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Tuesday, March 22, 2016

Correcting underestimated heat wave intensity - it's plants

Sou | 9:57 PM Go to the first of 8 comments. Add a comment

A new paper just published in the Nature open access journal Scientific Reports suggests that some plants may be exacerbating heat waves as climate change kicks in. As I read it, what happens is:

  • Certain plants will release less water in very hot weather, that means less evapotranspiration
  • Evapotranspiration has a cooling effect at the surface, because of the latent heat of evaporation (energy required to evaporate water comes from the surrounds, thus cooling the surface)
  • Because there is less water put out by some plants, there will be less of a cooling effect, therefore hotter days are even hotter than they would otherwise be.
The Australian research team, led by Jatin Kala, estimated that by the end of this century, that could add up to 5 °C to a heat wave day, and even more mid-century. From the press release the "biggest temperature changes were projected to occur over needleleaf forests, tundra and agricultural land used to grow crops".

In the paper the authors describe how heatwaves are expected to get more frequent and hotter and last longer in future, and how the land surface can affect them (my paras):
Future warming linked with increases in greenhouse gases is expected to increase the frequency, intensity and duration of heatwaves further5,12, particularly across the mid-latitudes including North America and Europe13,14. Heatwaves are associated with large-scale synoptic states15,16, which are influenced by modes of climate variability17.
However, it is now well established from observational18 and modelling studies19,20 that heatwaves are also strongly modulated by the land surface if the synoptic scale weather generates persistent anticyclonic patterns and the planetary boundary-layer strongly couples the land to the atmosphere over consecutive days21. Under these circumstances, heatwaves intensify as desiccated soils and a surface radiation balance dominated by the exchange of sensible heat is coupled with the boundary-layer to lead to events such as the “mega-heatwaves” experienced in Europe during 2003 and 201019,21.
Although the detailed role of the land surface on the exchange of water and energy during heatwaves remains uncertain22, there is evidence that capturing the detailed connection between the land and the atmosphere, and how soil moisture impacts the surface energy balance to moderate or intensify heat waves, is necessary to produce realistic simulations of these phenomena19.
Adjusting models to account for the lessening of transpiration improved the fit between models and observations in boreal regions, particularly over Eurasia in summer. The changed parametizations with reduced transpiration resulted in maximum temperatures up to 1 °C warmer (recent years).

Looking ahead, allowing for this reduced transpiration vs not, the biggest modeled differences are between 2040 and 2059, after which the difference decreases. The researchers put forward the idea that at high leaf temperatures, above 30 °C, which will be much more common in the second half of this century in these regions, "photosynthesis and stomatal conductance (and thus transpiration) are reduced due to photosynthetic inhibition (Fig. S2)".

The experiments indicated that allowing for leaf-level transpiration changes doesn't affect the duration or frequency of heat waves, but does affect the expected intensity. They'll be hotter in these regions. In some parts, namely western Europe, western Russia and eastern China, up 6 or 7 °C hotter. By the end of the century, even though the effect is reduced, some regions will still have heat extremes higher by around 5 °C because of this effect.

The lead author, Jatin Kala from Murdoch University, is quoted in the press release (at saying:
"These more detailed results are confronting but they help explain why many climate models have consistently underestimated the increase in the intensity of heatwaves and the rise in maximum temperatures when compared to observations."

How the dimwit deniers received this news

Anthony Watts seems to think this research is nonsense. He put forward two reasons, both illogical:

Anthony's not the brightest. Other than the above and his headline, Anthony had nothing to say except for his copy and paste of the press release. He didn't provide a link to the press release or to the paper itself. That's typical at WUWT.

From the WUWT comments

Marcus is the resident clown denier at WUWT, posting dozens of empty thoughts every day. His contribution was unrelated to the research, so not as funny as he probably thought:
March 21, 2016 at 3:24 pm
Well I guess, by their math, billions of Humans having sex every night must increase the temperature by at least 19 C…..

M Seward probably doesn't have a clue what the paper was about, but vents anyway:
March 21, 2016 at 3:28 pm
These clowns have thrown all caution to the wind now. They are basically on a war footing after the funding cutbacks to CSIRO and as they say ‘all’s fair in love and war’. Andy Pitman is fast approaching a Flannery rating of RED HOT RAVING FUNDAMENTALIST. Maybe he feels he is back in the pack after his colleague’s Antarctic Ship of Fools farcpedition and needs to put on a spurt, after all isn’t that what XSpurts do?

asybot says she or he is a farmer and just knows that CO2 has no impact on something or other:
March 21, 2016 at 10:47 pm
@Seth, as a farmer I have experienced heat waves on my fruit farms, plants react to heat by shutting down to save water, this rarely last for more than a few days and only during the day time. As such I, at those levels and the time frame involved, sincerely doubt CO2, in this context, has any long term, if any effect, on climate change as these people claim. 

dbakerber guessed correctly:
March 21, 2016 at 3:34 pm
I guess the concept of plants producing shade is irrelevant. 

Rob Potter confuses the temperature during heat waves with global mean surface temperature, and gets that wrong too:
March 21, 2016 at 3:34 pm
But the models don’t underestimate the warming – they overestimate. The whole study begins with a false premise. Junk. 

Tom Halla hasn't heard about the cooling effect of evaporation:
March 21, 2016 at 3:40 pm
I just don’t understand how variations in plant respiration increases local heating. I thought Australia was reducing finding for global warming research, anyway. This seems like a a lame attempt to tie plant biology to a funding source that is going away.

FJ Shepherd has limited "thought" capacity:
March 21, 2016 at 3:48 pm
FFS is all I can say.

Grady can only hold one simple thought at a time:
March 21, 2016 at 3:48 pm
If plants release less water there is more warming and a consequent increase in heat wave intensity.” … but wait a minute – don’t plants release the water as a vapor, not as a liquid? And doesn’t that water vapor enter the atmosphere? And isn’t water vapor a greenhouse gas?
Yup – another one from the “temperatures are always hotter no matter what” department :-(

ristvan misread the article, which was about less water coming from plants:
March 21, 2016 at 3:53 pm
This is how the CAGW house of cards collapses. In absurdity that even the uninitiated can grok.
As here. The satellite observed ‘greening’ (NVDI) is this stomata phenomenon for C3 plants, with an inverted result to this ridiculous Aussie paper. Bottom line in observational biology, C3 plants produce more photosynthesis biomass at higher CO2 while transpiring less water. 85% of all plants. C4 plants (15%) evolved to survive under low CO2 conditions the past 60 million or so years. Biology v. Warmunist climastrology. 

Okay, I've read enough. You get the picture. Anthony's fans almost to a man (very few women at WUWT) agree that climate science is a hoax. There were one or two reasonable comments that the mods overlooked, but most were of the sort you've come to expect from climate conspiracy theorists.

References and further reading

Jatin Kala, Martin G. De Kauwe, Andy J. Pitman, Belinda E. Medlyn, Ying-Ping Wang, Ruth Lorenz, Sarah E. Perkins-Kirkpatrick. "Impact of the representation of stomatal conductance on model projections of heatwave intensity." Scientific Reports, 2016; 6: 23418 DOI: 10.1038/srep23418 (open access)


  1. With apologies to Irving Berlin

  2. Sou my comment diisappeared

    1. Sorry about that PG. The blog was under spam attack so I turned on moderation for a bit. Back to normal again now.

  3. Is it just me or do the WUWT commentators seem even dumber today? I mean, cooling by evaporation is not exactly nuclear science.

    1. That's just short-term variation around a rising trend. It takes at least a fortnight's observations for the trend to become evident, and a month to see that it's accelerating.

  4. The plants, the trees, can modulate their evapotranspiration, but the soil in which they grow has no such options. The soil dries out, the tree fares badly, and after a few droughts, the whole tree dies.
    And then its transpiration factor drops to zero.

  5. Your 'News: Climate and Weather' links seem to have been hi-jacked, Sou.

    1. Thanks John. I've noticed it's been playing up the last few hours. It's done it before and righted itself, so I'll wait a bit before doing anything.


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