Temperature Changes

The concept of an average temperature for the entire globe may seem odd. After all, at this very moment, the highest and lowest temperatures on Earth are likely more than 100ºF (55ºC) apart. Temperatures vary from night to day and between seasonal extremes in the Northern and Southern Hemispheres. To speak of the “average” temperature, then, may seem like nonsense. However, the concept of a global average temperature is convenient for detecting and tracking changes in Earth’s energy budget, which can only be directly measured from space.

Temperature change has not been uniform across the planet, but more areas are warming than cooling. And the rate of warming has accelerated in recent decades. According to NOAA’s 2024 Annual Climate Report the combined land and ocean temperature has warmed at an average rate of 0.11ºF (0.06ºC) per decade since 1850 and more than three times that rate (0.36ºF, or 0.20ºC) per decade since 1975.

The amount of future warming Earth will experience depends on how much carbon dioxide and other greenhouse gases we emit in coming decades. According to the 2017 U.S. Climate Science Special Report, if yearly emissions continue to increase rapidly, as they have since 2000, models project that by the end of this century, global temperature will be at least 5ºF warmer than the 1901-1960 average, and possibly as much as 10.2º warmer. If annual emissions increase more slowly and begin to decline significantly by 2050,  models project global temperatures would still be at least 2.4º warmer than the first half of the 20^th century, and possibly up to 5.9º warmer.

Precipitation and Storm Events

Patterns of precipitation and storm events, including both rain and snowfall are likely to change. However, some of these changes are less certain than the changes associated with temperature. Projections show that future precipitation and storm changes will vary by season and region. Some regions may have less precipitation, some may have more precipitation, and some may have little or no change. The amount of rain falling in heavy precipitation events is likely to increase in most regions, while storm tracks are projected to shift towards the poles.

A warming climate implies a shift in the probability distribution such that hot extremes become more frequent and cold extremes become less frequent. This is what is currently observed and we can expect this trend to continue into the future. Due to the intensification of the hydrological cycle we can also expect more droughts and more floods. Changes in other extreme events are less well understood. Occurrences of total hurricanes and typhoons (tropical cyclones) are projected to decrease but the strongest hurricanes are projected to become more frequent. Hurricane development depends on warm ocean water as an energy source. Latent heat release is also an important fuel for hurricanes and other storms. Thus warmer sea surface temperatures and more latent heat release due to more water vapor in the warmer air will strengthen hurricanes, consistent with observations of increases in the destructiveness of tropical storms in the past decades. Another important factor for hurricane development, particularly in its early stages, is wind shear, how fast winds increase with elevation. Storms require low shear to develop into a coherent vortex, which may become less likely in the future. The end result of changes in wind shear and changes in temperatures is that in a warmer climate the total number of hurricanes will decrease, but strong hurricanes will become more frequent and they will cause more rainfall.

Sea Ice, Glacier, and Permafrost Melting

Arctic sea ice is already declining drastically. The area of snow cover in the Northern Hemisphere has decreased since 1970. Permafrost temperature has increased over the last century, making it more susceptible to thawing. Over the next century, it is expected that sea ice will continue to decline, glaciers will continue to shrink, snow cover will continue to decrease, and permafrost will continue to thaw. For every 2°F of warming, models project about a 15% decrease in the extent of annually averaged sea ice and a 25% decrease in September Arctic sea ice.

For example, many glaciers in Glacier National Park in Montana are retreating, a phenomenon known as glacier recession. In 1850, the area contained approximately 150 glaciers. By 2010, however, the park contained only about 24 glaciers greater than 25 acres in size. One of these glaciers is the Grinnell Glacier (Figure 18.1) at Mount Gould. Between 1966 and 2005, the size of Grinnell Glacier shrank by 40%. Similarly, the mass of the ice sheets in Greenland and the Antarctic is decreasing: Greenland lost 150–250 km³ of ice per year between 2002 and 2006. In addition, the size and thickness of the Arctic sea ice is decreasing.

Permafrost

An important component of the cryosphere (i.e., an Earth system containing frozen water) is permafrost, or frozen ground. Regions containing permafrost are often categorized by the quantity of total area of the region that is frozen. Continuous permafrost, as its name implies is nearly entirely frozen with 90-100% of the land area of the region containing permafrost. Discontinuous permafrost, as the name implies, is not a single, solid sheet of frozen land (less than 90% of the total area is frozen), and can be sporadic (10-50% frozen area) or isolated (less than 10% frozen area).

As with other components of the cryosphere, permafrost is particularly sensitive to warming. One mechanism for tracking changes in permafrost extent is to measure the depth of the active layer, or the unfrozen soil that overlays permafrost. The fate of permafrost in the future is dire. You may not think much about if you live outside the Arctic regions where permafrost is common. Scientists predict that 40% of the world’s permafrost could thaw if temperatures rise 2^oC (3.6^oF) due to global warming. Thawing of the permfrost could allow carbon that has been stored for thousands of years to be released into the atmosphere fueling additional warming. This would increase the respiratory activity of soil organisms, further increasing the release of CO₂ in Arctic ecosystems. This represents a concerning feedback loop by which warming caused by atmospheric greenhouse gases leads to thawing of permafrost, which releases more greenhouse gases into the atmosphere, exacerbating warming trends and thawing of permafrost.

In addition, thawing results in land subsidence and mass movement because of the destabilized soil profile – this will lead to sinkholes, mudslides, and collapse of infrastructure. Buildings and infrastructure, like roads, landing strips, and pipeleines, upon which residents depend have been constructed to account for the expansion and contraction that takes place in the active layer of permafrost. Rising temperature due to climate change threatens structures built under a different permafrost freeze-thaw regime.

The Arctic has experienced a significant rise in air temperature over the last few decades and the permafrost that underlies much of the surface is undergoing substantial changes. Continuous permafrost on Alaska’s North Slope has warmed 2.2^o-3.9^o C (4^o – 7^o F) over the last century making it more susceptible to erosion and mass movement. Some places in Alaska have subsided by 4.6 meters (15ft) due to thawing of the permanently frozen subsurface. Accompanied by rising sea level, Alaskan coastal communities near the Arctic Ocean and Bering Sea are being threatened.

Sea Level Change

A variety of factors affect the volume of water in the ocean, especially the temperature of the water (the density of water is related to its temperature: water volume expands as it warms, thus raising sea levels), as well as the amount of water found in rivers, lakes, glaciers, polar ice caps, and sea ice. As glaciers and polar ice caps melt, there is a significant contribution of liquid water that was previously frozen. Warming temperatures contribute to sea level rise by expanding ocean water, melting mountain glaciers and ice caps, and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean. Since 1870, global sea level has risen by about 8 inches. Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years. The contribution of thermal expansion, ice caps, and small glaciers to sea level rise is relatively well-studied, but the impacts of climate change on ice sheets are less understood and represent an active area of research. Thus, it is more difficult to predict how much changes in ice sheets will contribute to sea level rise. Greenland and Antarctic ice sheets could contribute an additional 1 foot of sea level rise, depending on how the ice sheets respond.

Regional and local factors will influence future relative sea level rise for specific coastlines around the world. For example, relative sea level rise depends on land elevation in addition to factors such as local currents, winds, salinity, water temperatures, and proximity to thinning ice sheets.

Clark et al. (2016) estimate that their low emission scenario (1,280 PgC) would submerge an area where currently 1.3 billion people live (19% of the global population) including 25 megacities such as Calcutta, New York, Tokyo, Shanghai, and Cairo.

Rising sea levels means that shorelines will be inundated, reducing island size, which will have an effect on some species, and a number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains—a cycle that has provided freshwater to environments for centuries—will also be jeopardized. This could result in an overabundance of salt water and a shortage of fresh water. 

Ocean Acidification

Ocean acidification is the process of ocean waters decreasing in pH. Oceans become more acidic as carbon dioxide (CO₂) emissions in the atmosphere dissolve in the ocean. This change is measured on the pH scale, with lower values being more acidic. The pH level of the oceans has decreased by approximately 0.1 pH units since pre-industrial times, which is equivalent to a 25% increase in acidity. The pH level of the oceans is projected to decrease even more by the end of the century as CO₂ concentrations are expected to increase for the foreseeable future. Ocean acidification adversely affects many marine species, including plankton, mollusks, shellfish, and corals. As ocean acidification increases, the availability of calcium carbonate will decline. Calcium carbonate is a key building block for the shells and skeletons of many marine organisms, including coral. If atmospheric CO₂ concentrations double, coral calcification rates are projected to decline by more than 30%. If CO₂ concentrations continue to rise at their current rate, corals could become rare on tropical and subtropical reefs by 2050.

Reading Question #4

Which of the following describes how climate change is projected to affect oceans?

A. Increasing temperature and acidity.
B. Increasing temperature and decreasing acidity.
C. Increasing temperature and no affect on acidity.
D. Decreasing temperature and acidity.

Mismatched Phenology

Temperature and precipitation play key roles in determining the geographic distribution and phenology of plants and animals. Phenology refers to the timing and cyclical patterns of biological life cycles, such as flowering in plants or migration in birds. Changing climates also throw off species’ delicate timed adaptations to seasonal food resources and breeding times. Many contemporary mismatches to shifts in resource availability and timing have already been documented. For example, researchers have shown that 385 plant species in Great Britain are flowering 4.5 days sooner than was recorded earlier during the previous 40 years. In addition, insect-pollinated species were more likely to flower earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect pollinators emerged earlier.

Changes in phenology can cause mismatch between organisms that interact with each other. For example, if insect-pollinated flowers are flowering earlier, but insects are not emerging earlier, then those flowers may not be pollinated and those pollinators may not have a food source. Another example is bird migration. Migratory birds rely on daylength cues, which are not influenced by climate change. Their insect food sources, however, emerge earlier in the year in response to warmer temperatures. As a result, climate change can decrease food availability for migratory bird species.

Migratory species rely on environmental cues, such as day length and temperature, as cues to trigger migration. But because different species rely on different environmental cues to time their life cycles (e.g. breeding), not all species will adjust to climate change at the same rate. Researchers have already seen signs of phenological mismatch: some migratory birds that overwinter in the tropics have started to migrate to their European breeding grounds at earlier dates than before (Both et al., 2006; Vickery et al., 2014). If these trends hold, they may soon start breeding before peak food availability, which could lead to lower fitness of offspring.

Range Shifts

As climates change, biomes will move polewards and upwards and the species that live in these biomes will have to move their ranges to stay within temperature and precipitation ranges they can survive. For example, present day tundra will be replaced by taiga. Species that used to live at high latitudes or on the upper elevations of mountains may be replaced by species moving in from further south or from lower altitudes. This can lead to species extinctions, especially when there is no farther north or higher elevation available and when climate change occurs faster than range can move. Especially vulnerable are species that live near the top of mountains such as the cute little pika, or near disappearing sea ice such as polar bears, walrus, and narwhales. Range shifts are already being observed: for example, some European bird species ranges have moved 91 km northward. The same study suggested that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough. Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, and mammals.

Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The accelerating rate of warming in the arctic significantly reduces snowfall and the formation of sea ice. Without the ice, species like polar bears cannot successfully hunt seals, which are their only reliable source of food. Sea ice coverage has been decreasing since observations began in the mid-twentieth century, and the rate of decline observed in recent years is far greater than previously predicted.

There is a danger that important mutualistic relationships might be pulled apart these during range adaptations. This is of concern for species with specialized feeding niches, as seen in some pollinators. For example, studies from South Africa have shown how necessary range adjustments under climate change threaten both sunbirds—which show low adaptability (Simmons et al., 2004)—and their host plants, if specialized pollinator niches are left vacant (Huntley and Barnard, 2012). Extinctions arising from this decoupling of mutualistic relationships are referred to as coextinction (Koh et al., 2004), while a series of linked coextinctions is called an extinction cascade.

The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range. One such unexpected species contact is between polar bears and grizzly bears (Figure 18.2). Previously, these two distinct species had separate ranges. Now, their ranges are overlapping and there are documented cases of these two species mating and producing viable offspring, which may be viable crossing back to either parental species (Figure 18.3).