Global Warming Natural Cycle

Global Warming in the natural cycle is well understood from paleo data covering the past 1 million years. To understand global warming and the natural cycle you need to understand the natural forcing, current forcing, greenhouse gases, and the Milankovitch cycles: The natural cycle refers to the long term forcing imposed on the earths climate system periodically (kiloyear cycles known as Milankovitch cycles). These cause earth climate to move in and out of ice ages and warm interglacial's with periodicity of around 100,000 years for the past 1 million years. This regulates natural greenhouse gases in that it imposes additional forcing on the earth climate when more solar energy is present. What would the forcing be if humans had not changed the amount of GHG? The natural cycle forcing ranges from around -3.4W/m2 during an ice age (glacial) to around +0.1W/m2 during an interglacial (warm period).

Is global warming a natural cycle? Or is global warming affected by human influence? What does the science say? Both are true. In the natural cycle, the world can warm, and cool, without any human interference. For the past million years this has occurred over and over again at 100,000 year intervals. About 80-90,000 years of ice age with about 10-20,000 years of warm period.

The difference is that in the natural cycle CO2 lags behind the warming because it is mainly due to the Milankovitch cycles. Now CO2 is leading the warming. This is clearly not natural cycle. The earths natural cycles, if human industrial output had not been involved would have us near or slight below thermal equilibrium, possibly slightly cooling.

In other words, if were were in the natural cycle without human influence, the forcing levels would likely be around 0W/m2 to -0.1W/m2. We are currently experiencing a positive forcing of around 3.6 to 3.8W/m2 and a human induced negative forcing of around 2W/m2. The resultant forcing, depending on current levels and the Schwabe cycle is around +1.6W/m2 above natural cycle as estimated.

Where are we currently in the natural Milankovitch cycle? Estimated around 0.0 to -0.1W/m2

The natural cycle is understood by examining the paleo records. The fact that the earth goes in and out of ice ages distinctly outlines the natural cycles of earths climate. This occurs about every 100,000 years. We are currently in a warm period. Generally, the earth spends about 80-90,000 years in an ice age and around 10-20,000 years (or so) in a warm period.

12,000 Years

Holocene Temperature Variations

The Holocene temperatures peaked around 8,000 years ago. This temperature peak was associated with the perihelion phase of the Milankovitch cycles. That was the time when the natural cycle climate forcing was at maximum. Since then the forcing levels have been slowly dropping and the climate forcing has been following the slope of forcing in line with the changes in the Milankovitch cycle forcing.

The current global mean temperature (GMT) is above the temperature peak associated with the forcing imposed on the climate system when we came out of the last ice age.

Source: http://www.globalwarmingart.com/wiki/Image:Holocene_Temperature_Variations_Rev_png

150 Thousand Years

GMT 150k

Source: http://www.ncdc.noaa.gov/paleo/globalwarming/paleobefore.html

450 Thousand Years

In the image below we can see that the cycle has been fairly regular for the past 450,000 years.

450k years

Source: http://eetd.lbl.gov/AQ/smenon/temperature-CO2.jpg

Source: http://eetd.lbl.gov/AQ/smenon/mainintro.htm

 

5 Million Years

The last 5 million years of climate history shows us settling in to our current 100k year cycles.

5 Million Year Temperature Record

65 Million Years

The image below shows the climate was much warmer prior to  7 million years ago. Here we can see the Eocine optimum and the PETM event, which is assumed to have involved a methane hydrate clathrate release that caused a temperature spike.

Eocine Optimum

542 Million Years

The past 542 million years of climate as it is currently understood.

542 Million Years Global Temperature Record

There are many reasons for the dramatic temperature differences and science is continually investigation to better understand past climate. Some of the reasons include Pangaea which is when the land mass of earth was connected 250 million years ago. Imagine the more land all facing the sun. Easy to understand how that can produce a warmer world. But that is an oversimplification of influences and science is never entirely settled but rather investigated further.

That does not mean that there are many things we understand fairly well or are even very certain of. Don't assume that because knowledge is not perfect that one can not have a very strong understanding of what influences climate. In our recent history, the past one million years, we have very strong understanding of climate influences.

Milutin Milankovitch calculated the cycles that influence the general climate of earth in the early part of the 20th century. It was not until after his death that his hypothesis was confirmed by the deep ocean sediment core studies.

The Milankovitch Cycles

 

Milankovitch ETP w/caption

These cycles increase and decrease the amount of solar forcing imposed within our climate system and that actually causes the temperature to rise and fall with calculable regularity. The more time the earth or land mass spends closer to the sun (at perihelion), the more energy it receives thus warming. The more time it spends farther form the sun (at aphelion) the less energy it receives and the earth cools.

  • The 'eccentricity' cycle period is around 100,000 years. This causes the orbit of the earth to elongate or become more elliptical. Imagine that the more elliptic it becomes, the less time during the year it spends near the sun. So the planet receives less solar energy and cools a bit.
  • The 'obliquity' cycle tilts the earth every 41,000 years and that causes the land mass of the norther hemisphere to face more towards the sun or less towards the sun.
  • The 'precession' cycle occurs about every 26,000 years and influences the wobble of the polar axis. This also influences earths climate by causing winters and summers to be warmer or colder depending on the amount of land surface being more or less exposed to the sun.

These are three main influences considered in the Milankovitch theory that regulate the general amount of energy received in our earth climate system. As we warm and cool, more or less of our natural greenhouse gases are released into the atmosphere, or stored in the oceans, ice and earth.

Solar Cycles

The sun also goes through cycles. Solar observations began in the time of Galileo. Even then they noticed that the number of sunspots changed and might be connected to climate.

Solar Sunspot Cycles

 

This sunspot cycle was detected as early as the 5th century. NASA began taking satellite readings of the total solar irradiance in 1978.

Solar Irradiance

Over the years, they refined their ability to calculate the total solar irradiance and we now have a very accurate measurement of the amount of energy that reaches our outer atmosphere.

A Change in the Atmospheric Composition

 

The main difference between the natural cycle and what is now called the anthropogenic cycle is that we have altered the atmospheric composition of greenhouse gases and therefore increased the climate forcing.

Greenhouse Gas Development

A Change in the Forcing Levels

This following image shows the last 800,000 years of temperature and forcing levels. Essentially, we have largely departed the climate forcing from the natural cycle.

Forcing Levels 800k-yrs

IPCC

 

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