Tropical forests are approaching critical temperature thresholds | Hacker News Reader

Tropical forests are approaching critical temperature thresholds

nature.com

84 pointsmyshpa24 comments

"The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages approximately 46.7 °C"

I know the tropics are hot, but the idea of accepting ~45c as "average" sounds insane.

It's the average temperature at which trees start to die, not the average temperature over a year.

Just a couple of weeks at that temp will do a lot of damage.

The point is that ~2C or so from the pre-warming baseline you start seeing signs of a mass die off. And ~4C or so the damage starts to become systemic and irreversible.

45C average is insane! but thankfully a worst-case scenario - not the current situation.

Here is the present day temperature readings, from the abstract:

>>> Here we found that pantropical canopy temperatures independently triangulated from individual leaf thermocouples, pyrgeometers and remote sensing (ECOSTRESS) have midday peak temperatures of approximately 34 °C during dry periods, with a long high-temperature tail that can exceed 40 °C.

> thankfully a worst-case scenario

Considering that the models predicting the worst cases have tended to be more accurate (eg https://www.nature.com/articles/nature24672), you may want to hold off on that thankfulness.

I live in a desert climate where it not only hits 46C, but does so for literally months on end. This article is utterly BS. Trees do fine, even an huge number of imported species from significantly cooler climates.

how long does it take for dessert plants to produce biomass compared to same species in temperate conditions?

Are you considering C-3 plants, C-4 plants, or CAM plants.

https://www.khanacademy.org/science/biology/photosynthesis-i...

https://www.frontiersin.org/articles/10.3389/fpls.2021.71539...

https://www.sciencedirect.com/topics/pharmacology-toxicology...

https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism

They survive, they're not 'doing fine', if they did you wouldn't live in a desert climate for long. High temperatures have a very visible effect on vegetation, so much so that you can trivially pick these out on satellite imagery.

leaf temperature is the issue. photosynthetic machinery begins to denature at 30C leaf temperature, this is physiologic break even point. plants become stunted and just survive, rather than produce net gain of biomass

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC129275/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10422931/

This is good data, but I think "approaching" is doing a lot of heavy lifting in this title - more than is supported by the abstract:

>>> Using an empirical model incorporating these dynamics (validated with warming experiment data), we found that tropical forests can withstand up to a 3.9 ± 0.5 °C increase in air temperatures before a potential tipping point in metabolic function, but remaining uncertainty in the plasticity and range of Tcrit in tropical trees and the effect of leaf death on tree death could drastically change this prediction.

3.9 ± 0.5 °C is a lot of runway; likely moreso than we have with other ecosystems. This isn't good by any means, but significantly less dire than other threats.

And when it's 45 degrees you'll be right there saying that there is nothing we can do about it now, so we should just accept it?

Pretty sure that what he's saying is that if we need a 4C increase to trigger mass die-off of tropical forests it's mostly irrelevant since at +4C we enter a lot of other catastrophic scenarios.

It's like if you discovered that when your house is on fire the windows will break. It's interesting on the scientific merits but if your house is on fire the whole building is at stake so it was already understood that the windows will break.

To be sure, when it's mass die off of one of the planets major carbon sinks we're talking about, it's good to pay more attention to the low end of the confidence interval...

No, it's not that. It's that there are multiple factors that put those forests at risk and all of them need to be mitigated to the point that they are safe for the foreseeable future. Deforestation is a huge problem, but this temperature effect has the potential to kill off entire forests in one go. So even if it is a little bit over the horizon assuming that you are going to be able to save some of the forests you'll still lose them. So you need to have multiple lines of attack at once, otherwise you will fail.

I plainly did not say that there was nothing to do about it, but I appreciate your enthusiastic defense of the forests. Unless we stop clear-cutting in the short term, there won't be any rainforests left by the time the temperatures hit 45C.

Both are a problem. The time to act is now not when the temperatures are so high that the trend alone + momentum will be enough to reach the point of no return. We really need those trees. So instead of trying to dismiss the article you could do a better job at making the case for action instead of pretending that there is 'plenty of room' when clearly there isn't. Because even in this comment you give another reason to say it's pointless and that we will hit 45 degrees.

Clearcutting rainforest is a huge problem, bigger so because the rich countries arguing that the poor countries shouldn't be doing this after clearcutting their own land is hypocritical to the extreme. But rising temperatures are by themselves also a huge problem and this is an 'either/or' situation, which means we need to tackle all of these in ways that last.

Genuine question: would you please quote where I dismissed the entire study? (note that I have slightly modified the wording of the first sentence since your original reply - the original phrasing was: >>> "approaching" is doing a lot of heavy lifting in this title - far more than is supported by the abstract (imo) )

I was critical of a single word in the title.

That's exactly the problem though: rather than to build upon the ideas in the article all you did was find fault with a single word, and that drove your comment. Instead, if you were to take this serious you could approach it constructively rather than destructively. It's maybe a bit more work but I think the cumulative effect of that would result in a much better discussion around the subject matter.

6 degrees difference sounds like it is huge, right? But: that's 6 degrees with a margin of error, and with a difference per species (it is an average!) that further enlarges that margin for error. Then there are possible feedback mechanisms to contend with, as well as various local effects. Rather than to just accentuate the fact that 'approaching' is doing the heavy lifting it would be trivial to point out that this is how scientists communicate around things they are not 100% sure of (and that includes most things), but meanwhile us silly laypeople should not assume that because the statement is made carefully that it still is a warning and so can't be dismissed out of hand. That is more typing but it takes you out of 'dismissal', which is how I read your original comment, especially with the 'far more' in there which is - ironically - doing a lot of heavy lifting in your original comment.

> clearcutting their own land is hypocritical to the extreme.

Hypocritical++ is cutting someone else's trees and shipping across an ocean to be burned for renewable subsidies. UK electricity plant converted from coal to burn wood from Canada.

https://www.bbc.com/news/science-environment-63089348

Talk about perverse incentives... wow. That takes the whole concept to new heights.

Yeah; by the time we are at 3.4C (the bottom of that range), we're already to the point where a large percentage of people are climate refugees. In addition to many currently-inhabited places being too hot for humans to survive, the Atlantic current will have already collapsed, so bug chunks of Western Europe will probably be under a glacier.

USGS has a decent and short write-up of glaciers across Europe. It says 20,000+ years ago.

https://www.usgs.gov/media/images/glaciers-extended-over-muc...

Yeah; crucially, we have some fossil record of the run up to that glacier, with 50 year resolution:

Sample 1: Normal

Sample 2: Atmospheric CO2 spike.

Sample 3: Glacier

Based on that, we are 0-75 years away from the next European ice age, which the maximum expectation estimate at ~25 years.

It’s not 4C when there’s this much uncertainty involved.

The concern isn’t hitting X temperature and everything instantly fails it’s looking for the lowest temperature where positive feedback loop occurs. That feedback temperature might be 3.4C or it might be 0.4C, this study hasn’t discovered a specific answer just a very critical threshold we are fairly close to where bad stuff occurs.

“Leaf thermocouple data from multiple sites across the tropics suggest that even within pixels of moderate temperatures, upper canopy leaves exceed Tcrit 0.01% of the time.” So it’s a rare event right now. Perhaps at +0.8C such extreme events become common enough for large local die offs to occur. When coupled with tree lifespan you actually get widespread cumulative damage much sooner than expected. Or perhaps it’s not a problem, we don’t have a definitive answer just one study.

>photosynthetic machinery in tropical trees begins to fail averages approximately 46.7 °C

>have midday peak temperatures of approximately 34 °C during dry periods, with a long high-temperature tail that can exceed 40 °C

So basically they have found absolutely nothing of concern

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