3.6: Scales of Measurement- Geography

Earth system forcings (events) happen at different geographic scales. That is, some have global effects while some have more regional or local effects. We will explore some of these below.

Global Effects

There have been many events throughout the Earth’s past that have affected change on a global scale. These tend to be the big events, though they may have begun as small events that cascaded into larger-scale forcings. There are many to choose from, but for this section we will discuss the forcing effects caused by the evolution of vascular plants during the Silurian Period.

Evolution of Vascular Plants

Vascular plants entered the scene on land about 420 Ma. One of the challenges plants would encounter as they evolved to colonize land would be how to get water to move up toward the top of a plant against the downward pull of gravity. When you consider the Giant sequoia or Redwood trees in the Pacific northwest of the United States, you can really tell that plants found a way to master this problem. However, it is also true that there are limits to this. Plants overcame gravity in part by taking advantage of some of water’s unique properties, such as capillary action. The evolution of specific tissues called xylem and phloem was the key development that allowed plants to harness this.

Xylem is a vascular tissue that transports water upward while phloem is a vascular tissue that transports photosynthetic cells to other parts of the plant for food or storage. The evolution of these tissues made it possible for plants to stand upright and, thus, colonize land.

Figure \(\PageIndex{1}\): Xylem transports water and nutrients from roots to stems and leaves. (CC BY-SA 3.0; Kelvinsong – Own work)

Figure \(\PageIndex{2}\): Phloem is living tissue in plants that transports soluble products of photosynthesis, such as sugars, where needed in plants. (CC BY-SA 3.0; Kelvinsong – Own work)

Why is this “event” a forcing mechanism? Well, consider the effect that photosynthetic vascular plants have on our atmosphere today. They are not only a critical part of the water cycle because of the transpiration of water through their tissues, but also for the removal of carbon dioxide from the atmosphere. Ultimately, plants’ ability to convert carbon in carbon dioxide into glucose is a critical process for all life on Earth. Once vascular plants evolved, they spread prolifically, as there was a nearly endless array of niches to fill on land. This proliferation of plant material certainly forced changes in the atmosphere. Plant photosynthesis would remove carbon dioxide and replace it with oxygen, leading to a cooling climate globally. Their effect was so impactful through their drawdown of atmospheric \(\ce{CO2}\) that it likely contributed to the Devonian extinction. They would not be outdone by their non-vascular cousins, who likely had the same impact on the end-Ordovician extinction.

Regional Effects

Some Earth system events create interesting regional dynamics. These are still affected by global changes, but the regionalism of impact simply means that global effects manifest themselves in different ways in different locations.

In terms of regional variation within Earth system, there are several examples that are important to understand today and that have applied or may have been different in the Earth’s past. In each of the examples used to discuss region effects, the biosphere will be the focus.

Latitudinal Biodiversity Gradients

Species richness is highest in the tropics and lowest at the poles. In a general sense, this gradient is

Figure \(\PageIndex{3}\): Modern latitudinal biodiversity gradient.

directly impacted by seasonality and the current, relatively moderate, climate. In Earth’s past, particularly during times like the late Paleocene, it is thought that the tropics were so hot that biodiversity was lower than at temperate latitudes. Diversity in polar regions was also lower, but the climate was warm enough to support palm trees and alligators above the Arctic Circle. Biodiversity gradients, then, were not only very different then, but have changed throughout Earth’s past. The Biodiversity gradient we see today from the tropics to the poles has not always looked the way it does today.

Why would latitudinal biodiversity gradients change over time? In the image below, from Mannion et al. (2013), one idea is that diverse tropics have not always been the norm. In cooler periods (blue bands), it may be, but during warmer global climates, the tropics may be much more inhospitable. This would lead to a situation where the biosphere may have had bands of high diversity at mid-latitudes that trended downward north to the poles and south to the equator (in the northern hemisphere).

Figure \(\PageIndex{4}\): Summary of changes in latitudinal biodiversity gradients throughout the Phanerozoic Eon, according to Mannion et al. (2013). The smooth red line indicates global temperature changes. Filled circles represent times when the fossil record indicates a biodiversity gradient similar to today (poleward decrease). Open circles occur at times when evidence suggests that greatest diversity existed at mid-latitudes. Red circles are terrestrial data and black are marine. Blue bands indicate global icehouse periods.

Mathematically, latitudinal biodiversity gradients are governed by a simple relationship:

\[S=cA^{z}\nonumber\]

Or

\[\log(S) = \log(c) + z\log(A) \nonumber\]

Figure \(\PageIndex{5}\): Species-area curve in arithmetic and log-log space. \(Z\) represents the slope of this curve. \(C\) is akin to the y-intercept, and \(A\) is the independent variable. The top image is the SAR curve with arithmetic axes and the bottom image is the SAR function in log-log space.

For any given area, \(S\) is the number of species predicted by the formula, per unit area. \(A\) is the area of the region of study, calculated in some unit for area. \(C\) and \(Z\) are constant values. In the equation above, \(C\) is the y-intercept and is typically a value that relates to the Area and defines the number of species that would exist in one square unit of area at that location. \(Z\) values represent the slope of the line. \(Z\) values are the most important factor in the equation and different environmental characteristics can be assigned different \(z\) values for the purposes of modeling. A region’s climate is one such characteristic, though other items as wide-ranging as reproductive capacity and life mode can be assigned \(z\) values also. The species-area relationship is typically used to describe individual species and not entire ecosystems. However, once you calculate the number of a given species using this formula, knowledge of population ecology and ecosystem dynamics can be used to express a more holistic view of an environment. Generally, as the size of an area increases, so does the number of species.

Because of the climate, a very small area in a tropical region like the Amazon can not only have a dizzying array of species, but a very small area might also serve as the only location where that species can be found! When using the species-area relationship to model species richness as a function of climate, the lack of diversity in polar regions is a predicted result of the modeling. In addition, a single species might cover a very large area, such as spruce forests or Arctic Willow do in portions of the Canadian wilderness.

It is reasonable to assume that mathematical relationships like the species-area relationship not only apply to environments from the Earth’s past as it does today. It should then be possible to model not only course global effects of forcings/events, but to examine regional variations also.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

When you consider today's latitudinal biodiversity gradient and the species-area relationship, where do we find most biodiversity on Earth today?

a. Canadian Arctic

b. Appalachian Mountains

c. Southeastern United States

d. Equatorial Tropical Rainforests

Answer

d. Equatorial Tropical Rainforests

Continental Interiors and Margins

The location on a continent is another important climate regionality. Generally, the interiors of continents have more extreme temperature and moisture differences during cold and warm periods (day/night, summer/winter). Areas along a coastline, by contrast, tend to have much more moderate temperature and moisture variation. This is due to the already discussed elevated specific heat of water and its ability to buffer high temperatures. Continental interiors also tend to have higher elevations. Such locations could be mountainous or high plains. In either case, the average temperature drops as altitude increases. Seasonally, this leads to much greater variation in temperatures between day and night or summer and winter.

Figure \(\PageIndex{6}\): Earth circa 500 Ma. Note the much different position of the continents and of sea level. (Maps by Christopher Scotese of the PALEOMAP project, with virtual globe by Ian Webster of Dinosaur Pictures.org; reproduced with permission.)

Throughout Earth’s past, the paleogeography has changed as plate tectonics forced continents into new positions. These forcings (events), though very long term in nature, led to significant changes in ocean current patterns and regional weather patterns. Short-term Earth system events, such as volcanic eruptions, might affect these patterns for a short time regionally or globally, but the return of the earlier dynamic equilibrium can typically be expected. Under average conditions, continental interior regions will have large range between moist and dry or warm or cold periods than their coastal counterparts.

Figure \(\PageIndex{7}\): Regional climate zones are determined by a variety of factors, including whether an environment is within a continental interior or along the margin. Altitude is another important factor. All of these variables lead to variation in precipitation and evapotranspiration. Thus, different locations experience the hydrologic cycle differently, leading to differing climates. (Wikipedia Commons)

Ultimately, all of these geographic variations lead to different areas experiencing the water cycle differently. The key climatic variables, independent of temperature, for any location that determine the kinds of ecosystems that can exist are evapotranspiration and precipitation. There are myriad natural biomes and climatic zones that can be described by these two variables.

Local Effects

Earth forcings (events) also have local effects that vary widely, including as these affect local climate. While detecting local changes due to particular Earth events in the rock record is usually very challenging or even impossible, it is important to note that we can use modern analogs (uniformitarianism) to hypothesize how a locality might have behaved. Global events, like large asteroid impacts, certainly had global impact and detectable regional impact. Localized impact can be harder to see, but even if we do not see evidence for it, we can know that it happened.

Orographic Effects and Slope Aspect

A good example of local climate effects can be found in the impact that mountains have on local (and regional) weather patterns. On a single northern hemisphere hillslope, the north side of a mountain or hill might be covered in tall pines while the south slope might be a grassland. The temperature and moisture effects of the degree of incident sunlight on a hillslope, referred to as slope aspect, can have a profound effect on the type of ecosystem and communities that it supports.

Figure \(\PageIndex{8}\): Slope aspect and its effect on vegetation near Anderson Ranch, Idaho. South-facing slopes get much more direct sunlight and less snow over. They sustain lower growing, desert flora. North-facing slopes hold snow cover and moisture longer and can support healthy forests.

Likewise, in situations where weather is moving from west to east, the eastern slope of a mountain range will typically experience a lower degree of moisture than the western slopes. This rain shadow effect is produced as orographic uplift causes weather systems to drain their energy and precipitation on one side of the mountain, leaving the leftovers (so to speak) to drop a pittance of their former moisture on the other side. Such local effects may or may not be recorded in the rock record. Once the position of ancient mountain ranges is determined and their latitude defined, it is then possible to craft hypotheses about how weather may have been manifest in a specific area at that time.

Forests

Forests can certainly extend over a very large area, but their effect is also very regional to local in terms of climate. Forests are excellent places for moisture to accumulate and carbon to be sequestered. Transpiration of water through leaves is one way that plants transport soil moisture back to the atmosphere, raising the local relative humidity. This has a cooling effect. Forests can then have a similar effect on land as water has on coastlines, producing a more even temperature and moisture profile and reducing the severity of swings for these two variables.