The World Meteorological Organization (WMO) recently officially confirmed that 2023 is the warmest year on record globally, with average Earth's surface temperatures 1 percent above pre-industrial levels45±0.12 degrees Celsius. However, in the context of global warming, extreme cold weather events such as extreme cold waves and snowstorms have also occurred from time to time in many parts of the world. How to understand the relationship between extreme cold events and global warming?
The first thing to do is to distinguish the relationship between weather and climate. Generally speaking, "weather" describes short-term atmospheric conditions, and "climate" describes long-term average atmospheric conditions, which are expressed in terms of short-term and long-term average temperature, pressure, wind, wind direction, humidity, and other parameters, respectively.
The sources of cold air for winter weather in the Northern Hemisphere are in the polar regions, the high latitudes of the North American continent, and the high latitudes of the Asian continent. In the middle and high latitudes of the Northern Hemisphere, there is a westerly circulation belt that circles the polar regions, and there is a strong jet flow area in the westerly circulation, and its wind speed can even reach more than 100 meters per second. When this jet stream forms the Arctic vortex, it tightly concentrates cold air in the polar regions. However, when the jet stream produces large-scale disturbances, the cold air tends to pour southward, forming cold waves and winter storms in the mid-latitudes, producing blizzards and freezing rain and snow. It's like a spinning top, and when it rotates at high speed, the top doesn't fall down easily; And when the speed of the gyroscope slows down, it is easy to fall to the ground.
The temperature difference between the polar regions and the tropics is the root cause of the formation of the westerly jet stream. The larger the temperature difference and the stronger the westerly jet, the more cold air stays at high latitudes, and the less extreme cold air activity is in the middle latitudes. Conversely, when the temperature difference weakens, the jet stream weakens, and the cold air tends to affect the mid-latitudes. In general, the jet stream is less frequent in January, when the jet stream is stronger, while in November and March, when the jet stream is not strong enough, the number of cold air events is the highest in the year. At present, the low temperature, rain, snow and freezing weather in southern China is mainly caused by the fluctuation of the westerly jet stream.
Short-term extremely cold weather events are not contradictory to long-term warming. Extreme cold events in one place are not contradictory to overall global warming. At the beginning of January 2024, Northern Europe experienced extreme weather, with temperatures as low as minus 40 degrees Celsius, and winter storms in Western and Central Europe. Since mid-January, temperatures have risen sharply in northern Europe, western Europe, and central Europe, and temperatures are now well above normal. In mid-January, the U.S. was hit by widespread winter storms; After the snowstorm ended, some places experienced rapid warming, with temperatures more than 10 degrees Celsius above normal.
It is important to note that extreme cold events are complexly linked to global warming. Although the world is warming, the warming is uneven, and the Arctic has the largest warming, about 2-3 times the global average, which is called the "Arctic amplification" phenomenon. This phenomenon attenuates the temperature difference between the polar and tropical regions, making the westerly jet stream more susceptible to instability, resulting in stronger weather processes. At the same time, due to global warming, there is more water vapor in the atmosphere, and when cold and warm air masses meet, more energy is released, making blizzards sometimes more intense.
In recent years, although severe cold spells have occurred from time to time, in general, the duration and frequency of cold spells are much lower than half a century ago. When people are accustomed to warm sunny weather, once there is a cold wave, the body will feel even colder. In the face of the compounding challenges brought about by global warming, the international community should take concrete actions to promote the full and effective implementation of the Paris Agreement and actively carry out green development cooperation.
The author is a researcher at the Institute of Atmospheric Physics, Chinese Academy of Sciences).