16.2: Evidence of Recent Climate Change

While climate has changed many times in the past due to natural causes, the scientific consensus is that human activity is causing the climate to change today more rapidly [11; 7]. While this seems like a new idea, it has been suggested for more than 75 years [12]. This section describes the evidence that scientists agree is most likely a result of anthropogenic climate change, or, human-caused climate change. For more information, watch this video on climate change by two professors at a North Carolina State University. Although the video is from 2015 and the data is not up to date, the trends have continued and still serve as evidence of climate change.

Global Temperature Rise

Since 1880, average global surface temperatures have trended upward and most of that warming has occurred since 1970 (see this NASA animation). Since the ocean is absorbing a lot of the additional trapped heat, surface temperatures include both land surface and ocean temperatures [13]. Changes in land-surface or ocean-surface temperatures compared to a reference period from 1951 to 1980, where the long-term average remained relatively constant, are called temperature anomalies. A temperature anomaly thus represents the difference between the measured temperature and the average value during the reference period. Climate scientists calculate long-term average temperatures over thirty years or more such as the reference period from 1951 to 1980. Another common period is the last century (1900-2000). Therefore, an anomaly of 1.25 ℃ (2.25°F) for 2015 means that the average temperature for 2015 was 1.25 ℃ (2.25°F) greater than the 1900-2000 average. In 1950, the temperature anomaly was -0.28 ℃ (-0.5°F), so this is -0.28 ℃ (-0.5°F) lower than the 1900-2000 average [3]. These temperatures are annual average surface temperatures.

Figure \(\PageIndex{1}\): Land-ocean temperature index, 1880 to present, with a base time 1951-1980. The solid black line is the global annual mean and the solid red line is the five-year Lowess smoothing. The blue uncertainty bars (95% confidence limit) account only for incomplete spatial sampling. The overall trend is that Earth's temperature is rising.

In addition to a rising average surface land temperature, the ocean has absorbed a lot of the heat. With oceans covering about 70% of the Earth’s surface, there is a lot of opportunities to absorb energy. The ocean has been absorbing about 80% to 90% of the additional heat added due to human activities. As a result, the top 2300 feet of the ocean has increased in temperature 0.17°C (0.3℉) since 1969 (external link to this 3-minute video by NASA JPL on heat capacity of the ocean) [3]. The reason the ocean has warmed less than the atmosphere, while still taking on most of the heat, is due to the very high specific heat of water, which means that water can absorb a lot of energy for a small temperature increase. In contrast, the atmosphere needs less energy to increase its temperature.

Some scientists suggest that anthropogenic greenhouse gases do not cause global warming since surface temperatures have not increased very much between 1998 and 2013, while greenhouse gas concentrations have continued to increase during that time period. However, since the oceans are absorbing most of the heat, decade-scale circulation changes in the ocean, similar to La Niña, push warmer water deeper under the surface [14; 15; 16]. Once the absorption and circulation of the ocean is accounted for and the heat added back into surface temperatures, then the temperature increases become apparent as shown in the above figure. Furthermore, this ocean heat storage is temporary, as reflected in the record-breaking warm years of 2014-2016. Indeed, with this temporary ocean storage effect, 15 of the first 16 years of the 21st century have been the hottest in recorded history.

Carbon Dioxide

Anthropogenic greenhouse gases, mostly carbon dioxide (CO₂), have increased since the Industrial Revolution when the burning of fossil fuels dramatically increased. These levels are unprecedented in the last 800,000-year Earth history as recorded in geologic sources such as ice cores. Carbon dioxide has increased by 40% since 1750 and the rate of increase has been the fastest during the last decade [3; 6]. For example, since 1750, 2040 gigatons of CO₂ have been added to the atmosphere, about 40% have remained in the atmosphere while the remaining 60% have been absorbed into the land (by plants and soil) or the oceans [6]. Indeed, during the lifetime of most young adults, the total atmospheric CO₂ has increased by 50 ppm or 15%.

Charles Keeling, an oceanographer with Scripps Institution of Oceanography in San Diego, California was the first person to make regular measurements of atmospheric CO₂. Using his methods, constant measurements of CO₂ in the atmosphere have been made at the Mauna Loa Observatory on Hawaii since 1958. These measurements are published regularly by NASA at this website: https://keelingcurve.ucsd.edu/. Go there now to see the very latest measurement. Keeling’s measured values have been posted in a curve of increasing values called the Keeling Curve. This curve varies annually up and down from summer (when the plants in the Northern Hemisphere are using CO₂) to winter when the plants are dormant, but shows a steady increase over the past several decades. This curve increases exponentially, not linearly indicating that the rate of increase of CO₂ is accelerating!

Figure \(\PageIndex{2}\): The Keeling Curve showing increasing atmospheric CO₂ since 1958. Note that the curve is exponential, not linear!

The following video shows how atmospheric CO₂ has varied recently and also over the last 800,000 years as determined by many CO₂ monitoring stations (shown on the insert map). It is also instructive to watch the CO₂ variation of the Keeling portion of the video by latitude. This shows that most of the human sources of CO₂ are in the Northern Hemisphere where most of the land is and where most of the developed nations are.

Melting Glaciers and Shrinking Sea Ice

Glaciers are large ice accumulations that exist year-round on the land's surface and ice sheets are thick glaciers that cover continents during ice ages. In contrast, icebergs are masses of floating sea ice, although they may have had their origin in glaciers. Alpine glaciers, ice sheets, and sea ice are all melting. Explore melting glaciers at NASA’s interactive Global Ice Viewer). Satellites have recorded that Antarctica is melting at 118 gigatons per year and Greenland is melting at 281 gigatons per year (1 gigaton is over 2 trillion pounds). Almost all major alpine glaciers are shrinking, deflating, and retreating and the rate of ice mass loss is unprecedented --never observed before-- since the 1940’s when quality records for most began.

Figure \(\PageIndex{3}\): Decline of Antarctic ice mass from 2002 to 2016.

Before anthropogenic warming, glacial activity was variable with some retreating and some advancing [17]. Now, the extent of spring snow cover has decreased. In addition, the extent of sea ice is shrinking. Most sea ice is at the North Pole which is only occupied by the Arctic Ocean and sea ice [3; 6]. The NOAA animation shows how perennial sea ice has declined from 2000 to 2022. The oldest ice is white and the youngest (seasonal) ice is dark blue. The amount of old ice has declined from 20% in 1985 to 3% in 2015.

Rising Sea Level

Sea level is rising 3.4 millimeters (0.13 inches) per year and has risen 0.19 meters (7.4 inches) from 1901 to 2010. This is thought largely to be from both the melting of glaciers and thermal expansion of sea water. Thermal expansion means that as objects such as solids, liquids, and gases heat up, they expand in volume. Since 1970, the melting of glaciers and thermal expansion account for 75% of the sea-level rise [6].

Classic video demonstration (30 seconds) on thermal expansion with brass ball and ring (North Carolina School of Science and Mathematics). 

Ocean Acidification

Since 1750, about 40% of the new anthropogenic carbon dioxide has remained in the atmosphere. The remaining 60% gets absorbed by the ocean and vegetation. Therefore, the ocean has absorbed about 30% of new anthropogenic carbon dioxide. When carbon dioxide gets absorbed in the ocean, it creates carbonic acid. This makes the ocean more acidic, which has an impact on marine organisms that secrete calcium carbonate shells. Recall that hydrochloric acid reacts by effervescing with limestone rock made of calcite, which is calcium carbonate. An increase in ocean acidity associated with climate change has been linked to the thinning of the carbonate shells of some sea snails (pteropods) and small protozoan zooplankton (foraminifera) and declining growth rates of coral reefs [6]. Small animals like protozoan zooplankton are an important component in the marine ecosystem. Ocean acidification combined with warmer temperature and lower oxygen levels is expected to have severe impacts on marine ecosystems and human-harvested fisheries, possibly affecting our ocean-derived food sources [6]. This video discusses the problem of ocean acidification. 

Extreme Weather Events

Occurrence and intensity of extreme weather events such as hurricanes, precipitation, and heatwaves are increasing [3; 6]. Since the 1980s, hurricanes, which are generated from warm ocean water, have increased in frequency, intensity, and duration and are likely connected to a warmer climate. Since 1910, average precipitation has increased by 10% in the contiguous United States, and much of this increase is associated with heavy precipitation events like storms [18]. However, the distribution is not even and more precipitation is projected for the northern United States while less precipitation is projected for the already dry southwest [3]. Further, heatwaves have increased and rising temperatures are already affecting crop yields in northern latitudes [6]. Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events [19].

Reference

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6. Pachauri, R. K. et al. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. (IPCC, 2014).

7. Oreskes, N. The scientific consensus on climate change. Science 306, 1686–1686 (2004).

11. Earle, S. Physical geology OER textbook. (BC Campus OpenEd, 2015).

12. Callendar, G. S. The artificial production of carbon dioxide and its influence on temperature. Q.J.R. Meteorol. Soc. 64, 223–240 (1938).

13. Hansen, J., Sato, M., Kharecha, P. & others. Earth’s energy imbalance and implications. Atmospheric (2011).

14. Foster, G. & Rahmstorf, S. Global temperature evolution 1979–2010. Environ. Res. Lett. 6, 044022 (2011).

15. Kosaka, Y. & Xie, S.-P. Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature 501, 403–407 (2013).

16. Easterling, D. R. & Wehner, M. F. Is the climate warming or cooling? Geophys. Res. Lett. 36, (2009).

17. Zemp, M. et al. Historically unprecedented global glacier decline in the early 21st century. J. Glaciol. 61, 745–762 (2015).

18. Karl, T. R. & Knight, R. W. Secular trends of precipitation amount, frequency, and intensity in the United States. Bull. Am. Meteorol. Soc. 79, 231–241 (1998).

19. Santer, B. D. et al. Identification of human-induced changes in atmospheric moisture content. Proc. Natl. Acad. Sci. U. S. A. 104, 15248–15253 (2007).