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.
Human Activity and current trend of Global Warming
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-20th century and proceeding at a rate that is unprecedented over decades to millennia1.
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 century2. 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 warming3.
The evidence for rapid climate change
i) Sea level rise
One of the most dire impacts of anthropogenic (man made) climate change is a rise in the global sea level caused by the melting of glaciers and land-based ice caps, as well as a smaller increase from expansion due to the higher temperature of the water itself. Unlike some other predicted effects of climate change, this impact has already been observed for some time. Indeed, not only is there evidence that sea levels are rising; there is also evidence both that the rate of sea level rise has been increasing in recent years and that it will continue to increase. 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 century4.
ii) 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 atmosphere5. 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 months6.
iii) 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 Sea surface temperature increased during the 20th century and continues to rise. From 1901 through 2015, temperature rose at an average rate of 0.13°F per decade. Sea surface temperature has been consistently higher during the past three decades than at any other time since reliable observations began in 1880. Based on the historical record, increases in sea surface temperature have largely occurred over two key periods: between 1910 and 1940, and from about 1970 to the present. Sea surface temperature appears to have cooled between 1880 and 1910. Changes in sea surface temperature vary regionally. While most parts of the world’s oceans have seen temperature rise, a few areas have actually experienced cooling—for example, parts of the North Atlanti.
iv) Shrinking ice sheets
The Greenland and Antarctic ice sheets have decreased in mass. Data from USA's 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. The polar ice sheets are indeed shrinking—and fast, according to a comprehensive new study on climate change. And the effects, according to an international team, are equally clear—sea levels are rising faster than predicted, which could bring about disastrous effects for people and wildlife.
v) Extreme events
Recent weather events such as deadly heat waves and devastating floods have sparked popular interest in understanding the role of global warming in driving extreme weather. These events are part of a new pattern of more extreme weather across the globe, shaped in part by human-induced climate change. As the climate has warmed, some types of extreme weather have become more frequent and severe in recent decades, with increases in extreme heat, intense precipitation, and drought. Heat waves are longer and hotter. Heavy rains and flooding are more frequent. In a wide swing between extremes, drought, too, is more intense and more widespread.
vi) Ocean acidification
Given that they cover 70% of the Earth’s surface—and provide about 90% of the planet’s habitable space by volume—the oceans tend to get short shrift when it comes to climate change. The leaked draft of the forthcoming coming new report from the Intergovernmental Panel on Climate Change highlighted the atmospheric warming we’re likely to see, the rate of ice loss in the Arctic and the unprecedented (at least within the last 22,000 years) rate of increase of concentrations of greenhouse gases like carbon dioxide and methane. Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30 percent11,12. 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 year13, 14.
vii) Declining Arctic sea ice
Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades8. September 2012 had the lowest sea ice extent ever recorded, 44 percent below the 1981–2010 average for that month. The September 2016 sea ice extent was more than 700,000 square miles less than the historical 1981–2010 average for that month (see Figure 1)—a difference more than two and a half times the size of Texas. March sea ice extent reached the lowest extent on record in 2015 and hit roughly the same low again in 2016—about 7 percent less than the 1981–2010 average.
IPCC Fifth Assessment Report, Summary for Policymakers
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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.
National Research Council (NRC), 2006. Surface Temperature Reconstructions For the Last 2,000 Years. National Academy Press, Washington, D.C.
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.
Levitus, et al, "Global ocean heat content 1955–2008 in light of recently revealed instrumentation problems," Geophys. Res. Lett. 36, L07608 (2009).
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
"Attribution of Extreme Weather Events in the Context of Climate Change," National Academies Press, 2016
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.
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C. Derksen and R. Brown, "Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections," GRL, 39:L19504
Rutgers University Global Snow Lab, Data History Accessed August 29, 2011.