefits relate to the role of ODSs as greenhouse gases.

ODSs (which include chlorofluorocarbons, also called CFCs) are compounds developed in the last century for industrial use as refrigerants and propellants.

The Montreal Protocol, which has now been signed by all 198 members of United Nations, regulated these compounds to preserve the ozone layer, which protects humans and the environment from harmful levels of ultraviolet radiation.

This effort has succeeded, with atmospheric concentrations of ODSs declining since the mid-1990s and signs that the ozone layer has started to heal.

However, research has suggested a slight rise in ODS concentrations from 2010-20, so Dr England said vigilance is still required.

Full article here.

Predictive power of climate models may be masked by volcanoes

From Phys.org

By National Center for Atmospheric Research

Simulated volcanic eruptions may be blowing up our ability to predict near-term climate, according to a new study published in Science Advances.

The research, led by the National Center for Atmospheric Research (NCAR), finds that the way volcanic eruptions are represented in climate models may be masking the models’ ability to accurately predict variations in sea surface temperatures in the tropical Pacific that unfold over multiple years to a decade.

These decadal variations in sea surface temperatures in the tropical Pacific are linked to climate impacts across the globe, including variations in precipitation and severe weather. Accurate predictions, therefore, could provide community leaders, farmers, water managers, and others with critical climate information that allows them to plan years in advance.

“Near-term climate prediction on annual to decadal timescales is a rapidly growing and important field in the climate community because it bridges the gap between existing seasonal forecasts and centennial climate projections,” said Xian Wu, who led the study as a postdoctoral researcher at NCAR. “When we rely on models to make these predictions, it’s important to carefully consider the model’s fidelity. In this case, we found that model errors in simulating the response to volcanic eruptions degraded our prediction skill.”

For the study, Wu and her colleagues relied on two parallel collections of climate simulations from the Decadal Prediction Large Ensemble, a dataset produced using the NCAR-based Community Earth System Model. These simulations were run as hindcasts and cover the years from 1954–2015, allowing scientists to compare the simulations with what really occurred and evaluate their skill at predicting the future.

One collection of simulations included the three major volcanic eruptions that occurred during the study period: Agung (1963), El Chichón (1982), and Pinatubo (1991). The other collection did not.

Because it is well established that large volcanic eruptions can have significant, long-term cooling effects on the climate, Wu and her colleagues expected that the collection of simulations that included the volcanic eruptions would produce more accurate multiyear and decadal climate predictions. Instead, they found that the inclusion of the eruptions degraded the model’s predictive capabilities, at least in the tropical Pacific, an area that is especially important because of the connections between sea surface temperatures and near-term climate events.

For example, the simulations that included the volcanoes predicted a subsequent cooling of the sea surface temperatures in the tropical Pacific after the eruptions. In reality, that region of the ocean warmed, a change that was well predicted by the simulations that did not include the volcanic eruptions.

These findings highlight the difficulty of accurately representing the complex climate impacts that follow a volcanic eruption in a model, a task made more challenging because researchers only have a few real-life examples in the observational record. Scientists know that volcanoes can loft sulfur gases high into the stratosphere where they can transform into sunlight-reflecting aerosols. But how the resulting cooling ultimately affects the entire Earth system, including sea surface temperatures, is not well understood.

“We just don’t have enough observations,” Wu said. “And our methods to observe what is happening in the stratosphere have only been available since the satellite era, which means we only have Chichón and Pinatubo.”

Still, Wu is hopeful that representations of volcanic eruptions and their impacts in models can be improved over time and that, ultimately, this work will improve our ability to forecast important climate events years in advance.

“Decadal variability in the tropical pacific is an important source of predictability worldwide,” Wu said. “It affects climate over the surrounding continents, as well as marine ecosystems. Better predictions will provide important information for stakeholders.”

More information: Xian Wu, Volcanic forcing degrades multiyear-to-decadal prediction skill in the tropical Pacific, Science Advances (2023). DOI: 10.1126/sciadv.add9364. www.science.org/doi/10.1126/sciadv.add9364

Journal information: Science Advances 

Provided by National Center for Atmospheric Research 

Is the Antarctic-driven abyssal ocean overturning doomed in 2050?

From Climate Etc.

by Frank Bosse

Probably not, in spite of the recent headlines.

A recent article in Nature Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater by England et al. (hereafter E23) caused quite a stir in the media.  The BBC wrote:Antarctic Ocean currents heading for collapse – report.

E23 built a model to describe the formation and behavior of abyssal water masses around Antarctica.  The Antarctic abyssal waters are important due to its impact on the overturning circulation (AOC) – the lower cell of the Meridional Overturning Curculation (MOC) – which overturns heat, freshwater, oxygen, carbon and nutrients in the abyssal ocean.  The AOC directly influences warming and the availability of nutrients to support marine life near the surface of the ocean.

Here is a schematic of the global MOC:

Fig.1.: The global MOC, a reproduction of Fig.1 of Marshall / Speer (2012).  The Antarctic Bottom Water (AABW) is shown on the left side in descending blue arrows.

E23 concludes:

“In particular, a net slowdown of the abyssal ocean overturning circulation of just over 40% is projected to occur by 205”

According to E23, this would also have some impact on the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for the vast majority of the northward heat transport on earth:

“As the meltwater release from Greenland and Antarctica increases over time, the AABW overturning and AMOC strength both weaken by 2050.” (AMOC by 19% shows Fig. 2).

The cause is the additional meltwater from the Antarctic ice shelves, which has a widespread impact on the Antarctic Bottom Water (AABW):

“First, the projected addition of Antarctic meltwater causes an anomalous freshening . . . which produces fresher and less dense AABW, and eventually reduced AABW volume, after the 2030s.”

The key figure of E23:

Fig.2: A reproduction of Fig. 3a, b in E23. The Antarctic melting will lead to a reduction under the influence of the anthropogenic forcing (aka “Climate Crisis”) of the AABW of 42% (a) in 2050, shown in red. In black: without this forcing.

In Fig. 2 (b) the AMOC shows a robust downward trend over 2004-2020; this is not the case in the observations of “Rapid” at 26.5N;  there is much internal variability, with a dip in 2010 and thereafter a slightly recovery.

Figure:  Observations of the AMOC 2004 to 2020 of “Rapid” at 26,5°N:  Source.

Let’s now have a look how the authors calculated the melting up to 2050, which is a crucial input of the described model for the AABW. From the Methods section of E23:

“…and the multi-model ensemble mean of CMIP6 models under a high- anthropogenic-emissions scenario, Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5), for the future climate component from 2020 until 2050.”

In a twitter thread the lead author stated (and provided a “SharedIt” link to read the full paper, thanks for this):

“…our projections were run under a ‘business as usual’ scenario. Deep and urgent emissions reductions will give us a chance of avoiding an ocean overturning collapse.”

Is SSP5-8.5 (or RCP 8.5 in IPCC AR5) “Business as usual”? Not so, stated this comment, also in “Nature”:

“Stop using the worst-case scenario for climate warming as the most likely outcome”.

Its projections of future greenhouse gas emissions are generally acknowledged to be unrealistic even on pessimistic assumptions.

Furthermore: is the Multi -Model Ensemble mean (MME) of the CMIP6-models appropriate for this approach? No, the MME mean is skewed high owing to a high climate sensitivity due to some models running much too hot. Gavin Schmidt:

“The default behavior in the community has to move away from considering the raw model ensemble mean as meaningful.”

This leads to an urgent need of a discussion of the choice of SPS5-8.5 and the CMIP6 ensemble mean in E23. Unfortunately the paper doesn’t do this, so I will do it in this blog post.

What effect does the choice of the projected temperatures in the Antarctic for 2020 to 2050 have, which in turn influences the expected melting?

With the help of the KNMI Climate explorer I investigated the expected trends, first for the settings used in E23:

Fig.3: The linear temperature trends in and around Antarctica for SPS5-8.5 and the MMM of the CMIP6’s, as it was estimated in E23.

In comparison, the not-so-skewed CMIP5’s MME mean for the more likely RCP4.5 scenario:

Fig.4: The linear temperature trends for RCP4.5 in and around Antarctica.

Note that the trend slopes in the crucial melting areas of the western Antarctic (including the Ross Sea and the Weddell Sea) are nearly 50% steeper in the Fig. 3 than in Fig. 4. This results in a warming in this area from 2020 to 2050 of 0.6 K (Fig.4) based on RCP 4.5 and the CMIP5 MME mean; in Fig. 3 it results in 1.3 K based on SSP5-8.5 and the CMIP6 MME mean.

However, these are climate model simulation results. Let’s compare the spatially resolved linear trends of grid cells for the area 60°S to 90°S in the time span 1990 to 2021, virtually the same length as the 30 years long time span 2020 to 2050 in E23 for the observations (GISS) and the “not so hot case” of Fig.4:

Fig.5: The spatial trend slopes of the gridded data for the CMIP5 models with the scenario RCP 4.5 (left) and observations, GISS (right) for the time span 1990-2021 (“Hindcast”)

Not only do the simulations warm far too quickly in Antarctica and its environs over the last 30 years,
but the modeled warming is poorly correlated with observed warming in most grid cells (Fig.5)

In wide and crucial areas for the forming of the AABW especially on the coastlines (with the exception of the Ross Sea on the bottom) the observed and modelled trends are quite different – in the observations the trends are near zero.

The more realistic scenario CMIP5 RCP4.5 shows a twice as fast warming in the Antarctic (60°S-90°S) as the observations, and the CMIP 6-SSP5-8.5 mean scenario shows an almost 3 times faster warming in the “hindcast” period 1990 to 2021 despite the fact that there is relatively little difference in greenhouse gas emissions and changes in other drivers of climate change between the SSP5-8.5 and RCP4.5 scenarios and observations during that period.

In the light uncertainty of the spatial resolved trends in the observations, I use the relation of the trends of the entire Antarctic region, estimating that the warming bias in models will persist to 2050. This would lead to an additional warming of only 0.3K for 2020 to 2050 in the Western Arctic, 23% of the estimation in E23.

Conclusion

Neither the sole warming scenario nor the multi model CMIP6 ensemble mean used by E23 to estimate the melting in Antarctica up to 2050 is appropriate. The resulting MME mean heavily overestimates the likely surface warming and hence the melting, making it “not meaningful” (see Gavin Schmidt’s citation) as input for the AABW-model used in E23.

For the crucial regions, the trend slopes 1990 to 2021 in the observations are only about one third of the simulations used by E23. Moreover, projected future greenhouse gas emissions and levels are unrealistically high in SSP5-8.5 scenario used by E23. This suggests that future surface warming in and around Antarctica is likely to be far lower than E23 assumes, which in turn means ice melting and hence the slowing of the abyssal ocean overturning would be much less than E23 projects.

E23 moreover concluded that the ocean freshening due to melting near parts of the western Antarctic (namely Ross Sea and Weddell Sea) will lead to the described reduction of the AABW within 30 years to 2050. I had a look at the observational “Argo” data, provided by the “Marine Atlas”. Until December 2021 there is no trend in the salinity data, here shown for the average 0-2000 m depth in the Weddell Sea:

Fig. 6: “Argo” observations of the ocean salinity near the Weddell Sea. In the area of the Ross Sea, there is also no trend (not shown) . The figure was generated with the “Marine Atlas”.

The problems with this paper are: reliance on the implausible SSP5-8.5 emissions scenario, use of the CMIP6 multi-model ensemble mean which is running too hot, and failure to critically evaluate the model simulations using recent observations.  Further failures by Nature’s review and editorial process, combined with uncritical and amplified media promotion,  have unnecessarily confused the science and public.

Acknowledgement: I thank Nic Lewis for very helpful comments on earlier draft versions.