Editing IPCC:AR6/WGII/Chapter-4 (section)

==== 4.2.1.1 Observed Changes in Precipitation ====

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AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) concluded that GHG forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas ( ''medium confidence'' ), with a detectable precipitation increase in the northern high latitudes ( ''high confidence'' ). GHG forcing has also contributed to drying in dry summer climates, including the Mediterranean, southwestern Australia, southwestern South America, South Africa and western North America ( ''medium to high confidence'' ) (Figure 4.3). AR6 WGI ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) also concluded that the frequency and intensity of heavy precipitation events have ''likely'' increased at the global scale over most land regions with good observational coverage. Heavy precipitation has ''likely'' increased on the continental scale over North America, Europe and Asia. Regional increases in heavy precipitation frequency and (or) intensity have been observed with at least ''medium confidence'' for nearly half of the AR6 WGI climatic regions (Figure 4.3). Human influence, in particular GHG emissions, is ''likely'' the main driver of the observed global-scale intensification of heavy precipitation in land regions

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'''[[File:0c19bfccc496c614c6eb3417eaa9c58e IPCC_AR6_WGII_Figure_4_003.png]] Figure 4.3 |''' '''Observed mean and extreme precipitation changes and people experiencing the emergence of historically unfamiliar precipitation and changes in extreme precipitation.'''

'''(a)''' Percentage changes in annual mean precipitation over land (1891–2019) per °C global warming in the Global Precipitation Climatology Centre (GPCC) v2020 data set ( [[#Schneider--2017|Schneider et al., 2017]] ; [[#Schneider--2020|Schneider et al., 2020]] ). Green shows increasing precipitation; orange shows decreasing precipitation.

'''(b)''' Levels of unfamiliarity of wetter and drier climates, classified in terms of the ratio of the signal S of change to the noise N of variability, where the latter is defined as one standard deviation in annual data with the trend removed, that is, occurs approximately one in 6 years. Grey regions are either unobserved (oceans) or deserts (&lt;250 mm year –1 ). Stippling indicates where the signal of change is not significant. See [[#Hawkins--2020|Hawkins et al. (2020)]] for further details.

'''(c)''' Population densities in regions with annual precipitation classified as “emerging”.

'''(d)''' Precipitation trends from the GPCC data set in December, January and February (mm day –1 decade –1 ).

'''(e)''' As (d) for June-July-August.

'''(f)''' Changes in annual maximum 1-day precipitation (Rx1day) in the HadEX3 data set ( [[#Dunn--2020|Dunn et al., 2020]] ).

'''(g)''' Trend in annual mean consecutive dry days (CDD), 1950–2018, in HadEX3.

'''(h)''' Population densities per grid box where the trend in Rx1day is significantly different from zero.

'''(i)''' Population densities per grid box where the trend in CDD is significantly different from zero. Stipples in (h) and (i) show where HadEX3 data is available. Population data in (c), (h) and (i) are for 2020 from CIESIN (2018a; 2018b).

Large numbers of people live in regions where the annual mean precipitation is now ‘unfamiliar’ compared to the mean and variability between 1891 and 2016 (Figure 4.3c). “Unfamiliar” is defined as the long-term change being greater than one standard deviation in the annual data (Figure 4.3b). In 2020, approximately 498 million people lived in unfamiliarly wet areas, where the long-term average precipitation is as high as previously seen in only about one in 6 years ( ''medium confidence'' ) (Figure 4.3c). These areas are primarily in mid and high latitudes ( [[#Hawkins--2020|Hawkins et al., 2020]] ). On the other hand, approximately 163 million people lived in unfamiliarly dry areas, mostly in low latitudes ( ''medium confidence'' ). Due to high variability over time, the signal of long-term change in annual mean precipitation is not distinguishable from the noise of variability in many areas ( [[#Hawkins--2020|Hawkins et al., 2020]] ), implying that the local annual precipitation cannot yet be defined ‘unfamiliar’ by the above definition.

Notably, many regions have seen increased precipitation for part of the year and decreased precipitation at other times ( ''high confidence'' ) (Figure 4.3d,e), leading to small changes in the annual mean precipitation. Therefore, the numbers of people seeing unfamiliar seasonal precipitation levels are expected to be higher than those quoted above for unfamiliar annual precipitation changes ( ''medium confidence'' ). Still, quantified analysis of this is not yet available.

The intensity of heavy precipitation has increased in many regions ( ''high confidence'' ), including much of North America, most of Europe, most of the Indian sub-continent, parts of northern and southeastern Asia, much of southern South America, parts of southern Africa and parts of central, northern and western Australia (Figure 4.3 f) ( [[#Dunn--2020|Dunn et al., 2020]] ; [[#Sun--2020|Sun et al., 2020]] ). Conversely, heavy precipitation has decreased in some regions, including eastern Australia, northeastern South America and western Africa. The length of dry spells has also changed, with increases in annual mean consecutive dry days (CDD) in large areas of western, eastern and southern Africa, eastern and southwestern South America, and Southeast Asia, and decreases across much of North America (Figure 4.3g). Precipitation extremes have changed in some places where annual precipitation shows no trend. Some regions such as southern Africa and parts of southern South America are seeing increased heavy precipitation and longer dry spells. Many regions with changing extremes are highly populated, such as the Indian sub-continent, Southeast Asia, Europe and parts of North America, South America and southern Africa (Figure 4.3h,i). Substantially more people (~709 million) live in regions where annual maximum one-day precipitation has increased than in regions where it has decreased (~86 million) ( ''medium confidence'' ). However, more people are experiencing longer dry spells than shorter dry spells: approximately 711 million people live in places where annual mean CDD is longer than in the 1950s, and ~404 million in places with shorter CDD ( ''medium confidence'' ) (Figure 4.3i).

In summary, annual mean precipitation is increasing in many regions worldwide and decreasing over a smaller area, particularly in the tropics. Nearly half a billion people live in areas with historically unfamiliar wet conditions, and over 160 million in areas with historically unfamiliar dry conditions ( ''medium confidence'' ). Over 700 million people experience heavy precipitation significantly more intense than in the 1950s, but less than 90 million experience decreased heavy precipitation. Compared to the 1950s, 711 million people now experience longer dry spells and 404 million experience shorter dry spells.

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