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 developments throughout 1988-2019 in sea floor temperature [SST], tropospheric temperature [TLT and TMT], and complete tropospheric water vapor [TWV] are typically inconsistent, by various quantities, with local weather mannequin developments over the identical interval. The examine makes use of ratios between noticed developments in these variables to discover how nicely the ratios match mannequin expectations, with the presumption that the fashions present “reality” 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 entire water vapor has risen quicker than one would count on for the weak price of satellite-observed tropospheric warming (however each are nonetheless lower than the common local weather mannequin developments 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 important of the satellite tv for pc temperature developments, and which ought to have been extra newsworthy. For instance: (1) SST warming is proven within the paper to be nicely beneath local weather mannequin expectations from each CMIP5 and CMIP6, which one may count on might have been a serious conclusion; (2) the chance that the satellite-based TWV is rising too quickly (admitted within the paper, and addressed beneath), and particularly (3) the chance that TWV will not be proxy anyway for mid- and upper-tropospheric warming (mentioned beneath).

As others have proven, free-tropospheric vapor (not nicely captured by TWV) could be the right proxy for free-tropospheric warming, and the truth that local weather fashions keep fixed relative humidity with altitude throughout warming will not 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 enhance 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 developments can also be addressed, which is per the opposite variables’ development conduct.

Proof Introduced in Santer et al. (2021)
I’ve been requested by a number of individuals 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 chance that satellite-based warming estimates of tropospheric temperature is perhaps too low. Primarily based upon my preliminary examination of the paper, I conclude that there’s nothing new within the paper that will forged doubt on the modest nature of tropospheric warming developments from satellites — except one believes local weather fashions as proof, during which case we don’t want observations anyway.

The brand new examine focuses on the interval 1988-2019 in order that complete built-in water vapor retrievals over the ocean from the SSM/I and SSMIS satellite-based devices can be utilized. Current 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.

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

Fig. 1. Tailored from Santer et al. (2021), comparability plots of tropical developments (1988-2019) in complete built-in water vapor, sea floor temperature, and tropospheric temperature, in local weather fashions versus observations. Notice in (A) and (D) the ocean floor temperature developments are nicely beneath the common mannequin developments, 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 developments are beneath the common local weather mannequin developments (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 beneath) because of some mixture of errors within the observations and errors within the local weather mannequin assumptions.

The Drawback with Utilizing Built-in Water Vapor Will increase as a Proxy for Tropospheric Warming
A central conclusion of the paper is that complete built-in water vapor has been rising extra quickly than SST developments recommend, whereas tropospheric temperature has been rising much less quickly (assuming the fashions are right that SST warming must 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 will not 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 environment.

Moreover, these retrievals are dependent upon an assumptions concerning the profile form of water vapor within the environment. 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 essential free-tropospheric humidity enhance won’t be as sturdy 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 power of the local weather system to chill to outer house 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 house.

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 modify 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 essentially 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 methods largely determines humidity. The fraction of condensed water vapor that’s eliminated by precipitation determines how a lot is left over to moisten the surroundings. The free-tropospheric air sinking in clear air even hundreds of km away from any precipitation methods had its humidity decided when that air ascended in these precipitation methods, days to weeks earlier than. As demonstrated by Renno, Emanuel, and Stone (1994) with a mannequin containing an express atmospheric hydrologic cycle, precipitation effectivity determines whether or not the local weather is cool or heat, by 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 sturdy water vapor suggestions is in the actual local weather system. We all know that tropical rain methods are extra environment friendly than increased latitude methods (as many people know anecdotally from visiting the tropics, the place even shallow clouds can produce torrential rainfall). It’s fully affordable to count on that world warming shall be accompanied by a rise in precipitation effectivity, and up to date analysis is beginning to assist this view (e.g. Lutsko and Cronin, 2018). This is able to imply that free-tropospheric absolute (particular) humidity won’t enhance 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’ conduct concerning temperature versus humidity within the free troposphere shouldn’t be used as “reality” when evaluating observations.

Whereas local weather fashions have a tendency to keep up fixed relative humidity all through the troposphere throughout warming, thus inflicting sturdy constructive water vapor suggestions (e.g. Soden and Held, 2006) and so leading to sturdy 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 price suggestions throughout fashions, which is a direct measure of how a lot tropospheric amplification there’s of floor warming (the so-called “hotspot”). That amplification is, in flip, instantly 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 sturdy 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 will not be so sure.

For instance, if we use satellite tv for pc measurements close to 183 GHz (e.g. accessible 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 (purple and orange), indicating precipitation methods in the actual world dry out the free troposphere with warming greater than happens in local weather fashions, doubtlessly lowering constructive water vapor suggestions and thus world warming.

Extra particulars concerning the leads to Fig. 3. may be discovered here.

Potential 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, delicate modifications within the vertical profile of water vapor throughout world warming can doubtlessly trigger biases within the TWV developments. The Santer et al. (2021) examine mentions the chance that the entire vertically-integrated atmospheric water vapor developments offered 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 might count on primarily based upon sea floor temperature (SST) warming developments 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 mirror form modifications throughout warming. If the overwhelming majority of the moistening with long-term warming happens within the boundary layer (see Fig. 2 above, beneath 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. It will 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 Workforce 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, it is usually price noting that the brand new examine doesn’t even reference our 2018 outcomes (Christy et al., 2018) displaying that probably the most secure radiosonde datasets assist the UAH satellite tv for pc temperature developments.

Conclusion
The brand new examine by Santer et al. doesn’t present convincing proof that the satellite tv for pc measurements of tropospheric temperature developments 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 assumption in local weather fashions for the way higher tropospheric warming pertains to growing complete 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 developments will happen (e.g. the “tropical hotspot”), TWV can not decide whether or not tropospheric temperature developments are life like or not.

Moreover, there’s some proof that the TWV developments are themselves biased excessive, which the examine authors admit is one doable 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 price that’s predicted by the newest local weather fashions, and that a lot of the disagreement between fashions and observations may 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 constructive water vapor suggestions in nature than in fashions

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

4) Current printed analysis (and preliminary proof proven in Fig. 3, above) assist the view that local weather mannequin water vapor suggestions is simply too sturdy, and so present fashions shouldn’t be used to validate observations on this regard.

5) Satellite tv for pc-based complete water vapor developments can’t be used to deduce water vapor suggestions as a result of they’re most likely biased excessive because of vertical profile assumptions and since they most likely don’t mirror how free-tropospheric water vapor has modified with warming, which has a big impression 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: Improve 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., Okay. C. Armour, C. M. Bitz, M. D. Zelinka, B. D. Santer, and Q. Fu, 2018: Sources of intermodel unfold within the lapse price and water vapor feedbacks. J. Local weather, DOI:https://doi.org/10.1175/JCLI-D-17-0674.1.

Renno, N. O., Okay. A. Emanuel, and P. H. Stone, 1994: Radiative-convective mannequin with an express 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. Okay. 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–environment 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.