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Indeed, we are "rebounding" from the LIA. But "rebounding" is a NOT a cause. It is an observation. That rebound happened for a reason that likely includes many contributing factors. CO2 is certainly among those factors and its significance became most acute after 1950. UHI effects are NOT ignored. Every conventional surface based database I'm aware of gives the UHI effect its well deserved consideration. Berkeley Earth has a great publication detailing how much the UHI bias impacts global mean temperatures (see Rohde 2013). "We observe the opposite of an urban heating effect over the period 1950 to 2010, with a slope of -0.10 ± 0.24°C/100yr (2σ error) in the Berkeley Earth global land temperature average.. The confidence interval is consistent with a zero urban heating effect, and at most a small urban heating effect (less than 0.14°C/100yr, with 95% confidence) on the scale of the observed warming (1.9 ± 0.1°C/100 yr since 1950 in the land average from Figure 5A)." As you can see their conclusion is that the UHI effect is more likely to have a negative bias than a positive bias on global mean temperature trends after 1950 when the warming became most acute. The observed trends in the global mean surface temperature datasets is NOT a result of the UHI effect. It is a result of the planet actually warming.

This actually comes from Myhre 1998. Well...the 5.35 sensitivity parameter anyway. Myhre used radiative transfer models (LBL, NBM, BBM) using data from the well known HITRAN database. The logarithmic behavior was first diagnosed by Arrhenius 1896 so he usually gets credit for this mind numbingly simple back-of-the-envelope model of the climate system's response to changes in CO2 concentration. This model is so well know that it is typically regarded as common knowledge and is referenced ubiquitously often without citations in the climate community. BTW...Arrhenius' final computation (which is said have been very laborious) from 1908 using his more complex 1896 model yields 4C of warming for 2xCO2 (see Worlds in the Making by Arrenhius 1908). It is interesting that this falls comfortably within the official IPCC range and despite his primitive (by today's standards) understanding of the climate system. Wouldn't that be a remarkable feat of brilliance if his 4C estimate turns out to be close to the observed value?

The data says the Earth's climate was that steady for almost 2000 years on a global scale. Global/hemisphere proxy data temperature reconstructions do not ignore the MWP or LIA. Remember, the MWP and LIA were names given to periods of climatic shifts primarily in the North Atlantic periphery area (see Lamb 1982) though most scientist do accept some global influence albeit by a smaller amount. These North Atlantic sites are included in Holocene temperature reconstructions regardless. CO2 did lead temperatures in the past. The PETM is probably the best analog. Nevermind that an agent does not have to lead a variable for it to be a significant contributing factor to the trajectory of that variable anyway. CO2 is leading now because it is being released in huge quantities independent of any climate modulation.

If CO2 is not a significant contributing factor then what is?

Actually...it's worse than that. Present for paleoclimatology is defined as 1950. The chart is missing the last 145 years of warming prior to 2000. If it had include the last 165 years of warming up to 2020 then you'd see that today's temperature is at least as high as the Minoan Warm Period and probably would exceed the boundaries of the y-axis. The x-axis is also not scaled linearly which makes the temperature changes in the past appear more rapid than they really are. For example, the abrupt change around 8000 years ago only had a warming rate of 0.1C/decade as compared to about 0.4C/decade observed today. The chart comes from Dr. Easterbrook who actually used it in congressional testimony (https://www.youtube.com/watch?v=ofXQdl1FDGk). I have no idea how a PhD geology professor could bungle something so badly. And his predictions are just as bad. He predicted 1998 to be the peak and his middle of the range estimate for the cooling by 2020 is 0.7C below 1998. In reality 2019 ended 0.3C above 1998. Dr. Easterbrook's prediction is off by an astonishing 1.0C in only 20 years!

Consider a forced induction combustion engine. Air induction can both lead and lag the power level. When the turbocharger is internally forced via the exhaust the induction lags the power increase. When the turbocharger is externally forced via electric boost technologies the induction leads the power increase. In both cases the turbocharger is a significant contributing factor to the final power level of the engine. Earth's carbon cycle behaves in this manner as well. I think those that understand forced induction combustion engines will find the lead/lag behavior of CO2 in the climate system intuitive. BTW...BorgWarner does produce dual modulated turbochargers with both internal forcing (exhaust) and external forcing (electric).

There have been a few comprehensive style studies that have been published recently which try to provide the best estimates of critical metrics related to climate change based on the large body of evidence available at the time of publication. I am but an amateur, but it is my belief that these publications are impactful and that they are of the type that may appear prominently in the forthcoming IPCC AR6 report. If anyone has noteworthy or impactful peer-reviewed publications then by all means please post them to this thread. If possible provide a link to an open access (non pay-walled) version of the manuscript. Temperature Reconstruction Kaufman 2020: Holocene global mean surface temperature, a multi-method reconstruction approach Kaufman 2020: A global database of Holocene paleotemperature records Summary: The rate of warming during the contemporary era is likely unprecedented during the Holocene. The global mean temperature is very likely to be much higher than at any point in the last 2000 years and possibly even exceeding the Holocene Climate Optimum 6000+ years ago. Climate Sensitivity Sherwood 2020: An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence Summary: For 2xCO2 they report 2.6 - 3.9C and 2.3 - 4.7C for 66% and 95% certainty respectively with a value of 3.1C being at the peak of the probability distribution curve. It is noted that the upper bound should be considered more cautiously. It is far easier to constrain the lower bound than the upper bound. Values <= 1.5C can now be confidently ruled out. Using the IPCC's standard 66% confidence window this study suggests a refinement of the "official" climate sensitivity to 2.6 - 3.9C as opposed to the current range of 1.5 - 4.5C. Earth Energy Imbalance Schuckmann 2020: Heat stored in the Earth system: where does the energy go? Summary: The EEI is +0.87 +- 0.12 W/m^2 and is increasing. 1% goes into the atmosphere. 4% goes into the cryosphere. 6% goes into the land. 89% goes into the oceans. To pull the EEI back to 0 would require a change in CO2 concentration from 410 ppm to 353 ppm. For those that don't know EEI is the amount of forcing still needing to be equilibriated via an increase in temperature. It should not be confused with effective radiative forcing (ERF) which is the cumulative forcing after fast feedbacks (like water vapor, etc.) have played out. EEI goes to zero after the climate system fully equilibrates to the ERF. This publication says the following of EEI.."This simple number, EEI, is the most fundamental metric that the scientific community and public must be aware of as the measure of how well the world is doing in the task of bringing climate change under control."

Carbon Brief has a couple of excellent articles as well. https://www.carbonbrief.org/qa-how-do-climate-models-work https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained

Atlantic Meridional Overturning Circulation. If ocean heat transport moving north increases/decreases then it only make sense that the atmospheric temperature response increases/decreases in the North Atlantic region as well. I'm having a hard time envisioning a mechanism by which the accumulation/depletion of a significant portion of heat in a particular region would not produce effects like the MWP and LIA. If anything the MWP and LIA are a testament to the fact that regions are more responsive to climate shifts as compared to the global scale. And we already know that the Earth is quite amendable to global scale climate shifts...a la...the PETM, other ETMx events, the glacial cycles, etc. Given the right nudge the Earth seems quite willing to undergo dramatic climate shifts that are magnified on regional scales. And we are giving the planet quite the nudge today; unprecedented at least through the Holocene.

People wonder why Dr. Spencer's reputation and legitimacy is being questioned. Well here you go... http://www.drroyspencer.com/2020/09/climate-hustle-2-premieres-this-evening-at-8-p-m/

In 1997 he put the 2xCO2 sensitivity at 0.3-0.5C (see Lindzen 1997). There is some equivocation in the paper above, but it sounds like he is now entertaining a value on the lower end of the IPCC range at about 1.5C. The rate at which his estimates have increased is about +0.45C/decade...faster than the actual warming rate. I will say that at least Lindzen proposes a legitimate hypothesis for supporting lower climate sensitivities...the iris effect. The question is...does Earth really have a mechanism like this that makes it resistant to climate change? The paleoclimate record seems to suggest that the Earth is quite amendable to large changes given the proper nudge and with each passing decade in the contemporary warming era we are constraining the lower the bound of sensitivities to higher and higher values. Each passing decade of warming is making Lindzen's iris effect and change resistant hypothesis appear less and less likely. BTW...A recent comprehensive style study puts the range at 2.6-3.9C and 2.3-4.7C for 1-sigma and 2-sigma confidence respectively (see Sherwood 2020 and free). Also, I just want to say that I do not condone the tone and rhetoric used against you. I do not think you are a liar, troll, or Russian (not that nationality matters). And while I do not think the body evidence supports the position you advocate for I still think you've handled yourself respectfully nonetheless. I think if you and I sat down for a beer (or coffee) we'd probably get along just fine. I still think you're wrong about climate science though

Here is Dr. Alley's data: ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/gisp2/isotopes/gisp2_temp_accum_alley2000.txt Notice that the rate of warming today is unprecedented during the Holocene per Dr. Alley. The warming rate is so high today that it smashes the record from that noticeable blip from 8210-7820 that admittedly looks pretty robust at first glance. The problem is that the x-axis on this specific graph has non-linear scaling (who does that?) and it is missing the contemporary warming era. This graph ends in 1855...a full 165 years in the past. If you stich on the last 165 years of warming we are at least close to the peaks of the Minoan warm period and Holocene climate optimum in the best case and may have already popped the bounds of the y-axis in the worst case. BTW...here's a good lecture by Dr. Alley if you want to hear his take on the science.

I was going through my Parametrization Schemes book by David Stensrud to try and better understand how GCMs handle water vapor. It's pretty complicated. Quite frankly...over my head. But I do see that many of the physics modules use the Clausius-Clapeyron and other relationships. I think if there was anything wrong with our understanding of the thermodynamic nature of water vapor and associated feedbacks then it would have had an impairing effect on weather forecasts and would have been noticed long ago.

It is an amplifying feedback only. Since it does not force the climate I believe its amplifying effect is included in the C per W/m^2 part. The best analogy here are audio amplifiers. There are many agents that can catalyze a sound: drummer, guitarist, vocalist, etc. Each agent has their effect amplified by the same fixed amount, but the amplifier does not catalyze a sound on its own. The final noise level is a product of all individually amplified agents. Like with climate forcing agents it is convenient to quantify each noise forcing agent's contribution to the final sound output with the amplification factor already included.

How do you know General Relativity is correct? It is but a model of gravity. I believe that model is correct because it makes predictions of the precession of Mercury, time dilation of the GPS satellites, etc. that match observations within a reasonable margin of error. It is the same with the consensus theory of climate change. We believe the models are correct because they makes predictions that match observations within a reasonable margin of error. The natural-only or natural-mostly hypothesis makes predictions that deviate from observations by an unacceptable amount. The predictions are bad enough that they are off by an order of magnitude in predicting the EEI at least after 1950. In fact, they are so bad that they often cannot even predict the sign of the temperature change. Ya know...I'm skeptical of quantum mechanics and general relativity. Between the two they make what is often called the worst prediction in all of science regarding the cosmological constant. One or both of them is wrong by an amount so astonishing that we cannot even fathom it. But I still think QM and GR are useful and they are certainly better than nothing. The same can be said for the consensus theory of climate change. The models deployed are not perfect. They never will be. But they are undeniably useful and represent the best of what we have. It is okay to advocate for natural-only or natural-mostly models for the post WWII era. But to convince cranky skeptics like me to use them in favor of what is already available you have to demonstrate that 1) they are testable, 2) they make useful predictions, and 3) that they match reality better and/or are simpler in answering certain questions than what we already have.

I'm sure there has been a positive energy imbalance since 1800. The question is...what contributing factors caused the imbalance and how and when did their contributions ebb and flow? Some of the warming is natural especially prior to 1950. But after 1950 the net of all naturally modulated factors is far too low to explain the warming. It is so low, in fact, that naturally modulated factors may even be working to lower the EEI and thus put a cooling pressure on the climate. But when scientists consider anthroprogenically modulated factors along with the naturally modulated factors we get a reasonable match between expectation and observation. Anthroprogenic factors dwarf natural factors by about an order of magnitude...at least after 1950 when the warming became most acute. We've already had about 1.2C of warming since about 1850 and that's with about 1.5xCO2 and a +0.87 W/m^2 EEI that still hasn't equilibrated yet. Even if CO2 concentrations stabilized at the current 410 ppm level we still have several tenths of degree C to warm to work off that EEI and measure the ECS. So the question for you is this...if CO2 is only 1/3'ish effective as the consensus then where are you going to get the other 2/3 energy required to produce the amount of warming we observe?

Even Arrhenius understood the water vapor feedback and considered it in his primitive 1896 model. The ERA5 850mb data is intriguing indeed. I'd like to learn more about this decline. I'm not as ready and willing to chalk up things I don't understand to the inadequacies of the data. ERA5 is considered to be the best of the best. I'll see if I can do some digging on that 850 mb specific humidity decline from 1980-2000.

I'm just being pedantic here...the word "runaway" actually has special meaning in climate science. It is unlikely that Earth can achieve "runaway" greenhouse warming in the strictest sense. This is due primarily because the Simpson-Nakajima limit (and related Komabayashi-Ingersoll limit) on Earth is sufficiently high and because the primary feedback driver would be water vapor which is a condensing gas. The SN limit is the clamp on the outgoing longwave radiation (OLR) in a saturated atmosphere. The SN limit is thus the point at which absorbed solar radiation (ASR) must exceed OLR to bootstrap the runaway phase. I believe the SN limit is around 290 W/m^2, but OLR is currently 240 W/m^2 so there is ~50 W/m/2 of buffer before the SN limit is even reached. The consensus seems to be that there aren't enough non-condensing GHGs to get us anywhere close to this SN limit. However, a "moist" greenhouse has a lower limit. The moist greenhouse is characterized by a state at which the Earth's cold trap near the stratosphere can no longer stop the leaching of water vapor (which would typically condense out and drop back to the surface) into the stratosphere where it would then slowly deplete to space. Aside from the obvious fact that the atmosphere would be in a perpetual moist state this scenario would likely have other undesirable consequences like the destruction of the ozone layer and evaporation of the oceans. There is still considerable debate regarding whether Earth can support a moist greenhouse state. Assuming that it can the estimates I've seen of the CO2 required to bootstrap this process could be 10,000 ppm (give or take) given the current solar output. I will add the caveat that there are notable scientists (namely James Hansen) that aren't quite so confident that a "runaway" phase is not achievable. Here is a pretty good summary style publication that explains things better than I can. Goldblatt & Waston 2012: The runaway greenhouse: implications for future climate change, geoengineering and planetary atmospheres

That first paper says the water vapor feedback is strongly positive at about 2 W/m^2 per C. They end with "The existence of a strong and positive water‐vapor feedback means that projected business‐as‐usual greenhouse‐gas emissions over the next century are virtually guaranteed to produce warming of several degrees Celsius." Unfortunately the second paper is paywalled and I cannot find an open copy. I will say that I took the opportunity to read other publications by the lead author Dr. Sullivan. So far I've not seen anything that leads me to believe he doubts the water vapor feedback or GHGs contribution to warming.

UKMET says it may do something as well.

I agree. I think the aerosol hypothesis can be at least partially falsified by the OHC data. If aerosol increases were playing a significant factor we should have observed a decrease in the EEI and thus a reduction in OHC uptake. I think ENSO or other natural cycles better explain the atmospheric warming hiatus. Cryosphere declines also seem (to me) like a reasonable hypothesis for explaining the hiatus as well, but I haven't seen any literature that I can use to support it at the moment.

Right. No disagreement with the uptake of carbon by the ocean. But oceanic heat content (OHC) is a measure of heat uptake; not carbon uptake. It is directly related to the Earth Energy Imbalance (EEI). The EEI is dispatched into the ocean, air, ice, and land. The hiatus period is characterized by a general pause in heat uptake by the air despite heat uptake proceeding in the ocean. ENSO cycles likely played a role in the waning of the transfer of heat to the atmosphere. I was wondering how much of a role the cryosphere played in that as well. The puzzle is that increased aerosol loading brings down the EEI and thus should have reduced the rate of OHC increases. This is what makes me think aerosol loading may not be as important in explaining the hiatus period as some have hypothesized. @donsutherland1 found the following study which estimates that value at 89%. It appears to be one of those comprehensive style studies that attempts to provide the best estimate of EEI with the lowest uncertainty from the vast body of evidence available at the time of publication. Schuckmann et al. 2020: Heat Stored in the Earth System: where does the energy go? There are some big names in the author list and the bibliography at the end is huge. Keeping in mind that I'm but an amateur...this looks like something that will be heavily relied upon in the upcoming IPCC AR6 report.

Aerosol loading probably explains part of the warming hiatus. One thing that puzzles me is that oceanic heat content kept marching upward. It makes me wonder if the typical transfer of heat into the atmosphere waned during the period only to be taken up by the cryosphere. The post 1998 El Nino period was about the time where cryosphere declines became most acute.

That is a reasonable, rational, and well thought out post @skierinvermont It is important for people to understand that while many of us do advocate for AGW in general we're not all blinded by the fact that there are many things regarding the state of climate science that lead to frustratingly large uncertainty envelopes, annoyingly large spreads between prediction and observation, poor explanations of past observations, etc. I'm always open to discussing climate science's shortcomings. I have a list of pet peeves, concerns, and questions myself actually. My only request is that all discussions be done with an evidence centric approach. Claims on either side of the spectrum should be reasoned, measured, and backed by peer reviewed literature, repeatable experimentation, and verifiable observations. Unsubstantiated claims of fraud, manipulation, conspiracy, the general "nuh-uh" class of arguments, and hostility towards science in general are unsatisfying and unconvincing and do little if anything to move the discipline forward. For those contrarians out there...don't hear what I'm not saying. I'm not saying that criticism isn't welcome. What I'm saying is that if that criticism does not provide a means or path by which our models of the climate system can improve then what good is it really? If you ask me to abandon a model, which I fully understand isn't perfect, for no model at all then I'm going to push back. But if you instead accompany your criticism or insight with a statement of "here's how your model could be better" then I'll be all over it.

How much stronger? How much slower? How much wetter? Those are tough questions to answer on an individual storm basis. I will say that the Siberian heat wave this summer was made vastly more likely by global warming than it would have been otherwise. The shifting of the bell curve and skewing of the probabilities was extreme enough in this particular case that it's hard to claim that the event was merely a fluke. I'm just saying in general it's a stretch to attribute all of an individual event's magnitude on global warming. Even if Harvey were a bit faster or had slightly less water content to work with Houston still would have experienced catastrophic flooding. In that regard I don't think global warming tipped the scale that caused Harvey to transition from a nuance to a catastrophe. But it may have shortened the recurrence interval between storms of similar magnitude.