Climate Change Caused by Humans
Climate Change: Natural Causes

When one examines the natural causes for climate change it becomes quite apparent that none of these can explain the modern day climate change.

Tectonic-Scale Climate Change (Millions of Years):

Figure 4.1 (Ruddiman, 2008) shows how carbon is cycled into and out of the atmosphere by plate tectonics over millions of years.

Tectonic-Scale Climate Change
Figure 4.1: Tectonic-Scale Climate Change

Through a process known as chemical weathering, rainwater combines with CO2 gas in the air and forms carbonic acid. Carbonic acid "attacks" the silicate bedrock and creates carbon-containing ions that are carried to the ocean by rivers. This carbon is ultimately stored in the shells of marine plankton. When marine plankton die, they fall to the sea floor where their carbon gets buried in the sediment. Therefore, chemical weathering removes atmospheric carbon and causes a cooler climate.

During the process of plate tectonics the sea floor spreads. As the sea floor spreads, sediment containing carbon is forced into the earth's interior (by a process known as subduction) and is melted. When magma rises and is ejected by volcanoes, the carbon is released back into the air. During increased periods of plate tectonics (more volcanism) there are higher levels of CO2 in the atmosphere and the climate warms.

Approximately 55 million years ago, India slammed into Asia and began to build the Himalayan Mountains. These mountains are still rising today. Due to the massive amount of material being uplifted by this collision, chemical weathering rates over the past 55 million years have been very high resulting in a gradual tectonic cooling since that time. Recall Figure 1.4:

Deep water oxygen 18 ratios vs. time
Figure 1.4: Deep water δ18O values vs. time (Ruddiman, 2001)

Because plate tectonics changes climate over millions of years, it cannot be responsible for the rapid climate change observed today.

Orbital-Scale Climate Change (Tens to Hundreds of Thousand Years):

Orbital-Scale Climate Change
Figure 4.2: Orbital-Scale Climate Change (Ruddiman, 2001)

Figure 4.2 shows that climate also cycles along a 100,000 year and 41,000 year cycle. These climate changes are the result of changes in the shape of the orbit of Earth around the sun and also the change of the tilt of Earth over time. The cycle is often referred to as the Milankovic Cycle named after Serbian engineer and mathematician Milutin Milankovic who proposed the theory.

The tilt of the Earth varies between 22.2o and 24.5o over a period of 41,000 years. This change in tilt causes long-term variations in the amount of seasonal radiation received on Earth, especially at higher latitudes. Increased tilt amplifies the seasonal differences while decreased tilt reduces the seasonal differences.

The shape of the Earth's orbit around the sun is measured by eccentricity (denoted by ε) and changes over time. When ε = 0 the orbit would be a perfect circle. ε values for Earth's orbit range between 0.005 and 0.0607. Today, Earth's ε = 0.0167 which is close to being a circular orbit. There is a cycle (period) between maximum and minimum ε values every 100,000 years.

When the Earth is closest to the sun it is at the perihelion position and when Earth is farthest away it is at the aphelion position. The tilt of the Earth is much more important than the distance to the sun (in fact present day Earth is actually 3 million miles closer to the sun in January). However, when the tilt is at a minimum and the Earth is at aphelion, climate is at its coolest as shown by Figure 4.3 (Ruddiman, 2001).

Milankovitch Cool Climate
Figure 4.3: Milankovic Cool Climate (Ice Growth)

When the tilt is at a maximum and the Earth is at perihelion, climate is at its warmest as shown by Figure 4.4 (Ruddiman, 2001).

Milankovitch Warm Climate
Figure 4.4: Milankovic Warm Climate (Ice Decay)

Because Milankovic cycles cause climate to change over tens to hundreds of thousand years, it cannot be responsible for the rapid climate change observed today.

Shorter-Scale Climate Change (Decades or Less):

The Pacific Decadal Oscillation (PDO) is a long-lived El Ni�o-like pattern of Pacific climate variability. PDOs typically last between 20 and 30 years while El Ni�o/Southern Oscillation (ENSO) events last between six to 18 months. PDOs influence the sea surface temperature and wind patterns in the North Pacific with secondary influences existing in the tropics - the opposite is true for ENSO. Figure 4.5 illustrates the warm and cool phases of the PDO. (Mantua, 2000)

PDO Warm vs. Cool Phase
Figure 4.5: Warm (positive) and cool (negative) phases of the PDO (ibid)

Several independent studies find evidence for just two full PDO cycles in the past century: cool PDO regimes prevailed from 1890-1924 and again from 1947-1976, while warm PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. (ibid) Figure 4.6 (ibid) shows the PDO index since 1900.

PDO Index
Figure 4.6: PDO index since 1900

Causes for the PDO are not currently known. Likewise, the potential predictability for this climate oscillation are not known. Some climate simulation models produce PDO-like oscillations, although often for different reasons. However, like ENSO, the warm and cool phases typically cancel out in the long run so the PDO cannot explain the long term warming trend observed in the modern climate record. Figure 4.7 (Cook, 2008) illustrates this quite well:

PDO Index vs. Warming Trend
Figure 4.7: PDO index vs. warming trend

Because the sun is the primary heat source for the Earth-atmosphere system, changes in incoming sunlight can influence climate. Sunspot numbers correlate well with solar irradiance - more sunspots means a stronger sun while fewer sunspots means a weaker sun. Figure 4.8 (Ruddiman, 2008) shows this relationship quite well. A sunspot cycle is one in which numbers go from high to low and back to high. This cycle averages about 11 years and is known as the sunspot cycle.

Sunspot Cycle and Solar Irradiance
Figure 4.8: Sunspot Cycle and Solar Irradiance

Figure 4.8a (Cook, 2010) shows that total solar irradiance cannot explain the rise in temperatures since 1980. Furthermore, solar activity has been very low in the second half of the 2000s yet temperatures are still higher than the previous few decades. 2009 was the 2nd warmest year on record according to NASA GISS data while the sun was at its weakest in over 100 years!

Total Solar Irradiance vs. Global Temperatures
Figure 4.8a: Total Solar Irradiance vs. Global Temperatures

According to the IPCC (2007) current estimates suggest that only 0.1 oC of the 0.8 oC of warming since the late 1800s is due to solar irradiance. More importantly, since direct satellite measurements (1980 � present) solar contribution to the observed rapid warming is negligible. There is no evidence that variations in the strength of the sun are the cause of the modern day climate change.

What would happen if the Earth entered a prolonged solar minimum period such as that during the Maunder Minimum? Feulner and Rahmstorf (2010) asked that question and modeled the results based on two GHG emission scenarios. Scenario A1B has emissions eventually declining by the year 2100 while scenario A2 has emissions increasing throughout the century. Figure 4.8b (Cook, 2010) shows that in either scenario, the warming from GHGs dwarfs changes in solar radiation. There is little difference in the climate change whether there is the normal 11 year sunspot cycle (indicated by solid lines) or a Maunder Minimum type scenario (indicated by dashed lines).

Maunder Minimum and Climate Change
Figure 4.8b: Global mean temperature anomalies 1900 to 2100 relative to the period 1961 to 1990 for the A1B (red lines) and A2 (magenta lines) scenarios and for three different solar forcings

As John Cook states in a wonderful blog post on the subject:

For both the A1B and A2 emission scenario, the effect of a Maunder Minimum on global temperature is minimal. The TSI reconstruction with lesser variation shows a decrease in global temperature of around 0.09oC while the stronger variation in solar forcing shows a difference of around 0.3oC. Compare this to global warming between 3.7oC (A1B scenario) to 4.5oC (A2 scenario). Considering the less variable solar reconstruction shows such strong agreement with past temperature, the authors conclude the most likely impact of a Maunder Minimum by 2100 would be a decrease in global temperature of 0.1oC . With all uncertainties taken into account, the estimated maximum decrease in global temperature is 0.3oC.
No Solar Influence Peter Sinclair's Climate Crock of the Week: Solar Schmolar
Watch this video that illustrates why the sun cannot be causing the current global warming.

Figure 4.9 (Ruddiman, 2008) shows the influence of volcanic eruptions and El Niño events.

Influence of Volcanoes and El Nino
Figure 4.9: Influence of volcanoes and El Niño on climate

Volcanoes spew ash and sulfuric gases which are converted into sulfate aerosols. These substances reduce incoming solar radiation and can linger in the atmosphere for several years causing a cooler climate. The current global warming was greatly reduced by the 1991 eruption of Mount Pinatubo in the Philippines. However, the record shows that the influence was short-lived (a few years).

El Niño events cause global warming on a short time scale. Notice the year 1998 in which a particularly strong El Niño caused a record spike in global temperatures.

Because volcanic eruptions and El Niño events cause climate change on a scale of one to three years, these natural forcing mechanisms cannot be responsible for the global warming observed today.

A recent paper by noted skeptics McLean, de Freitas, and Carter (2009) titled Influence of the Southern Oscillation on Tropospheric Temperature, received much attention. The authors concluded that much of the recent trend in global warming was caused ENSO events. Unfortunately, their methodoology was severely flawed because their analysis effectively removed any trends related to non-ENSO global warming. A paper yet to be put into print details these flaws. See Foster et al. (2009).

Next: Global Cooling?

Scott A. Mandia
Professor - Physical Sciences
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Last Updated: 11/1/18