| With the continuous development of human industry,the impact of human activities on climate can not be ignored,which has led to the emergence of a new research direction in climatology to explore how human activities affect to climate.In the second half of the last century,the global greenhouse gases(GHGs)concentration continued to rise rapidly,and the global average surface air temperature(SAT)maintained an overall upward trend,accompanied by a series of climatic problems,such as sea level rise,Plateau snow cover melting,polar sea ice recession,and sea surface air temperature rising.By the beginning of the 21 st century,the theory of global warming has taken shape,and the relevant research has been strongly supported and accepted by governments of all countries and the public.At that time,it is almost no longer a controversial fact that greenhouse gases emitted by human activities will lead to global warming.Under this historical background,slowdown of the global warming was suddenly appeared after the year of 1998.Smith et al.(2008),as well as Easterling and Wehner(2009)found that,it seems global warming has paused in a decade since 1998,although GHGs are still breaking record year by year.Periods of time exhibiting GHGs increases without simultaneous global SAT warming are known as global warming slowdown or global warming hiatus.Existing climatological theories can not fully explain the phenomenon of global warming slowdown,so this phenomenon has attracted widespread attention from both the scientific community and the public at the same time,and triggered scientific discussions,which also led to public and government concerns and doubts about global warming.This,of course,also enthusiasm of scientists for research.Based on the research status of global warming and its slowdown,we have developed and improved the method of climate trend turning detection.Then,combined with the existing statistical methods,we have carried out a variety of studies from different perspectives,such as global and regional,and found some new features of global surface air temperature trend turning.We hope these new features can provide useful help for deepening the understanding of global warming.The main conclusions of this doctoral dissertation are as follows:1)Many characteristics of trend turning is discussed in this paper,and the trend turnings are divided into three basic types according to different combinations of trends before and after turning: trend reversal(TR),trend generation or disappearance(TN,Trend vs.No trend)and trend identical but significant difference in trend value(TD,Trends are different in Degree,and are in the same sign),being subdivided into the corresponding 10 sub-types.We find that the TD trend turning is often neglected in the current trend turning research and the existing trend turning detection methods have low recognition ability to it.However,as a reality,TD trend turning is very important in the study of global surface air temperature trends.For the problem of trend turning,the significant trend difference between pre-turning and post-turning subsequences is the common feature in all types of trend turning.According to this point,a new method of trend turning detection,called running slope difference t-test(RSD t-test),is developed in this paper.Using Monte Carlo simulation,the new detection method is evaluated with the false rejection rate,false acceptance rate and the ability to identify multiple trend turnings.Then,we compare the RSD t-test method with some other existing trend turning detection methods in an idealized time series case and several climate time series cases.The RSD t-test method has a better ability to identify all three types of trend turning,especially the stronger ability to identify the trend turning of TD type.2)Whether the global average surface air temperature has slowed down in the first decade of the 21 st century has attracted much more attention in the study of climate change.In this paper,we use multi-data and multi-methods to discuss the trend characteristics of global mean surface air temperature at this stage.We find that in the first decade of the 21 st century,the trend of global average surface air temperature has slowed down to a certain extent compared with the previous period,but the slowdown is not statistical significant.Further,we designed two sets of Monte Carlo simulation experiments to study whether random disturbances may lead to a relative slowdown of warming in the first decade of the 21 st century.On the basis of a large number of experiments,we find,it only has a very low probability that random perturbations in data could cause a relatively slowdown in the first decade of the 21 st century.we speculate that the trend change of global surface air temperature field during this period may have strong regional characteristics.3)By using the method of regional trend turning characteristics division based on running slope difference t-test,we studied the spatial-temporal characteristics of the trend turning of global SAT field.It was found that there were four main trend turning processes in global SAT field since 1880,namely,the warm trend turning dominated process from 1902 to 1915,the cool trend turning dominated process from 1930 to 1950,the warm trend turning dominated process from 1964 to 1975,and the cool-warm trend turning matched process from 1980 to 2001.Further,by analyzing the spatial-temporal distribution of the cool-warm trend turning matched process from 1980 to 2001,we identified six major cooling zones,namely,the Southern Ocean region,the Southern Hemisphere Tropical East Pacific region,the Northern Hemisphere Tropical East Pacific region,the North Atlantic region,the Kuroshio region and the Northeast Asian continent.In addition,we calculated the relative contribution rates of each region to the recent warming slowdown.The total relative contribution rates of the six major cooling zones are about 71.32% to 82.34%.This indicates that the turning point of SAT cool trend turning in these six major cooling zones is probably the main reason for the slowing down of global warming.4)Based on the cool-warm trend turning matched process from 1980 to 2001,we find the Middle East arid region with the Arabian Peninsula as the core had a warming turning point with a larger turning range.This as a clue,the SAT trend turning characteristics of global arid regions of the world are researched.It is found that in the last 40 years,the global land average air temperature has increased by about 1oC;while in the global arid regions average air temperature has increased by nearly 1.5oC.The average SAT of the global land and the global arid regions showed a trend of accelerated warming around 1980.The warming rate of the average SAT of the global land reached 0.28oC/10 years after the turning point,and the global arid regions reached the level of 0.36 to 0.38 oC /10 years.The warming rates of these two regions was higher by about 3.5 times and 6 times after trend turning.The acceleration of explosive warming in global arid regions areas mainly contributes to Sahara and Arabian Peninsula. |
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