On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added “At a glance” section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a “bump” for our ask. This week features “Tree-ring proxies and the divergence problem“. More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.
At a glance
“Trees tell of past climates: but are they speaking less clearly today?” That was the intriguing title of a paper published in the peer-reviewed journal, Philosophical Transactions of the Royal Society of London, in 1998. The authors discussed various aspects of the use of tree-rings as representatives – or proxies – of past climatic patterns.
Tree growth is sensitive to temperature. Because of that, warmer wetter periods produce wider tree-ring patterns. When it’s colder and dryer, the rings are narrower, indicating slow growth. In this way, the width and density of tree-rings in ancient trees serve as a proxy for temperature. That makes it possible to reconstruct temperature records going back over many centuries. For example, historic events such as major volcanic explosions tend to lead to global cooling. Such events show up very well in tree-ring reconstructions.
In more recent times, since the late 19th century, we also have the observational temperature record to compare with tree-ring reconstructions. Agreement between the two datasets is at first close. However, in middle and especially high latitude sites, the correlation breaks down after 1960. At this point, while temperatures rise, tree-ring width shows a falling trend – a decline. This divergence between temperature and tree growth is called, imaginatively enough, the divergence problem.
The decline, or divergence problem had been recognised, regularly discussed and written about since around 1995. That’s 14 years before anyone had ever heard of ‘climategate’ and the song and dance that the science–deniers made of ‘hide the decline’. If anything should serve as a quality-control alert for the output of science-deniers, it’s right there.
Now, everything that happens has a cause, but not all causes are straightforward. There are plenty of ways to put stress on plants and stress makes growth-patterns abnormal. The trouble is that such stress-factors often vary in an irregular fashion and independently of one another. Even in a 2023 paper, the detailed cause of the divergence problem was described as being accompanied by ‘significant controversies’. It’s real, but it’s complicated, in other words.
Temperature-induced drought stress and changes in seasonality are likely to be relevant here. Also likely to have had a role is the phenomenon of ‘Arctic dimming’. The term ‘dimming’ refers to reduced sunshine reaching the surface in some circumpolar regions, due to industrial aerosol pollution. Northern Hemisphere pollution tends to accumulate over the Arctic. Reduced sunshine affects photosynthesis and in return that impacts upon plant health and growth. Indeed, a 2021 paper ominously commented that the effects of Arctic aerosols on net primary production – growth – were particularly important in light of the current race to exploit natural resources north of the Arctic circle.
The extensive boreal forests of the north play a major role in shaping Earth’s carbon cycle and climate system. In the divergence problem, they have raised a red flag. Something up there is wrong and it should come as no surprise that, once again, our reckless misuse of our planet is high on the list of suspects for that.
Please use this form to provide feedback about this new “At a glance” section. Read a more technical version below or dig deeper via the tabs above!
Click for Further details
In case you’d like to explore more of our recently updated rebuttals, here are the links to all of them:
Myths with link to rebuttal
Short URLs
Ice age predicted in the 1970s
sks.to/1970s
It hasn’t warmed since 1998
sks.to/1998
Antarctica is gaining ice
sks.to/antarctica
CRU emails suggest conspiracy
sks.to/climategate
What evidence is there for the hockey stick
sks.to/hockey
CO2 lags temperature
sks.to/lag
Climate’s changed before
sks.to/past
It’s the sun
sks.to/sun
Temperature records are unreliable
sks.to/temp
The greenhouse effect and the 2nd law of thermodynamics
sks.to/thermo
We’re heading into an ice age
sks.to/iceage
Positives and negatives of global warming
sks.to/impacts
The 97% consensus on global warming
sks.to/consensus
Global cooling – Is global warming still happening?
sks.to/cooling
How reliable are climate models?
sks.to/model
Can animals and plants adapt to global warming?
sks.to/species
What’s the link between cosmic rays and climate change?
sks.to/cosmic
Is Al Gore’s An Inconvenient Truth accurate?
sks.to/gore
Are glaciers growing or retreating?
sks.to/glacier
Ocean acidification: global warming’s evil twin
sks.to/acid
The human fingerprint in global warming
sks.to/agw
Empirical evidence that humans are causing global warming
sks.to/evidence
How do we know more CO2 is causing warming?
sks.to/greenhouse
Explaining how the water vapor greenhouse effect works
sks.to/vapor
Is extreme weather caused by global warming?
sks.to/extreme
How substances in trace amounts can cause large effects
sks.to/trace
How much is sea level rising?
sks.to/sealevel
Is CO2 a pollutant?
sks.to/pollutant
Does cold weather disprove global warming?
sks.to/cold
Do volcanoes emit more CO2 than humans?
sks.to/volcano
How do human CO2 emissions compare to natural CO2 emissions?
sks.to/co2
Climate scientists could make more money in other careers
sks.to/money
How reliable are CO2 measurements?
sks.to/co2data
Do high levels of CO2 in the past contradict the warming effect of CO2?
sks.to/pastco2
What is the net feedback of clouds?
sks.to/cloud
Global warming vs climate change
sks.to/name
Is Mars warming?
sks.to/mars
How the IPCC is more likely to underestimate the climate response
sks.to/underestimat
How sensitive is our climate?
sks.to/sensitivity
Evidence for global warming
sks.to/warming
Has the greenhouse effect been falsified?
sks.to/falsify
Does breathing contribute to CO2 buildup in the atmosphere?
sks.to/breath
What is causing the increase in atmospheric CO2?
sks.to/CO2increase
What is methane’s contribution to global warming?
sks.to/methane
Plants cannot live on CO2 alone
sks.to/plant
Is the CO2 effect saturated?
sks.to/saturate
Greenhouse warming 100 times greater than waste heat
sks.to/waste
How will global warming affect polar bears?
sks.to/bear
The runaway greenhouse effect on Venus
sks.to/venus
What climate change is happening to other planets in the solar system?
sks.to/planets
Has Arctic sea ice returned to normal?
sks.to/arctic
Was Greenland really green in the past?
sks.to/green
Is Greenland gaining or losing ice?
sks.to/greenland
Human activity is driving retreat of Arctic sea ice
sks.to/arcticcycle
The albedo effect and global warming
sks.to/albedo
Does CO2 always correlate with temperature?
sks.to/correlate
Human fingerprints on climate change rule out natural cycles
sks.to/cycle
Global warming and the El Niño Southern Oscillation
sks.to/elnino
The Pacific Decadal Oscillation (PDO) is not causing global warming
sks.to/pdo
Is the science settled?
sks.to/settled
The difference between weather and climate
sks.to/weather
Clearing up misconceptions regarding ‘hide the decline’
sks.to/decline
Tree-ring proxies and the divergence problem’
sks.to/diverge
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