Claim: Global Warming will Increase the Threat of Weeds

Flat weed or Catsear (Hypochaeris radicata). A weed of lawns, pastures roadsides and waste places. Como NSW Australia, December 2008. John Tann from Sydney, Australia / CC BY

Guest essay by Eric Worrall

Despite the fact tropical countries do just fine controlling agricultural weeds, computer models suggest global warming will cause weeds to be a big problem in the USA, though unusually for a climate study the authors admit there are serious limitations to their modelling technique.

Global warming will boost agriculture weed threat

Date: June 2, 2020
Source: Flinders University

Summary:Invasive weeds pose a significant threat to global agriculture productivity — and their threat will become more pronounced if the Earth’s climate is affected by increased greenhouse gas concentration, according a climate researcher.

Working with computer models to predict the likely impact of climate change on invasive weed propagation, Dr Farzin Shabani from Flinders University’s Global Ecology Lab found a likely increase in areas of habitat suitability for the majority of invasive weed species in European countries, parts of the US and Australia, posing a great potential danger to global biodiversity.

In predicting the impact of climate change on current and future global distributions of invasive weed species, Dr Shabani also found that existing attempts to eradicate invasive populations are inadequate.

Dr Shabani and an international team of researchers investigated 32 globally important Invasive Weed Species to assess whether climate alteration may lead to spatial changes in the overlapping of specific IWS globally.

“We aimed to evaluate the potential alterations — whether that be a gain, loss or static — in the number of potential ecoregion invasions by IWS, under climate change scenarios,” says Dr Shabani. “We utilised all possible greenhouse gas concentration to examine a range of possible outcomes.”

The paper — Invasive weed species’ threats to global biodiversity: Future scenarios of changes in the number of invasive species in a changing climate, by Farzin Shabani, Mohsen Ahmadi, Lalit Kumar, Samaneh Solhjouy-fard, Mahyat Shafapour Tehrany, Fariborz Shabani, Bahareh Kalantar and Atefeh Esmaeili — has been published in the journal Ecological Indicators.

The abstract of the study;

Invasive weed species’ threats to global biodiversity: Future scenarios of changes in the number of invasive species in a changing climate

Author links FarzinShabaniabc Mohsen Ahmadide Lalit Kumara Samaneh Solhjouy-fardf Mahyat Shafapour Tehranyg Fariborz Shabanih Bahareh Kalantari Atefeh Esmaeilia

Invasive weed species (IWS) threaten ecosystems, the distribution of specific plant species, as well as agricultural productivity. Predicting the impact of climate change on the current and future distributions of these unwanted species forms an important category of ecological research. Our study investigated 32 globally important IWS to assess whether climate alteration may lead to spatial changes in the overlapping of specific IWS globally. We utilized the versatile species distribution model MaxEnt, coupled with Geographic Information Systems, to evaluate the potential alterations (gain/loss/static) in the number of potential ecoregion invasions by IWS, under four Representative Concentration Pathways, which differ in terms of predicted year of peak greenhouse gas emission. We based our projection on a forecast of climatic variables (extracted from WorldClim) from two global circulation models (CCSM4 and MIROC-ESM). Initially, we modeled current climatic suitability of habitat, individually for each of the 32 IWS, identifying those with a common spatial range of suitability. Thereafter, we modeled the suitability of all 32 species under the projected climate for 2050, incorporating each of the four Representative Concentration Pathways (2.6, 4.5, 6.0, and 8.5) in separate models, again examining the common spatial overlaps. The discrimination capacity and accuracy of the model were assessed for all 32 IWS individually, using the area under the curve and true skill statistic rate, with results averaging 0.87 and 0.75 respectively, indicating a high level of accuracy. Our final methodological step compared the extent of the overlaps and alterations under the current and future projected climates. Our results mainly predicted decrease on a global scale, in areas of habitat suitable for most IWS, under future climatic conditions, excluding European countries, northern Brazil, eastern US, and south-eastern Australia. The following should be considered when interpreting these results: there are many inherent assumptions and limitations in presence-only data of this type, as well as with the modeling techniques projecting climate conditions, and the envelopes themselves, such as scale and resolution mismatches, dispersal barriers, lack of documentation on potential disturbances, and unknown or unforeseen biotic interactions.

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What does this mean for US agriculture?

Probably not a lot. My personal series of tests suggests weed killer works just as well in the tropics as it does in temperate climates. Local farmers also seem to manage. Ponds and waterways in the tropics are left to their own devices, or managed with an occasional bag of copper sulphate.

Worst case someone might have to order a few ship loads of tropical strength agricultural chemicals

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Dan Sudlik
June 6, 2020 6:19 am

“Working with computer models”… The End!!!

old white guy
Reply to  Dan Sudlik
June 6, 2020 7:50 am

Lack of weed killer increases the “threat” of weeds.

Reply to  old white guy
June 7, 2020 3:23 am

overuse of weedkiller has given us superweeds, that ever more toxic chem is required to kill
or revert to the safer pre chem till n burn
the fact the big aggro corps push notill and plenty of gm plants etc as green
should be a wakeup call.

Carbon Bigfoot
Reply to  old white guy
June 7, 2020 5:44 am

Mile-a-Minute Weed obliterated by ROUNDUP —-using by directions and safety protocols for 30 years good stuff.

Walter Sobchak
Reply to  Dan Sudlik
June 6, 2020 10:02 am

Mathematical onanism.

Richard (the cynical one)
June 6, 2020 6:20 am

There would also be the threat (gasp!) of increased crops, and that would inevitably lead to an increase in people! And that would be unacceptable to any acolyte of Malthus.

June 6, 2020 6:21 am

EAT WEEDS! As AG pours on more ‘weed killer’ (or ‘pours in’ as GMO), cancer and other autoimmune will increase. Eating weeds, especially dandelions or other health-giving non-GMO varieties, may be essential to health.

Reply to  Ge0ld0re
June 6, 2020 8:10 am

You forgot to take your meds this morning, didn’t you.

Reply to  Ge0ld0re
June 7, 2020 3:25 am

dandelions burdock are ok
the one pictured looks like dandelion to the less aware, dunno about humans but it gives horses guts an upset

June 6, 2020 6:24 am

Weeds must be overwhelming Florida by now. But here in Maryland, where temperatures are normally lower, we are doing OK. Maine must be in really good shape.

Bruce Cobb
June 6, 2020 6:25 am

More junk science, based on fantasy models, based on carbon myths. It’s junk all the way down.

Just Jenn
June 6, 2020 6:36 am

what 32 important invasive species? How do they differ from each other? What is the ecology in their native habitat like? AND MOST IMPORTANTLY: ARE THEY ALREADY THERE AND WHAT MAKES THEM THE IMPORTANT ONES?

Basic freaking questions.

At least they had the decency to say there is inherent uncertainty.

June 6, 2020 6:44 am

I thought we were woke. Isn’t calling some plants “weeds”, like, racist or something?

Reply to  BallBounces
June 6, 2020 10:00 pm


Reply to  BallBounces
June 7, 2020 3:29 am

a weed is just a plant growing where you didnt plant it;=-)
learning to use them properly could be beneficial
you can spray nettles or market them to other sources instead
I enquired about sellling healthy nettles and was asked how many acres i could supply, and what time contract was I looking at?
I had a half acre;-/ too little for their needs.

June 6, 2020 6:49 am

CO2 plant growth enhancement should increase growth rates in all plants. Weeds are unwanted plants.

When there is more biomass produced from more plant growth the soils everywhere incorporate more of that mass into the soil improving soil quality. So, more and bigger weeds is good for building top soils.

I have gardened for 50+ years, and it took me less than half of the first year to learn that weed control requires REMOVING THE Damn Things BEFORE THEY GET BIG.

It’s just as easy to remove 1/4″ high weeds at 100 weeds/square foot as 1 weed per square foot.

I never gave much thought to how big a dead weed could have gotten.

Reply to  DocSiders
June 6, 2020 6:49 pm

My botany teacher many moons ago defined a weed as any plant growing where it wasn’t wanted. So, a cabbage in a rose garden is a weed, and vice versa.

Reply to  DocSiders
June 7, 2020 3:31 am

yeah, slide a sharp hoe over 2 leaf stage easy peasy
dig/pull the ones you didnt.. hours and bags worth later

On the outer Barcoo
June 6, 2020 6:54 am

Thoreau once remarked that a weed is a plant whose virtues have yet to be discovered.

Ron Long
June 6, 2020 6:56 am

Eric, you must not have been watching too much TV the last few days, as there has been a never-ending parade of apparent volunteers to smoke these weeds. Problem solved. Next?

Justin Burch
June 6, 2020 7:07 am

What is a weed? It is a plant that does not require seeding, pruning, fertilizing, harvesting, tilling, application of pesticides and/or herbicides, and staking to grow. Add computer models and I just yawn.

Andre Lauzon
June 6, 2020 7:26 am

I know computer models do not work. Many political parties use them to choose their leaders.

Tom Abbott
June 6, 2020 7:28 am

From the article: “incorporating each of the four Representative Concentration Pathways (2.6, 4.5, 6.0, and 8.5) in separate models,”

Well, at least they used all four of them. Usually these studies only use the worst-case scenario, 8.5, when reporting their results.

From the article: “Our results mainly predicted decrease on a global scale, in areas of habitat suitable for most IWS, under future climatic conditions, excluding European countries, northern Brazil, eastern US, and south-eastern Australia.”

This is a little confusing. Why the difference? Why wouldn’t the whole world show the same result?

June 6, 2020 7:32 am

Alarmists claim global warming will cause many bad things. Just for once I’d like them to name one thing global warming won’t cause, anything.

Tiger Bee Fly
Reply to  ScienceABC123
June 6, 2020 7:44 am

Yep – there were already thousands of such claims. I guess if they had to answer, they’d say, “anything positive you can think of is not the result of global warming; anything negative is.”

Moderately Cross of East Anglia
Reply to  ScienceABC123
June 6, 2020 7:45 am


Alasdair Fairbairn
June 6, 2020 7:43 am

It is amazing what dastardly tricks CO2 gets up to once it invades a computer model.

Coeur de Lion
June 6, 2020 7:49 am

But this ‘warming’ is about 1.3degsC a century! What’s going to change that? Not CO2 if we are to believe the latest sensitivity numbers. During the prosperous MWP people had to do a lot of hoeing, I expect.

Tom in Florida
June 6, 2020 8:01 am

32 species? It must be hard to weed through all that info.

Rich Davis
Reply to  Tom in Florida
June 6, 2020 1:32 pm

groan 🙂

June 6, 2020 8:02 am

Why is it that the plants that we who garden ( I am too incompetent to call myself a gardener) call weeds are always the most succesful whatever the weather or soil . Is there something in their genome that makes them so sturdy . Why can’t that element be extracted and our useful or decorative , but less sturdy , plants be modified with it to make them bigger, more resilient , more productive of fruit or flowers , etc ?

Pat from Kerbob
Reply to  mikewaite
June 6, 2020 8:18 am

Need more GMO
Generally desirable plants seem to have the resiliency bred out of them lest they become invasive.
I too am an amateur gardener and I can attest that a plant is one that will die no matter how much care it gets while a weed won’t die no matter what I do to it

June 6, 2020 8:02 am

One person’s weed is another’s natural medicine.

Sweet Old Bob
Reply to  Scissor
June 6, 2020 5:32 pm

Exactly !
A study of the uses of plants by “Native ” Americans can be very useful .
I now grow some “weeds” for animal and human use .
No , not THAT weed !
Ambrosia trifida for example .
AKA giant ragweed .

June 6, 2020 8:11 am

Do weeds do better with enriched CO2 than do food crops? If not, why worry?

Rich Davis
Reply to  MarkW
June 6, 2020 1:34 pm

Because there is a very lucrative worrying industry, Mark.

Lawrence E Todd
June 6, 2020 8:13 am

maybe the increase of weeds is because of the false science being used to stop the use of pesticides.

Pat from kerbob
Reply to  Lawrence E Todd
June 6, 2020 8:21 am

That is true, many herbicides have been banned here in Canada.
In Calgary we are now in that wonderful period where all public spaces are overrun with dandelions.
Because that is “natural”

Reply to  Pat from kerbob
June 7, 2020 3:46 am

you need to understand plants better
dandelions bring up calcium/minerals rom deep down
their strong taproot breaks down hardpan soils allows moisture to get in deeper and creates a richer soil
as the soil changes and gets richer?
other plants then become more prevalent utilising the improved soils
every single year an entirely different plant colony appears in any soils
temperature means some sprout faster than others, moisture levels also affect that.
3 yrs ago after decent rains i had wild geranium seriously nasty barbed seeds real problem plant
following year(yes i did hand remove a lot) nowhre in the yard at all could i find more that 3 or 4 of them ie a tiny fraction of ONE seedhead .
marshmallow and horehound last yr but lesser than prior events too.
capedaisy the inbetween yr
so far its cape daisy in moderation and some nettles the rest is now self seeding pasture grases using the better richer soils the rotted down weeds created.
NO WASTE should ever leave the property its ALL nutrients brought up from deeper that boosts soils. really savage seeded weeds can be burnt if needed that also help soils.

Reply to  Lawrence E Todd
June 7, 2020 3:37 am

get a gong for that logic
so called weeds tend to also be resistant to bugs by design
no labcoated heros required.
our hybrid lab created wonderplants are weak and fall victim to every bug n virus/mould etc going.
Heritage plants that breed true have survived thousands of yrs in some cases
our wussy new stuff wont survive a season or two without human input
thatsa pretty poor plant

June 6, 2020 8:21 am

The definition of a weed according to out crop husbandry lecturer was a plant in the wrong place,
A strongly growing crop or grass competes more effectively with weeds, so more CO2 should help the crop out-compete the weeds, possibly with a little help from a hoe or herbicide.

June 6, 2020 8:30 am

The weed shown at the top looks a bit like the UK dandelion (dent-de-lion) which has leaves that can be used in salads. However they are a mild diuretic and were known by the French as piss-en-lit.
In the UK their peak flowering is on St. George’s day, April 23rd.
As children we were supposed to be able to tell the time by how many puffs of breath it took to blow all the seeds off the seed-head.

Rich Davis
Reply to  StephenP
June 6, 2020 3:22 pm

I knew that they were called Löwenzahn in German, meaning lion’s tooth, which I understood to refer to the jagged leaves. Dent-de-lion certainly seems to be the etymology of the English name. Interesting!

It’s dente di leone in Italian, diente de león in Spanish, dente-de-leão in Portuguese, Løvetann in Norwegian, and dant y llew in Welsh, but a paardebloem (horse flower) in Dutch, voikukka (butter flower) in Finnish, pitypang (blow ball) in Hungarian, kiaulpienė (pig’s milk?) in Lithuanian. It seems that our language almost always prefers a loan word while everybody else sticks to their own words. Why not lion’s tooth?

I waste a lot of time playing with Google Translate. 🙂

Gregory Woods
June 6, 2020 8:41 am

Old Folk Saying: The only good weed is a dead weed.

Gordon A. Dressler
June 6, 2020 9:03 am

It’s a vicious cycle: more weeds = increased use of gasoline-powered lawn mowers and string trimmers = increased CO2 dumped into the atmosphere = increased global warming (per the AGW meme, yet unproven) = more weeds (per claim in above article).

Heaven help those of us that don’t really appreciate weeds!

June 6, 2020 9:41 am

There is a slight grain of truth to this, because of CO2 increase increasing growth of most but not all plants. Grasses and grains are mostly C4 plants, which have evolved to flourish in lower CO2 conditions, especially the mostly 180-280 PPMV of the Pleistocene (which we have yet to establish that we got out of). Their growth is not improved (at least not significantly) by increasing CO2 past 280 PPMV. (Although some of these probably have some improvement of drought tolerance from more CO2 due to less need of respiration of air.) Most other plants (including most weeds) are C3 plants, whose growth is significantly increased by increasing CO2 above Pleistocene levels. Their growth improvement from increase of CO2 level continues until CO2 reaches levels intentionally achieved in some greenhouses by means of CO2 generators, about 1000 PPMV.

Gordon A. Dressler
Reply to  Donald L. Klipstein
June 6, 2020 11:52 am

“. . . a slight grain of truth . . .” Groan.

June 6, 2020 3:57 pm

As a farmer who has been controlling weeds for over 60 years I would say that this study is about as worthless as a weed,
Weeds are plants in the wrong place ,such as rye grass in wheat crops or when the early settlers to New Zealand brought in gorse seed for hedges and now thousands of acres are covered in gorse and the cost to control it is a huge ongoing cost .
We have a huge problem with wilding pines in the South Island mountains .Pinus Contorta was planted to stabilize eroding gullies but they have spread over thousands of acres of productive sheep grazing land .
Our government in their eternal wisdom is allocating millions of dollars to eradicate these weed trees and at the same time have a plan to plant a billion trees on other land .
I have farm forestry blocks on my farm and forestry does have a place on steep gorse infected land .
When good farms are being brought up and planted in Pinus Radiata and we as tax payers are funding the investors carbon credits we as farmers object .
You can’t eat wood.
Proud to be farming feeding the world.

Reply to  Gwan
June 7, 2020 3:54 am

yeah they removed the feral…willows in sth aus riverbanks
the next decent rains the banks caved in
and willows provided shade shelter for native critters
aussie version the broughton river willow is a woody weed grows in masses that make access to water near impossible and they burn so well too and theyre a magnet for the traveller caterpillars that denude everything in sight and cause grief with the fine hairs to birds etc silly enough to eat em
woods useless it stinks when burnt provides little heat and rots fast if used for anything else.

June 6, 2020 4:21 pm

Heigh-Ho, Heigh-Ho, computer modelling we will go!
We did dig dig dig dig dig dig dig
In our Lab the whole day through
To dig dig dig dig dig dig dig dig
It’s what we like to do
It’s quite a trick
To get rich quick
If you dig dig dig
With a Graphic or a Hockey stick.

June 6, 2020 6:53 pm

Must be true. I’ve been pulling weeds for years and they only seem to grow.

Bruce of Newcastle
June 6, 2020 8:18 pm

I was wondering what would happen in in July.
We’ve had Chinese crud and murder hornets.
The asteroids all missed.
So now we have the answer: giant mutant weeds from planet CO2.

Mark Pawelek
June 7, 2020 2:19 am

Weeds are not a threat to agricultural productivity because modern farmers kill weeds with herbicides which food crops are resistant to. Unless we ban herbicides – which modern “do-gooders” try to.

June 7, 2020 8:50 am

Gwan, a farmer has it right. Investments should be in food and not taxing farmers to fund useless schemes.

I wonder if there are more weeds now than in 500BC….

June 7, 2020 12:43 pm

What about the threatened insects? Maybe a weed diet will save them? Insects deserve diversity in diet….insect lives matter too.

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