Leveraging a Climate-neutral Society
Strategic Research Network

Hiroyuki Enomoto
Vice Director, Arctic Environment Research Center (AERC) of the National Institute of Polar Research (NIPR)
Lead Author, Chapter 1, IPCC Special Report on the Ocean and Cryosphere

Kainuma: Dr. Enomoto, I understand that you were in charge of writing Chapter 1 of the IPCC Special Report on the Ocean and Cryosphere (SROCC). Can you please tell us what you particularly wanted to tell us through this report?

Enomoto: Experts from the natural sciences, humanities, and social sciences agree that changes in the cryosphere and the oceans are already occurring and that we estimate significant changes in the future.
I specialise in the natural sciences, and in terms of the IPCC, I was very familiar with the research findings of Working Group I (WGI, The Physical Science Basis) , but did not know much about the work of Working Group II (WGII, Impacts, Adaptation, and Vulnerability) or Working Group III (WGIII, Mitigation of Climate Change). For the SROCC, each chapter included researchers from WGI, WGII, and WGIII and the content was considered from scratch. There were some struggles, but I learned a lot.

Kainuma: I, on the other hand, know quite a lot about the work of WGIII and am more or less familiar with WGII because I work with researchers involved in WGII at the National Institute for Environmental Studies. However, I have very limited knowledge about WGI. So this time, I learned a great deal from reading the SROCC, which contained some extremely important insights into the natural sciences.

Enomoto: Between the natural scientists and the cultural studies and social scientists, there were many points of agreement, but I also experienced the dejection of the barriers between us. We have achieve a certain level of success, even though discussions have not been completed and certain parts are still in progress.
For a long time, I have been making observations of the Antarctic and Arctic regions.  Since 2016, I have been working together with humanities and social scientists who focus on the changes in the Arctic and the people and communities who live and experience the Arctic. Since I’ve experienced  such research interactions, I was somewhat prepared to work together with researchers in the humanities and social sciences for the SROCC. Taking my projects in the Arctic as an example, we are conducting research on Arctic communities, as well as Japanese industries in the Arctic, through a humanities and social science lens. In a sense, it is like a miniaturised version of the IPCC.

Kainuma: What do you mean by the involvement of “Japanese industries”?

Enomoto: This includes sea routes, fisheries, and resource development.
Moreover, regarding the SROCC, I think we included a lot of the implications from a humanities and social science perspective with respect to the natural science of the oceans, cryosphere, and the high mountains. I realised that it is important to consider the issue of global warming from both the natural science and social science perspectives.

Kainuma: So do you mean to say that it is important to understand this issue not only from the perspective of the natural sciences, but also in terms of the humanities and social sciences?

Enomoto: Regarding the SROCC’s regions of focus, the oceans, for example, are so wide. The cryosphere includes mountain ranges like the Himalayas and the Andes. Even the Polar Regions are far removed from the regions of ordinary life. While seas near Japan seem like topics close to us, when we think about what is happening to the world’s oceans, I think this topic is far from our ordinary consciousness.
I would like many people to understand what is happening in far-away regions as problems that are close to us. The Polar Regions, the Himalayas, and the Andes are all parts of the planet. I felt this way quite acutely while writing the report.
On the other hand, the SROCC’s time scale is up to 2100 and up to 2300 in some cases. In day-to-day life, the time scales we deal with are typically in the realm of, “what was last week’s weather?” or “this year’s summer was hot”. But I guess people do not think that what will happen 10 years from now, or in the year 2030, or in 2050, or in 2100, is relevant to them.

Kainuma: That’s true. There are a lot of people who think it has nothing to do with them.

Enomoto: And how much can we empathise with problems that are temporally distant from us? With this, the SROCC has extended the time scale up to the year 2300. Right now, inspired by Greta Thunberg’s activism, youth around the world are speaking out, saying that the greenhouse gases that humans are emitting are related to future climate change. Global warming will transcend generations, impacting our children, our children’s children, and their children… The SROCC is trying to show that we will not make it if we do not take immediate action.
Personally, as I do research on the Polar Regions, I get told that these are “worlds far away”, but I respond by saying, “No, they are actually close to you. We need to think about this issue as people living in the same world as those living in such regions.”
The SROCC is quite a lengthy publication, so it can be challenging to share the report’s messages with many people, but I think it could be good if I can share its insights via this interview today. I hope this will be a good opportunity to encourage everyone to “transcend time and geography, and think together about this as a problem that is close to us.” In addition, normally, there aren’t many opportunities to think about the natural sciences, and the humanities and social sciences, so I hope I can link all these disciplines as well.

Kainuma: Thank you.

Enomoto: The SROCC contains six chapters. The six chapters have the following titles : “Framing and Context of the Report”, “High Mountain Areas”, “Polar Regions”, “Sea level Rise and Implications for Low-Lying Islands, Coasts, and Communities”, “Changing Ocean, Marine Ecosystems, and Dependent Communities”, and “Extremes, Abrupt Changes and Managing Risks”.
Insights that are shared overall are summarised in the Cross-Chapter Boxes. Every chapter is related to both the natural sciences and the humanities and social sciences, but we focused on “risk” as a common cross-cutting theme. Moreover, Cross-Chapter Box 2 explains the key concepts of risk, in relation to adaptation, resilience, and transformation.
You will see the word “community” throughout the SROCC. The IPCC is an intergovernmental panel, so it plays a role in providing information to political decision-makers, but the idea is to be of use not only to governments but to communities as well. There is a term, “community-based,” which has not spread completely, but its definition and examples are introduced in the report. In the Arctic region, community-based management and community-based experiments are critical.
I have also explained this report to fellow NIPR researchers and have particularly recommended them to read parts that are not within their area of specialty or interest.
Since the SROCC is divided into chapters, there are researchers who download and read just the chapters that are relevant to their specialty, but I tell people that it states on the report that we should read parts that our area of research does not cover. In short, I would like to emphasise this idea of overcoming siloes, of learning beyond what the SROCC directly mentions and not limiting our understanding of the oceans and cryosphere as simply regions or natural phenomena.
The SROCC is characterised by the interwoven insights of researchers from WGI, WGII, and WGIII of the IPCC, and the mentions of “risk” and “community” throughout the report. The natural sciences, humanities, and social sciences perspectives are indispensable to understand and solve global warming.

Kainuma: The report states that over 90% of thermal energy on Earth due to anthropogenic greenhouse gas emissions is stored in the oceans. Temperature rise on land is an issue, but this is the effect of less than 10% of the additional thermal energy on Earth. Given that the rest of the energy is stored in the oceans, I am concerned that the effects on the oceans are astronomical. Is the increasing intensity of typhoons in recent years affected by the stored thermal energy in the oceans? I am worried that there will be more super-typhoons in the future because removing heat trapped in the oceans is thought to be difficult.

Enomoto: We need to be very careful about releasing heat trapped in the oceans. Soon after the release of the SROCC on 25 September, Typhoon Hagibis made landfall in Japan and the relationship between the effects of global warming and changes in the marine environment became news.
With respect to impacts on typhoons, no consensus has been reached yet among researchers. To give you some context, warm sea water, which makes it easier for typhoons to form, has come near Japan, and the temperature of the sea is higher than normal. Given how typhoons can more easily form with high sea temperature, we can say that such effects exist. If we look at each case individually, there are other factors, and we can’t really say which is affected and which is not, but we can say that we are entering an era where strong typhoons can form more easily.

Kainuma: Last year, the western part of Japan was impacted by large typhoons, and this year, the typhoons especially devastated the eastern region. I worry greatly if this is going to continue every year.

Enomoto: We are probably approaching a time where such large typhoons are no longer infrequent events, but can possibly occur regularly. Contextually, this is the effect of the oceans.

Kainuma: Speaking of Typhoon Hagibis, it was first reported on the news as a “once-in-a-decade typhoon”, but afterwards, news media began reporting it as “a typhoon that had never before existed.” It brought record-breaking rainfall and maximum wind speed. Would it be possible that next year will bring yet another “record-breaking” typhoon?

Enomoto: I think so. Perhaps this is an unfamiliar term, but in the section where sea level rise is addressed in the SROCC, the term “historical centennial events” (HCE) is used. It is reported that a centennial event – one that had historically occurred once in a hundred years – may occur annually in many parts of the world by 2100. It is estimated that an event that would occur once in a hundred years would, by 2050, be annually experienced in large cities located in the lowlands and in many small island states. It is alarming that such rare phenomena would occur much more frequently.
Regarding Typhoon Hagibis, regular typhoons form in water with temperatures between 26℃ and 27℃, but the seas around Japan were between 26℃ and 27℃ even in October. Conventionally, the intensity of the typhoon should wane as it travels north, but because of the high sea temperature, it traveled northward retaining energy, and became a super-typhoon.
I think that this was affected by climate change. Like the IPCC, the SROCC uses confidence levels throughout the report, with labels like “high confidence” and “very high confidence”. There are various confidence levels, and they are getting higher. Perhaps the relationship between global warming and typhoons has been noted earlier, but the evidence and observations have been put together and we can now see this relationship more clearly.

Kainuma: In the case of the IPCC, the confidence level must be expressed scientifically and accurately. Moreover, the concept of ‘likelihood’ is used. Reports use expressions such as “likely” and “virtually certain”. For instance, “very likely” refers to a probability of 90-100%. Even if the likelihood is very high, I think that there may be some people who will not see the sense of urgency. Stating that “the probability of this event occurring is over 90%” is the same as saying that “the probability that this event will not occur is still 10%”, so still, there may be some people who do not fully appreciate the gravity of this issue. What do you think?

Enomoto: Similar to evacuation warmings, there is a tendency to just think, “Sure, there is an evacuation warning, but we’ll be fine.” One takeaway from Typhoon Hagibis is that we experienced the importance of immediately taking action when given the warning, “the likelihood of disaster has increased, so people are recommended to evacuate.”
The IPCC is “ringing the bell” in various ways, but some may see it as a concern far into the future or think, “Even if it happens, we can get through it”. However, I can say that we are expected to react to these warning bells.

Kainuma: There are some people who are skeptical about global warming, but isn’t it apparent that temperatures are increasing?

Enomoto: Global and local changes interact, so there may be some uncertainty, but atmospheric and sea temperatures have clearly risen. It is my ninth year living in Tokyo, and I have wondered why it is so hot. One time, I even took a thermometer with me and drove all over the city. The temperature rose to nearly 40℃… that’s an anomaly. With this example alone, we might think it’s just local, but high temperatures have been observed in Europe and other regions around the globe. Such global changes are happening.

Kainuma: Certain areas, such as Kumagaya City, Saitama Prefecture, have exceeded temperatures of 40℃. I would think that it is not normal for us to still observe typhoons at the end of October.

Enomoto: From NIPR, we can see Mount Fuji very well. Yesterday (24 October 2019), we observed the first layer of snow on the mountain this year. The first snowfall was on 23 October, which is 26 days later than last year. I would say that this is a reflection of change at a much larger scale, not limited to the local level.
With respect to the question, “Are typhoons increasing in strength because of heat that is accumulating in the ocean?”, we can say that the evidence is coming together on the effects of global warming on this process. As stated at the press conference when the SROCC was launched, effects “are already observed”. The report describes various dangers, and within them, some areas of concern “which were expected but not observable are now emerging”. This is true of places like Antarctica, but while the effects have been talked about in the past, there had not been enough proof until now. We can now say that these effects have also been included in the forecast.

Kainuma: What about the destabilisation of Antarctic ice? While it has been reported in the past, I heard that it was hard to show definitive evidence for this.

Enomoto: In the 1980s, there were reports and discussions on the destabilisation of West Antarctica’s ice. I would say most people thought that it was theoretically predictable but did not see it as something that had happened or will happen in the future. Also, some thought that we may only see its probability in models. However, recently, this is starting to become observable. More evidence is being collected, and the phenomenon has been added to predictions and is being considered. The reality is that this is becoming a large factor.

Kainuma: I am thinking that heat that has been absorbed by the oceans cannot be removed. So if we cannot remove this heat, won’t the effects persist?

Enomoto: Yes, I think that they will persist for a long time. On the other hand, some claim that reversing this is possible. For example, the sea ice in both Poles is shrinking. The important point here is whether this phenomenon is reversible or irreversible. Moreover, some believe that, even if it were to be reversible, it could be hysteresis (when the state of a system is dependent on its history). Using magnets as an example, if we expose objects to a magnetic field, those that are easily magnetised become magnets. However, even if we were to then remove the magnetic field, the objects will not return to their original state and the remnants of the effect of the magnetic field will remain. To return things to the way they were, we would need to expose the objects to a large magnetic field that opposes the original magnetic field. It means that we need a lot of power to reverse the effect. We can think of it as taking a certain path to get somewhere and using a different route to return to where we were. In terms of the distribution of sea ice, which is said to affect the sea temperature, if we can decrease temperatures in the Northern Hemisphere, then it can be thought of using the same road to go and come back. If the temperature increases by 2℃, then the ocean will decrease by X%, and if the temperature decreases by 2℃, then the ocean will increase by X%… in this way, the equivalent sea ice mass will come and go, and we can use the same path that shrinks the ice to gain it back.

Kainuma: So we need to decrease the temperature by 2℃? That is a challenge.

Enomoto: It means that temperature changes and changes in ice are proportional. If we work hard, we get back as much as the effort we put in.
On the other hand, ice in the Southern Hemisphere is predicted to not return, even if we try to restore it. If we work extremely hard, it will come back, but it will be a large detour. Decreasing as much as we increased will not guarantee that things will go back to the way things were. But we can say that if we wait a very long time, the ice will come back.

Kainuma: By “a very long time”, is this over centuries? Is the scale around 100 or 200 years?

Enomoto: For sea ice, the time scale could be in the decades. If continental ice becomes runoff and flows into the ocean, it may take several centuries or even longer. Also, there are some places that could look like they have been restored, and there are other places that are fundamentally difficult to restore. Since these are model outputs, we do not have enough evidence yet to see if these predictions will actually come true. I do think that we can restore the ice, but it will require a huge effort.

Kainuma: So immense  efforts will be needed to reverse this change.

Enomoto: Since the average temperature has risen by 1.5℃, does that mean we can immediately return to the original state if we lower the temperature by 1.5℃? No, it does not work that way. It is possible that things will return via a different transition pathway that takes more time. But of course, limiting warming to 1.5℃ is in and of itself a challenge.

Kainuma: As for the impacts, heat trapped in the oceans has the potential to increase the intensity of typhoons and rainstorms with strong wind. Aside from the impacts of heat, what other effects exist? The report states that there are impacts associated with ocean acidification and deoxygenation, but what are your thoughts on this?

Enomoto: Acidification is becoming a pertinent topic in the Arctic Ocean. Due to acidification, organisms are unable to make calcium shells, and this is affecting the ecosystem’s food web. Populations of plankton that make shells out of calcium are decreasing, which is affecting the fish that feed on them. This also affects food for humans. There are such reports already for the North Pole. On top of this, in Southern oceans, the effect of acidification on coral reefs is severe.

Kainuma: I do not know much about the mechanism by which oxygen is depleted from the oceans. Can you please explain this?

Enomoto: The SROCC has made careful mentions of hypoxia.

Kainuma: On land, since carbon dioxide concentrations are increasing, the concentration of oxygen is decreasing. While the concentration of oxygen is decreasing, this is not to the point where it would affect human wellbeing. But for the oceans, it is necessary to transport oxygen at the top layer of the ocean to deeper zones. When this cycle is stopped, the oxygen supply in deep oceanic zones will be depleted. What do you think about this? In Lake Biwa, oxygen cycling was not observed for the very first time this year. Is this related to global warming?

Enomoto: Lake Biwa and the oceans work similarly, and if circulation declines, then the top and bottom layers will stop mixing. Only the surface layer contains oxygen, but oxygen is not sinking down to the bottom layers. Meanwhile, the nutritional salts at the bottom are not ascending to the surface. Because of this, the ecosystem is being damaged.
This phenomenon is affected by warming in the Pacific, sea temperature rise, and marine heatwaves. The layers become stable due to the warm water sitting on the surface. Warm, light water is covering the surface and does not move. It does not sink. Deep water doesn’t rise either. This is what impacts ocean circulation and causes hypoxia. This is covered by the SROCC.
The Summary for Policymakers (SPM) also mentions hypoxia as one of the major impacts, together with sea temperature rise and acidification.

Reference: IPCC SROCC Box 1.1, Figure 1
Figure 1. Elements related to the ocean and cryosphere and their changes, as well as interactions with the planetary system via heat, water, and carbon.

Kainuma: I have heard a lot about the sea temperature rise and ocean acidification, but would you be able to tell us a little more about marine heatwaves?

Enomoto: Marine heatwaves are a topic that first was added in the IPCC 5th Assessment Report. Even among marine researchers, I can’t say that the topic of marine heatwaves has reached everyone, and, even if the term is familiar, different definitions exist. Even the phenomenon of sea surface temperature rise isn’t as well known, and the general public may not know even what “sea surface temperature rise” means. I get asked, “In the summer, we are told to be careful about heat stroke caused by heatwaves, but how are marine heatwaves different?”

Kainuma: That’s true, I share the same level of understanding. 

Enomoto: I frequently get asked, “How are marine heatwaves different from heat waves and cold waves on land?” For atmospheric temperature, we use the term “heat wave”. The term “Marine heatwaves” is new, and while publications do come up in search results if you use the term, few publications define the term specifically. Even in the SROCC’s glossary, it is very simply explained in only two lines. Over the past month, multiple people have asked me, “What are marine heatwaves?”

When atmospheric temperatures are high for 4-5 days or even longer, we call that a heatwave. But a similar phenomenon occurs in the ocean as well. “Marine heatwave” is explained as, “a period of extreme warm near-sea surface temperature (SST) that persists for days to months and can extend up to thousands of kilometers”. When we agreed on this new term and its definition and reexamined what is happening globally, we realised that these events are happening quite frequently. Moreover, these events are said to become more frequent in the future, so this time, we were able to identify high-risk areas. One part compares what happens under RCP8.5 (the scenario in which we continue to emit greenhouse gases and are estimated to experience a 2.6-4.8℃ rise in temperature by 2100） and under RCP2.6 (the scenario in which we limit warming to 2℃ or less）. What happens when we take action? What happens when we don’t? This difference is described in the report, as well as in the following figure.

Reference: IPCC SROCC Figure 6.4
Note: The probability ratio is the fraction by which the number of MHW days yr-1 has changed since 1850-1900.
Figure 2. Global and regional changes in the probability ratio of marine heatwaves (MHWs).

Kainuma: Is coral bleaching caused by marine heatwaves? Or is it caused by something else?

Enomoto: In the same chapter (Chapter 6), there’s a similar figure on coral bleaching. I think it is also related to ocean acidification.

Reference: IPCC SROCC Figure 6.3b
Figure 3. Examples of recent marine heatwaves (MHWs) and their observed impacts: the record warming years 2015 and 2016 and the global extent of mass bleaching of corals during these years.

By the way, there are reports around the world stating that, because of marine heatwaves, they are no longer able to catch salmon, and fish that normally live in the area are gone. This is serious damage. There have been so many reports about atmospheric phenomena and sea surface temperature rise. Now, the focus is leaning towards how these phenomena affect organisms. In Alaska, they are reporting this as a critical situation, as barely any salmon could be caught in the past two years. Marine heatwave effects on ecosystems are being observed even in Tasmania and along the Australian coastline. The point of focus is moving from physical phenomena to effects on ecosystems.

Kainuma: The increasing sea level has previously been seen as an issue, but are there any updates on this?

Enomoto: Firstly, I think it was the value “1.1 m” in the SROCC that attracted a lot of attention. 1.1 meters is the maximum value for the range in predicted sea level rise, but it is over 10 centimeters higher than previous estimates.
I was asked, “Why did the estimate suddenly go up?” It has been said for a while that the shrinking of Greenland’s ice sheets and mountain glaciers has been accelerating. An even bigger reason is that some say that the Antarctic ice sheet may possibly melt as well. As with Greenland, temperature rise in Antarctica brings with it both the dynamics of ice becoming water – in other words, ice melting – and also the dynamics of ice cracking open and becoming runoff, as well as a combination of the two.
People have said that Antarctica is cold enough and the ice will not melt. The loss of most ice sheets begins through cracks at the edges, but there has not been a large increase in temperature, so most people thought that there was no substantial change. However, in reality, the effect from the bottom part that is in contact with the ocean is becoming apparent. This is what is causing the ice sheet to destabilise. As the bottom part melts and gets thinner, the part that is touching the ocean will float up, creating a gap below. This leads to further destabilisation. In other words, like dominoes, the ice is melting, thinning, destabilising, and further destabilising. Some people put forward the opinion that a chain reaction may be happening. But this isn’t just an opinion anymore – we are collecting more and more evidence that it is indeed happening. This is known as “marine ice sheet instability” (MISI), pointing to the destabilisation of ice sheets that are in contact with the ocean. This has now become a topic of great concern and warrants serious attention.
When the state of the ice switches to one of instability, it will disappear very rapidly. There is a possibility that this will persist until maybe 2100. On the other hand, if we’re asked, “What about after 2100?” we don’t know the answer.

Kainuma: At the time when the IPCC 3rd Assessment Report was released, it was said that the Antarctic was not contributing to sea level rise.

Enomoto: At that time, the Antarctic ice sheet was increasing in volume due to snowfall and working in the opposite direction as sea level rise. However, over the long term, the Antarctic snow will also start melting, so its effect will be in the same direction as sea level rise.

Kainuma: Yes. Later, the IPCC 4th Assessment Report showed a change in this understanding: the directionality of the effect was reversed. At the time of the Third Assessment, the amount of snowfall on the continent of Antarctica was increasing and piling up, and it was argued that, because snowfall is on land, this would not affect sea level rise. But in reality, the accumulated snow is what melts, leading to the destabilisation and increased melting of the Antarctic ice sheet. Is that correct?

Enomoto: At the time of the IPCC 3rd Assessment report, it was said that the Antarctic was still cold enough and the falling snow will accumulate and not melt. It will just keep piling up, they said. Yet, it was also said that, if global warming accelerates in the future and the snow begins to melt, then it is possible that the snow that falls will melt and become runoff.

Kainuma: Are we already living in the “future”?

Enomoto: It had been previously estimated that the surface of ice sheets may melt and become runoff in the future, but apart from the runoff at the surface, there has been more evidence of the bottom part of the ice sheet melting, becoming thinner, and causing instability. These have been highlighted in the SROCC.
It is mostly in West Antarctica where MISI is estimated to drive changes in the ice sheets. Yet, on top of this, there are also similar concerns in parts of East Antarctica. In the region of East Antarctica where the Japanese research station Showa Station is located, snow accumulates more often than not. If we only look at the observation data around the Japanese research station, the amount of snow is increasing, but, including the East Antarctic glaciers where the Japanese observation teams are active, Antarctica as a whole is experiencing decreases in the mass of ice sheets.
With respect to glaciers, there are areas where the glaciers are increasing, but there are more areas where the glaciers are decreasing. In the past, glaciers have been said to be increasing. Moreover, at one point in time, there were glaciers that were decreasing, but these effects were more or less balanced against some glaciers that were increasing. We now know that losses and gains of glaciers are not balanced: glaciers as a whole are decreasing. This understanding that glaciers are decreasing is becoming clear.

Kainuma: So there is such a thing as “West Antarctica” and “East Antarctica”?

Enomoto: If we split the continent at 0°longitude, we have two sides: West Antarctica and East Antarctica. Antarctica does not have a “North” or “South”, but it does have an “East” and “West”. Visual records of ice shelves being broken and becoming runoff in West Antarctica’s peninsula are often reported. This is another relatively unstable region. Ice shelves are originally floating, so they will not impact sea levels if they melt. However, there are glaciers descending from the mountains right behind the floating ice shelves, so there is fear that the phenomenon of ice flow will accelerate.
The foundation underneath West Antarctica’s ice is mostly below sea level. This means that, as the ice thins and begins to float, sea water will intrude underneath the ice. This will cause more melting, more ice loss due to the increased buoyancy of the ice, and make it easier for the ice to collapse. That is what is causing the destabilisation. Since the ice sheet is below sea level, even in places far inland, this process will not stop.
On the other hand, in East Antarctica’s case, most of the foundation is above sea level. Where glaciers retreat, the ground surface will appear and the ice sheet will no longer have contact with the ocean. Even in East Antarctica, the foundation of Totten Glacier lies below sea level and is under the same condition as West Antarctica. There are concerns that the retreat of ice sheets will accelerate.
The Greenland Ice Sheet is experiencing both of these conditions, and parts of its current edges lie below sea level, so they will be affected by the ocean. If the process in West Antarctica were to happen in Greenland, the loss of the ice sheet will accelerate. There are also parts of Greenland’s surface that is melting. Moreover, there are places where the ice sheet edge is cracking and collapsing. There, two processes are happening. What has been happening in Greenland in recent years is starting to happen in Antarctica as well.
The mechanisms of change are shared between the regions. But if we look at which mechanisms will proceed in which area, the changes in Greenland are leading the way.

Kainuma: Greenland’s ice is melting first – is this observation related to the fact that global warming is more intense in the Arctic?

Enomoto: Yes, the temperature at the North Pole is rising by 2-3 times as much as the global average. This is having a major effect. The reason for accelerated warming is a mechanism called ice-albedo feedback. Snow ice will reflect sunlight, so as long as the light is being reflected, there is no warming. If the snow melts and water or ground surface appears, the substance with the highest reflectivity – albedo – on the Earth’s surface is being replaced with something with a lower albedo, so it will begin absorbing more and more radiation. When there is more absorption, the sea temperature increases, causing more ice to melt, and sets this feedback loop in motion. This is happening in the North Pole, and it is said that the ice-albedo feedback is accelerating warming. Periods of snowfall are becoming shorter and sea ice is shrinking, exposing more ocean. This is a major reason why the North Pole is experiencing warming right now. It is this warming that is causing the Greenland Ice Sheet to change.
Antarctica, on the other hand, is very large, so it had not been affected as much by these processes. For this reason, it was called the “sleeping giant.”

Kainuma: But once the “giant” begins to wake up, the effects will be huge.

Enomoto: The North and South Poles are often compared, with respect to topics such as ice sheets, sea ice, and warming.
I’ve reiterated for a long time that ice in the North Pole is decreasing, but certain estimates show that the Arctic ice will completely disappear by 2050. What about in Antarctica? We have observed that both the amount of ice and sea ice coverage are increasing over time.
In the North Pole, the sea ice is shrinking and the ice sheet is melting, but in the South Pole, the ice sheet and the sea ice are both growing. Because of this, people made claims like, “Maybe warming isn’t really happening”, “Maybe warming isn’t happening in the Southern Hemisphere”, and “Maybe there are no effects of warming.”
In reality, the increase in sea ice in Antarctica peaked at around 2015-2016, and has been decreasing rapidly since then. Moreover, all-time record-breaking lows were observed in 2016, 2017, and 2018. Because we only have data from the most recent three years, the SROCC states that it is still unclear whether it is a short-term trend, an effect of climate change, or the effect of anthropogenic warming. Nevertheless, we need to observe this phenomenon cautiously.
Moreover, while the Antarctic sea ice flipped from growth to shrinkage, it was reported that Antarctica has lost significant ice surface area within the last three years – an area even larger than how much the Arctic has lost over the past 30 or so years combined. At the same time as when we began observing changes in the Antarctic Ice Sheet and Antarctic sea ice, monumental impacts began arising all of a sudden.
As I said before, regarding the Antarctic Ice Sheet, there is snowfall around area surrounding the Japanese Showa research station in East Antarctica. Even in places of great concern in West Antarctica, there is a lot of snowfall, so if we just see the surface, snow is steadily accumulating. However, the amount of ice lost at the bottom far exceeds the amount of accumulated snow on the surface. When we just look at the surface, we do not observe anything. But when we factor in the bottom portion of the ice sheet as well, we have come to understand that the volume of ice in the ice sheet is decreasing overall. There is a need to think not only about the surface, but also what is underneath.

Kainuma: It has been said that global warming will be accelerated by phenomena such as the release of methane due to permafrost thaw and changes to albedo due to melting ice sheets and shelf ice. What is the level of risk?

Enomoto: I get a lot of questions about permafrost thaw. It is necessary to think about melting and permafrost temperature increase. The temperature of permafrost is increasing. Although it hasn’t started melting yet, we can say that there are many places where the temperature has increased to the point where it could begin melting at any moment.

Kainuma: Does this mean that in 2100, the temperature will increase and cause permafrost to melt? Would it be correct to think that the permafrost will melt, which will release methane, which will accelerate global warming?

Enomoto: Permafrost in some regions have already begun melting, releasing methane. Craters are being formed. These craters are thought to be the result of methane explosions after their release.

Kainuma: So this is already happening?

Enomoto: Yes, permafrost is already melting in some places and other places are nearing that critical point as well.

Kainuma: Methane has an even greater greenhouse effect than CO2 (In fact, methane’s greenhouse effect is about 25 times that of CO2). Does this mean that global warming will accelerate even faster?

Enomoto: The methane released from permafrost has a greater greenhouse effect than CO2, so it is referred to as a “methane bomb”. It is as if a bomb is buried inside permafrost, and if it explodes – in other words, large quantities of methane are released – then this will trigger a chain reaction of releasing more methane. This will accelerate global warming, and there is nothing we can do to stop it.
Let me explain this with Figure 1. This figure contains terms like “marine heatwave” and “permafrost thaw”.

Kainuma: By the way, can you also explain a little more about tipping points?

Enomoto: Tipping points, also known as “transition points” and “critical points”, are points after which changes will continue and create a new state of the system, which cannot be reversed. For the Arctic sea ice, some have claimed that the tipping point is close or has already been reached. For the latter, we use the expression, “we have reached a point of no return”.
However, some estimates show that it may not be totally impossible to restore the original state. Some calculations show that, if CO2 concentrations can be reduced, then freezing will occur again and ice will form in the same shape as before.

Kainuma: Currently, the temperature is increasing, but can we restore the system to its original state if the temperature returns to the way it used to be?

Enomoto: Without first creating conditions to decrease the temperature, we won’t be able to restore the system.

Kainuma: Are you saying that, even if we were to limit warming to 1.5℃ — if we were to keep the 1.5℃ level forever — the sea ice would not return?

Enomoto: That’s right. Current conditions will not cut it.
There are various definitions and lists of tipping points in the literature. For example, one German institute has put together a list of assumed tipping points from around the world. Various tipping points, such as plants, droughts, melting of sea ice, and reduced snowfall, have been summarised.
The SROCC’s Chapter 1 contains a figure that explains tipping points (shown below).

Reference: IPCC SROCC Figure 1.1c
Figure 4. Schematic of key concepts associated with changes in the ocean and cryosphere: tipping points and the change of their behaviour through tie in response to, for example, anthropogenic change (adapted from Lenton et al., 2008).

The two minima represent two stable fixed points, separated by a maximum representing an unstable fixed point, acting as a tipping point. The ball represents the state of the system with the red dash line indicating the stability of the fixed point and the system’s response time to small perturbations.

The state of the system will fluctuate within a certain range, but if there is a drastic change, it will leave the range of fluctuation and move to a state that will not be able to return to the original.

The next figure shows that the red response (linear, in sync with forcing) will just keep following the factor in red (forcing). On the other hand, the green response (abrupt, non-linear) shows that there will be no change until a certain point, but when this point is reached, an explosive change occurs. As shown, this threshold is the tipping point. If the point has not yet been reached, it is possible to return to the base state, but once this point is reached, it is impossible to return. The challenge is to control ourselves to not reach this point. There are concerns that it may already be too late, as also expressed by Dr. Hans-Otto Pörtner (Co-chair, IPCC WGII) at the SROCC press conference on 25 September 2019.

Reference: IPCC SROCC Figure 1.1a
Figure 5. Schematic of key concepts associated with changes in the ocean and cryosphere: differing responses of systems to gradual forcing (e.g. linear, delayed, abrupt, nonlinear).

Kainuma: In relation to tipping points, I hear that large-scale oceanic circulation may stop. Is this true?

Enomoto: Apart from the fast surface ocean currents, the ocean has another type of current – the deep ocean currents – about 3000-4000 m below sea-level. In the sea near Northeastern Greenland, the seawater is very saline, heavy, and cold. This heavy seawater will sink deeply and go southward. This water mixes with deep water around Antarctica and circulates at a global scale, taking 1500-2000 years to complete one round.
If this circulation stops, this will reduce heat transport by the ocean across the equator, and is thought to cool the Northern Hemisphere and warm the Southern Hemisphere. It will create one half of the world where it is always hot and the other half which is always cold and freezing.
One such large-scale oceanic circulation is known as the Atlantic Meridional Overturning Circulation (AMOC). Observations show that the AMOC has weakened. The weakening of the circulation is not very noticeable, but it is clear that we need to be cautious, because heat transport by oceanic circulation plays a critical role in controlling the global climate.
From the Greenland Ice Sheet, vast amounts of meltwater are flowing into the ocean. When freshwater is added to ocean surfaces, the ocean’s circulation stops. The atmospheric temperature rise, the melting of ice on land, and oceanic circulation are all connected.

Kainuma: What about effects on Japan? I think that, unless we personalise the issue, it is difficult to raise consciousness about the issue. The Arctic and the Antarctic are easily thought of as worlds far away from us, but what is happening in Japan?

Enomoto: One issue of concern is whether or not we will experience typhoons like ones we experienced recently more frequently in the future. I worry about the climate-related disasters. There are predictions that we may experience heavy rains and strong wind at magnitudes we have never experienced before.
Some research suggests that the number of typhoons may decrease, but the ones that form will be more intense. There are predictions that we will see an increase in powerful, ferocious typhoons. This is one concern I have with respect to typhoons.
When the recent typhoon was approaching the Tokyo Bay, it just so happened that the tide was low. It was speculated that because of this, coastal areas were able to avoid damage from the storm surge. This means that it was possible that the damages would have been more catastrophic. In the future, if sea levels rise, the damage is estimated to be greater.
Moreover, because the Arctic sea ice is shrinking, there are changes to atmospheric circulation, which are impacting cold waves and snowstorms during winters in Japan. Japan is located in the mid-latitudes, between the tropics and the Arctic, and the impacts of climate change in the Arctic may manifest in the weather in Japan.

Kainuma: Yes, people often ask, “Since winter is so cold, is global warming not happening?” Last year, a cold wave hit us, but this phenomenon of cold winters can also be explained by the phenomenon of global warming.

Enomoto: I think that there may be some cases that can’t be solely attributed to global warming, but as the sea ice melts in the Arctic Ocean, winter temperatures are increasing. Moreover, this influences continental pressure patterns, which may be one factor that brings cold waves to Japan.
In the SROCC this time, effects through the troposphere are described with middle confidence. With respect to stratospheric and such planetary impacts, there is some advancement in research, but not a lot of evidence yet. For effects through the stratosphere, a little more time will be needed to elucidate these effects and reach a high level of confidence in accordance with the IPCC.
Even in the winter, it rains, rather than snows, in the Arctic Ocean. We are coming to a point where there is less ice on the sea. This problem of “Who got rid of the ice?” – it’s a problem of global warming.

Kainuma: What other impacts should we be worried about?

Enomoto: The glaciers melting in the Himalayas is deeply related to our lives. It is thought that warming has cascading impacts that will appear after a time lag. For example, more glaciers melt due to warming, the glaciers at lower altitudes will melt first and the water volume in the river will increase. If there is a lot of melting, there is a risk of flooding. If there is further melting, glaciers will only be in the high altitudes, and the mass will shrink as well. At this time, the meltwater volume will become smaller, and regions that depend on meltwater from mountainous regions will experience water shortages. Moreover, the resulting water insecurity may cause the foundations of life and industry to become unstable or cause conflict. And if people cannot lead their lives, they will be forced to migrate elsewhere. Just like this, we can imagine that society will become unstable.
The oceans have done their very best behind the scenes. However, they are reaching their limit, and the effects from the oceans are starting to be seen. I am worried that we may potentially enter a new state that has never before been experienced.

Kainuma: Thank you for sharing some interesting implications from the natural sciences point of view. It was striking to learn that, once the “sleeping giant” is stirred awake, we can never go back to the way things were. Thank you very much for your time today.

Interview on 25 October 2019 at the National Institute of Polar Research (NIPR)