Climate Change Banter

[ORH_wxman]

 

Do we have any long term statistics on heatwaves in the U.S.? How much has the frequency of extreme heatwaves increased since AGW began?

 

The first portion of this paper gives a look at heat waves and cold waves in the U.S.

http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-12-00066.1

[tamarack]

So: 1) you are calling me a denier, which shows complete ignorance of my views on this issue, and 2) as much as you might like the world to be a simple black/white place, there are nuances and shades of gray. This is a complex issue, with complex personal views on it. That's not me "framing the discussion like a politician", that's just reality.

 

I'm sure it's easier to just frame everything as "denier vs. AGW believer".

 

Given tacoman's posts on CC, A-L-E-K's "denier" post would seem to place all the 3s,4s,5s in the denier camp, which would be classic "shift the middle" tactics,

[Snow_Miser]

I'm a 4 on Tacoman's list. I don't deny warming, and I don't deny a human component to that warming. However, the more important questions are,

• How much of the warming is due to the Anthropogenic Forcing?
• What implications might this have for climate sensitivity?
• How much warmer will we get?
• To what degree can we attribute climate change to extreme weather?
• Why do GCMs overestimate temperature changes over the last 20 years?
• Why can't GCMs accurately simulate the early-20th Century warming trend?

A lot of these are open questions that have not been resolved. The "pause" in the warming has shown how poor our understanding of the climate system is. To claim that the science is setlled and we know all, is ignorant and arrogant.

[Isotherm]

We've been over this before in other threads, but I think most people fall into the following categories (NOTE: there are more than two!):

 

1. Extreme alarmist - AGW is absolutely going to be catastrophic, if mankind doesn't act now, civilization as we know it is over, the IPCC is way too conservative, any and all extreme weather events can probably be pinned on AGW

 

2. Alarmist - AGW is a very real and present danger, always looking for a way to connect AGW to natural disasters, favors the higher end IPCC estimates, downplays other factors that might be playing a role in things like Arctic sea ice

 

3. Middle of the roader - Believes that AGW is definitely real and probably poses significant risks, favors middle or lower end IPCC estimates, not convinced AGW will be catastrophic, looks for other factors besides just AGW at play in climate/weather trends

 

4. Skeptic - Thinks that AGW probably exists on some level but is skeptical that it is significant or poses any risk, places much more emphasis on natural factors, believes the IPCC is mostly out to lunch

 

5. Denier - Believes only in natural climate change, AGW is a hoax

 

 

I would say 90% of the people on this forum fall somewhere between #2 and #4.

 

 

I have a different categorization and think you can create more groups.

 

 

 

Extreme alarmist: 100% human induced

Alarmist: > 75% human induced, < 25% natural

 

Denier: 100% Natural

Skeptic: > 75% natural, < 25% human induced

 

Middle of the road can be broken down IMO.

 

High end Middle of the roader: 25-50% naturally induced, 50-75% human induced

Central Middle of roader: Around a 50/50 split for natural and human factors

Low end Middle of roader: 25-50% human induced, 50-75% naturally induced

 

 

So 7 categories total.

[Snow_Miser]

I have a different categorization and think you can create more groups.

Extreme alarmist: 100% human induced

Alarmist: > 75% human induced, < 25% natural

Denier: 100% Natural

Skeptic: > 75% natural, < 25% human induced

Middle of the road can be broken down IMO.

High end Middle of the roader: 25-50% naturally induced, 50-75% human induced

Central Middle of roader: Around a 50/50 split for natural and human factors

Low end Middle of roader: 25-50% human induced, 50-75% naturally induced

So 7 categories total.

The extreme alarmist claims that there is actually more warming than observed that is due to humans, but some of that additional warming is being masked by anthropogenic aerosols.

[ORH_wxman]

The extreme alarmist claims that there is actually more warming than observed that is due to humans, but some of that additional warming is being masked by anthropogenic aerosols.

 

 

This doesn't make any sense...if the aerosols are anthropogenic

[Snow_Miser]

This doesn't make any sense...if the aerosols are anthropogenic

 

I was on my phone typing that, so the grammar was likely not great.

 

What I am saying is that the extreme alarmist makes a claim that more warming has occurred than observed. This is because anthropogenic aerosols have masked some of the warming. Because anthropogenic aerosols have masked warming, this means that more warming should have occurred without the presence of anthropogenic aerosols, and thus, climate sensitivity is higher.  

[skierinvermont]

? 

 

I've never gotten any indication from Hoerling's papers that he has an agenda. If he does, he seems to be good at rounding up sympathetic coauthors from a variety of respected institutions to go along with his campaign. Romm ripped him apart as a useful idiot or denier for criticizing Hansen's drought statements in the media, but if he's an idiot and has many of his papers debunked after they're published, it's a scary thought that he's gotten so many through peer review.

 

From what I can tell, he's only been attacked for the Hansen drought incident. I don't think that meets the definition of "schtick". 

 

I'm not saying his science is wrong, but he ripped into Hansen pretty hard (probably a little harder than was scientifically deserved given the great uncertainty in this field) and the way he went about it was beyond normal criticism you see between scientists. For example, when Trenberth criticized Hoerlings studies, he wasn't quite so belligerent about it. i think Hoerling and his co-authors are making good faith efforts to interpret evidence objectively and they are doing good science.

 

I'm just saying that if anybody could find evidence that central U.S. drought would not increase, it would be Hoerling, but even he concludes that there will be a modest increase. Other studies find a larger increase. We shouldn't be so quick to assume either party is right. And we definitely should not ignore the overall probability that drought in the central U.S. will increase with AGW. Rising temperature will cause an increase in drought without a pretty solid increase in rainfall to go with it, which does not appear likely. 

[skierinvermont]

Yes in the future...but not currently. Or at least there has been no current trend frequency or intensity of central U.S. drought over the past century. If anything, there has been a slight decrease.

 

Another paper this summer on the 2012 drought:

 

http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-13-00055.1

 

 

Basically concluding that natural variability in SSTs combined with shorter term variability in weather was responsible for the central U.S. drought. Little if any evidence was found that increased GHGs were a significant factor in the drought. Which supports my original assessment of media blaming the 2012 drought on climate change is ignorant and/or misleading.

 

At the very least, the media who mention climate change and the 2012 drought should inform of how likely small an impact it had on it (if it did at all in any significant manner) and how uncertain the attribution studies are. Most point toward southwest U.S. drought increasing with only modest increases in the central US, and most are future projections and do not reflect current longterm trends.

 

The long-term trend on drought in the central U.S. is close to zero. The trend during the period of fastest warming, since 1970, is strongly towards more drought. Without the warming in this period we would probably be closer to 1940s-1960s drought frequency and the long-term trend due to the massive 1930s aberration (which might also have had something to do with land-use feedbacks) would be even more negative.

 

The effect on frequency and intensity thus far is likely quite small, given the increase after much more warming may still only be modest according to some (Hoerling and peers). Reporting on the issue should always stress this using the 'loaded dice' paradigm instead of blaming the event entirely on AGW. Actually "loaded" might still be too strong of a word in this case, but you get the idea and I think we pretty much agree. 

 

I just don't agree blaming the drought on AGW is ignorant or false. Depending on exactly how the blame is ascribed, it would be more like an exaggeration.

[skierinvermont]

The extreme alarmist claims that there is actually more warming than observed that is due to humans, but some of that additional warming is being masked by anthropogenic aerosols.

 

How is acknowledging the obvious fact that human aerosol emissions have caused very large negative forcing and cooling "extreme" or "alarmist?"

 

Do you deny that a significant fraction of the sun's SW radiation does not even reach the surface of the earth anymore because of these aerosols, a phenomenon known as "global dimming" which has gone so far as to even slow the growth of plant life on earth?

 

You cannot simply wish these facts away. It is yet another nail in the coffin of denialism. The earth would have warmed far more if we were not literally blocking out the sun with aerosols. 

[tacoman25]

I'm not saying his science is wrong, but he ripped into Hansen pretty hard (probably a little harder than was scientifically deserved given the great uncertainty in this field) and the way he went about it was beyond normal criticism you see between scientists. For example, when Trenberth criticized Hoerlings studies, he wasn't quite so belligerent about it. i think Hoerling and his co-authors are making good faith efforts to interpret evidence objectively and they are doing good science.

 

I'm just saying that if anybody could find evidence that central U.S. drought would not increase, it would be Hoerling, but even he concludes that there will be a modest increase. Other studies find a larger increase. We shouldn't be so quick to assume either party is right. And we definitely should not ignore the overall probability that drought in the central U.S. will increase with AGW. Rising temperature will cause an increase in drought without a pretty solid increase in rainfall to go with it, which does not appear likely. 

 

I think we also have to keep in mind that at this point, this is all theoretical, since the observed trends within the warming time frame have not yielded these results yet.

 

EDIT: I see your follow up post now, and while there could be different reasons for the shorter term increase in U.S. drought (although not in the central U.S. as was primarily being discussed here), the long term trend during warming is still close to zero as you said.

[skierinvermont]

I think we also have to keep in mind that at this point, this is all theoretical, since the observed trends within the warming time frame have not yielded these results yet.

 

True, which is why "low to medium confidence" in these conclusions should be stressed. As I pointed out above though, the period since 1970 has seen a strong increase in droughts concurrent with the period of fastest warming, so we may be beginning to witness the increase. If climate models are to be believed, drought frequency/intensity since 1980 would probably have been slightly lower.

[f2tornado]

The increased temperature trend likely outweighs any small increase in precipitation. At least this is the conclusion of most climate models, climate experts, and the IPCC which gives low to medium confidence of increased drought risk in the central U.S. I see little reason to disagree with this conclusion. Even Hoerling whose "schtick" seems to be trying to one-up and attack Hansen and others forecasting dire consequences acknowledges CO2 forcing probably drives a "modest" increase in drought risk.

 

There has been no increased temperature trend east on the Rockies over the last 100+ years. There has actually been surface cooling. Temperature anomalies were pushing -1C until the past couple decades yet are still around -0.3C. These authors blame aerosols http://www.sciencedaily.com/releases/2012/04/120426155117.htm but I think the the much larger player is land use change which is strongly supported here: http://www.bama.ua.edu/~jcsenkbeil/appclim/mahmood%20et%20al.pdf All that irrigated corn might be the impetus for the increasing trend of heavy rain events in the Plains on east as well. Some quotes about it from researchers here: http://usatoday30.usatoday.com/weather/news/2007-07-23-crops_N.htm

[tacoman25]

How is acknowledging the obvious fact that human aerosol emissions have caused very large negative forcing and cooling "extreme" or "alarmist?"

 

Do you deny that a significant fraction of the sun's SW radiation does not even reach the surface of the earth anymore because of these aerosols, a phenomenon known as "global dimming" which has gone so far as to even slow the growth of plant life on earth?

 

You cannot simply wish these facts away. It is yet another nail in the coffin of denialism. The earth would have warmed far more if we were not literally blocking out the sun with aerosols. 

 

As has been discussed before, global temp trends do not match long term global aerosol concentration as well as other significant factors like the PDO. To say the earth would have warmed "far more" without aerosols is not supportable, in my opinion. Compare current aerosol concentrations to those during Pinatubo.

[tacoman25]

There has been no increased temperature trend east on the Rockies over the last 100+ years. There has actually been surface cooling. Temperature anomalies were pushing -1C until the past couple decades yet are still around -0.3C. These authors blame aerosols http://www.sciencedaily.com/releases/2012/04/120426155117.htm but I think the the much larger player is land use change which is strongly supported here: http://www.bama.ua.edu/~jcsenkbeil/appclim/mahmood%20et%20al.pdf All that irrigated corn might be the impetus for the increasing trend of heavy rain events in the Plains on east as well. Some quotes about it from researchers here: http://usatoday30.usatoday.com/weather/news/2007-07-23-crops_N.htm

 

Doesn't irrigated corn also lead to much higher dew points, which would hold down high temps in many places, but also hold up low temps?

[skierinvermont]

Climate sensitivity (warming per doubling of Co2 concentration) is probably the best way to break down peoples opinions into categories. Since you've all used loaded language in you labels, I'll continue with the tradition:

 

1. Extreme alarmist: believes or focuses solely on the high end or even higher than the scientifically accepted climate sensitivity (>4C). Focuses solely on the worst case consequences and denies or ignores all benefits.

 

2. Alarmist: believes or focuses on the high end of scientifically accepted climate sensitivity (3-4.5C). Focuses usually on the worst case consequences and acknowledges few of the benefits. 

 

3. Scientifically grounded: acknowledges the full range of scientifically accepted climate sensitivity (1.5-4.5C). May lean slightly one direction or another within that range based on a good-faith effort to objectively interpret the evidence with assistance of peer-reviewed literature, but acknowledges all of the uncertainty and the lack of concrete evidence. Has a balanced understanding and acceptance of the various consequences and benefits and the evidence that on net the consequences will be negative. A moderate to high level of mitigation is warranted, and adaptation cannot be relied upon solely.

 

4. The biased/arrogant/confused/misled lukewarmer category: believes or focuses solely on the low end or slightly below scientifically accepted climate sensitivity (1-2C). Ignores the evidence that climate sensitivity is probably higher than 2C. Often an undue focus on the benefits of AGW, or downplaying of the consequences. Possibly believes that the benefits of AGW will outweigh the consequences.

 

5. Denier/stupid: believes in a climate sensitivity below the scientifically accepted range (0-1.5C). Usually focuses on the benefits of warming and downplays the consequences. Probably believes that the benefits of warming will outweigh the consequences.

 

6. Extreme denier/stupid: believes CO2 has little to no warming effect. If warming did occur, it would be good.

[skierinvermont]

As has been discussed before, global temp trends do not match long term global aerosol concentration as well as other significant factors like the PDO. To say the earth would have warmed "far more" without aerosols is not supportable, in my opinion. Compare current aerosol concentrations to those during Pinatubo.

 

It doesn't matter whether the temperature correlates or not. There are a million factors interacting all at the same time, so expecting any single one or two of them (say aerosols+CO2) to correlate perfectly to temperatures is stupid. The endless teasing out and eyeball comparison of charts that goes on here and across the blogosphere is completely scientifically useless. Aerosols are literally blocking out the sun. This causes cooling. This is an indisputable fact.

 

Aerosols have increased rapidly throughout the last century, until 1970 when their rise slowed. The period of fastest rise was the 1950s and 1960s relative to CO2 concentration, which may have had something to do with the lack of warming in this period. There could have been 100 other factors at play as well. 

 

We may never know exactly how all of these factors interacted in the 1950s and 1960s. But it remains indisputable that the blocking out of the sun by aerosols this century has caused major cooling. 

[Snow_Miser]

How is acknowledging the obvious fact that human aerosol emissions have caused very large negative forcing and cooling "extreme" or "alarmist?"

 

Do you deny that a significant fraction of the sun's SW radiation does not even reach the surface of the earth anymore because of these aerosols, a phenomenon known as "global dimming" which has gone so far as to even slow the growth of plant life on earth?

 

You cannot simply wish these facts away. It is yet another nail in the coffin of denialism. The earth would have warmed far more if we were not literally blocking out the sun with aerosols. 

 

I'm not saying that human induced aerosols are not important, but saying that the contribution of Greenhouse Gases to recent warming trends is over 100% because aerosols are masking some of that warming is most certainly alarmist. In addition, aerosol forcing isn't understood all that well, and is probably one of the largest uncertainties in Climate Science. Aerosols have an impact, but my stance is that anthropogenic aerosols are likely not as significant as you assert.

 

For example, the average lifespan of an anthropogenic aerosol is quite a bit less than the average lifespan of a molecule of CO2, and thus the effects from aerosols are likely to be much more local than the effects from CO2. Most of the anthropogenic aerosols are found in the Northern Hemisphere as a result of this local effect.

 

 

Yet, the Northern Hemisphere has actually warmed faster than the Southern Hemisphere according to both satellite measurements and surface temperature measurements. While this doesn't disprove an effect from aerosols, it suggests that the forcing from aerosols may lean more towards 0 in the IPCC's aerosol error range.

 

Also, do you deny that a significant increase in SW radiation has been absorbed at the Earth's surface over the last 30 or so years? This would seem to suggest that anthropogenic aerosols have not masked much warming during the late-20th Century warm period.

 

http://www.atmos-chem-phys.net/13/8505/2013/acp-13-8505-2013.html

 

From the paper:

 

"The 340 nm LER is highly correlated with cloud and aerosol cover because of the low surface reflectivity of the land and oceans (typically 2 to 6 RU, reflectivity units, where 1 RU = 0.01 = 1.0%) relative to the much higher reflectivity of clouds plus nonabsorbing aerosols (typically 10 to 90 RU). Because of the nearly constant seasonal and long-term 340 nm surface reflectivity in areas without snow and ice, the 340 nm LER can be used to estimate changes in cloud plus aerosol amount associated with seasonal and interannual variability and decadal climate change. The annual motion of the Intertropical Convergence Zone (ITCZ), episodic El Niño Southern Oscillation (ENSO), and latitude-dependent seasonal cycles are apparent in the LER time series. LER trend estimates from 5° zonal average and from 2° × 5° , latitude × longitude, time series show that there has been a global net decrease in 340 nm cloud plus aerosol reflectivity. The decrease in cos²(latitude) weighted average LER from 60° S to 60° N is 0.79 ± 0.03 RU over 33 yr, corresponding to a 3.6 ± 0.2% decrease in LER. Applying a 3.6% cloud reflectivity perturbation to the shortwave energy balance partitioning given by Trenberth et al. (2009) corresponds to an increase of 2.7 W m⁻² of solar energy reaching the Earth's surface and an increase of 1.4% or 2.3 W m⁻² absorbed by the surface, which is partially offset by increased longwave cooling to space."

[Jonger]

Most warming from 1970 to 1995 was almost completely as a result of the Clean Air Act. If CO2 is the main culprit, its warming would have been spread more evenly without the aerosols, instead it was seen after the fog cleared.

 

I wouldn't be surprised if we get a second warming bounce due to particulate filters being mainstream since 2007 in many of the worlds diesel burning engines, a huge source of aerosol production. 

 

Disclaimer: I'm not disputing CO2 being a GHG, I'm just agreeing with the surfer theory of global dimming during the period of 1945-1990s.

[Snow_Miser]

3. Scientifically grounded: acknowledges the full range of scientifically accepted climate sensitivity (1.5-4.5C). May lean slightly one direction or another within that range based on a good-faith effort to objectively interpret the evidence with assistance of peer-reviewed literature, but acknowledges all of the uncertainty and the lack of concrete evidence. Has a balanced understanding and acceptance of the various consequences and benefits and the evidence that on net the consequences will be negative. A moderate to high level of mitigation is warranted, and adaptation cannot be relied upon solely.

 

That's interesting. Only a few months ago, you were saying that the accepted climate sensitivity was 2-4.5 Degrees C, and that anyone who would dare put out sensitivity values lower than 2 Degrees C were, according to your definition, a biased ignoramus. Unfortunately for your definition, there are actually a lot of recent peer reviewed studies focusing on sensitivity values between 1-2 Degrees C. Skeie et al. is one of the most recent examples. Ring et al. and Asten are also recent examples as well, finding sensitivities of 1.77 Degrees C and 1.1 Degrees C respectively. Now you say that the accepted values is now 1.5-4.5 Degrees C, which I guess had to do with the latest IPCC report lowering the lower bound of the sensitivity from 2 to 1.5 Degrees C, which is a nod to the with many recent studies finding an ECS of 1-2 Degrees C.

[Snow_Miser]

Most warming from 1970 to 1995 was almost completely as a result of the Clean Air Act. If CO2 is the main culprit, its warming would have been spread more evenly without the aerosols, instead it was seen after the fog cleared.

 

I wouldn't be surprised if we get a second warming bounce due to particulate filters being mainstream since 2007 in many of the worlds diesel burning engines, a huge source of aerosol production. 

 

Part of the Global Brightening trend since 1979 may also have to with the +PDO. There was some rough speculation by Roy Spencer that the mechanism for which the PDO can cause warming on multidecadal timeframes is through a decrease in Cloud Cover.

[Jonger]

Part of the Global Brightening trend since 1979 may also have to with the +PDO. There was some rough speculation by Roy Spencer that the mechanism for which the PDO can cause warming on multidecadal timeframes is through a decrease in Cloud Cover.

 

I think the land temperature "Pause" or "Decline" is just an side effect of the aerosol reduction plateau, the bulk of the brightening has taken place...

 

Like I posted previously, there might be a second bounce because of a major reduction of diesel particulate matter.

[skierinvermont]

That's interesting. Only a few months ago, you were saying that the accepted climate sensitivity was 2-4.5 Degrees C, and that anyone who would dare put out sensitivity values lower than 2 Degrees C were, according to your definition, a biased ignoramus. Unfortunately for your definition, there are actually a lot of recent peer reviewed studies focusing on sensitivity values between 1-2 Degrees C. Skeie et al. is one of the most recent examples. Ring et al. and Asten are also recent examples as well, finding sensitivities of 1.77 Degrees C and 1.1 Degrees C respectively. Now you say that the accepted values is now 1.5-4.5 Degrees C, which I guess had to do with the latest IPCC report lowering the lower bound of the sensitivity from 2 to 1.5 Degrees C, which is a nod to the with many recent studies finding an ECS of 1-2 Degrees C.

 

Please show these posts. I've been aware of these studies for quite some time, and have been suggesting that there is some possibility for an ECS <2C for quite some time. 

 

I have said, and still would say, that anybody saying that climate sensitivity is probably <2C is biased and not looking at the full body of evidence. Personally I think the most likely range is 2-3C, but there's no evidence that can rule out anywhere from 1.5-4.5C.

[skierinvermont]

Most warming from 1970 to 1995 was almost completely as a result of the Clean Air Act. If CO2 is the main culprit, its warming would have been spread more evenly without the aerosols, instead it was seen after the fog cleared.

 

 

That's not true, if you think about it. There was a lot of CO2 warming built up in the industrialized areas by 1980. With the brightening that occurred, this CO2 warming simply became apparent. Many of the areas that warmed pre-1980 because of CO2 and their lack of industrialization are now seeing industrialization and a pause in warming. 

[skierinvermont]

To introduce some facts into this discussion, the IPCC gives the RF from well-mixed GHGs as 2.8+/-.3Wm/2. The RF for aerosols is -.7W/m2, and the AF is -.9W/m2 (-.3 to -1.5W/m2).

 

They say there is "high confidence" that aerosols have offset a substantial portion of GHG forcing. Best guess appears to be 25%. 

[Snow_Miser]

Please show these posts. I've been aware of these studies for quite some time, and have been suggesting that there is some possibility for an ECS <2C for quite some time. 

 

I have said, and still would say, that anybody saying that climate sensitivity is probably <2C is probably biased and not looking at the full body of evidence.

 

I linked you to one of your previous posts suggesting the accepted sensitivity to be 3 Degrees C with a lower bound of 2 Degrees C. The models, which have a ECS of 3.2 Degrees C do generally simulate too much cooling when compared to observations during cooling events like Mt. Pinatubo. In addition, the models underestimate the early-20th Century temperature trend, which suggests they are too sensitive to changes in radiative forcing. In addition, in the GCMs, with just changes in Water Vapor, Lapse Rate, and Surface Albedo only produce an ECS of 1.9 Degrees C. This means that if the Cloud Feedback is negative, ECS can easily be lower than 2 Degrees C.

 

 

A lot of recent studies, as I've said have gradually been shifting away from the 2-4.5 Degree C figure. Quite a few have high probability densities between 1-2 Degrees C, with some even lower than that.

[ORH_wxman]

That's not true, if you think about it. There was a lot of CO2 warming built up in the industrialized areas by 1980. With the brightening that occurred, this CO2 warming simply became apparent. Many of the areas that warmed pre-1980 because of CO2 and their lack of industrialization are now seeing industrialization and a pause in warming. 

 

 

Actually the largest regions that lack warming are the already-industrialized regions...and the lack of warming is almost exclusively coming from a cooling trend in winter in these regions.

 

http://web.mit.edu/jlcohen/www/papers/Cohenetal_GRL2012.pdf

 

 

Essentially the opposite of what you'd expect.

[Jonger]

http://en.wikipedia.org/wiki/Pan_evaporation

 

This almost seals the deal on the global dimming hypothesis.

[Snow_Miser]

Actually the largest regions that lack warming are the already-industrialized regions...and the lack of warming is almost exclusively coming from a cooling trend in winter in these regions.

 

http://web.mit.edu/jlcohen/www/papers/Cohenetal_GRL2012.pdf

 

 

Essentially the opposite of what you'd expect.

 

To back up your point:

 

 

You can see how far off the models simulated the observed trends in Dec-Feb especially in recent years.

 

 

http://klimazwiebel.blogspot.com/2013/09/the-intriguing-stagnation.html

[f2tornado]

Doesn't irrigated corn also lead to much higher dew points, which would hold down high temps in many places, but also hold up low temps?

 

Correct. And these higher dewpoints increase SBCAPE (surface based convective available potential energy) for thunderstorms to potentially get juiced like Arod. The bulk of research suggests nocturnal temperatures are increasing at roughly twice the rate of afternoon temperatures on a global scale. This doesn't seem to apply to the irrigated Plains perhaps because of the artificially high peak transpiration of plants occurs during the afternoon. Clouds may very well have some role as well but I don't have any paper on that subject at my immediate disposal. Whatever is resulting from AGW in the Plains isn't coming from positive temp anomalies in the region unless you ignore all the data prior to circa 1980.