12.2: Increasing Temperatures

Global temperatures are rising (Figure \(\PageIndex{1}\)), and there is no doubt that humans are part of that rise, mostly because of our use of fossil fuels. Over a 60-year period (from 1960 to 2020) the global mean annual temperature in both land and sea areas rose from 14° C to 15° C. Although the temperature rise was erratic in the early part of the 20th century, it has been quite consistent since 1960, increasing at a rate of 0.16° C per decade over the 60-year period, and by about 0.25° C per decade since 2000. If that continues, we are on track to exceed the Paris agreement’s aspirational goal of limiting warming to 1.5° C above pre-industrial levels by 2050.

Figure \(\PageIndex{1}\): Variations in the Global Mean Annual Temperature, 1880 to 2020

As shown on Figure \(\PageIndex{2}\), most of the warming over the past 40 years has been in the northern hemisphere, and especially the far north. That is partly because of the much greater landmass of the northern hemisphere (since land areas warm faster than ocean areas). The far north is particularly affected by warming because of the significant loss of sea ice in the Arctic Ocean. Late summer Arctic Ocean sea ice extent is now typically about 60% of what it was in 1980, and the large areas of open water absorb a great deal more solar energy than the snow-covered ice that used to be there, adding to the warming. Another positive feedback from warming in the Arctic is the melting of permafrost, releasing stored methane and carbon dioxide.

In addition to the far north, central and eastern Europe and the Arabian Peninsula have seen very strong warming, and there has also been strong warming in the rest of Europe, northern China, much of Africa, Brazil, the southeastern USA and eastern Australia.

The significant warming of our climate has resulted in increased evaporation, and that is one reason why forests are so dry in many regions, leading to the increased incidence of wildfires. The other reason is localized and periodic reduced rainfall.

Figure \(\PageIndex{2}\): Spatial Pattern of Differences in Mean Annual Temperatures. 1979 to 2019

While the heat has made many places drier, some regions have become wetter because warmer air can hold more moisture than cold air. For example, much of the interior of British Columbia has seen higher levels of precipitation in recent decades. A typical British Columbia interior precipitation record is shown on Figure \(\PageIndex{3}\). The average precipitation in Kaslo in the period 1915 to 1925 was 51 mm/month and the monthly amount never exceeded 70 mm. In the period from 1997 to 2007 the average was 72 mm/month (a 40% increase), and it only dropped below 60 mm for five months during that decade. This significant increase in precipitation has not been without consequences. In July 2012 part of a slope near to Kaslo failed, leading to a debris flow that killed four people in the community of Johnsons Landing.

Figure \(\PageIndex{3}\): Monthly Precipitation at Kaslo, British Columbia,1915 to 2007

The excess heat that exists within the atmosphere is slowly being transferred to the oceans and this is illustrated on Figure \(\PageIndex{4}\). Since the early years of the 20th century there has been a 1° C increase in the temperature of the surface waters of the ocean. Because warm water takes up more space than cold water, this has contributed to sea level rise and to an increased incidence of tropical cyclones. Warm water can hold less carbon dioxide than cold water, so some of the ocean’s CO₂ is being transferred to the atmosphere. Warm water can also hold less oxygen than cold water, and that is slowly contributing to lower oxygen levels in the oceans and in lakes.

Figure \(\PageIndex{4}\): Global Average Ocean Surface Temperatures, 1900 to 2015, Compared With the Average for 1900 to 1920

An important implication of warming ocean water is the breakdown of reef ecosystems. Damage to corals within tropical reefs occurs when the water temperature exceeds their range of tolerance and the symbiotic relationship between the coral structures and the algae living within their tissues breaks down. The result is bleached—and therefore, likely dead—coral. Coral reefs are a vital component of ocean ecosystems, and their breakdown also has implications for marine sedimentation and the stability of the sea floor.

The famous Great Barrier Reef in Australia was suffering from successive disturbances. Marine heatwaves in 2016 and 2017 resulted in coral bleaching. Cyclones caused further damage resulting in widespread death of many corals. In the early 2020s the Reef rebounded. Generally cooler La Niña conditions mean corals made significant recovery, regrowing from very low levels to record high levels in 2022 across two-thirds of the reef. What happens in the future is yet to be seen. Higher global temperatures will cause further destruction of the reef, though it has shown it can recover significantly in short periods of time.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0, based on data from NASA Goddard Institute for Space Studies (accessed April, 2021), public domain, https://data.giss.nasa.gov/gistemp/t...LB.Ts+dSST.txt
• Figure \(\PageIndex{2}\): Making Sense of climate Sensitivity from NASA, public domain, https://climate.nasa.gov/blog/3017/m...e-sensitivity/
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0, based on Open Government Licence – Canada data from Environment Canada.
• Figure \(\PageIndex{4}\): Steven Earle, CC BY 4.0, based on data from National Oceanic and Atmospheric Administration (NOAA) (2016). Extended reconstructed sea surface temperature (ERSST.v4). National Centers for Environmental Information/ Environmental Protection Agency. https://www.epa.gov/climate-indicato...mperature#ref6