Earth system science is the study of how scientific data stemming from various fields of research, such as the atmosphere, oceans, land ice and others, fit together to form the current picture of our planet as a whole, including its changing climate.
Climate scientists separate factors that affect climate change into three categories: forcings, feedbacks, and tipping points.
Forcings: The initial drivers of climate.
- Solar Irradiance. Solar radiation is the source of heat for planet Earth. Scientists also use evidence from proxy measurements, such as sunspot counts going back centuries and ancient tree rings, to measure the amount of Sun that reaches Earth’s surface. The Sun has an 11-year sunspot cycle, which causes about 0.1% of the variation in the Sun’s output.1 The solar cycle is incorporated into climate models.
- Greenhouse gas emissions. Since the industrial revolution, concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have risen in the atmosphere. Burning fossil fuels such as coal, oil and gas has increased the concentration of atmospheric carbon dioxide (CO2) from 280 parts per million to 393 parts per million.2 These greenhouse gases absorb and then re-radiate heat in Earth’s atmosphere, which causes increased warming.
- Aerosols, dust, smoke, and soot. Very small airborne particles come from both human and natural sources and have various effects on climate. Sulfate aerosols, which result from burning coal, biomass, and volcanic eruptions, tend to cool the Earth. Other kinds of particles such as black carbon have a warming effect.3 The global distribution of aerosols is being tracked from the ground and from satellites.
Climate feedbacks: processes that can either amplify or diminish the effects of climate forcings. A feedback that increases an initial warming is called a "positive feedback." A feedback that reduces an initial warming is a "negative feedback."
- Clouds. Clouds have an enormous impact on Earth's climate, reflecting about one-third of the total amount of sunlight that hits the Earth's atmosphere back into space. Even small changes in cloud amount, location and type could have large consequences. A warmer climate could cause more water to be held in the atmosphere, leading to an increase in cloudiness and altering the amount of sunlight that reaches the surface of the Earth. Less heat would get absorbed, which could slow the increased warming.
- Precipitation. Global climate models show that precipitation will generally increase due to the increased amount of water held in a warmer atmosphere, but not in all regions. Some regions will dry out instead. Changes in precipitation patterns, such as increased water availability, may cause an increase in plant growth, which in turn could potentially remove more carbon dioxide from the atmosphere.
- Greening of the forests. Natural processes, such as tree growth, remove about half of human carbon dioxide emissions from the atmosphere every year. Scientists are currently studying where this carbon dioxide goes. The delicate balance between the absorption and release of carbon dioxide by the oceans and the world’s great forested regions is the subject of research by many scientists. There is some evidence that the ability of the oceans or forests to continue absorbing carbon dioxide may decline as the world warms, leading to faster accumulation in the atmosphere.
- Ice albedo. Ice is white and very reflective, in contrast to the ocean surface, which is dark and absorbs heat faster. As the atmosphere warms and sea ice melts, the darker ocean absorbs more heat, causes more ice to melt, and makes the Earth warmer overall. The ice-albedo feedback is a very strong positive feedback.
Climate tipping points: When Earth’s climate abruptly moves between relatively stable states.
- Ocean circulation. As Arctic sea ice and the Greenland ice sheet melt, ocean circulation in the Atlantic may divert the Gulf Stream. This and/or other changes would significantly change regional weather patterns. A change in the Gulf Stream could lead to a significant cooling in Western Europe. This highlights the importance of ocean circulation in maintaining regional climates.
- Ice loss. Due to the strong positive feedback of the ice albedo, if enough ice melts, causing Earth’s surface to absorb more and more heat, then we may hit a point of no return. Shrinking ice sheets contribute to sea level rise. Many hundreds of millions of people live near a coast, so our ability to predict sea level rise over the next century has substantial human and economic ramifications.
- Rapid release of methane. Deposits of frozen methane, a potent greenhouse gas, and carbon dioxide lie beneath permafrost in Arctic regions. About a quarter of the Northern hemisphere is covered by permafrost. As the environment warms and the permafrost thaws, these deposits can be released into the atmosphere and present a risk of enhanced warming.4
Claus Frohlich and Judith Lean, “Solar radiative output and its variability: evidence and mechanisms,” The Astronomy and Astrophysics Review, 2004, doi:10.1007/s00159-004-0024-1.
Trends in Atmospheric Carbon Dioxide, National Oceanic & Atmospheric Administration, 2013. http://www.esrl.noaa.gov/gmd/ccgg/trends/
NASA's Earth Observatory, "Aerosols: Tiny Particles, Big Impact," 1999.
A. Vaks et al, "Speleotherms Reveal 500,000-Year History of Siberian Permafrost," Science, April 12, 2013: Vol. 340 no. 6129 pp. 183-186, doi: 10.1126/science.1228729