The rising average temperature of Earth's climate system, called global warming, is driving changes in rainfall patterns, extreme weather, arrivals of seasons, and more. Collectively, global warming and its effects are known as climate change. While there have been prehistoric periods of climatic change, observed changes since the mid-20th century have had such a devastating impact or progressed at such an unprecedented rate and scale.
Evidence
Global Temperature Rise
The planet's average surface temperature has risen about 1.62 degrees Fahrenheit (0.9 degrees Celsius) since the late 19th century, a change driven largely by increased carbon dioxide and other human-made emissions into the atmosphere [4]. Most of the warming occurred in the past 35 years, with the six warmest years on record taking place since 2014.
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 more than 0.4 degrees Fahrenheit since 1969 [6].
Shrinking Ice Sheets and Glacial Retreat
The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost an average of 286 billion tons of ice per year between 1993 and 2016, while Antarctica lost about 127 billion tons of ice per year during the same time period [7]. The rate of Antarctica ice mass loss has tripled in the last decade. Glaciers are retreating everywhere in the world including the Alps, Himalayas, Andes, Rockies, Alaska, and Africa.[8]
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 and is accelerating slightly every year [10].
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 [12].
Ocean Acidification
Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30 percent [13, 14]. 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 [15,16].
Causes
The Greenhouse Effect
Scientists attribute global warming trend to human's exacerbation of the "greenhouse effect" - the process by which greenhouse gases like water vapor, nitrous oxide, carbon dioxide, and methane trap solar radiation and warm the planet's surface.
On Earth, human activities are changing the natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2. To a lesser extent, the clearing of land for agriculture, industry, and other human activities has increased concentrations of greenhouse gases.
Solar Irradiance
It is reasonable to believe that changes in the Sun's output could cause climate change since it drives the greenhouse effect. However, evidence shows that this global warming is not a result of the Sun's energy output variability.
Since 1750, the average amount of energy coming from the Sun either remained constant or increased slightly.
If the warming were caused by a more active Sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere due to the greenhouse gas effect.
Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.
Effects
Change Will Continue Through This Century and Beyond
Global climate is projected to continue to change over this century and beyond. The magnitude of climate change depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions.
Temperatures Will Continue to Rise
Because human-induced warming is an expansion on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.
Changes in Precipitation Patterns
Projections of future climate suggest that the recent trend towards increased heavy precipitation events will continue. This trend is projected to occur even in regions where total precipitation is expected to decrease, such as the Southwest.
More Droughts and Heat Waves
Droughts and heat waves (periods of abnormally hot weather lasting days to weeks) everywhere are projected to become more intense, and cold waves less intense everywhere. Summer temperatures are projected to continue rising, and a reduction of soil moisture, which worsens heat waves, is projected.
Hurricanes Will Become Stronger and More Intense
The intensity, frequency and duration of hurricanes, as well as the frequency of the strongest hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.
Sea Level Rise
Global sea level has risen by about 8 inches since records from 1880. It is projected to rise another 1 to 8 feet by 2100. This is the result of added water from melting land ice and the expansion of seawater as it warms. In the next several decades, storm surges and high tides could combine with sea level rise and land subsidence to further increase flooding in many regions.
To prevent further climate change, we must reduce our global carbon footprint. The whole world must act in an environmentally conscious manner to reduce emissions by switching to low carbon and zero carbon alternatives in power, industry, agriculture, and transportation. The transition to clean power must be made now. To learn more, learn about how renewables can help mitigate emissions.
References
IPCC Fifth Assessment Report, Summary for Policymakers
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. http://earthobservatory.nasa.gov/Features/GlobalWarming/page3.php
Levitus, S.; Antonov, J.; Boyer, T.; Baranova, O.; Garcia, H.; Locarnini, R.; Mishonov, A.; Reagan, J.; Seidov, D.; Yarosh, E.; Zweng, M. (2017). NCEI ocean heat content, temperature anomalies, salinity anomalies, thermosteric sea level anomalies, halosteric sea level anomalies, and total steric sea level anomalies from 1955 to present calculated from in situ oceanographic subsurface profile data (NCEI Accession 0164586). Version 4.4. NOAA National Centers for Environmental Information. Dataset. doi:10.7289/V53F4MVP
National Snow and Ice Data Center Robinson, D. A., D. K. Hall, and T. L. Mote. 2014. MEaSUREs Northern Hemisphere Terrestrial Snow Cover Extent Daily 25km EASE-Grid 2.0, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/MEASURES/CRYOSPHERE/nsidc-0530.001. [Accessed 9/21/18]. http://nsidc.org/cryosphere/sotc/snow_extent.html Rutgers University Global Snow Lab, Data History Accessed September 21, 2018.
R. S. Nerem, B. D. Beckley, J. T. Fasullo, B. D. Hamlington, D. Masters and G. T. Mitchum. Climate-change–driven accelerated sea-level rise detected in the altimeter era. PNAS, 2018 DOI: 10.1073/pnas.1717312115
USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp, doi: 10.7930/J0J964J6
C. L. Sabine et.al., “The Oceanic Sink for Anthropogenic CO2,” Science vol. 305 (16 July 2004), 367-371