Causes

The Causes of Climate Change

Human activities are driving the global warming trend observed since the mid-20th century.
Takeaways

  • The greenhouse effect is essential to life on Earth, but human-made emissions in the atmosphere are trapping and slowing heat loss to space.
  • Five key greenhouse gases are CO2, nitrous oxide, methane, chlorofluorocarbons, and water vapor.
  • While the Sun has played a role in past climate changes, the evidence shows the current warming cannot be explained by the Sun.

The Greenhouse Effect

Increasing Greenhouses Gases Are Warming the Planet

Scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the "greenhouse effect" — warming that results when the atmosphere traps heat radiating from Earth toward space.

Life on Earth depends on energy coming from the Sun. About half the light energy reaching Earth's atmosphere passes through the air and clouds to the surface, where it is absorbed and radiated in the form of infrared heat. About 90% of this heat is then absorbed by greenhouse gases and re-radiated, slowing heat loss to space.

Gases that Contribute

The Role of Humans

Human Activity Is the Cause of Increased Greenhouse Gas Concentrations

Over the last century, burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This increase happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2. To a lesser extent, clearing of land for agriculture, industry, and other human activities has increased concentrations of greenhouse gases.

The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels by nearly 50% since 1750. This increase is due to human activities, because scientists can see a distinctive isotopic fingerprint in the atmosphere.

In its Sixth Assessment Report, the Intergovernmental Panel on Climate Change, composed of scientific experts from countries all over the world, concluded that it is unequivocal that the increase of CO2, methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the principal driver of many changes observed across the atmosphere, ocean, cryosphere and biosphere.

"Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact."
- Intergovernmental Panel on Climate Change

The panel's AR6 Working Group I (WGI) Summary for Policymakers report is online at https://www.ipcc.ch/report/ar6/wg1/.

Is the Sun to Blame?

Evidence Shows That Current Global Warming Cannot Be Explained by Solar Irradiance

Scientists use a metric called Total Solar Irradiance (TSI) to measure the changes in energy the Earth receives from the Sun. TSI incorporates the 11-year solar cycle and solar flares/storms from the Sun's surface.

Studies show that solar variability has played a role in past climate changes. For example, a decrease in solar activity coupled with increased volcanic activity helped trigger the Little Ice Age.

temperature vs solar activity updated July 2020
The above graph compares global surface temperature changes (red line) and the Sun's energy that Earth receives (yellow line) in watts (units of energy) per square meter since 1880. The lighter/thinner lines show the yearly levels while the heavier/thicker lines show the 11-year average trends. Eleven-year averages are used to reduce the year-to-year natural noise in the data, making the underlying trends more obvious.

The amount of solar energy that Earth receives has followed the Sun’s natural 11-year cycle of small ups and downs with no net increase since the 1950s. Over the same period, global temperature has risen markedly. It is therefore extremely unlikely that the Sun has caused the observed global temperature warming trend over the past half-century. Credit: NASA/JPL-Caltech

But several lines of evidence show that current global warming cannot be explained by changes in energy from the Sun:

  • Since 1750, the average amount of energy from the Sun either remained constant or increased slightly.
  • If a more active Sun caused the warming, scientists would expect warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere and a warming at the surface and lower parts of the atmosphere. That's because greenhouse gases are slowing heat loss from the lower atmosphere.
  • Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.
References

References

  1. IPCC Fifth Assessment Report, 2014

    United States Global Change Research Program, "Global Climate Change Impacts in the United States," Cambridge University Press, 2009

    Naomi Oreskes, "The Scientific Consensus on Climate Change," Science 3 December 2004: Vol. 306 no. 5702 p. 1686 DOI: 10.1126/science.1103618

  2. Friedlingstein et al 2021: “The mass of carbon in the atmosphere increased by 48% from 590 GtC in 1750 to 876 GtC in 2020.https://essd.copernicus.org/preprints/essd-2021-386/

  3. U.S. Environmental Protection Agency: "Climate Impacts on Agriculture and Food Supply"

  4. Mike Lockwood, “Solar Change and Climate: an update in the light of the current exceptional solar minimum,” Proceedings of the Royal Society A, 2 December 2009, doi 10.1098/rspa.2009.0519;

    Judith Lean, “Cycles and trends in solar irradiance and climate,” Wiley Interdisciplinary Reviews: Climate Change, vol. 1, January/February 2010, 111-122.