At a glance - What caused early 20th Century warming?

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 "What caused early 20th Century warming?". 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

The Twentieth Century climate was a veritable Smorgasbord of natural and manmade factors. They vied with or against one another, driving temperatures in various directions at different times. Conditions were on the cool side up until the 1920s. That was followed by a sustained warming through to the late 1940s. From then until the late 1970s, periods of warmer and cooler conditions alternated. Thereafter, we saw the erratic but one-way climb of temperatures that has persisted through to the present day.

This was also the period during which CO2 emissions from use of fossil fuels really took off, from 1.5 billion tonnes a year in 1900, through 6 billion tonnes in 1950, 25 billion by 2000 and 44 billion now. How well do those figures correlate with what was going on in Earth's climate system at the time?

Planetary climate is affected by a number of factors. Examples include solar variability, the amount of volcanic activity, fluctuating ocean currents, cyclic variations in Earth's orbit, the strength of the Greenhouse Effect and the amount of sunlight reflected by Earth's surface. Some are forcings - factors that dictate and change the total energy within the climate system. Others are feedbacks - they respond in various ways to those changes.

Both forcings and feedbacks operate on a variable timescales - from years to millennia. That variability means they may sometimes reinforce one another and at other times cancel each other out. To assess why the climate behaved in a certain way at a certain time, all must be examined.

In the case of the 1920-1940 period, the increase in global temperature is thought to have been largely caused by three of the above factors. The amount of solar energy reaching the top of Earth's atmosphere rose steadily from 1920-1940. Although the amount of change was small, it was certainly not negligible. Volcanic activity produces atmospheric aerosols that can have a cooling effect by partly blocking out the sun. Lower than normal volcanic activity, as was the case in the 1920-1940 period, would result in less of those airborne coolants. Our early, albeit relatively low greenhouse gas emissions also contributed to the warming. Regional factors played a role in increasing temperatures in some parts of the world, too. Most notably, changes in ocean currents led to warmer-than-normal sea temperatures in the North Atlantic.

Anyone who has been around for long enough to recall the chemical smogs of the 1950s and early 1960s will know that pollution can be deadly. In one smog, in London in December 1952, between 4,000 and 12,000 people died, victims of severe respiratory disease. Governments responded. Over the next two decades numerous Clean Air Acts were passed. Pollution levels fell accordingly. But such pollutants, just like volcanic gases, are aerosols that have that same cooling effect. Clearing them away removed that effect and tore off the mask behind which CO2 emissions had been hiding - and off we went on our warming journey.

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
It hasn't warmed since 1998
Antarctica is gaining ice
CRU emails suggest conspiracy
What evidence is there for the hockey stick
CO2 lags temperature
Climate's changed before
It's the sun
Temperature records are unreliable
The greenhouse effect and the 2nd law of thermodynamics
We're heading into an ice age
Positives and negatives of global warming
The 97% consensus on global warming
Global cooling - Is global warming still happening?
How reliable are climate models?
Can animals and plants adapt to global warming?
What's the link between cosmic rays and climate change?
Is Al Gore's An Inconvenient Truth accurate?
Are glaciers growing or retreating?
Ocean acidification: global warming's evil twin
The human fingerprint in global warming
Empirical evidence that humans are causing global warming
How do we know more CO2 is causing warming?
Explaining how the water vapor greenhouse effect works
The tricks employed by the flawed OISM Petition Project to cast doubt on the scientific consensus on climate change
Is extreme weather caused by global warming?
How substances in trace amounts can cause large effects
How much is sea level rising?
Is CO2 a pollutant?
Does cold weather disprove global warming?
Do volcanoes emit more CO2 than humans?
How do human CO2 emissions compare to natural CO2 emissions?
Climate scientists could make more money in other careers
How reliable are CO2 measurements?
Do high levels of CO2 in the past contradict the warming effect of CO2?
What is the net feedback of clouds?
Global warming vs climate change
Is Mars warming?
How the IPCC is more likely to underestimate the climate response
How sensitive is our climate?
Evidence for global warming
Has the greenhouse effect been falsified?
Does breathing contribute to CO2 buildup in the atmosphere?
What is causing the increase in atmospheric CO2?
What is methane's contribution to global warming?
Plants cannot live on CO2 alone
Is the CO2 effect saturated?
Greenhouse warming 100 times greater than waste heat
How will global warming affect polar bears?
The runaway greenhouse effect on Venus
What climate change is happening to other planets in the solar system?
Has Arctic sea ice returned to normal?
Was Greenland really green in the past?
Is Greenland gaining or losing ice?
Human activity is driving retreat of Arctic sea ice
The albedo effect and global warming
Does CO2 always correlate with temperature?
Human fingerprints on climate change rule out natural cycles
Global warming and the El Niño Southern Oscillation
The Pacific Decadal Oscillation (PDO) is not causing global warming
Is the science settled?
The difference between weather and climate
Clearing up misconceptions regarding 'hide the decline'
Tree-ring proxies and the divergence problem'
What ended the Little Ice Age?
How does the Medieval Warm Period compare to current global temperatures?
What is the link between hurricanes and global warming?
CO2 is main driver of climate change
Does positive feedback necessarily mean runaway warming?
Was 1934 the hottest year on record?
What caused early 20th Century warming?


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Posted by John Mason on Tuesday, 25 June, 2024

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