While climate has changed many times in the past (see chapter 14.5.1 and chapter 15.3), 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 . 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 six-minute video on climate change by two professors at a North Carolina State University.
15.2.1: 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 . Changes in land surface or ocean surface temperatures can be expressed as temperature anomalies. A temperature anomaly is the difference in average temperature measurement from a predetermined datum. This datum is the average temperature of a particular date range, for example, 1951 to 1980. Another common datum is the last century (1900-2000). Therefore, an anomaly of 1.25 ℃ for 2015 (last century datum) means that the average temperature for 2015 was 1.25 ℃ greater than the 1900-2000 average. In 1950, the temperature anomaly was -0.28 ℃, so this is -0.28 ℃ lower than the 1900-2000 average . These temperatures are annual average surface temperatures.
This video figure shows worldwide temperature changes since 1880. The more blue, the cooler; the more yellow and red, the warmer.
In addition to a rising average surface land temperature, the ocean has absorbed a lot of the heat (remember that the specific heat of water is unusually large). 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 2,300 feet of the ocean has increased in temperature 0.3℉ since 1969 (external link to this 3-minute video by NASA JPL on heat capacity of the ocean) . 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 (similar to La Niña) in the ocean 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.
15.2.2: 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 (or speed) 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 . Indeed, during the lifetime of most young adults, the total atmosphere 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: scripps.ucsd.edu/programs/keelingcurve/. 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 itself increasing!
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.
15.2.3: Melting Glaciers and Shrinking Sea Ice
Glaciers are ice on top of the land. 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. Before anthropogenic warming, glacial activity was variable with some retreating and some advancing . The extent of spring snow cover has decreased. In addition, the extent of sea ice is shrinking. Sea ice is ice floating in the ocean (not on land like a glacier). Most sea ice is at the North Pole which is only occupied by the Arctic Ocean and sea ice [3; 6]. Below, the NOAA animation shows how perennial sea ice has declined from 1987 to 2015. 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.
15.2.4: 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. 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 .
Classic video demonstration (30 seconds) on thermal expansion with brass ball and ring (North Carolina School of Science and Mathematics).
15.2.5: 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 which 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. Ocean acidification associated with climate change has been linked to the thinning of the carbonate walls of some sea snails (pteropods) and small protozoan zooplankton (foraminifera) and declining growth rates of corals . Small animals like protozoan zooplankton are an important component in the marine ecosystem. Acidification combined with warmer temperature and lower oxygen levels is expected to have severe impacts on marine ecosystems and human-used fisheries, possibly affecting our ocean-derived food sources .
15.2.6: 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 connections to a warmer climate are likely. 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 . 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 . Further, heatwaves have increased and rising temperatures are already affecting crop yields in northern latitudes . Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events .
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).