Biased Media Reporting on the New Santer et al. Study Regarding Satellite Tropospheric Temperature Trends « Roy Spencer, PhD

Government Abstract
A brand new paper by Santer et al. in Journal of Local weather reveals that noticed tendencies throughout 1988-2019 in sea floor temperature [SST], tropospheric temperature [TLT and TMT], and whole tropospheric water vapor [TWV] are usually inconsistent, by various quantities, with local weather mannequin tendencies over the identical interval. The examine makes use of ratios between noticed tendencies in these variables to discover how nicely the ratios match mannequin expectations, with the presumption that the fashions present “fact” in such comparisons. Particular emphasis is positioned on the inconsistency between TWV moistening charges and the satellite tv for pc tropospheric temperature warming charges: the overall water vapor has risen quicker than one would count on for the weak charge of satellite-observed tropospheric warming (however each are nonetheless lower than the typical local weather mannequin tendencies in both CMIP5 or CMIP6).

Whereas the paper itself doesn’t single out the tropospheric temperatures as being in error, widespread reporting of the paper used the identical biased headline, as an illustration this from DailyMail.com: “Satellites may have been underestimating the planet’s warming for decades”. The reporting largely ignored the majority of what was within the paper, which was a lot much less crucial of the satellite tv for pc temperature tendencies, and which ought to have been extra newsworthy. For instance: (1) SST warming is proven within the paper to be nicely under local weather mannequin expectations from each CMIP5 and CMIP6, which one may count on might have been a serious conclusion; (2) the likelihood that the satellite-based TWV is rising too quickly (admitted within the paper, and addressed under), and particularly (3) the likelihood that TWV shouldn’t be a great proxy anyway for mid- and upper-tropospheric warming (mentioned under).

As others have proven, free-tropospheric vapor (not nicely captured by TWV) could be the correct proxy for free-tropospheric warming, and the truth that local weather fashions preserve fixed relative humidity with altitude throughout warming shouldn’t be primarily based upon primary bodily processes (because the authors suggest), however as a substitute upon arbitrary moistening assumptions implicit in mannequin convective parameterizations. Observational proof is proven that free-tropospheric humidity doesn’t improve with tropospheric temperature as a lot as within the GFDL local weather mannequin. Thus, weak tropospheric warming measured by satellites may very well be proof of weak water vapor suggestions within the free troposphere, which in flip might clarify the weaker than (mannequin) anticipated floor warming. A possible cause for a excessive bias in TWV tendencies can be addressed, which is per the opposite variables’ development habits.

Proof Offered in Santer et al. (2021)
I’ve been requested by a number of folks to touch upon a brand new paper in Journal of Local weather by Santer et al. (Using Climate Model Simulations to Constrain Observations) that has as considered one of its conclusions the likelihood that satellite-based warming estimates of tropospheric temperature is perhaps too low. Based mostly upon my preliminary examination of the paper, I conclude that there’s nothing new within the paper that might solid doubt on the modest nature of tropospheric warming tendencies from satellites — until one believes local weather fashions as proof, by which case we don’t want observations anyway.

The brand new examine focuses on the interval 1988-2019 in order that whole built-in water vapor retrievals over the ocean from the SSM/I and SSMIS satellite-based devices can be utilized. Latest floor and tropospheric warming has certainly been accompanied by growing water vapor within the troposphere, and the quantitative relationship between temperature and vapor is utilized by the authors as a information to assist decide whether or not the tropospheric warming charges from satellites have been unrealistically low.

Many of the pertinent conclusions within the new paper come from their Fig. 9, which I’ve annotated for readability in Fig. 1, under.

Fig. 1. Tailored from Santer et al. (2021), comparability plots of tropical tendencies (1988-2019) in whole built-in water vapor, sea floor temperature, and tropospheric temperature, in local weather fashions versus observations. Observe in (A) and (D) the ocean floor temperature tendencies are nicely under the typical mannequin tendencies, which curiously was not a part of the media-reported outcomes. These plots present that in all 4 of the properties chosen for evaluation (SST, TLT, TMT, and TWV) the noticed tendencies are under the typical local weather mannequin tendencies (the latter of which decide world coverage responses to anthropogenic GHG emissions). The actual fact the observations fall off of the model-based regression strains is (as mentioned under) because of some mixture of errors within the observations and errors within the local weather mannequin assumptions.

The Downside with Utilizing Built-in Water Vapor Will increase as a Proxy for Tropospheric Warming
A central conclusion of the paper is that whole built-in water vapor has been rising extra quickly than SST tendencies counsel, whereas tropospheric temperature has been rising much less quickly (assuming the fashions are appropriate that SST warming needs to be considerably amplified within the troposphere). This pushes the observations away from the local weather model-based regression strains in Fig. 1a, 1b, and 1b.

The difficulty with utilizing TWV moistening as a proxy for tropospheric warming is that whereas TWV is certainly strongly coupled to SST warming, how nicely it’s coupled to free-tropospheric (above the boundary layer) warming in nature may be very unsure. TWV is dominated by boundary layer water vapor, whereas it’s mid- to upper-tropospheric warming (and thus within the TMT satellite tv for pc measurements) which is strongly associated to how a lot the humidity will increase at these excessive altitudes (Po-Chedley et al., 2018).

This high-altitude area shouldn’t be nicely represented in TWV retrievals. Satellite tv for pc primarily based retrievals of TWV use the comparatively weak water vapor line close to 22 GHz, and so are primarily delicate to the water vapor within the lowest layer of the ambiance.

Moreover, these retrievals are dependent upon an assumptions concerning the profile form of water vapor within the ambiance. If world warming is accompanied by a preferential moistening of the decrease troposphere (because of elevated floor evaporation) and a thickening of the moist boundary layer, the exceedingly vital free-tropospheric humidity improve won’t be as robust as is assumed in these retrievals, that are primarily based upon regional profile variations over totally different sea floor temperature regimes.

As proven by Spencer & Braswell (1997) and others, the flexibility of the local weather system to chill to outer area is strongly dependent upon humidity modifications within the higher troposphere throughout warming (see Fig. 2). The higher troposphere has very low ranges of water vapor in each relative and absolute phrases, but these low quantities of vapor within the higher 75% of the troposphere have a dominating management on cooling to outer area.

Fig. 2. Tailored from Spencer & Braswell, 1997: The speed of humidity will increase within the free troposphere (above the boundary layer) with long-term floor warming can dominate water vapor suggestions, and thus free-tropospheric warming (e.g. from satellite-based TMT), in addition to floor warming. The precipitation processes which govern the humidity on this area (and particularly how they alter with warming) are very unsure and solely crudely dealt with in local weather fashions.

As indicated in Fig. 2, water vapor within the lowest ranges of the troposphere is basically managed by floor evaporation. If the floor warms, growing evaporation moistens the boundary layer, and fixed relative humidity is a fairly good rule of thumb there. However within the mid- and upper- troposphere, detrained air from precipitation techniques largely determines humidity. The fraction of condensed water vapor that’s eliminated by precipitation determines how a lot is left over to moisten the setting. The free-tropospheric air sinking in clear air even 1000’s of km away from any precipitation techniques had its humidity decided when that air ascended in these precipitation techniques, days to weeks earlier than. As demonstrated by Renno, Emanuel, and Stone (1994) with a mannequin containing an specific atmospheric hydrologic cycle, precipitation effectivity determines whether or not the local weather is cool or heat, via its management on the primary greenhouse gasoline, water vapor.

Importantly, we have no idea how precipitation effectivity modifications with warming, due to this fact we don’t know the way robust water vapor suggestions is in the actual local weather system. We all know that tropical rain techniques are extra environment friendly than increased latitude techniques (as many people know anecdotally from visiting the tropics, the place even shallow clouds can produce torrential rainfall). It’s solely affordable to count on that world warming will likely be accompanied by a rise in precipitation effectivity, and up to date analysis is beginning to help this view (e.g. Lutsko and Cronin, 2018). This may imply that free-tropospheric absolute (particular) humidity won’t improve as a lot as local weather fashions assume, resulting in much less floor warming (as is noticed) and fewer tropospheric amplification of floor warming (as is noticed).

As a result of local weather fashions don’t but embrace the precipitation microphysics governing precipitation effectivity modifications with warming, the fashions’ habits concerning temperature versus humidity within the free troposphere shouldn’t be used as “fact” when evaluating observations.

Whereas local weather fashions have a tendency to keep up fixed relative humidity all through the troposphere throughout warming, thus inflicting robust optimistic water vapor suggestions (e.g. Soden and Held, 2006) and so leading to robust floor warming and even stronger tropospheric warming, there are distinction between fashions on this respect. Within the CMIP5 fashions analyzed by Po-Chedley et al. (2018, their Fig. 1a) there’s a issue of three variation within the lapse charge suggestions throughout fashions, which is a direct measure of how a lot tropospheric amplification there may be of floor warming (the so-called “hotspot”). That amplification is, in flip, immediately associated (they get r = -0.85) to how a lot additional water vapor is detrained into the free troposphere (additionally of their Fig. 1a).

What Occurs To Free Tropospheric Humidity within the Actual World?
In the actual world, it’s not clear that free-tropospheric water vapor maintains fixed relative humidity with warming (which might end in robust floor warming, and even stronger tropospheric warming). We would not have good long-term measurements of free-tropospheric water vapor modifications on a world foundation.

Some researchers have argued that seasonal and regional relationships can be utilized to infer water vapor suggestions, however this appears unlikely. How the entire system modifications with warming over time shouldn’t be so sure.

For instance, if we use satellite tv for pc measurements close to 183 GHz (e.g. obtainable from the NOAA AMSU-B devices since late 1998), that are very delicate to higher tropospheric vapor, we discover within the tropics that tropospheric temperature and humidity modifications over time seem like fairly totally different in satellite tv for pc observations versus the GFDL local weather mannequin (Fig. 3).

Fig. 3. Zonal averages of gridpoint regression coefficients between month-to-month anomalies in 183.3 GHz TB and TMT throughout 2005-2015 in observations (blue) and in two GFDL local weather fashions (pink and orange), indicating precipitation techniques in the actual world dry out the free troposphere with warming greater than happens in local weather fashions, doubtlessly lowering optimistic water vapor suggestions and thus world warming.

Extra particulars concerning the ends in Fig. 3. might be discovered here.

Attainable Biases in Satellite tv for pc-Retrieved Water Vapor Traits
Whereas satellite tv for pc retrievals of TWV are identified to be fairly correct when in comparison with radiosondes, refined modifications within the vertical profile of water vapor throughout world warming can doubtlessly trigger biases within the TWV tendencies. The Santer et al. (2021) examine mentions the likelihood that the overall vertically-integrated atmospheric water vapor tendencies supplied by satellites since mid-1987 is perhaps too excessive, however doesn’t deal with any explanation why.

This is a matter I’ve been involved about for a few years as a result of the TWV development since 1988 (solely retrievable over the ocean) has been rising quicker than we’d count on primarily based upon sea floor temperature (SST) warming tendencies mixed with the belief of fixed relative humidity all through the depth of the troposphere (see Fig. 1a, 1b, 1c above).

How may such a retrieval bias happen? Retrieved TWV is proportional to warming of a passive microwave Tb close to the weak 22.235 GHz water vapor absorption line over the radiometrically-cold (reflective) ocean floor. As such, it relies upon upon the temperature at which the water vapor is emitting microwave radiation.

TWV retrieval relies upon upon assumed shapes of the vertical profile of water vapor within the troposphere, that’s, what altitudes and thus what temperatures the water vapor is emitting at. These assumed vertical profile shapes are primarily based upon radiosonde (climate balloon) information from totally different areas and totally different seasons having totally different underlying sea floor temperatures. However these regionally- and seasonally-based form variations won’t replicate form modifications throughout warming. If the overwhelming majority of the moistening with long-term warming happens within the boundary layer (see Fig. 2 above, under 800 hPa stress altitude), with perhaps slight thickening of the boundary layer, however the higher troposphere experiences little moistening, then the retrieved TWV may very well be biased excessive as a result of the additional water vapor is emitting microwave radiation from a decrease (and thus hotter) altitude than is assumed by the retrieval. This may result in a excessive bias in retrieved water vapor over time because the local weather system warms and moistens. Because the NASA AMSR-E Science Staff chief, I requested the developer of the TWV retrieval algorithm about this risk a number of years in the past, however by no means obtained a response.

The New Santer at al. Examine Ignores Radiosonde Proof Supporting Our UAH Satellite tv for pc Temperatures

As an apart, additionally it is value noting that the brand new examine doesn’t even reference our 2018 outcomes (Christy et al., 2018) exhibiting that essentially the most steady radiosonde datasets help the UAH satellite tv for pc temperature tendencies.

Conclusion
The brand new examine by Santer et al. doesn’t present convincing proof that the satellite tv for pc measurements of tropospheric temperature tendencies are unrealistically low, and the media reporting of their examine on this regard was biased. Their conclusion (which they admit is equivocal) relies upon upon the idea in local weather fashions for the way higher tropospheric warming pertains to growing whole tropospheric water vapor (TWV) quantities. Since TWV doesn’t present a lot sensitivity to higher tropospheric water vapor modifications, and people modifications largely decide how a lot tropospheric amplification of floor temperature tendencies will happen (e.g. the “tropical hotspot”), TWV can not decide whether or not tropospheric temperature tendencies are reasonable or not.

Moreover, there may be some proof that the TWV tendencies are themselves biased excessive, which the examine authors admit is one attainable rationalization for the development relationships they’ve calculated.

The prevailing observations as offered within the Santer et al. examine are largely per the view that world warming is continuing at a considerably decrease charge that’s predicted by the newest local weather fashions, and that a lot of the disagreement between fashions and observations might be traced to improper assumptions in these fashions.

Particularly:

1) SST warming has been significantly much less that the fashions predict, particularly within the tropics

2) Tropospheric amplification of the floor warming has been weak or non-existent, suggesting weaker optimistic water vapor suggestions in nature than in fashions

3) Weak water vapor suggestions, in flip, helps clarify weak SST warming (see [1]).

4) Latest revealed analysis (and preliminary proof proven in Fig. 3, above) help the view that local weather mannequin water vapor suggestions is just too robust, and so present fashions shouldn’t be used to validate observations on this regard.

5) Satellite tv for pc-based whole water vapor tendencies can’t be used to deduce water vapor suggestions as a result of they’re in all probability biased excessive because of vertical profile assumptions and since they in all probability don’t replicate how free-tropospheric water vapor has modified with warming, which has a big affect on water vapor suggestions.

REFERENCES

Christy, J. R., R. W. Spencer, W. D. Braswell, and R. Junod, 2018: Examination of space-based bulk atmospheric temperatures utilized in local weather analysis.
Intl. J. Rem. Sens., DOI:https://doi.org/10.1080/01431161.2018.1444293

Lutsko, N. J. and T. W. Cronin, 2018: Enhance in precipitation effectivity with floor warming in radiative-convective equilibrium. J. of Adv. Mannequin. Earth Sys., DOI:https://doi.org/10.1029/2018MS001482.

Po-Chedley, S., Ok. C. Armour, C. M. Bitz, M. D. Zelinka, B. D. Santer, and Q. Fu, 2018: Sources of intermodel unfold within the lapse charge and water vapor feedbacks. J. Local weather, DOI:https://doi.org/10.1175/JCLI-D-17-0674.1.

Renno, N. O., Ok. A. Emanuel, and P. H. Stone, 1994: Radiative-convective mannequin with an specific hydrologic cycle: 1. Formulation and sensitivity to mannequin parameters, J. Geophys. Res. – Atmos., DOI:https://doi.org/10.1029/94JD00020.

Santer, B. D., S. Po-Chedley, C. Mears, J. C. Fyfe, N. Gillett, Q. Fu, J. F. Painter, S. Solomon, A. Ok. Steiner, F. J. Wentz, M. D. Zelinka, and C.-Z. Zou, 2021: Utilizing local weather mannequin simulations to constrain observations. J. Local weather, DOI:https://doi.org/10.1175/JCLI-D-20-0768.1

Soden, B. J., and I. M. Held, 2006: An evaluation of local weather feedbacks in coupled ocean–ambiance fashions. J. Local weather, DOI:https://doi.org/10.1175/JCLI3799.1.

Spencer, R.W., and W.D. Braswell, 1997: How dry is the tropical free troposphere? Implications for world warming concept. Bull. Amer. Meteor. Soc., 78, 1097-1106.