Evidence of Climate Change

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The main sources for evidence of global climate change are temperature increase in the atmosphere, rising sea levels, and the shrinking of Earth’s glaciers and ice caps.

Temperature Increases

Climate studies focused on the most recent centuries have evidenced a warming trend in the surface of the Earth. Between the years 1880 and 2012 the average temperature of the Earth’s surface has risen by 0.85°C (1.53°F), as shown in Figure 7. Additionally, this warming trend has been accelerating over time. For example, the rate of warming over the 50 years from 1956 to 2005 is 0.128°C per decade―nearly twice that of the 0.074°C per decade rate of increase for the 100 years between 1906 and 2005. This increase in Earth’s average surface temperature is what we call global warming.

global mean temperature

Figure 7: Annual global average observed temperatures (black dots) along with simple fits to the data. The left hand axis shows anomalies relative to the 1961 to 1990 average and the right hand axis shows the estimated actual temperature (°C). Linear trend fits to the last 25 (yellow), 50 (orange), 100 (purple) and 150 years (red) are shown. Note that for shorter recent periods, the slope is greater, indicating accelerated warming.1

  • 1.

    PCC, 2013: Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, FAQ 2.1, Figure 1; FAQ 3.1, Figure 1. [Stocker,T.F., D.Qin, G.-K. Plattner, M.Tignor, S.K.Allen, J.Boschung, A.Nauels, Y.Xia, V.Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, UK and New York, USA.

The global temperature increase is greater at higher northern latitudes, as shown by the red-orange shading in Figure 8. This is due to several causes, including the greater buffering effect of larger, more extensive oceans in the southern latitudes, and the higher incidence of positive feedbacks in northern latitudes.

Temperature Anomaly

Figure 8: Global temperature anomalies for 2000 to 2009. Temperature anomalies do not depict absolute temperature, but rather how much warmer or colder a region is compared to the norm for that region from 1951 to 1980. Global temperatures from 2000–2009 were on average about 0.6°C higher than they were from 1951– 1980. The Arctic, however, was about 2°C warmer.1

One of those feedbacks is referred to as “Arctic Amplification.” This phenomenon occurs because warmer temperatures cause snow to melt in northern regions. Loss of snow cover on the surface of the Earth due to snowmelt changes the color of the Arctic from white to dark, which decreases Earth’s albedo―the reflection of solar energy by Earth’s surface back to outer space. As a result, more sunlight is absorbed into the surface of the Earth, which causes further heating of the land and air in polar and mountainous regions. The positive feedback loop causes an amplification of polar warming. People living in northern countries, such as the Mongolian herders discussed is the case study at the beginning of this chapter, are trying to cope with the results of Arctic Amplification today.

Sea Level Rise

Global Mean Sea Level

Figure 9. Changes in global average sea level since 1880. Data from coastal tide gauges and satellite altimeter observations were combined to provide the blue line (averages) and shaded blue area (depicting the variability). Changes since 1993 from satellite altimeter data alone are in red.1

The increase in sea level is another source of empirical evidence of climate change which is consistent with global warming. In the last century, the global average sea level rose 1.7 millimeters (0.067 inches) per year.

Looking Ahead

In the Global Climate Change and Action section you will learn what the islanders of Kiribati are doing to prepare for the day when their nation will drown under rising sea level.

Since the 1990s, the rate of sea level rise has increased to 3.1 millimeters (mm) per year (Figure 9). This increase has been caused by two factors: 1) thermal expansion of the oceans as the water becomes warmer, and 2) added water from the melting of land ice, including glaciers and ice caps. Both of these factors are linked to global warming.

Receding Ice Caps

Receding glaciers and ice caps are the third line of evidence of climate change. Recent satellite data shows that each year the area of Arctic sea ice has been shrinking, with larger decreases observed in the northern hemispheric summer (Figure 10).

Arctic Ice Caps

Figure 10. Decline of Arctic ice cap from 1980 (left) to 2012 (right). The bright white central mass shows the perennial sea ice while the larger light blue area shows the full extent of the winter sea ice, including the average annual sea ice during the months of November, December and January.1

The average rate of Arctic ice loss since 2000 is about four times higher than the rate seen in the 1990s (Figure 11). Arctic ice loss is caused by the warming of ocean water, which has made ice sheets unstable. For example, the Thwaites Glacier, which holds the massive West Antarctic Ice Sheet together, has started to melt and collapse.

Figure 11. Decline of Arctic summer sea ice extent since 1900.1

The collapse of this glacier is irreversible and will produce a massive domino-effect collapse to other ice structures. Thwaites Glacier collapse is predicted to be complete within the next 200-plus years.1 This will inevitably result in the full-scale collapse of the whole West Antarctic Ice Sheet, which will further accelerate sea level rise.

Looking Ahead

Later in this chapter, you will learn about the close relationship between climate, culture, and spirituality--and the devastating effects that climate change can have on a culture’s spiritual traditions.

Glaciers and snow cover have also declined in the Northern Hemisphere since the mid-twentieth century. For example, the Northwestern Glacier in Alaska has diminished dramatically during that time. In addition, both the thickness and the extent of permafrost have experienced considerable reductions in the tundra of Northern Alaska and the Russian European North since the 1970s.

Permafrost is any soil or rock that remains frozen—below 0°C or 32°F—throughout the vast majority of the year, and thaws only at the very surface for a short growing season. For a soil to be considered permafrost, it must be frozen for two consecutive years or longer. Permafrost can be found in cold climates where the average annual temperature is less than the freezing point of water. Such climates are found near the North and South poles. In the northern hemisphere they occur as far south as 50° N throughout most of Siberia, Northern Europe, Mongolia, Alaska, parts of Canada, and in alpine regions including the Himalayas.

Inspired People

Balog [photo]

James Balog is an American photographer who has risked his life to explore the impact of climate change on the world’s glaciers. In 2007 he began the Extreme Ice Survey, the most extensive photographic study of the receding glaciers ever conducted. His award-winning 2012 film Chasing Ice tells the story of his adventurous photography on the world’s ice sheets. Learn more about his work at TedTalk.

Tundras are northern “wetlands” whose organic carbon-rich soils are permanently frozen. The thaw of permafrost caused by climate change, further accelerates climate change because the thaw of these high organic content soils results in the occurrence of anaerobic decomposition. Anaerobic decomposition releases methane, a GHG that is 20 times more potent at trapping heat than carbon dioxide. The release of methane from the thawing permafrost has a positive feedback because methane enhances more warming in the atmosphere, resulting in greater melting of the tundra. Permafrost thaw and subsequent methane release is another component of Arctic Amplification.

Questions to Consider

Imagine you are a Sherpa guide who makes a living taking climbers to the top of Mount Everest. Do you think the receding snow and ice on Mount Everest will make your job easier, or more difficult? Consult this recent discussion among Sherba guides for ideas.