By accurately measuring global climate change, scientists have arrived at a surer understanding of what causes this phenomenon. Today, scientists know that climate variation has both natural and anthropogenic (human-induced) causes.

Natural Causes

The natural causes of normal climate variations include changes in solar activity, volcanic activity, variations in Earth’s orbit, and the role of the oceans. Among these, the variations in Earth’s orbit is the major driver of glacial and interglacial periodicity. It is important to note that the climate change we are experiencing today is a level of variation that far exceeds the normal climate variations caused by these natural causes.

Solar Activity

Closer Look


Learn more about the sun’s fascinating sunspots.

Solar activity determines the amount of solar radiation that the sun emits. Sunspots are storms on the sun’s surface that are accompanied by intense magnetic activity; the storms and magnetic activity affect the output of solar radiation. There is an 11- to 22-year cycle of sunspots, which causes the total solar irradiance to vary within the cycle, and affects Earth’s climate. However, the variation in solar radiation caused by sunspot cycles is relatively small compared to total solar output (~0.1%) and far too low to cause the temperature changes observed by climate scientists today.

Volcanic Activity

Volcanoes emit large amounts of ash that can remain in the atmosphere for long time periods, blocking solar radiation and effectively lowering the solar input to Earth, which causes a cooling period. The long period of cooling between 1500 and the end of the 1800s known as the “Little Ice Age” is now considered to be the result of a sizable increase in world-wide volcanic activity. There is some evidence for at least four major volcanic eruptions that initiated this cooling period.

Earth's Orbit

Milutin Milankovitch, a Serbian astronomer and mathematician, suggested that variations in Earth’s orbit affect both the amount and the distribution of sunlight received at the Earth’s surface, which directly impacts the warming of Earth. These variations are called the “Milankovitch Cycles” (Figure 12) and are caused by three forces;

Milankovitch cycles
Figure 12. The Milankovitch cycles. 1
  • changes in the shape of Earth’s orbit around the sun (eccentricity),
  • the tilt of Earth on its axis (obliquity), and
  • the wobbling of the Earth’s axis (precession).

Milankovitch’s theory explains the timing of the past ice ages and major continental glaciations according to paleoclimatology studies, but these cycles occur over tens of thousands of years and longer, and cannot account for the rapid temperature changes observed in the last few decades.

The Role of the Oceans in Moderating the Climate

The oceans cover 70% of the earth’s surface and because of their great depth and the high specific heat capacity of water, oceans retain much more heat than land surfaces. The 2013 IPCC report indicates that 90% of the net energy increase in the climate system between 1971 and 2010 is stored in the Earth’s oceans; with 60% being stored in the upper ocean (0-700 meters depth), and 30% being stored in depths below 700 meters.

Closer Look


Learn more about ENSO and learn more about how climate variability has been determined by scientists, click here.

There is a natural interactive 3-6 year cycle that involves both the oceans and the atmosphere that has be termed the El Niño-Southern Oscillation (ENSO). ENSO has a major impact on regional climate, often with disastrous consequences. Every few years, it brings flooding to some areas and drought to others. It is believed that as more heat becomes stored in the waters of the ocean, the ENSO effect will become more extreme. This means that ocean storms, including typhoons and hurricanes, will become more intense and frequent. Read more about El Niño at the National Oceanic and Atmospheric Administration webpage. As the examples above demonstrate, natural factors do affect Earth’s climate. However, the changes in climate that have taken place since the 1900s do not fit the patterns of natural variability in climate as caused by these natural factors alone.

Anthropogenic Causes

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There is overwhelming scientific evidence that the significant climate changes that are currently taking place are due to anthropogenic causes. While the natural causes discussed above can help explain historic climate variations, they cannot account for the dramatic warming that the Earth has been experiencing since the 1950s. As stated in the 2007 IPCC report, “Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

carbon cycle
Figure 13: The carbon cycle diagram diagram. White numbers in parenthesis indicate how much carbon (in gigatons) is stored in the carbon reservoirs such as the oceans, atmosphere, and land. The arrows show the movement of carbon between different reservoirs. Yellow numbers are natural fluxes of carbon and red are human contributions in gigatons of carbon per year. 2

Looking Back

To review photosynthesis, go back to the Energy Chapter.

The greenhouse gas emitted into the atmosphere in the highest quantity, due to human activity is carbon dioxide. Carbon is naturally cycled through the Earth’s oceans, land, biosphere, and atmosphere (Figure 13). In the atmosphere, carbon is present primarily as carbon dioxide gas (CO2). Carbon dioxide is released into the atmosphere from natural sources such as plant, microbe, and animal respiration. Alternately, carbon dioxide is absorbed out of the atmosphere through natural sinks, such as the process of photosynthesis used by plants and algae. Through this natural cycle, the amount of CO2 released through respiration balances the amount consumed by photosynthesis. In this way, the atmosphere neither accumulates too much CO2 nor becomes depleted of CO2. Here, the amount of CO2 released through respiration balances the amount consumed by photosynthesis.

Coalbrookdale by Night painting
Figure 14: The artist Philip James de Loutherbourg painted this picture of Coalbrookdale by Night in 1801. It depicts an example of Industrial Revolution carbon emission, in this case from the ‘Bedlam’ iron smelting furnaces of England’s Coalbrookdale Company. 3

However, as you recall from the Energy Chapter, since the Industrial Revolution, humans have extracted large amounts of carbon-based resources, such as wood, peat, and fossil fuels from within the Earth’s crust (Figure 14). Fossil fuels, such as coal, oil, and natural gas are created from buried accumulations of decaying plant and animal organic compounds that have gradually been transformed.

Once extracted, humans burn or combust these carbon-based resources for fuel. The carbon that is emitted as a result of burning these fuels reacts with the oxygen that naturally occurs in the atmosphere and releases both heat and carbon dioxide in a process called a combustion reaction. The amount of carbon dioxide added to the atmosphere through combustion reactions far exceeds the ability of plants and ocean algae to take-up through photosynthesis. This pushes the natural carbon cycle significantly out of balance.

For example, the atmospheric concentration of CO2 rose from 280 ppm (parts per million) in 1850 to over 400 ppm in 2014 (a 30% increase) because carbon dioxide was added to the atmosphere through fossil fuel emissions faster than it was removed by plants and algae. The maximum carbon dioxide level for Earth to maintain balances among its many systems and natural processes is <350 ppm. This is often referred to as the “tipping point”.

rise in CO2 graph
Figure 15: Rise in CO2 ppm since 2005. [eftn_note] Source: [/efn_note]

Questions to Consider

Imagine you are talking to a friend who is convinced that global climate change has natural causes and that human actions have not significantly contributed to this phenomenon.

What scientific evidence could you offer to challenge your friend’s point of view?

If your friend does not accept your scientific evidence, what point would you make next to try and help your friend understand the problem of global climate change?

The burning of fossil fuels is not the only anthropogenic driver of global climate change. As noted in the discussion of greenhouse gases earlier in this section, climate change has also been accelerated by industrial agricultural practices and deforestation. In fact, industrial agricultural practices produce more greenhouse gas emissions than any other human activity. Additionally, deforestation is largely linked to industrial agriculture to expand land under cultivation.