3. What we must avoid
Although the scale of climate change is large, it is hard to see. Therefore, people often think that the increase in temperature and its impacts are gradual and so may also be stopped at any time. Sadly, however, such is not the case.
Let's think about a ball rolling down a moderate incline. The ball moves forward gradually. However, if a cliff is ahead, what would happen? The ball would roll over the cliff and drop at a tremendous speed, never being able to return to where it was.
The changes caused by global warming are similar to this. At a specific temperature, change stops occurring gradually and starts occurring too rapidly to stop. This critical threshold is called the tipping point.
From among the significant changes that occur beyond the tipping point, Professor Schellnhuber chose ten large-scale phenomena with the potential to exert a decisive impact on the global environment. Such drastic, relatively rapid and potentially irreversible changes are called tipping elements, and they are all likely significant threats to human survival.
For example, the majority of Greenland, the large island located between the Arctic Ocean and the North Atlantic Ocean, is covered by ice sheets. However, the ice sheets are predicted to melt as the temperature increases. If all the ice sheets there were to melt, the oceans would rise by as much as 7 meters.
Furthermore, there is also a risk to the gigantic ice sheets in the Antarctic. The western part is more at risk than the eastern part is. Ice sheets surround the Antarctic with an average thickness of 1.6 kilometers. The bedrock under the ice sheets in the West Antarctic are more than several hundred meters below the surface. If water seeps in between the ice sheets and bedrock, the melting may accelerate. Compared with this, the East Antarctic is more stable although there is a risk of partial destruction there as well.
If all the ice sheets around Greenland and the Antarctic were to melt, the oceans would rise 70 meters, which would inundate most of the current coastal areas.
The melting of the ice sheets around Greenland may also trigger another tipping elements, the deceleration of the ocean circulation system.
The waters around Greenland are cold and high in salinity. Because of this, the water is dense and so tends to sink to the bottom. This movement creates the oceanic current that circulates on the earth like a conveyor belt.
However, if the fresh-water ice sheets were to melt, the water around Greenland would become less dense and not sink to the bottom. As a result, general ocean circulation may decelerate or stop. The oceanic current carries warm and cool air that adjust the climate. If it stops, the warm current does not reach the higher latitudes and temperatures in Europe would drop rapidly. The world climate could change completely.
This climate change would have a severe impact on ecosystems too. The tropical rainforests in the Amazon account for about a half of the tropical rainforests in the world. However, deforestation and forest fires have decreased the area of rainforests, and there is concern about the reduced amount of rainfall caused by global warming. Rain in the tropical rainforests is derived from moisture. Therefore, acceleration of deforestation would decrease rainfall, which would disrupt circulation and have a significant influence on the climate of the entire world.
Coral reefs are an ecosystem; however, they are susceptible to changes in water temperature and acidification. The bleaching and death of coral reefs have already been observed in many places around the world.
A slight increase in average temperature may cause one unpredicted phenomenon after another.
Professor Schellnhuber compares tipping elements to major body organs such as the heart and kidneys. Although a slight cut on the skin won't kill us, stab wounds penetrating heart will. Tipping elements are as dangerous for the earth as being stabbed in the heart is for humans.
Preventing such threats requires that we maintain the temperature at a certain level and avoid triggering tipping elements.
What limiting the temperature increase to 2 degrees C means
It is, however, difficult to determine how much of an increase in temperature is permissible. The chart shown below shows the possibility of triggering a tipping element through temperature change.
The permissible temperature is dependent upon conditions. For example, we think a 1- to 4- degree C increase in air temperature may cause the ice sheets in Greenland to melt. Fortunately, ice sheets will not melt completely until the temperature increases by 4 degrees C; but, unfortunately, they will start melting if the temperature increases by only 1 degree C.
In reality, it is desirable to keep the temperature at the current level. However, the temperature has continued to increase. Strictly speaking, it is not possible to prevent global warming entirely, but it is possible to restrain or slow it. What we can and have to do is to suppress global warming within a permissible range.
Taking into account the facts above mentioned, the COP21 UN climate change agreement signed in Paris included the goal of limiting global warming by 1.5 to 2 degrees C.
The gray area shown as the "Paris range" in the middle of the chart is between 1.5 and 2 degrees C. By this chart, if we restrain global warming by 1.5 to 2 degrees C, the eight tipping elements on the right, including the impact on the Amazon rainforest, could be prevented.
Meanwhile, even if we can limit global warming to between 1.5 and 2 degrees C, the five tipping elements on the left, including the melting of ice sheets in the West Antarctic may not be prevented. Even if we can achieve the 2-degree C target, it is predicted that most of the coral reef ecosystems will disappear.
However, the more we limit the increase in temperature, the higher the possibility of preventing these threats becomes.
The right side of the chart shows the results of simulation of the future temperature increase in four different scenarios. If people continue depending on fossil fuels and discharging carbon dioxide at the current level, we will be subject to the RCP8.5 (red) scenario, in which the temperature increases more than 8 degrees C and all tipping elements will be triggered.
If we try our best to reduce the carbon dioxide emissions, we will experience the RCP2.6 (green) scenario. If we can achieve this, the temperature will not increase by 1.5 degrees C, but decrease at a certain point. If so, we will be able to realize our 2-degree C target.