Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.1.2 Temperatures During the Instrumental Period – Free Atmosphere ====

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The AR5 reported that it was ''virtually certain'' that tropospheric temperatures have risen, and stratospheric temperatures fallen, since the mid-20th century, but that assessments of the rate of change and its vertical structure had only ''medium confidence'' in the NH extratropics and ''low confidence'' elsewhere. In particular there was ''low confidence'' in the vertical structure of temperature trends in the upper tropical troposphere.

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===== 2.3.1.2.1 Dataset developments =====

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There have been updated radiosonde estimates from the University of Vienna (RAOBCORE and RICH; [[#Haimberger--2012|Haimberger et al., 2012]] ) and a new dataset from the State University of New York (UAHRD, [[#Zhou--2020|Zhou et al., 2020]] ). There are new versions of AMSU products from the University of Alabama in Huntsville (UAHv6.0; [[#Spencer--2017|Spencer et al., 2017]] ) and Remote Sensing Systems (RSSv4.0; [[#Mears--2017|Mears and Wentz, 2017]] ). These updates have led to convergence in the lower stratosphere layer ( [[#Maycock--2018|Maycock et al., 2018]] ); in particular, the move to UAHv6.0 has addressed homogeneity issues identified by [[#Seidel--2016|Seidel et al. (2016)]] , although residual differences remain ( [[#Christy--2018|Christy et al., 2018]] ). Reanalyses products had identified limitations near the 300 hPa level where the contribution of aircraft observations has increased rapidly in recent years ( [[#Dee--2011|Dee et al., 2011]] ; [[#Gelaro--2017|Gelaro et al., 2017]] ), leading to identified biases ( [[#Dee--2009|Dee and Uppala, 2009]] ), that have been addressed in ERA5 ( [[#Hersbach--2020|Hersbach et al., 2020]] ). Modern reanalyses are generally well aligned with radiosonde and satellite observations in the middle and lower troposphere and lower stratosphere. A new operational mid- and upper-stratospheric dataset (STAR) has been developed by [[#Zou--2016|Zou and Qian (2016)]] , merging the previous 1979–2006 SSU dataset ( [[#Zou--2014|Zou et al., 2014]] ) with a dataset from 1998 onwards drawn from relevant AMSU channels ( [[#Wang--2014|Wang and Zou, 2014]] ). Further stratospheric satellite-based datasets from various combinations of satellites have been developed by [[#McLandress--2015|McLandress et al. (2015)]] and [[#Randel--2016|Randel et al. (2016)]] .

New assessments of free-atmosphere temperature are available through radio occultation (RO) and Atmospheric Infrared Sounder (AIRS) products which begin in the early 2000s ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.1.1|Section 1.5.1.1]] ). Global Navigation Satellite System (GNSS)-RO datasets have been compared against AMSU data records, finding almost identical trends ( [[#Khaykin--2017|Khaykin et al., 2017]] ). Comparison of RO with collocated radiosondes, Vaisala RS90/92 and GCOS Reference Upper Air Network data (RS92-GDP; [[#Dirksen--2014|Dirksen et al., 2014]] ), show very good correspondence with global annual mean differences of less than 0.2°C in the upper troposphere and lower stratosphere. Radiosonde daytime radiation biases were identified at higher altitudes ( [[#Ladstädter--2015|Ladstädter et al., 2015]] ; [[#Ho--2017|Ho et al., 2017]] ). The stability of RO makes this data a useful comparator for AMSU ( [[#Chen--2014|Chen and Zou, 2014]] ) and radiosondes ( [[#Ho--2017|Ho et al., 2017]] ; [[#Tradowsky--2017|Tradowsky et al., 2017]] ), as well as anchoring post-2006 reanalyses datasets and improving their consistency in the lower and middle stratosphere ( [[#Long--2017|Long et al., 2017]] ; [[#Ho--2020|Ho et al., 2020]] ). The effective vertical resolution of RO measurements in the upper troposphere and lower stratosphere was found to be up to 100 m at the tropical tropopause ( [[#Zeng--2019a|Zeng et al., 2019a]] ), which is favourable for resolving atmospheric variability ( [[#Scherllin-Pirscher--2012|Scherllin-Pirscher et al., 2012]] ; [[#Wilhelmsen--2018|Wilhelmsen et al., 2018]] ; [[#Stocker--2019|Stocker et al., 2019]] ). Temperature trends in RO products are most consistent with each other and with other observations between 8 km and 25 km ( [[#Ho--2012|Ho et al., 2012]] ; [[#Steiner--2013|Steiner et al., 2013]] , 2020a). The uncertainty increases above 25 km for the early RO period, for which data are based on the single-satellite CHAMP mission, but data at higher altitudes become more reliable for later missions based on advanced receivers ( [[#Steiner--2020a|Steiner et al., 2020a]] ), along with the application of corrections for ionospheric effects ( [[#Danzer--2020|Danzer et al., 2020]] ). The uncertainty due to the changing number of observations is reduced by correcting for the sampling uncertainty in RO climatological fields (e.g., [[#Scherllin-Pirscher--2011|Scherllin-Pirscher et al., 2011]] ). For AIRS, thus far, stability of the instrument has been constrained to less than 0.03°C per decade for selected window channels in a comparison to SSTs measured by ocean buoys ( [[#Aumann--2019|Aumann et al., 2019]] ). Trends were inter-compared with trends in RO data and reanalysis data to assess systematic uncertainties ( [[#Leroy--2018|Leroy et al., 2018]] ).

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===== 2.3.1.2.2 Assessment of trends =====

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Warming has continued in the lower troposphere according to all radiosonde, reanalyses and satellite datasets, with a rate over 1980–2019 similar to surface warming rates (Table 2.5; c.f. Table 2.4). Radiosonde-based products generally show greater warming rates for 1980–2019 than satellite-based products and reanalyses. They also extend further back to the 1950s and trends since quasi-global coverage around 1960 also show warming (Table 2.5). Trends in RO and AIRS data, supported by radiosonde datasets, exhibit a warming trend in most of the mid- to upper- troposphere at all non-polar latitudes over 2002–2019. These also exhibit faster warming rates in the tropics in the upper troposphere than those observed at or near the surface (Figure 2.12); with the lowermost stratosphere also warming while above it is cooling. There is some spread between different data types in the tropics near the 15km level, although these differences are reduced to near zero if a subset of radiosonde data, using only high-quality instruments, is used ( [[#Steiner--2020b|Steiner et al., 2020b]] ). AMSU tropical middle troposphere data also show that warming rates are near or above those in the lower troposphere, but they are measuring much broader layers which greatly complicates interpretation ( [[#Steiner--2020b|Steiner et al., 2020b]] ).

Temperatures averaged through the full lower stratosphere (roughly 10–25 km) have decreased over 1980–2019 in all data products, with the bulk of the decrease prior to 2000. The decrease holds even if the influence of the El Chichon (1982) and Pinatubo (1991) volcanic eruptions on the trend, found by [[#Steiner--2020a|Steiner et al. (2020a)]] to have increased the 1979–2018 cooling trend by 0.06°C per decade, is removed. Most datasets show no significant or only marginally significant trends over 2000–2019, and the results of [[#Philipona--2018|Philipona et al. (2018)]] show weak increases over 2000–2015 in the very lowermost stratosphere sampled by radiosondes.

The STAR dataset shows cooling in the middle and upper stratosphere with a trend of –0.56°C ± 0.16°C per decade for the mid-stratosphere and –0.62°C ± 0.29°C per decade for the upper stratosphere over 1980–2019, although both cooling rates have slowed substantially since the mid-1990s. The overall post-1980 trend is reduced in magnitude by about 0.10°C per decade at both levels if the influences of the El Chichon and Pinatubo eruptions, and the solar cycle, are removed ( [[#Zou--2016|Zou and Qian, 2016]] ). The results obtained by [[#McLandress--2015|McLandress et al. (2015)]] for 1980–2012, [[#Randel--2016|Randel et al. (2016)]] for 1979–2015, and [[#Maycock--2018|Maycock et al. (2018)]] for 1979–2016 are broadly consistent with this.

A rise in the tropopause height of 40 to 120 m per decade between 1981 and 2015 was determined from both radiosonde and reanalysis datasets ( [[#Xian--2019|Xian and Homeyer, 2019]] ). Local studies (e.g., [[#Tang--2017|Tang et al., 2017]] ; X. [[#Chen--2019|]] [[#Chen--2019|Chen et al., 2019]] ) found stronger trends in some regions near the subtropical jet linked to tropical expansion ( [[#2.3.1.4.1|Section 2.3.1.4.1]] ). Whilst [[#Seidel--2006|Seidel and Randel (2006)]] found that the tropopause height was more closely coupled with temperatures in the stratosphere than those in the troposphere, it is not yet clear whether the rate of increase in tropopause height has experienced a similar recent slowdown to that of the cooling of the lower stratosphere, as short-period trends are typically inconclusive due to significant natural variability ( [[#Scherllin-Pirscher--2021|Scherllin-Pirscher et al., 2021]] ). RO data ( [[#Gao--2015|Gao et al., 2015]] ) indicate little change in tropopause height over the short period from 2006 to 2014, but a warming below the tropopause is observed over 2002 to 2019 (Figure 2.12).

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[[File:87a7746f1162a91715bb84e29f811c36 IPCC_AR6_WGI_Figure_2_12.png]]

'''Figure 2.1''' '''2 |''' '''Temperature trends in the upper air. (a)''' Zonal cross-section of temperature anomaly trends (2007–2016 baseline) for 2002–2019 in the upper troposphere and lower stratosphere region. The climatological tropopause altitude is marked by a grey line. Significance is not indicated due to the short period over which trends are shown, and because the assessment findings associated to this figure relate to difference between trends at different heights, not the absolute trends. '''(b, c)''' Trends in temperature at various atmospheric heights for 1980–2019 and 2002–2019 for the near-global (70°N–70°S) domain. '''(d, e)''' as for (b, c) but for the tropical (20°N–20°S) region. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, the troposphere has warmed since the mid-20th century. There is ''medium confidence'' that temperatures in the tropical upper troposphere have warmed faster than those at the surface since 2001, but ''low confidence'' in changes prior to 2001. It is ''virtually certain'' that the lower stratosphere has cooled since the mid-20th century. However, most datasets show that lower stratospheric temperatures have stabilized since the mid-1990s with no significant change over the last 20 years. It is ''likely'' that middle and upper stratospheric temperatures have decreased since 1980, but there is ''low confidence'' in the magnitude. It is ''virtually certain'' that the tropopause height has risen over 1980–2019 but there is ''low confidence'' in the magnitude of this rise, or whether the rate of change has reduced commensurate with stabilized lower stratospheric temperatures.

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Table 2.5 '''|''' '''Observed change (°C) in free atmospheric temperatures in various datasets, for the lower tropospheric and lower stratospheric layers.''' Numbers in square brackets indicate 5–95% confidence ranges. Trend values are calculated with ordinary least squares following ( [[#Santer--2008|Santer et al., 2008]] ) and are expressed as a total change over the stated period. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

{| class="wikitable"
|-
| '''Diagnostic/Dataset'''

| '''Trend'''

'''1960–2019'''

| '''Trend'''

'''1980–2019'''

| '''Trend'''

'''2000–2019'''

|-
| colspan="4"| '''Lower troposphere'''

|-
| '''RAOBCORE'''

| 1.08

[0.94 to 1.23]

| 0.74

[0.57 to 0.91]

| 0.52

[0.26 to 0.78]

|-
| '''RICH'''

| 1.20

[1.06 to 1.34]

| 0.79

[0.63 to 0.96]

| 0.53

[0.28 to 0.77]

|-
| '''UAHRD'''

| 0.97

[0.80 to 1.13]

| 0.91

[0.76 to 1.05]

| 0.53

[0.35 to 0.72]

|-
| '''UAH'''

|
| 0.51

[0.37 to 0.65]

| 0.29

[0.07 to 0.50]

|-
| '''RSS'''

|
| 0.79

[0.66 to 0.92]

| 0.41

[0.24 to 0.58]

|-
| '''ERA5.1'''

|
| 0.68

[0.52 to 0.84]

| 0.55

[0.34 to 0.75]

|-
| Average

| 1.08

| 0.74

| 0.47

|-
| colspan="4"| '''Lower stratosphere'''

|-
| '''RAOBCORE'''

| –1.37

[–1.80 to –0.93]

| –1.00

[–1.56 to –0.45]

| –0.05

[–0.20 to 0.09]

|-
| '''RICH'''

| –1.45

[–1.99 to –0.92]

| –1.19

[–1.95 to –0.42]

| 0.02

[–0.20 to 0.23]

|-
| '''UAHRD'''

| −1.25

[−1.51 to −0.98]

| −0.79

[−1.16 to −0.43]

| −0.11

[−0.25 to 0.03]

|-
| '''UAH'''

|
| –1.14

[–1.61 to –0.67]

| –0.24

[–0.37 to –0.12]

|-
| '''RSS'''

|
| –0.90

[–1.37 to –0.43]

| –0.14

[–0.26 to –0.03]

|-
| '''STAR'''

|
| –0.97

[–1.45 to –0.49]

| –0.17

[–0.29 to –0.04]

|-
| '''ERA5.1'''

|
| –1.19

[–1.87 to –0.50]

| –0.01

[–0.13 to 0.10]

|-
| Average

| –1.36

| –1.03

| –0.10

|}

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