Climate Change: How Do We Know?

NASA

This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO₂ has increased since the Industrial Revolution. (Credit: Vostok ice core data/J.R. Petit et al.; NOAA Mauna Loa CO2 record.) Find out more about ice cores (external site).

The Earth's climate has changed throughout history. Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 7,000 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth's orbit that change the amount of solar energy our planet receives.

Scientific evidence for warming of the climate system is unequivocal.
- Intergovernmental Panel on Climate Change

The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20^th century and proceeding at a rate that is unprecedented over decades to millennia.¹
Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.
The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century.² Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause the Earth to warm in response.
Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth's climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly ten times faster than the average rate of ice-age-recovery warming.³

The evidence for rapid climate change is compelling:

Sea level rise Image: Republic of Maldives: Vulnerable to sea level rise Global sea level rose about 8 inches in the last century. The rate in the last two decades, however, is nearly double that of the last century.4 Links Earth's vital signs: Sea Level Quiz: Sea level	Global temperature rise The planet's average surface temperature has risen about 2.0 degrees Fahrenheit (1.1 degrees Celsius) since the late 19th century, a change driven largely by increased carbon dioxide and other human-made emissions into the atmosphere.5 Most of the warming occurred in the past 35 years, with 16 of the 17 warmest years on record occurring since 2001. Not only was 2016 the warmest year on record, but eight of the 12 months that make up the year — from January through September, with the exception of June — were the warmest on record for those respective months.6 Links Earth's vital signs: Global temperature Global Climate Change: Causes Graphic: Global warming from 1880 to 2015
Warming oceans The oceans have absorbed much of this increased heat, with the top 700 meters (about 2,300 feet) of ocean showing warming of 0.302 degrees Fahrenheit since 1969.7 Links Climate Kids: What is happening in the ocean? Video: Oceans of climate change Article: Warming ocean causing most Antarctic ice shelf mass loss	Shrinking ice sheets Image: Flowing meltwater from the Greenland ice sheet The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost 150 to 250 cubic kilometers (36 to 60 cubic miles) of ice per year between 2002 and 2006, while Antarctica lost about 152 cubic kilometers (36 cubic miles) of ice between 2002 and 2005. Links Vital Signs: Land Ice Global Ice Viewer
Declining Arctic sea ice Image: Visualization of the 2007 Arctic sea ice minimum Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades.8 Links Earth's vital signs: Sea ice Interactive: Global ice viewer NASA's Operation Icebridge mission	Glacial retreat Image: The disappearing snowcap of Mount Kilimanjaro, from space. Glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska and Africa.9 Link Interactive: Global ice viewer
Extreme events The number of record high temperature events in the United States has been increasing, while the number of record low temperature events has been decreasing, since 1950. The U.S. has also witnessed increasing numbers of intense rainfall events.10 Links Precipitation Measurement Missions Precipitation Quiz	Ocean acidification Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30 percent.11, 12 This increase is the result of humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the oceans. The amount of carbon dioxide absorbed by the upper layer of the oceans is increasing by about 2 billion tons per year.13, 14 Links How does climate change affect coral reefs The Other Carbon Dioxide Problem
Decreased snow cover Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and that the snow is melting earlier.15 Links National Snow and Ice Data Center Earth Observatory: Snow Cover

References

1. IPCC Fifth Assessment Report, Summary for Policymakers
   B.D. Santer et.al., "A search for human influences on the thermal structure of the atmosphere," Nature vol 382, 4 July 1996, 39-46
   Gabriele C. Hegerl, "Detecting Greenhouse-Gas-Induced Climate Change with an Optimal Fingerprint Method," Journal of Climate, v. 9, October 1996, 2281-2306
   V. Ramaswamy et.al., "Anthropogenic and Natural Influences in the Evolution of Lower Stratospheric Cooling," Science 311 (24 February 2006), 1138-1141
   B.D. Santer et.al., "Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes," Science vol. 301 (25 July 2003), 479-483.
2. In the 1860s, physicist John Tyndall recognized the Earth's natural greenhouse effect and suggested that slight changes in the atmospheric composition could bring about climatic variations. In 1896, a seminal paper by Swedish scientist Svante Arrhenius first predicted that changes in the levels of carbon dioxide in the atmosphere could substantially alter the surface temperature through the greenhouse effect.
3. National Research Council (NRC), 2006. Surface Temperature Reconstructions For the Last 2,000 Years. National Academy Press, Washington, D.C.
   http://earthobservatory.nasa.gov/Features/GlobalWarming/page3.php
4. https://www.ipcc.ch/pdf/assessment-report/ar5/syr/AR5_SYR_FINAL_SPM.pdf
   Church, J. A. and N.J. White (2006), A 20th century acceleration in global sea level rise, Geophysical Research Letters, 33, L01602, doi:10.1029/2005GL024826.
   The global sea level estimate described in this work can be downloaded from the CSIRO website.
5. https://www.ncdc.noaa.gov/indicators/
   http://www.cru.uea.ac.uk/cru/data/temperature
   http://data.giss.nasa.gov/gistemp
6. https://www.giss.nasa.gov/research/news/20170118/)  
7. Levitus, et al, "Global ocean heat content 1955–2008 in light of recently revealed instrumentation problems," Geophys. Res. Lett. 36, L07608 (2009).
8. L. Polyak, et.al., "History of Sea Ice in the Arctic," in Past Climate Variability and Change in the Arctic and at High Latitudes, U.S. Geological Survey, Climate Change Science Program Synthesis and Assessment Product 1.2, January 2009, chapter 7
   R. Kwok and D. A. Rothrock, "Decline in Arctic sea ice thickness from submarine and ICESAT records: 1958-2008," Geophysical Research Letters, v. 36, paper no. L15501, 2009
   http://nsidc.org/sotc/sea_ice.html
9. National Snow and Ice Data Center
   World Glacier Monitoring Service
10. "Attribution of Extreme Weather Events in the Context of Climate Change," National Academies Press, 2016
    https://www.nap.edu/read/21852/chapter/1
    Kunkel, K. et al, "Probable maximum precipitation and climate change," Geophysical Research Letters, (12 April 2013) DOI: 10.1002/grl.50334
    Kunkel, K. et al, "Monitoring and Understanding Trends in Extreme Storms: State of the Knowledge," Bulletin of the American Meteorological Society, 2012.
    http://lwf.ncdc.noaa.gov/extremes/cei.html
11. http://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidification%3F
12. http://www.pmel.noaa.gov/co2/story/Ocean+Acidification
13. C. L. Sabine et.al., "The Oceanic Sink for Anthropogenic CO2," Science vol. 305 (16 July 2004), 367-371
14. Copenhagen Diagnosis, p. 36.
15. National Snow and Ice Data Center
    C. Derksen and R. Brown, "Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections," GRL, 39:L19504
    http://nsidc.org/cryosphere/sotc/snow_extent.html
    Rutgers University Global Snow Lab, Data History Accessed August 29, 2011.