Drought in the north and heavy rain in the south, has the earth entered the era of extreme weather?

No, in the weather distribution in 2022, it is indeed drought in the north and heavy rain in the south. Especially the rainstorm in the south seems to be getting worse and worse. Many people can't help but sigh: "The earth has entered an era of extreme weather." In fact, this exclamation is a bit exaggerated. The climate of the earth is more and more elusive, but it is far from "extreme", and even the concepts used here are a little incorrect.

First, emphasize the concepts of weather and climate. Climate is different from weather. More precisely, climate refers to the long-term pattern of weather in a specific area. Weather units can be hours to hours, days to days, months to months and years to years. Therefore, in the same area, there may be many different weather patterns in a year, and the weather with the longest duration is climate, which cannot be easily judged as an "era" just by the continuous rainstorm in a certain period of time. To give a very simple example, the desert area has little rainfall and is in a dry climate for a long time. Although there will be rain in a certain period of time, even the rain can last for a week, we will not define the desert as entering a "wet era" because it has rained for a week.

In recent ten years, people have become more and more interested in the possible relationship between global warming and individual extreme climate events in history, which is based on scientific and practical motives. First of all, extreme weather is the basis of serious pressure on natural and human systems, so it is very important to understand the impact of historical warming on extreme events for detecting the impact of climate change. Secondly, the frequency or intensity of extreme events is increasing, which may lead to unprecedented events around the world in the future. But again, the earth's climate will not change easily, and short-term extreme weather changes do not mean that we have entered an "extreme era". The main reason is that the earth's climate will not change easily, and greenhouse gas emissions will only change slightly, as follows.

The periodic changes of the earth's climate occur on multiple time scales, from decades to hundreds, thousands and tens of thousands of years. The period of each scale is caused by many physical mechanisms. The climate in any particular period is the expression of the interaction of all these nested mechanisms and cycles. The main glacial periods (cold) and interglacial periods (warm) are caused by the change of the earth's orbit around the sun, which is called Milankovic cycle. These periods appear with different intensities during the period of 10000- 100000. Orbital changes occur slowly with time, affecting the position of the earth's surface receiving solar radiation in different seasons. In itself, these changes in the distribution of solar radiation are not enough to cause large temperature changes. However, they can activate a powerful feedback mechanism and amplify the slight warming or cooling effect caused by Milankovitch cycle.

One kind of feedback is caused by the change of global surface reflectivity (also called albedo). At north latitude, even a slight increase in solar radiation will increase the melting of ice. Due to the loss of ice, the bright white ice surface reflects less sunlight, and the earth absorbs more sunlight, thus aggravating the overall warming. The second feedback mechanism involves the concentration of greenhouse gases in the atmosphere, such as carbon dioxide. Slight warming caused by changes in the earth's orbit warms the oceans, enabling them to release carbon dioxide. We can see that more carbon dioxide in the atmosphere will lead to more warming, which will produce amplification effect (Hansen 2003). Feedback 2 of different concentrations of atmospheric CO may lag behind the warming or cooling caused by orbital changes by as much as 1000 years. In this way, a small change in orbit at the beginning can produce the ice age and interglacial cycle of the past 800 thousand years. One of the main problems of current climate change is that similar feedback mechanism will cause modern "runaway" warming effect, which is extremely difficult to stop or reverse.

In the century-scale climate cycle, besides the glacial and interglacial cycles of 10000, there are also short-term cold and warm cycles, which occur on the time scale of about 200 to 1500. The mechanism leading to these cycles is not completely clear, but people think that it is driven by several corresponding changes, such as the changes of the solar and ocean circulation models. For example, the warm period in medieval Europe (AD 900- 1300) and the Little Ice Age in Ming and Qing Dynasties (AD 1450- 1900) are examples of cold and warm periods in a cycle. Even some of these cycles, such as the warm period in medieval Europe, may still be regional and may not necessarily reflect the great changes in the global average. To sum up, there is no extreme weather era on the earth, and man-made climate warming can not affect the climate cycle of the solar system where the earth is located. Compared with the whole universe, human behavior is actually very small.