Scientific Answers to Climate Change Questions
This is an excerpt from the report below, focused on the topic above, edited by the author of the blog.
National Academy Press
Climate Change — Evidence and Causes
An overview from the Royal Society and the US National Academy of Sciences
Updated 2020
Marcia McNutt, President, National Academy of Sciences
Venki Ramakrishnan, President, Royal Society
Edited by
The Health Strategist
the blog of Joaquim Cardoso MSc
Q&A2: How do scientists know that recent climate change is largely caused by human activities?
Scientists know that recent climate change is largely caused by human activities from an understanding of
- basic physics,
- comparing observations with models, and
- fingerprinting the detailed patterns of climate change caused by different human and natural influences.
Since the mid-1800s, scientists have known that CO2 is one of the main greenhouse gases of importance to Earth’s energy balance.
Direct measurements of CO2 in the atmosphere and in air trapped in ice show that atmospheric CO2 increased by more than 40% from 1800 to 2019.
Measurements of different forms of carbon (isotopes, see Question 3) reveal that this increase is due to human activities.
Other greenhouse gases (notably methane and nitrous oxide) are also increasing as a consequence of human activities.
The observed global surface temperature rise since 1900 is consistent with detailed calculations of the impacts of the observed increase in atmospheric greenhouse gases (and other human-induced changes) on Earth’s energy balance.
Different influences on climate have different signatures in climate records.
These unique fingerprints are easier to see by probing beyond a single number (such as the average temperature of Earth’s surface), and by looking instead at the geographical and seasonal patterns of climate change.
The observed patterns of surface warming, temperature changes through the atmosphere, increases in ocean heat content, increases in atmospheric moisture, sea level rise, and increased melting of land and sea ice also match the patterns scientists expect to see due to human activities (see Question 5).
The expected changes in climate are based on our understanding of how greenhouse gases trap heat.
Both this fundamental understanding of the physics of greenhouse gases and pattern-based fingerprint studies show that natural causes alone are inadequate to explain the recent observed changes in climate.
Natural causes include variations in the Sun’s output and in Earth’s orbit around the Sun, volcanic eruptions, and internal fluctuations in the climate system (such as El Niño and La Niña).
Calculations using climate models (see infobox, p. 20) have been used to simulate what would have happened to global temperatures if only natural factors were influencing the climate system.
These simulations yield little surface warming, or even a slight cooling, over the 20th century and into the 21st. Only when models include human influences on the composition of the atmosphere are the resulting temperature changes consistent with observed changes.
Citation
“Q&A.” National Academy of Sciences. 2020. Climate Change: Evidence and Causes: Update 2020. Washington, DC: The National Academies Press. doi: 10.17226/25733.
Originally published at
Edited by:
The Health Strategist
the blog of Joaquim Cardoso MSc
