The above image shows relatively low anomalies over the Arctic Ocean, with a relatively cool area persisting in the North Atlantic, south of Greenland. This appears to reflect heavy melting, slowing down of the Atlantic Meridional Overturning Circulation (AMOC) and strong evaporation followed by more rainfall further down the track of the Gulf Stream, as illustrated by the image on the right.
The above image also shows very high anomalies over Antarctica and over the Antarctic sea ice. This appears to reflect a reversal of the Southern Meridional Overturning Circulation (SMOC).
Covering more than 70% of Earth’s surface, our global ocean has absorbed 90% of the warming that has occurred in recent decades due to increasing greenhouse gases, and the top few meters of the ocean store as much heat as Earth's entire atmosphere, as described by a NASA post. A small change in this ratio could result in a huge rise in the global air temperature, and studies warn about changes that are occurring in the AMOC and SMOC, as discussed in earlier posts such as this one.
A 2024 study led by Judd finds that climate sensitivity has historically been about 8°C. According to James Hansen, equilibrium global warming for today’s amount of greenhouse gases is 10°C, which includes a 2°C rise that would eventuate by the falling away of the aerosols that currently mask the temperature rise.
[ NOAA ENSO outlook ]
Meanwhile, the IPCC keeps down-playing the potential impact of feedbacks such as changes to ocean currents, wind patterns, clouds, water vapor, sea ice and permafrost, and the temperature is likely to increase strongly with a new El Niño. For now, temperatures remain suppressed due to borderline La Niña conditions (image right).
The image below shows monthly temperature anomalies through June 2025, based on ERA5 anomalies vs 1951-1980 from Jan 2014-June 2025 (red circles).
In the above image, data are adjusted by 1°C to reflect a pre-industrial base (black circles). Cubic trends are added to show that 3°C could be crossed late 2028 (red) or early 2027 (black).
A 3°C rise (inset above image) constitutes an important threshold, since humans will likely go extinct with such a rise. As illustrated by the image below, we may already be more than 2°C above pre-industrial and face a potentially huge temperature rise over the next few years.
Recent research led by David Fastivich finds that, historically, vegetation responded at timescales from hundreds to tens of thousands of years, but not at timescales shorter than about 150 years. It takes centuries for tree populations to adapt - far too slow to keep pace with today’s rapidly warming world. Note that vegetation depends on the presence of a lot of things including healthy soil, microbes, moisture, nutrients and habitat.
A 2018 study by Strona & Bradshaw indicates that most life on Earth will disappear with a 5°C rise (see box on the right). Humans, who depend on a lot of other species, will likely go extinct with a 3°C, as discussed in the earlier post When Will We Die?
Climate Emergency Declaration
The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.
A double Blue Ocean Event could occur in 2024. Both Antarctic sea ice and Arctic sea ice could virtually disappear in 2024. A Blue Ocean Event (BOE) occurs when sea ice extent falls to 1 million km² or less, which could occur early 2024 for Antarctic sea ice and in Summer 2024 in the Northern Hemisphere for Arctic sea ice.
Antarctic sea ice loss
The situation regarding Antarctic sea ice extent is pictured in the image below, which shows that on December 12, 2023, Antarctic sea ice extent was 9.499 million km², a record low for the time of year.
Antarctic sea ice extent was 1.788 million km² on February 21, 2023. Antarctic sea ice extent may well be much lower in February 2024, with sea ice loss fuelled by several self-reinforcing feedback loops, as discussed in an earlier post.
Arctic sea ice loss
The situation regarding Arctic sea ice extent is pictured in the image below.
The above image shows that on December 12, 2023, Arctic sea ice extent was 9.499 million km², third lowest low for the time of year, behind 2016 and 2020.
Temperature November 2023
The above image shows the November 2023 temperature anomaly compared to a 1951-1980 base. The image below also shows the November 2023 temperature anomaly, but it is not compared to a 1951-1980 base (NASA's default), it is instead compared to a 1900-1923 base.
Of course, the temperature anomaly will be much higher when compared to pre-industrial. Further adjustments are required, because the NASA data are for sea surface temperatures (rather than temperatures of the air 2 meters above the sea surface). Also note the grey areas on the above map, signifying that no data are available for earlier years. This especially affects the Arctic, where the anomalies are highest, so disregarding these data is not appropriate. In the image below, data are adjusted by 0.99°C to reflect all this, as discussed at the pre-industrial page.
[ click on images to enlarge ]
The above image is created with NASA Land+Ocean monthly mean global temperature anomalies vs 1900-1923, adjusted by 0.99°C to reflect ocean air temperature, higher polar anomalies and a pre-industrial base. Blue: Polynomial trend based on Jan.1880-Nov. 2023 data. Magenta: Polynomial trend based on Jan. 2010-Nov. 2023 data.
The above images illustrate that temperatures are rising strongly in the Arctic, which gives a dire warning that a Blue Ocean Event could occur in Summer 2024 in the Northern Hemisphere that could further speed up global temperatures, as illustrated by the magenta-colored trend in the above image.
The situation is dire
Temperature anomalies in the Northern Hemisphere were more than 2°C above 1951-1980 recently (2.024°C in October 2023 and 2.058 in November 2023), as illustrated by the above image. Note that anomalies on the image are calculated from 1951-1980 and that anomalies from pre-industrial are higher.
Land-only temperature anomalies can be much higher than land+ocean anomalies, since oceans act as a buffer. It is therefore most important to look at the land-only temperature anomaly in the Northern Hemisphere, since that is where the highest anomalies occur, at the very places where most people live. Furthermore, as temperatures keep rising, more extreme weather events occur, with an increase in intensity, frequency, duration and area covered by such events. The urban heat island effect can further add to the rising high temperature peaks reached in cities.
The precautionary principle urges the world to closely watch peak hourly local wet-bulb globe temperatures, rather than to hide the full wrath of the temperature rise by focusing on global temperature anomalies that are compared to recent base periods and that are averaged over periods going back ten years or longer.
Temperatures are rising most rapidly in the Arctic, which contributes to the occurrence of more extreme weather events. Low temperatures in Winter in the Arctic are essential to build up ice thickness to preserve sea ice as the melting season starts.
[ Climatology temperatures are 1979-2000 averages and anomalies are calculated from 1979-2000 averages. Black line: 2023. Orange line: 2022. Grey line: 2016. ]
Arctic temperature hit a record high for the time of year on December 15, 2023, and an anomaly of 5°C, as the above image shows. Arctic anomalies are the highest in the world, as illustrated by the record 8.3°C anomaly that was reached on November 18, 2016. Since the chance that the current El Niño will slow down soon is minimal, Arctic anomalies could reach even higher records in the next few months.
On December 12, 2023, as said, Arctic sea ice extent was third lowest for the time of year, i.e. only 2016 and 2020 were lower. The years 2016 and 2020 had the highest annual temperature (a tie) on record and this annual temperature record is likely to be surpassed in 2023, while 2024 may be even worse, as the chance that the current El Niño will slow down soon is minimal.
[ Water Vapor tipping point ]
In the video below, Anton Petrov discusses the runaway greenhouse effect.
This is important, as a very small increase in solar irradiation – leading to an increase of the global Earth temperature, of only a few tens of degrees – would be enough to trigger an irreversible runaway process on Earth and make our planet as inhospitable as Venus, a recent study concludes, as discussed at this post.
A temperature rise of more than 10°C could unfold as early as by end 2026, due to contributions of gases (including water vapor), aerosols, albedo changes and further elements, in the process causing the clouds tipping point to get crossed, which could add a further 8°C to the rise.
This rise could in turn cause the water vapor tipping point to be crossed. The rise in water vapor alone could from then on suffice to push temperatures up further, in a runaway greenhouse process in which evaporation causes a global surface temperature rise of several hundred degrees Celsius.
Arctic sea ice could have been even lower in extent, had the Atlantic meridional overturning circulation (AMOC) not been slowing down. As a result of AMOC's slowing down, less ocean heat is reaching the Arctic Ocean. Instead, a huge amount of ocean heat has been accumulating in the North Atlantic and much of this heat could soon be pushed abruptly into the Arctic Ocean as storms temporarily speed up currents that carry ocean heat into the Arctic Ocean.
Arctic sea ice volume is getting very low, as illustrated by the image on the right, adapted from dmi.dk.
Meanwhile, Earth's radiation imbalance is very high, emissions are high and rising, and politicians refuse to act responsibly, all contributing to further deterioration of the situation, with the danger that ocean heat will reach and destabilize methane hydrates that are contained in sediments at the seafloor of oceans, resulting in massive methane eruptions, further pushing up global temperatures, as discussed in many earlier posts such as this one and this one.
As more people become aware of the dire situation, widespread panic may set in, as this 2007 post warned about. People may stop showing up for work, resulting in a rapid loss of the aerosol masking effect, as industries that now co-emit cooling aerosols (such as sulfates) grind to a halt. Many people may start to collect and burn more wood, resulting in an increase in emissions that speed up the temperature rise. As temperatures rise, more fires could also break out in forests, peatlands and urban areas including landfills and waste dumps, further contributing to emissions that speed up the temperature rise.
Ominously, the highest methane levels on record (surface flasks) were recently reached at Barrow, Alaska, U.S., as illustrated by the image below.
Climate Emergency Declaration
The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.
On July 25, 2023, the North Atlantic sea surface reached a record high temperature of 24.9°C. The previous record was in early September 2022, when the temperature peaked at 24.89°C, according to NOAA scientist Xungang Yin and as illustrated by the image below.
In previous years, a La Niña was suppressing temperatures, whereas El Niño is now pushing up temperatures. Arctic sea ice typically reaches its minimum extent about half September. We are facing huge sea ice loss over the coming weeks.
Temperatures are very high (and rising) and the following eight points contribute to this rise:
1. Emissions are high and greenhouse gas levels keep rising, and this is increasing Earth's Energy Imbalance. Oceans take up 89% of the extra heat.
2. El Niño is pushing up temperatures, whereas in previous years La Niña was suppressing temperatures. Moving from the bottom of a La Niña to the peak of a strong El Niño could make a difference of more than half a degree Celsius, as discussed in an earlier post.
In February 2016, when there was a strong El Niño, the temperature on land was 3.28°C (5.904°F) hotter than 1880-1896, and 3.68°C (6.624°F) hotter than February 1880 on land. Note that 1880-1896 is not pre-industrial, the difference will be even larger when using a genuinely pre-industrial base.
The above image, from an earlier post discussing extreme heat stress, adds a poignant punchline: Looking at global averages over long periods is a diversion, peak temperature rise is the killer!
[ click on images to enlarge ]
3. Sunspots in June 2023 were more than twice as high in number as predicted, as illustrated by the image on the right, from an earlier post and adapted from NOAA.
If this trend continues, the rise in sunspots forcing from May 2020 to July 2025 may well make a global temperature difference of more than 0.25°C, a recent analysis found.
4. A submarine volcano eruption near Tonga in January 2022 did add a huge amount of water vapor to the atmosphere, as discussed in an earlier post and also at facebook.
Since water vapor is a potent greenhouse gas, this further contributes to speeding up the temperature rise. A 2023 study calculates that the eruption will have a warming effect of 0.12 Watts/m² over the next few years.
5. Aerosol changes are also contributing to the temperature rise, such as less Sahara dust than usual and less sulfur aerosols that are co-emitted with fossil fuel combustion, which previously masked the full impact of greenhouse gases.
6. The Jet Stream is getting increasingly deformed as the temperature difference between the Arctic and the Tropics narrows, and this can strongly increase the intensity, duration and frequency of extreme weather events in the Northern Hemisphere.
The image on the right shows North Atlantic sea surface temperatures as much as 8.2°C or 14.7°F higher than 1981-2011 (green circle) on July 24, 2023. The image also shows that the Jet Stream is very deformed and features many circular patterns that contribute to stronger heating up of the North Atlantic, especially along the path of the Gulf Stream where the Jet Stream has a strong presence.
Deformation of the Jet Stream can also lead to stronger heatwaves on land that extend over the Arctic Ocean, which in turn can also strongly heat up the water of rivers that end in the Arctic Ocean. The image on the right shows huge amounts of heat surrounding Arctic sea ice and also shows that on July 28, 2023, the sea surface was as much as 19.7°C or 35.4°F hotter than 1981-2011 at an area where the Ob River meets the Kara Sea (green circle).
7. AMOC (the Atlantic meridional overturning circulation) is slowing down, further contributing to more hot water accumulating in the North Atlantic. Instead of reaching the Arctic Ocean gradually, a huge part of this heat that is now accumulating in the
North Atlantic may abruptly be pushed into the Arctic Ocean by strong storms that gain strength as the Jet Stream gets increasingly deformed. This danger grows as more ocean heat is accumulating in the North Atlantic and this situation threatens to cause huge eruptions of methane from the seafloor.
8. Increased stratification, as temperatures rise, combines with increased meltwater and with stronger evaporation over the North Atlantic and stronger precipitation further down the path of the Gulf Stream. This threatens to result in the formation of a freshwater lid on top of the North Atlantic, enabling more hot water to flow underneath this lid into the Arctic Ocean, further increasing the methane threat.
Arctic reaches record high air temperature
The Arctic reached a record high 2-meter air temperature of 5.81°C on July 27, 2023, almost 2°C higher than the daily mean for the period 1979-2000, as illustrated by the image below. Arctic sea ice typically reaches its minimum extent half September, when the temperature in the Arctic falls below 0°C and water at the surface starts refreezing.
One danger is that, as more heat is reaching sediments at the seafloor of the Arctic Ocean, hydrates will be destabilized, resulting in eruption of huge amounts of methane from the seafloor.
As sea ice melts away, less sunlight gets reflected back into space, so more heat will reach the Arctic ocean and heat up the water, as discussed at the albedo page.
Furthermore, Arctic sea ice is already very thin, as illustrated by the image on the right. The thinner the sea ice, the less heat can be consumed in the process of melting the ice, as discussed at the latent heat page.
These are just three out of numerous developments that could unfold in the Arctic soon, such as tipping points getting crossed and feedbacks starting to kick in with greater ferocity, as discussed in an earlier post.
Syee Weldeab et al., in a 2022 study, looked at the early part (128,000 to 125,000 years ago) of the penultimate interglacial, the Eemian, when meltwater from Greenland caused a weakening of the Atlantic meridional overturning circulation (AMOC).
“What happens when you put a large amount of fresh water into the North Atlantic is basically it disturbs ocean circulation and reduces the advection of cold water into the intermediate depth of the tropical Atlantic, and as a result warms the waters at this depth,” he said. “We show a hitherto undocumented and remarkably large warming of water at intermediate depths, exhibiting a temperature increase of 6.7°C from the average background value,”
Weldeab said.
Weldeab and colleagues used carbon isotopes (13C/12C) in the shells of microorganisms to uncover the fingerprint of methane release and methane oxidation across the water column.
“This is one of several amplifying climatic feedback processes where a warming climate caused accelerated ice sheet melting,” he said. “The meltwater weakened the ocean circulation and, as a consequence, the waters at intermediate depth warmed significantly, leading to destabilization of shallow subsurface methane hydrates and release of methane, a potent greenhouse gas.”
Furthermore, more methane over the Arctic would push up temperatures locally over the Arctic Ocean as well as over permafrost on land. A 2020 study by Turetsky et al. found that Arctic permafrost thaw plays a greater role in climate change than previously estimated.
Ominously, some very high methane levels were recorded recently at Barrow, Alaska, as illustrated by the NOAA image below.
Further feedbacks can make the situation even more threatening. As an example, dissolved oxygen in oceans decreases as the temperature rises, further pushing up the temperature rise, as discussed, e.g., in a 2022 study by Jitao Chen et al. As the temperature rises, soil moisture content decreases, further pushing up temperatures, as discussed in an earlier post.
• A Prehistoric Climate Feedback Loop - Paleoclimatologist uncovers an ancient climate feedback loop that accelerated the effects of Earth's last warming episode (news release)
Arctic sea ice extent was 5.88 million km² on August 21, 2022, larger in extent than in any of the years from 2010 through 2021 at this time of year, as illustrated by the NSIDC image below.
At first glance, one might think that this relatively large extent was a sign of healthy sea ice. After all, the larger the sea ice, the more sunlight gets reflected back into space. At the same time, however, the situation is very dangerous, as there is a growing risk that large eruptions of methane will occur from the seafloor of the Arctic Ocean.
Why is the situation so dangerous? There are many contributors to the danger, three of them are:
1. Ice acts as a seal
Ice acts as a seal, insulating the soil from warmer air and holding the soil together, like a glue. A 2022 study by Elizabeth Webb et al. concludes that rainwater carries heat into the soil and accelerates permafrost thaw, and the glue that holds the soil together disappears. This can open up underground channels that drain the surface.
Rainwater can also travel along cracks deeper into sediments, where the heat can destabilize methane hydrates, resulting in the release of large amounts of methane into the atmosphere from hydrates and from gas underneath hydrates. As temperatures rise in the Arctic, more rain will fall over the Arctic, increasing this danger.
Where rain falls onto the sea ice, the rainwater also adds heat to the sea ice, speeding up its demise, and stronger winds can further accelerate this. The compound impact is that such feedbacks accelerate the pace at which the Arctic is warming, but as long as air temperatures are low enough, there will continue to be sea ice that acts as a seal, impeding transfer of ocean heat from the Arctic Ocean to the atmosphere.
Temperatures in the Arctic are rising faster than in the rest of the world. As temperatures rise in the Arctic, increased precipitation, meltwater and runoff from land, and flow of freshwater from rivers all decrease salinity of the water in the Arctic Ocean. Lower salinity makes it harder for sea ice to melt.
As temperatures in the Arctic are rising faster than in the rest of the world, the Jet stream is getting deformed. Deformation of the Jet Stream causes more wind to go over the Arctic Ocean, which can cool down the sea surface, resulting in more extensive sea ice.
Furthermore, we're currently in the depth of a persistent La Niña (NOAA image on the right), and the associated lower air temperatures further contribute to a relatively larger extent of the sea ice.
More extensive sea ice in turn makes it harder for ocean heat to be transferred to the atmosphere, thus instead raising the temperature of the water of the Arctic Ocean.
The larger the sea ice is in extent, the less ocean heat can be transferred from the Arctic Ocean to the atmosphere, which means that more heat will remain in the Arctic Ocean.
2. Lid on North Atlantic
Ocean stratification is increasing globally, as ocean warming is stronger for upper layers versus the deep ocean. Stratification increased from 1960 to 2018 by 5.3% for the upper 2000m and by as much as 18% for the upper 150m, while salinity changes also play an important role locally, a 2020 study finds.
[ SSTA (left) and SST (right), August 23, 2022 - click on image to enlarge ]
Deformation of the Jet Stream can at times strongly increase evaporation over the North Atlantic with more precipitation further down the path of the Atlantic meridional overturning circulation (AMOC).
Deformation of the Jet Stream can also increase runoff from land (including from melting glaciers).
In both these cases, this can contribute to the formation and growth of a relatively cold, freshwater lid at the surface of the North Atlantic.
This lid on the North Atlantic reduces transfer of ocean heat to the atmosphere and enables large amounts of salty, warm water to enter the Arctic Ocean, diving under the sea ice.
This lid also increases the risk of a sudden, large influx of hot, salty water. Slowdown of AMOC causes ocean heat to accumulate, while more warm water travels underneath this lid (instead of at the sea surface) toward the Arctic Ocean. As the Jet Stream gets more deformed, strong winds along the path of AMOC can at times speed up the flow of water that travels underneath this cold freshwater lid over the North Atlantic, suddenly pushing large amounts of salty, warm water into the Arctic Ocean.
3. Latent heat buffer loss
The navy.mil combination image below has three panels. The left panel shows the sea ice on August 30, 2012, the center panel shows the sea ice on August 30, 2015, and the right panel shows a forecast for the sea ice for August 30, 2022, run on August 22, 2022.
The image illustrates that Arctic sea ice is currently larger in extent than it was in 2012 and 2015 at this time of year, while there has been a dramatic reduction in thickness of the sea ice over time.
Sea ice acts as a buffer that absorbs heat, while keeping the temperature at zero degrees Celsius. As long as there is sea ice in the water, this sea ice will keep absorbing heat, so the temperature doesn't rise at the sea surface. The amount of energy absorbed by melting ice is as much as it takes to heat an equivalent mass of water from zero to 80°C.
This ice has meanwhile all but disappeared, so without this latent heat buffer further incoming heat must go elsewhere, i.e. the heat will further raise the temperature of the water of the Arctic Ocean.
Compound impact
The danger is that, as more salty, warm water keeps arriving in the Arctic Ocean while the latent heat buffer has largely disappeared and while sea ice extent is relatively large, this will raise the temperatures and salinity levels at the bottom of the Arctic Ocean enough to destabilize hydrates in sediment at the seafloor of the Arctic Ocean, resulting in methane eruptions both from these hydrates and from free gas underneath these hydrates.
[ The Buffer has gone, feedback #14 on the Feedbacks page ]
Very high methane levels
The Copernicus image below shows a forecast of high levels of methane over the Arctic on August 28, 2022 18:00 UTC at 500 hPa.
Methane levels are already at record high and growth is accelerating, even without an extra burst of seafloor methane. The NOAA record shows that methane grew by 18.31 ppb in 2021, the highest annual growth on record.
The most recent monthly NOAA data are for the globally averaged marine surface mean for April 2022, which was 1909.9 ppb. This is 18.7 ppb higher than April 2021, as illustrated by the image on the right, from an earlier post.
NOAA's data are for marine surface measurements. More methane tends to accumulate at higher altitudes, as illustrated by the two data images on the right.
The top data image on the right shows methane recorded by the MetOp satellite on August 22, 2022 am. The image shows means of 1972 ppb at five pressure levels (of 280 mb and less), with a peak level of 2543 ppb, the highest that day, occurring at 218 mb.
The second data image on the right shows methane means recorded by the MetOp satellite on August 25, 2022 pm of 1975 ppb at four pressure levels (at 254 mb, 266 mb, 280 mb and 283 mb).
The image underneath on the right shows a methane peak of 2622 ppb (marked by the red oval), recorded by the N20 satellite on August 20, 2022 am at 399.1 mb. High methane levels are visible north of Siberia, indicating that much of the methane may originate in the Arctic.
Another N20 satellite image is added underneath showing high methane concentrations over the Arctic, also on August 20, 2022 am, but at 695.1 mb, which is much closer to sea level. This confirms that much of the methane may have originated in the Arctic.
An image is added underneath from another satellite, the MetOp satellite, also showing high methane concentrations over the Arctic, also on August 20, 2022 am, this time at 586 mb, further confirming that much of the methane may have originated in the Arctic.
A large abrupt methane release could double the methane in the atmosphere. Methane releases from the seafloor of the Arctic Ocean are very dangerous because there is very little hydroxyl in the atmosphere over the Arctic to break down the methane.
A level twice as high as that 1975 ppb mean is a mean of 3950 ppb, and when using a 1-year GWP of 200, this translates into 790 ppm CO₂e, i.e. only 410 ppm away from the 1200 ppm clouds tipping point.
The average monthly CO₂ at Mauna Loa, Hawaii, was 420.99 ppm both in May and in June 2022. As illustrated by the image on the right, average daily CO₂ hasn't been below 416 ppm in July and August 2022, while some hourly measurements were around 425 ppm CO₂.
On August 25, 2022 16:30 UTC, CO₂ at the North Pole was 422 ppm, as illustrated by the nullschool.net image on the right.
In other words, a large eruption of methane from the seafloor of the Arctic Ocean could abruptly cause the joint CO₂e of just two greenhouse gases, i.e. methane and CO₂, to cross the 1200 ppm clouds tipping point globally and trigger a further 8°C global temperature rise, due to the clouds feedback alone. When adding further forcers, a huge temperature rise could be triggered even with far less methane erupting from the seafloor.
Conclusion
In conclusion, there is a growing danger that methane will erupt from the seafloor of the Arctic Ocean and cause a dramatic rise in temperature.
Even without such eruption of methane from the seafloor of the Arctic Ocean, temperatures look set to rise strongly soon, as we move into an El Niño and face a peak in sunspots.
Either way, the resulting temperature rise could drive humans extinct as early as in 2025 with temperatures continuing to skyrocket in 2026.
This makes it in many respects rather futile to speculate about what will happen beyond 2026. At the same time, the right thing to do now is to help avoid the worst things from happening, through comprehensive and effective action as described in the Climate Plan.
• Understanding the Permafrost–Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf - by Natalia Shakhova et al. (2019) https://www.mdpi.com/2076-3263/9/6/251