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 11,700 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 the Earth’s orbit that alter the amount of solar energy our planet receives.
The current warming trend is of particular importance because it is most likely (more than 95% probability) the result of human activities since the mid-20th century.
Earth orbiting satellites and other technological advances have allowed scientists to see the big picture and gather many different types of information about our planet and its climate on a global scale. These data, collected over many years, show the signals of a changing climate.
The heat-trapping nature of carbon dioxide and other gases was discovered in the mid-19.2 Their ability to affect the transmission of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that elevated greenhouse gases must cause the Earth to warm in response.
Ice cores from Greenland, Antarctica, and tropical mountain glaciers show that 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 shows that current warming is occurring about ten times faster than the average rate of ice age recovery warming. Carbon dioxide from human activity is increasing more than 250 times faster than from natural sources after the last ice age.
Global Temperature Rise
Since the beginning of the 21st century, the planet’s average surface temperature has increased by about 2.12 degrees Fahrenheit (1.18 degrees Celsius), a change caused primarily by increased carbon dioxide emissions to the atmosphere and other human activities.4 Most of the warming has occurred in the last 40 years, with the last seven years being the warmest. The years 2016 and 2020 are tied for the warmest year on record.
The ocean has absorbed much of this increased heat, with the top 100 meters (about 328 feet) of ocean showing a warming of more than 0.6 degrees Fahrenheit (0.33 degrees Celsius) since 1969 Earth is storing 90% of the extra energy in the ocean.
Shrinking ice floes
The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA’s Gravity Recovery and Climate Experiment show that Greenland lost an average of 279 billion tons of ice per year between 1993 and 2019, while Antarctica lost about 148 billion tons of ice per year.
Glaciers are retreating almost everywhere in the world-including the Alps, Himalayas, Andes, Rockies, Alaska, and Africa.
Satellite observations show that spring snowpack in the Northern Hemisphere has decreased over the past five decades, and snow is melting earlier.
Global sea level rose 20 centimeters over the last century. However, the rate over the past two decades is nearly double that of the last century and is accelerating slightly each year.10