Is it true that the global climate will get warmer in the future?

There is no doubt that the mother of global warming.

What is the evidence of global warming? In the past, we always discussed the problem of climate warming around the potential residual of temperature recorded by ground weather stations. These records are particularly important, but they only represent a single indicator of climate system change. More evidence of global warming comes from the extensive measurement results of many other elements in the climate system, which are independent, physically consistent and closely related.

In climate change, the global average surface temperature rise is a well-known index, which has a linear trend for more than 30 years in the sense of climate statistics. Although the temperature every year or even every 10 year is not always higher than that of the previous year or the last ten years, the global surface temperature has risen sharply since 1900. From 1880 to 20 12 years, the average surface temperature of the earth has increased by 0.85℃. Climate warming does not mean that any part of the earth is warming at any time, but the linear trend of the earth's surface temperature is rising for at least 30 years. Therefore, in the past hundred years or more, it is normal for the temperature to drop and slow down in a certain decade, twenty years, a certain season or a certain area. Since 1850, the interdecadal variability of global average surface temperature has appeared in 1870s- 1900s, 1940s- 1970s, but this does not affect the overall trend of global average surface temperature rising continuously. In addition, the "slowing down" of global warming since 1998 is mainly manifested in the change of global average surface temperature, but from the change of the whole climate system, global warming has not "slowed down". Due to the rapid development of urbanization, the warming of cities and urban areas is higher (called urban heat island effect), but it is limited to a certain spatial range. The research shows that even if it is not revised, its impact on global temperature will not exceed 10%, and East Asia may reach more than 20%. The measurement of global land temperature is based on the observation data of thousands of stations around the world, which is obtained through the homogenization of mathematical statistics, that is to say, the factors such as uneven space and urbanization of stations are excluded.

The warming of land temperature is closely consistent with the observed warming trend of ocean temperature. Many independent analysis results prove that the rise of ocean temperature measured from ships is synchronous with the rise of sea surface temperature.

Both the atmosphere and the ocean are fluids, so surface warming can also be seen in the lower atmosphere and the upper ocean, and the observation results confirm that it is true. From the observation data analysis of radio sounding balloons and satellites, it can be seen that tropospheric warming can make the weather layer in the atmosphere active. At least since 1970s, more than 90% of the heat has been absorbed by the climate system, which can be confirmed by the records of global ocean heat content since 1960s.

As the ocean warms, the ocean itself will expand. This expansion is also one of the main factors of sea level rise independently observed in the past century. The melting of glaciers and ice sheets is also one of the reasons for sea level rise, and the storage and utilization of land water have also changed accordingly.

A warm world is also a humid world, because warm air can lock in more water vapor. Global analysis shows that this specific humidity, which is used to measure atmospheric water vapor content, has increased on land and in the ocean.

As the frozen part of the whole planet-always called the cryosphere-it is not only controlled by the local temperature, but also affects the local temperature. The amount of ice in global glaciers is decreasing every year, which has been going on for more than 20 years. The melting amount of ice is partly responsible for the rise of sea level. Snow is very sensitive to temperature changes, especially when it melts in spring. Since 1950s, Chun Xue in the whole northern hemisphere began to shrink. Since the satellite records, a great deal of melting of Arctic sea ice has been observed, especially in the smallest ice period, that is, at the end of the annual melting season in September. In contrast, the increase of Antarctic sea ice is small.

Individually, any single analysis result may be unconvincing, but the analysis results of these different indicators and different data sets have led many research groups to reach the same conclusion: from the deep sea to the top of the troposphere, all the evidences of air or ocean warming, melting of ice and snow and sea level rise show exactly one thing, that is, the world has been warming since the late19th century.

In a word, the evidence of global warming comes from many complex and independent climate indicators, from the upper atmosphere to the bottom of the ocean. These indicators include changes in the earth's surface temperature, atmospheric temperature and ocean temperature, as well as changes in glaciers, snow, sea ice, sea level and atmospheric water vapor. Scientists around the world have independently verified this evidence for many times, confirming that global warming undoubtedly began in the19th century. In order to better explain this conclusion, this paper compares the historical climate with the modern climate.

Similarities and differences between modern climate and historical climate change

First of all, answer: what are the reasons for the ice age before the industrial age and other important climate changes?

The climate of the earth has changed on all time scales, including long before human activities came into play. People have made great progress in understanding the causes and mechanisms of these climate changes. The change of the earth's radiation balance is the main driving factor of climate change in the past, but the reasons for these changes are different. In any case, whether it is the ice age, the warm period of the dinosaur age, or the past 1000 years of fluctuations, the specific reasons should be determined separately. In many cases, the causes can now be determined with confidence, and many past climate change processes can be reconstructed by using various quantitative models.

From the beginning of the Ice Age to the past 3 million years, there is ample evidence that these changes are related to the periodic changes of the Earth's orbit around the sun, which is also called the Milankovic cycle (Figure 3). These periods change the solar radiation received at each latitude and season (but have little effect on the global annual average), and we can calculate these periods with astronomical accuracy. At present, there are still discussions about when the ice age will start and end, but many studies show that the summer sunshine on all continents in the northern hemisphere is very important: if it is below a certain threshold, the snow in the past winter will not melt in summer, and with more and more snow accumulation, the ice sheet will begin to thicken. The simulation results of climate model confirm that the ice age can indeed start like this, and at the same time, some simple conceptual models are used to successfully "post-report" the beginning of the past ice age on the basis of orbital changes. Similar to the beginning of previous ice ages, the next sharp decrease in summer solar radiation in the northern hemisphere will begin in 30,000 years.

Although carbon dioxide (CO2) in the atmosphere is not the main cause, it also played an important role in the ice age. The Antarctic ice core data show that the carbon dioxide concentration is very low (~ 190 ppm) during the cold ice period and high (~280ppm) during the warm interglacial period. Atmospheric carbon dioxide changes with the change of Antarctic temperature, with a lag period of about several hundred years. Because the climate change from the beginning to the end of the ice age lasted for thousands of years, most of these changes were affected by the positive feedback of carbon dioxide; That is, the initial small cooling caused by Milankovitch cycle is amplified with the decrease of carbon dioxide concentration. Only when carbon dioxide is taken into account can the simulation of ice age climate produce satisfactory results.

During the last ice age, there were 20 sudden and drastic climate changes, especially recorded near the North Atlantic. These changes are different from the interglacial period, during which there was no big global average temperature change: the changes in Greenland and Antarctica were not synchronous, while the changes in the South Atlantic and the North Atlantic were in opposite directions. This means that the above changes can be triggered without major changes in the global radiation balance; The redistribution of heat in the climate system is enough to cause this change. In fact, there is conclusive evidence that the changes of ocean circulation and heat transport can explain many characteristics of these emergencies; Sediment data and model simulation show that some of these changes may be caused by the instability of the ice sheet around the Atlantic Ocean at that time and the release of fresh water to the ocean.

In the past 500 million years, there have been warmer periods in the history of climate, and the earth may have no ice sheet at all (geologists can tell from the traces of ice leaves on rocks), unlike Greenland and Antarctica, which are covered by ice sheets today. There is still considerable uncertainty in the greenhouse gas data dating back to/kloc-0.0 million years ago, that is, the data dating back to the Antarctic ice core. However, geological sampling analysis shows that the ice-free warm period is consistent with the high concentration of carbon dioxide in the atmosphere. On the time scale of millions of years, the change of carbon dioxide concentration is caused by crustal tectonic activity, which affects the exchange of carbon dioxide between the ocean and the atmosphere and the solid earth.

Another possible cause of climate change in the past is the change of solar energy output. Observations in recent decades show that the solar energy output has little change (close to 0. 1%) during1year. Sunspot observation (dating back to17th century) and isotope data produced by cosmic radiation prove the long-term change of solar activity. The results of data correlation and model simulation show that solar change and volcanic activity may be the main reasons for climate change before industrialization/kloc-0.0 million years.

These examples show that different climate changes in the past had different reasons. In the past, natural factors caused climate change, but this does not mean that the current climate change is natural. By analogy, forest fires have been caused by lightning strikes for a long time in nature, but it does not mean that the fire cannot be caused by a careless camper.

The global climate is determined by the radiation balance of the earth. There are three basic ways to change the radiation balance of the earth and lead to climate change: (1) changing the incident solar radiation (such as the change of the earth's orbit or the sun itself); (2) change the reflectivity of solar radiation (this reflectivity is called albedo, which can be changed by changes in clouds, aerosols or particles in the earth layer. ); (3) changing the long-wave energy radiated back into space (for example, by changing the concentration of carbon dioxide). In addition, the local climate also depends on how the wind and ocean currents distribute heat. All these factors have played a role in the past climate change.

The second answer: Compared with the changes in the early history of the earth, is the current climate change abnormal?

Throughout the history of the earth, the climate has been changing. The current climate change is not abnormal in some aspects, but it is abnormal in others. Compared with the past 500,000 years, the concentration of CO2 in the atmosphere has reached the highest level in history, and this trend is continuing at an unusually fast speed. Compared with at least the past 500 years or even 1000 years, the global temperature is warmer at present. If this warming continues, the climate change in this century will be extremely abnormal from a geological point of view. Another unusual aspect of current climate change is its cause: the past climate change originated from natural causes, while the warming in the past century, especially in the past 60 years, was mostly caused by human activities.

When comparing the current climate change with the early natural change, we must distinguish three differences. First of all, which variables must be clearly compared: greenhouse gas concentration, or temperature (or other climate parameters), their absolute values, or their rate of change? Secondly, don't confuse local changes with global changes. Local climate change is usually much greater than global climate change, because local factors (such as changes in ocean or atmospheric circulation) can transfer heat and water vapor from one place to another, and local feedback will have an impact (such as sea ice feedback). In contrast, a large change in the global average temperature requires some kind of global coercion (such as changes in greenhouse gas concentration or solar activity). Third, time scales must be distinguished. Compared with the time scale of 100 years, the climate change in millions of years may be much greater and have different causes (such as continental drift).

At present, the main reason for concern about climate change is the increase of atmospheric carbon dioxide (CO2) (and other greenhouse gases), which was very rare in the Quaternary (about two million years ago). The carbon dioxide concentration in the past 650,000 years is accurately known from the Antarctic ice core. During this period, the CO2 concentration varied between the low value of 180 ppm in the cold ice age and the high value of 300ppm in the warm interglacial age. In the past century, the concentration value has increased rapidly and greatly exceeded this range, and now it is about 397ppm. In contrast, at the end of the last ice age, it took more than 5,000 years for the concentration of carbon dioxide to rise by about 80ppm. The concentration value higher than the present value only appeared millions of years ago.

Temperature is a variable that is more difficult to reconstruct than CO2 (a completely mixed gas in the world), because it has different meanings in different parts of the world, so a single record (such as ice core) has limited value. The change of local temperature, even in just a few decades, may reach several degrees Celsius, which is far greater than the global warming signal of about 0.8 degrees Celsius in the past century.

Large-scale (global or hemispheric) average analysis is more meaningful to global change. At this scale, most of the changes reach the average, and the variation is very small. Records of instruments with sufficient coverage can only be traced back to about 150 years. Further back to the time, compile alternative data from the aspects of annual rings and ice cores. Tracing back to 1000 years ago, its spatial coverage is reduced. Although there are differences and obvious uncertainties in those reconstruction results, all published reconstruction results show that the temperature in the Middle Ages was warm, which cooled to a low value in 17, 18 and 19 centuries, and then warmed rapidly. The degree of warmth in the Middle Ages is uncertain, but it may have reached this level in the middle of the 20th century, and only then will it be possible to surpass it. These conclusions are also supported by climate simulation. The temperature changes more than 2,000 years ago were not systematically arranged into a large-scale average, but no evidence was provided to prove that it was warmer than the current global annual average temperature and could be traced back to the Holocene (nearly 1 1 600 years ago). There are strong indications that a warmer climate prevailed until about 3 million years ago, which greatly reduced the global ice sheet and made the sea level higher. Therefore, the current warming seems abnormal in the past few thousand years, but it is not abnormal on a longer time scale. At this scale, the change of tectonic activity (which may be driven by the natural and slow change of greenhouse climate concentration) becomes relevant.

Another different problem is the current warming rate. Is the global climate change recorded by proxy data fast? The biggest temperature change in the last million years occurred in the ice age, during which the global average temperature changed from 4℃ in the ice age to 7℃ in the warm interglacial period (the local change was much larger, such as near the continental ice sheet). However, the data show that global warming at the end of the ice age is a gradual process, which lasted for nearly 5000 years. So obviously, according to the past changes, the current global climate change rate is much faster, which is very abnormal. The abrupt climate change during the ice age, which is often discussed, is by no means a counterexample, because such abrupt changes are likely to be caused by heat transport in the ocean, and they are unlikely to affect the global average temperature.

Going back to the time before the ice core data, the time resolution of sediments and other archives can't distinguish the current rapid warming. Therefore, although there have been great climate changes in the past, there is no evidence that these changes occur faster than the current climate warming. If we realize that the prediction of global warming in this century is about 5℃ (the upper limit of the range), then the earth will probably experience the same global average warming as at the end of the last ice age. There is no evidence that any global warming in the past 50 million years can match the possible global change rate in the future.

To sum up, there is no doubt that the contemporary global climate is warming, both at the rate of warming and on a global scale. Then, everyone is looking for the cause of global warming.