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A detailed look at the Little Ice Age

Here’s a key point that the media has failed to explain and the the anti-science disinformers refuse to accept: The Earth’s overall temperature does not change randomly on a decadal scale — it changes when it is driven to do so by an external forcing.

The Little Ice Age is a case in point, as Skeptical Science explains in this repost:

The argument that we’re simply “coming out of the Little Ice Age (LIA)” makes one of two assumptions:

  1. The planet oscillates around some natural equilibrium temperature such that after it cools, it must warm to return to this temperature, and vice-versa.
  2. Whatever caused the LIA cooling has reversed phase and is now causing global warming.

The first assumption demonstrates a lack of understanding regarding what causes planetary temperature changes.  The second does not hold up under scrutiny of the empirical data.


Climate Change Causes

A long-term increase in the Earth’s average temperature is caused by a change in the planetary energy balance (incoming vs. outgoing energy), also known as a ‘radiative forcing.’  If the amounts of incoming and outgoing energy are equal, the planet is in equilibrium and its temperature will not increase on average.

Note that over short periods of time, energy can be exchanged between the Earth’s oceans and surface air through natural cycles such as the Pacific Decadal Oscillation, which can result in a short-term warming of the surface oceans and air at the expense of a cooling of the deeper oceans, or vice-versa.  However, these cycles oscillate between positive and negative states, which over the long-term cancel each other out and do not cause significant temperature trends.  These oscillations neither create nor retain heat; they simply move it around and thus physically cannot cause global warming or cooling.  Further, if these cycles were causing the surface to warm, they would be causing the oceans to cool, which is the opposite of what we observe.

There are many different factors which can cause a planetary energy imbalance.   Some of the most common examples are changes in solar activity, atmospheric greenhouse gases, volcanic activity, the Earth’s overall reflectivity, and variations in the Earth’s orbit around the Sun (also known as ‘Milankovitch cycles’).  However, the key point is that the planet will not warm or cool over the long-term unless there is a radiative forcing causing a planetary energy imbalance.  Thus the planet will not warm simply because it had previously cooled, and the notion that the planet is just “recovering” from the LIA makes no physical sense.

Little Ice Age Causes

Therefore, the only way the current warming could be because “we’re coming out of the LIA” would be if whatever caused the energy imbalance resulting in the LIA cooling had since changed state to cause a positive radiative forcing, thus resulting in the global warming we’ve observed over the past century.   Climate scientists have proposed numerous factors which likely contributed to the global cooling of the LIA.

Decreased Solar Activity

The LIA occurred from approximately the 16th to 19th centuries.  During this time frame, there were periods of significantly diminished solar activity known as the Spörer Minimum (1460-1550) and Maunder Minimum (1645-1715).  These minima were discussed in a seminal study by Eddy (1976).  Additionally, the Dalton Minimum was a less prominent period of diminished solar activity observed from 1790 to 1830.

Figure 1: Reconsructed total solar irradiance (Delaygue and Bard 2010)

These periods of diminished solar activity were likely significant contributors to the LIA cooling.  And solar activity has increased since the end of the Dalton Minimum to the current ‘Modern Maximum’.  However, solar activity has not increased on average since the mid-20th century.


Figure 2: Total Solar Irradiance as measured by satellite from 1978 to 2010

Studies quantifying the contributions of various radiative forcings to the global temperature change have estimated that over the past century, the increase in solar irradiance is responsible for about 15-20% of global warming (Meehl 2004). However, it cannot explain the accelerated warming over the past 50 years.

Figure 3: Solar radiative forcing temperature change vs. observed global surface temperature increase (Meehl 2004)

Increased Volcanic Activity

The Earth experienced heightened volcanic activity throughout the LIA.  Volcanic eruptions release aerosols into the atmosphere which diffuse sunlight, causing global dimming and cooling.  According to Crowley et al. (2000),

“over the interval 1400–1850, the volcanic contribution [to the decadal-scale variance in global temperatures] increases to 41 to 49% (P < 0.01), thereby indicating a very important role for volcanism during the Little Ice Age.”

However, volcanic activity has had a net negative forcing (cooling effect) over the past century, particularly since 1950, and thus cannot explain the global warming over this period.

Figure 4: Volcanic radiative forcing temperature change vs. observed global surface temperature increase (Meehl 2004)

Ocean Conveyor Slowdown

Another proposed contribution to the LIA cooling is a slowdown of the thermohaline circulation through an introduction of a large amount of freshwater into the North Atlantic Ocean, potentially as a result of melting ice from Greenland due to warming temperatures during the Medieval Warm Period.  The Gulf Stream is part of the thermohaline circulation, and transports warm water from the equator polewards toward Europe.  If the North Atlantic Ocean becomes diluted with fresh water, this current could potentially become slowed or even shut down entirely.  Broecker (2000) proposed this mechanism as a contributor to the LIA cooling.

However, since the Greenland Ice Sheet has been declining due to the global warming over the past century, and the slowdown and potential shutdown of the thermohaline circulation has become a concern as a result, quite obviously the ocean conveyor has not had a warming effect over the past century.

Decline in Human Population

The Black Death caused a decrease in the human populations of Europe, East Asia, and the Middle East during the 14th century, and a consequent decline in agricultural activity.  A similar effect occurred in North America after European contact in the 16th century.  Ruddiman (2003) suggests reforestation took place as a result of this reduced human population and agricultural activity, allowing more carbon dioxide uptake from the atmosphere to the biosphere, thus having a cooling effect.  Ruddiman concluded as follows.

“If the 10-ppm CO2 decreases are caused by plague-induced reforestation events, they would cool northern hemisphere temperatures by ∼0.17°C, assuming a 2 × CO2 sensitivity of 2.5°C.”

Of course, we’ve seen the exact opposite effect in recent centuries, as the human population has continued to grow, increasing deforestation, and of course anthropogenic CO2 emissions through burning fossil fuels.  However, it’s difficult to classify increased human greenhouse gas emissions as a “recovery from the LIA.”

The Planet is not Recovering from the LIA

To sum up, with the exception of the human population, the factors which contributed to the LIA cannot account for the global warming of the past 50-100 years.  Further, it is not physically accurate to claim that the planet is simply “recovering” from the LIA.  This argument is akin to saying that when you drop a ball off a cliff, it falls because it used to be higher.  There is a physical mechanism for these changes.  In the case of the ball, it falls because of the gravitational pull at the Earth’s surface.  In the case of the global temperature, it is warming from the increased greenhouse effect due to human activities.

– dana1981, reposted via Skeptical Science.

JR:  There is actually a 2009 study that provides clearcut support for the above analysis, “Temperature response in the Altai region lags solar forcing” (subs. req’d).  It makes use of “ice core oxygen isotope record from the continental Siberian Altai, serving as a high-resolution temperature proxy for the last 750 years.”  It found that:

The strong correlation between reconstructed temperature and solar activity suggests solar forcing as a main driver for temperature variations during the period 1250–1850 in this region.

It also concluded that

during the industrial period (1850-2000) solar forcing became less important and only the CO2 concentrations show a significant correlation with the temperature record.

I interviewed the lead author Anja Eichler, Senior Scientist at the Switzerland’s Paul Scherrer Institute (see Scientist: “Our conclusions were misinterpreted” by Inhofe, CO2 — but not the sun — “is significantly correlated” with temperature since 1850).  Eichler told me:

We did a strong differentiation between preindustrial (1250-1850) time and the last 150 years. In the preindustrial time we found a strong correlation between the solar activity proxy and our temperature, suggesting solar forcing as a main force for temperature change in this time. However, the correlation between the solar activity proxy and Altai temperature is NOT significant anymore for the last 150 years. In this time the increase in the CO2 concentrations is significantly correlated with our temperature.

And that brings me back to the key point that the disinformers try to obfuscate:   The Earth’s overall temperature does not change randomly on a decadal scale — it changes when it is driven to do so by an external forcing.

Yes, the Earth has had brief warming and cooling periods since 1250. But those temperature changes were not random. They were largely responses to changes in the solar radiation hitting the earth (which is itself affected by volcanoes).

The Naval Research Laboratory and NASA reported in2009 that, “if anything,” the sun contributed “a very slight overall cooling in the past 25 years.” The study, “How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006,” found:

According to this analysis, solar forcing contributed negligible long-term warming in the past 25 years and 10% of the warming in the past 100 years.

A major 2007 study concluded:

Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

Now human-caused emissions are driving climate change to dangerous levels with forcings that dwarf previous natural forcings both in speed and scale (see “Humans boosting CO2 14,000 times faster than nature, overwhelming slow negative feedbacks“). And that’s why the time to act is now.

Related studies can be found on Skeptical Science:

  • Ammann 2007: “Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century.”
  • Foukal 2006 concludes “The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years.”
  • Usoskin 2005 conclude “during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source.”
  • Solanki 2003 concludes “the Sun has contributed less than 30% of the global warming since 1970.”
Joseph Romm's picture

Thank Joseph for the Post!

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