After crossing the critical point of climate

On July 3, 219, in Irulisat, Greenland, climate change is having a far-reaching impact on Greenland. The local weather is unusually warm and many glaciers are melting. (vision china/Photo)

People all over the world are witnessing with their own eyes how climate change can damage the earth. The steadily rising global average temperature has intensified the increasingly intense wildfires, hurricanes and other disasters, which can not be ignored now.

An important report issued by the United Nations Intergovernmental Panel on Climate Change (IPCC) in August, 221 depicts a disastrous climate change scene. Humans have approached or passed some critical points, and some irreversible things are happening quietly. What will the earth do after crossing the critical point?

for more than 3 years, scientists have known that there is a threshold in the climate system. If this threshold is crossed, the earth's climate system will cause irreversible changes on the human time scale. The IPCC introduced the concept of climate critical point 2 years ago. At that time, it was thought that these "large-scale discontinuous" events in the climate system could only occur when global warming was 5 times higher than the pre-industrial level. As time goes on, scientists have made new discoveries. The IPCC concluded in two reports released in 218 and 219 that even between 1 and 2, it may exceed the critical point.

In 219, an article published in Nature listed nine critical points of climate faced by human beings, and pointed out that the world may have crossed a series of critical points of climate, and some impacts caused by global warming are irreversible. The author's team discovered a series of potential critical points in the earth system more than ten years ago, and now there is evidence that more than half of them are active. In August 221, the IPCC report listed a series of impacts of climate change on the earth. Judging from the irreversible effects of climate change, mankind is approaching the critical point infinitely, and some effects have already occurred and are irreversible, such as the continuous rise of sea level, global heat wave and drought.

by 21, if the most active emission reduction method is adopted, the temperature of the earth may be about 1.4 higher than that before industrialization, but if the negative emission reduction method is adopted, it may be more than 4 higher than that before industrialization. However, even at the low end of this range, irreversible changes may occur in every corner of the earth, and with the intensification of climate warming, the risks become greater and more difficult to avoid.

The carbon legacy left by the lifestyle based on fossil fuels is bringing climate crisis to the future of billions of people. However, when we talk about the "future", how far should we consider? By 25? In 21? Or a little further, maybe 25 years or more?

the future climate prediction used by international scientific and political groups is usually only until 21. The reason is simple. In the next few centuries, it is impossible for us to accurately predict how much greenhouse gases will be released and how much impact this will have on the climate.

What is certain now is that with the temperature rising in the future, extreme weather events will become more and more serious. According to the data in the IPCC report, if the earth's temperature is 2 degrees higher than that before industrialization, the extreme temperature events that occurred every 5 years in the past few centuries may occur every 3-4 years. The world will also face more complex events, and climate change will aggravate the water cycle, which will bring more rainfall and related floods and more serious droughts to many areas. Climate change will also affect rainfall patterns. At high latitudes, precipitation may increase, while it is expected to decrease in most parts of the subtropics. Throughout the 21st century, the sea level in coastal areas will continue to rise, leading to more frequent and serious coastal floods and coastal erosion in low-lying areas.

by the end of this century, extreme sea level events that happened only once in the previous 1 years may occur every year. Further warming will aggravate the melting of permafrost, the decrease of seasonal snow, the melting of glaciers and ice sheets, and the decrease of Arctic sea ice in summer. Ocean changes, including warming, more frequent ocean heat waves, ocean acidification and declining oxygen levels, affect both marine ecosystems and people who depend on them, and will continue at least for the rest of this century. For cities, some aspects of climate change may be magnified, including floods caused by high temperature and heavy precipitation events and sea level rise in coastal cities. In June 221, a study published in Nature Climate Change attributed more than one-third of high temperature-related deaths to climate change. In some countries where there is no air conditioning or other factors that make people more vulnerable to high temperatures, the death toll is even higher.

on September 5, 221, a joint editorial was published in the New England Journal of Medicine, urging world leaders to take action. "There is no doubt about science," they wrote. "The global temperature is 1.5 higher than the pre-industrial average, and the continuous loss of biodiversity may cause catastrophic damage to health, which is irreversible." Despite the serious danger posed by COVID-19, the author of this article wrote that governments around the world "can't wait until the pandemic is over to reduce emissions quickly". On the contrary, they believe that everyone must treat climate change as urgently as COVID-19.

Generally speaking, scientists believe that by 21, the temperature of the earth will rise by more than 3.5 to 8, but the most likely situation is in the middle of this range. But no matter where we are after 79 years, no matter what the thermometer reading is, the impact will definitely be severe.

A study published in Nature in August, 221 pointed out that the carbon dioxide emitted from fossil fuels today will still affect the climate for thousands of years to come. Most of the carbon dioxide emitted by burning a ton of coal or oil today will be absorbed by the ocean and vegetation in a few centuries, and the remaining 25% will still affect the climate in 1 years. So far, the most comprehensive report on climate change focuses on potential irreversible climate risks, from sea level rise to ocean circulation slowdown, but these risks can still be avoided through strong climate action.

IPCC lists five potential climate scenarios according to different socio-economic trends, including the prediction of population and economic growth, geopolitical trends and technological development. In the first and most optimistic scenario, human beings have taken immediate and active measures to reduce global carbon emissions to zero by 25. This is the only plan that can achieve the goal of the Paris Agreement, that is, to control global warming at 1.5. The report predicts that global warming will fall back to 1.4 by the end of this century. In the second case, mankind will reach net zero after 25, but the global warming will remain below 2, and will stabilize at around 1.8 by 21. In the third case, the world socio-economic trend will not really deviate from its historical model, and mankind will not reach net zero in 21, so the global warming will reach 2.7 at the end of this century. In the fourth case, global warming will reach 3.6 by the end of this century. Finally, in the worst case: not only can we not limit the current emission level, but it will double by the middle of this century. By the end of this century, the global temperature will rise by 4.4.

At present, the earth is in the middle stage. By 21, global warming will reach 2.7. This scenario is consistent with many countries' commitments to climate change in 23 under the Paris Agreement. Therefore, if greenhouse gas emissions are reduced immediately, rapidly and on a large scale, human beings can prevent and weaken disasters. In the final analysis, it depends on how human beings, as the greatest uncertainty, carry out climate action.

Southern Weekend Special Contributor Zhu Yehua