Myth busters 2. Was it really hotter in the Medieval Warm Period?

May 5, 2024

Woman clothed in sun – original image from rawpixel-id-14246134

As we move into a period in which the powers that be will be retreating and retrenching on climate policy, we can expect discredited arguments and factoids to be churned out with dreary regularity; so that anyone who does not want to confront the realities we face has a set of one liners to trot out to deflect thought and effort.

This series of blogs is aimed at giving workers and activists the information we need to debunk these claims if a workmate, friend or relative comes out with one of them.

Many denialist arguments rest aggressively asserted half truths on the sorts of information most of us will vaguely remember from secondary school science lessons, with a rhetorical torque that’s just enough to create an element of doubt, as long as no one checks.

Here is an example from a recent leaflet that is fairly characteristic of the genre.

“The World is not “heating up” rapidly

  1. As climate scientist Tim Ball’s chart below shows, current temperature levels are far from exceptional. Temperatures were higher in the Medieval Warm period – without the current level of emissions. Temperatures were also higher in the 1940s. There is no cause for panic!
  2. By excluding the Medieval Warm Period, Michael Mann’s alarmist “hockey stick” chart has fraudulently covered up evidence against man made global warming. Temperatures have hardly changed since 1998, but Mann’s chart shows a spike anyway. This is a blatant attempt to support the climate alarmists’ narrative and create fear.

All of the statements in these two paragraphs are untrue.

Global temperatures are rising rapidly and the eight years leading up to 2023 were the hottest on record

This graph shows how rapidly they have risen in recent decades, how far above 1998 and the 1940s they now are, and how fast this trajectory is increasing.

Monthly global average temperatures since June 2023 have set record highs

Hold onto your hats for 2025, and beyond.

Temperatures in the 1940s were lower than they are today. Indeed, there was a mid century drop in the otherwise rising trend throughout the rest of the 20th Century that lasted until 1970. Other deniers have argued that this shows that carbon emissions cannot be the cause of a global temperature rise. 

That mid century dip, and the relentless rise since, can be seen here.

Global Temperatures Graph

Graph from New Scientist

In an article from 2007 New Scientist explains the countervailing factors behind this drop, most of them the result of human activity, but also natural factors. The essential point is that the deniers tend to put forward one factor at a time without taking into account how it reacts with other factors, and often get these wrong too because their aim is not to get at the truth but to obscure itT.

The mid-century cooling appears to have been largely due to a high concentration of sulphate aerosols in the atmosphere, emitted by industrial activities and volcanic eruptions. Sulphate aerosols have a cooling effect on the climate be cause they scatter light from the Sun, reflecting its energy back out into space.

The rise in sulphate aerosols was largely due to the increase in industrial activities at the end of the second world war. In addition, the large eruption of Mount Agung in 1963 produced aerosols which cooled the lower atmosphere by about 0.5°C, while solar activity levelled off after increasing at the beginning of the century

The clean air acts introduced in Europe and North America reduced emissions of sulphate aerosols. As levels fell in the atmosphere, their cooling effect was soon outweighed by the warming effect of the steadily rising levels of greenhouse gases. The mid-century cooling can be seen in this NASA/GISS animation, which shows temperature variation from the annual mean for the period from 1880 through 2006. The warmest temperatures are in red.

They go on to note that Climate models that take into account only natural factors, such as solar activity and volcanic eruptions, do not reproduce 20th century temperatures very well. If, however, the models include human emissions, including greenhouse gases and aerosols, they accurately reproduce the 1940 to 1970 dip in temperatures.

This notes the cooling effect of volcanic eruptions, which has had significant impacts at times in human history, like the “year without a Summer” in 1816 after a massive eruption of Mount Tambora, which temporarily reduced global temperatures by between 0.4C and 0.7C. They also note the impact of solar activity, which deniers often argue is the sole cause of global temperature variations. For example the leaflet we are looking at argues

The sun drives the climate, not human activity

The climate has always changed, from the ice ages to the Medieval Warm Period. Temperatures on Planet Earth are determined by a complex set of solar cycles. Human impact is insignificant by comparison.

Again, we have this assertion that solar cycles NOT human activity have an impact on climate. In fact BOTH do. As do volcanic eruptions and factors like variations in the Earth’s orbit around the Sun. All these factors interact.

The variations in Earth’s orbit are cyclical, are known as Milankovitch cycles) take place over tens of thousands of years and have been the fundamental drivers of Ice Ages and periods that have been far hotter than today. That can be explored here. That is not an explanation for our current period of rapidly increasing temperatures.

These orbital cycles should not be confused with the cycles of solar activity in the Sun itself, which are far shorter, but don’t explain the current sharp temperature increases either. As NASA notes “We know subtle changes in Earth’s orbit around the Sun are responsible for the comings and goings of the past ice ages. But the warming we’ve seen over the last few decades is too rapid to be linked to changes in Earth’s orbit, and too large to be caused by solar activity.” In fact “the current scientific consensus is that long and short-term variations in solar activity play only a very small role in Earth’s climate. Warming from increased levels of human-produced greenhouse gases is actually many times stronger than any effects due to recent variations in solar activity. Exactly the opposite of what the deniers claim (with no evidence). 

The evidence in fact shows that For more than 40 years, satellites have observed the Sun’s energy output, which has gone up or down by less than 0.1 percent during that period. Since 1750, the warming driven by greenhouse gases coming from the human burning of fossil fuels is over 270 times greater than the slight extra warming coming from the Sun itself over that same time interval.

Turning reality on its head, another denialist argument is this one.

Higher air and ocean temperatures lead to higher CO2 levels

There is indeed a proven correlation between temperatures and CO2 levels in the atmosphere – but its the opposite of what we are being told: 500 – 800 years after warm periods, oceans warm up and expel some of the CO2 they are holding into the air. This means the higher atmosphere CO2 levels of the last 100 years are an after – effect of the Medieval warm period. CO2 changes follow temperature changes, they don’t cause them.

The core irrationality is in the last four words. The “proven correlation” between CO2 concentrations and temperature only works in one direction it seems. This is simplistic linear thinking.  What they seem to have missed is that at the end of the Ice Age there was no industrial human activity that was pouring quantities of CO2 into the atmosphere well above anything that could be created by natural causes. Now there is. As the NOAA notes “The annual rate of increase in atmospheric carbon dioxide over the past 60 years is about 100 times faster than previous natural increases, such as those that occurred at the end of the last ice age 11,000-17,000 years ago.” 

Yet, the denialists expect us to believe that this has been done with no impact whatsoever. The rise in emissions has increased especially in the last seven decades from 6 billion tonnes in 1950, to 20 billion tonnes in 1990 to 35 billion tonnes in 2024 (From Our World in Data). This has increased the concentration of CO2 from 280 parts per million in 1750 to 419 parts per million now. This graph shows the correlation between the increasing emissions and the atmospheric concentration as about as exact as you could reasonably expect.

Global atmospheric carbon dioxide compared to annual emissions graph

But, what about that Medieval Warm Period?

An awful lot of weight is being put on some pretty thin ice here.

As we can see here, Global average temperatures were not actually that warm in the Medieval Warm Period, and were well below what we have already reached today. Looks like a hockey stick to me.

Global Average Temperature Change

By RCraig09 – Own work, CC BY-SA 4.0,

It is perhaps characteristically Global North centric that a period of warmer temperatures in the North Atlantic resulting from a combination of solar activity, decreased volcanic activity and shifts in ocean circulation patterns bringing warmer water into the North Atlantic (famously allowing the Vikings to settle in Greenland) are falsely taken as a global phenomena.

But, even in the Northern Hemisphere, we are already above where we were then and heading up fast. The speed with which the ice shelf on Greenland is melting is testament to that.

Northern Hemisphere Temperatures (5-year averages)

This can be explored more fully here.

To summarise,

  1. Short term solar activity has a limited impact on climate change and assessment of recent solar cycles shows no relation between it and the overall trajectory of rising temperatures, as solar radiation has decreased somewhat in the last 40 years.
  2. Orbital cycles have a massive impact, but these take place on a much larger timescale than the current rapid shift, tens to hundreds of thousands of years, not decades and a couple of centuries, so do not explain it. In fact, according to NASA, “”If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago.
  3. Volcanic activity can, and  does, have a short sharp downward impact. Air pollution also has a downward impact on a longer term basis, so, neither of these would explain why temperatures are rising, but do explain dips in the rising curve.
  4. This curve is, however, consistent with the current rising curve of greenhouse gas emissions.