Changes in the Cryosphere are an Important Piece of Evidence of Global Warming
Professor, Department of Civil Engineering, College of Engineering
Shibaura Institute of Technology
Lead Author, Chapter 2, IPCC Special Report on the Ocean and Cryosphere
Kainuma: Professor Hirabayashi, can you first tell us what you particularly wanted to convey with the IPCC Special Report on the Ocean and Cryosphere (SROCC)?
Hirabayashi: Changes in the cryosphere are important evidence of global warming, and we can get a very good idea of the impacts of global warming by observing the cryosphere. The term “cryosphere” may be unfamiliar to some, but it refers to areas with remarkable landscape features, namely glaciers, snow, and frozen soil. Meteorological phenomena, like rainfall, change quickly, but the cryosphere supposedly changes very slowly, over a long time. The fact that we can actually observe glacier retreats and other such phenomena in this region demonstrates that global warming is actually happening.
For this report, I was in charge of assessing the impact on the high mountain regions, which are highly elevated regions. In high mountain regions, we have already observed the melting of glaciers. As a result, the absolute amount of water resources is changing in the river basins with glaciers. When glaciers melt, the amount of water temporarily increases, but, as the glaciers continue to melt and shrink, the amount of water paradoxically decreases. This change has a particularly large impact on downstream regions that rely on water from glaciers for most of their day-to-day water supply. While temporary increases in water volume causes floods and other disasters, the decrease in water volume after the peak has been reached will decrease electricity production by hydropower generation and destabilise food production and water security.
Japan is not included in the “high mountains,” but similar phenomena apply to Japanese snowy mountainous regions. The winter snowfall brings benefits to our lives as snowmelt in early spring. However, reduced snowfall brings reduced water resources, and this may potentially impact agriculture, such as rice farming. Moreover, there are economic consequences for regions that rely on snow tourism. For instance, a paper referenced in the SROCC has reported that some regions where past Olympic Games had been held may not be able to host in the future.
There are concerns, not only for society, but also for the ecosystem. In particular, we believe there will be impacts for populations of animals such as rabbits and foxes whose fur changes color in the winter, populations of snowbirds and other aviaries, and the ecology of alpine plants.
Being involved this time in the SROCC as an author, I felt the need to accurately communicate how the environment will change due to global warming.
Kainuma: Professor Hirabayashi, you attended the 51st Session of the Intergovernmental Panel on Climate Change (IPCC) and were present for the approval of the SROCC’s Summary for Policymakers. During this approval process, what was mainly discussed? Please share with us any particularly interesting discussions you observed.
Hirabayashi: I was mostly in charge of the impact assessment in the high mountain regions, and the findings are summarised in Figure 1 (SPM2). This figure was created by reviewing relevant publications for each of the three regions (high mountain regions, oceans, and the polar regions) and summarises the level of confidence in the evidence and the direction of the impact. The review was conducted by several authors, so we were meticulous about reaching a consensus on the decision-making criteria with respect to the confidence level and direction of impact, and made adjustments. Afterwards, at the SPM Approval Session, country representatives engaged in discussion.
Some places in the figure are white, meaning “no assessment,” but this does not mean that there are no impacts. For the most part, it means that there are no English-language publications that serve as evidence. For instance, many high mountain regions are in Spanish-speaking regions and Asia, and, while we employed the help of lead authors with knowledge of these languages to search as much as possible, there were few peer-reviewed papers written in many cases. It was therefore not possible to report these results. Some people expressed views that “no assessment” would hinder actions in each country, but we cannot just show baseless data either. We struggled greatly with interpretation.
Japan is no exception to the lack of English-language publications. For example, Figure 2.2 in Chapter 2 shows the trends in the annual average temperatures in mountainous regions, but only one data point was available for Japan. I believe that they are recording temperature data in mountainous regions, but because there were no English-language publications on the latest changes, these data could not be included.
Kainuma: People may not be so familiar with the term “cryosphere,” but which region does it refer to exactly?
Hirabayashi: In this report, “cryosphere” is defined as areas with remarkable landscape features, namely glaciers, snow, and frozen soil. However, the polar regions are described in detail in a separate chapter, so it is not included in Chapter 2, for which I served as lead author. Moreover, regions with snow coverage only in the winter, like mountainous regions in Japan, were not included in the cryosphere because there would be too many regions to cover. So the focus was limited to regions where there is frozen soil or snow even in summer.
Kainuma: When thinking about changes in the cryosphere, “decreases in glaciers and ice sheets”, “decreases in ice coverage”, and “temperature rise” come to mind, but does the SROCC report on any new effects or phenomena in which the level of confidence has increased?
Hirabayashi: Compared to the IPCC 5th Assessment Report, the SROCC focuses more on regional changes and assesses the impacts more closely by region for many topics. As well as changes in upstream regions of the water cycle including mountainous regions, there have been downstream regions whose water resources and agricultural productivity have already been affected. Such research findings were also cited.
Kainuma: In mountainous regions like the Himalayas, there have been reports of melting glaciers in recent years. What are the effects? Please tell us about the current as well as future effects.
Hirabayashi: There is fear that the melting of glaciers in Nepal and Bhutan could lead to depleted water resources, but the reality is that we do not yet know exactly what is going on. Are the glaciers melting? Or is there less rainfall? We do not know enough about this yet. The citations made in the SROCC are focused on research on the effects from the cryosphere.
Kainuma: In terms of effects on our oceans, “marine heatwaves increase”, “sea level rise”, “ocean heat content increase”, “ocean acidification”, and “ocean deoxygenation” come to mind. What are the impacts on our day-to-day lives, specifically?
Hirabayashi: The melting of glaciers explains one-fourth of the reason why the sea level is rising. For example, if the sea level were to rise by a meter, around 25 centimeters is due to the melting of snow and ice, and the rest (the three-fourths) is due to the thermal expansion caused by ocean warming. Japan is an archipelago surrounded by ocean, so we can say that it will be greatly impacted by sea level rise.
Kainuma: At this rate, the sea level will rise up to 1.1 meters. How will Japan be affected?
Hirabayashi: Up until now, no studies have estimated a sea level rise that exceeds one meter. The SROCC is the first to do so, with 1.1 meters, and this has been reported widely by the media. To be clear, this 1.1-meter value is the maximum value until 2100. However, even within this century, it has been estimated that the sea level will rise by roughly 85 centimeters. If the sea level truly rises by this much, the impact of storm surges and high waves will be that much more intense in various parts of the world.
Moreover, it has been said that Japan’s sandy beaches will start to disappear. Rivers and oceans are connected, so sea level rise will cause river water level rise, elevating the risk of floods in river basins. During Typhoon Hagibis, the flow of the main river increased, and the tributaries connecting to the main river could not flow through, which resulted in backflow. Because of this, a series of bank breaks and floods were observed. This became known as a “backwater phenomenon,” and it became well-known by the general public because of large coverage on television. We can say that the same thing would happen where the oceans and rivers meet.
Kainuma: The SROCC claims that ocean deoxygenation will continue for more than another century. There have been concerns that the deoxygenation in the Sea of Japan will worsen. In the future, if global warming exacerbates further and bottom water formation completely stops, then the Japanese deep ocean is estimated to be anoxic in roughly 100 years (Aramaki, 2018). I imagine that this would affect ecosystems. What are your thoughts?
Hirabayashi: Regarding ocean deoxygenation, apart from the effects of climate change, it is sometimes caused by material circulation due to increases in nutrients from rivers. So we still do not know how much each cause can be attributed to this phenomenon. Because of this, the level of confidence regarding its impacts of climate change has been reportedly lower.
Kainuma: Question 6.1 of the SROCC’s Frequently Asked Questions reads, “How can risks of abrupt changes in the ocean and cryosphere related to climate change be addressed?” I think that the answer to this question is that there is no other way but to “ultimately reduce emissions.” What are your views?
Hirabayashi: Changes in the ocean and cryosphere are extremely significant in terms of both time and space, and it has been reported that the effects are detrimental if global warming continues. Yes, like you said, I think that reducing emissions is critical.
Kainuma: Last but not least, would you tell us what topics you’d like to explore next?
Hirabayashi: I think that I should write more about the synergies and tradeoffs between the Sustainable Development Goals (SDGs) and climate action. Several parts of the SROCC did try to address this, but I don’t think it is enough. There is limited research on this as well.
Being involved in this report, I realised that I need to make further progress on this point. For example, I am thinking that it would be good to describe the synergies and tradeoffs for each continent.
Kainuma: That’s wonderful. I am looking forward to it. Thank you so much for today.
Interview on 28 October 2019 at the Shibaura Institute of Technology
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