3.7: Scales of Measurement - Time

Earth system events and forcing mechanisms also vary across time. Some events are very short in duration but bring with them an intense amount of energy. This leads to massive and rapid change and a completely new dynamic equilibrium, eventually. Some events are much more gradual.

Daily/Diurnal Events

If you stop and think, you can certainly come up with a list of daily environmental “events” that go by without much notice. The Sun rises and sets while the Earth rotates on its axis. Tides come in and go out along coastlines. You sleep, you awaken. Temperatures rise and fall with the Sun. Such events are not always recorded in the rock record, yet we know they played an important role through time.

The nature of the term diurnal itself has variation across Earth’s past. The length of a diurnal cycle today is just under 24 hours. However, this has not always been the case and will not remain this way into the future. Using fossil coral and fine layering in some sedimentary deposits that can be attributed to changes in lunar cycles, it is possible to extrapolate the change in diurnal cycles over time. Every 100 years, the length of a day is increased by 0.0024 seconds.

Figure \(\PageIndex{1}\): Changes in the length of a year and day on Earth over time. (NASA graphic)

In the graph above, note that while a day has gotten longer over time, the number of days in a year has decreased. This is because it is assumed that the Earth revolves around the Sun in a fixed amount of time, 365.25 days.

Why does this matter? Many, perhaps most, species today are very much connected to the diurnal cycle. The entire biosphere then is very much linked with this cycle, as it is with seasonal cycles throughout a year.

Annual and longer events

Earth system changes are ideally measured in terms of years. This can be a shorter term event that affects only a given year or two (small volcanic eruption, earthquake, forest fires). Or, important Earth system events can be measured over millions of years. A great example of this is silicate weathering.

Figure \(\PageIndex{2}\): The silicate weathering cycle.

When two continents collide, mountains are raised up. We call this orogeny. As you drive those silica-rich rocks into the atmosphere they begin to weather, or break down. This is done through a process called hydrolysis, where weakly acidic rain (carbonic acid due to carbon dioxide) breaks down feldspar minerals in the rocks. In the image below, the role of silicate weathering in carbon regulation is highlighted. The long term effect of building up a mountain range is to force the climate to cool over a long period of time. Chemical weathering by hydrolysis breaks up silicate minerals and pulls carbon dioxide out of the air. Two by-products of the process are free calcium ions (aqueous) and aqueous bicarbonate. These new inputs into nearby basins can lead to a flourishing of carbonate life forms, who use these two substances to build their shells. Over time, they die, and their shells and the carbon they contain become limestone, a very effective sink for carbon storage.

Anomalies (Comparison to past trends/normals)

When analyzing data from any particular time period, modern or ancient, how do we know whether or not it is unusual or typical?

In order to define any variable within a system as complex as the Earth system at any given moment, it is useful to define anomalies. Simply put, an anomaly is a departure from normal behavior. The degree of anomaly can help scientists define the intensity of a forcing event and the state of the system at the time.

A very useful modern example, again taken from climate studies, comes from ground-based NASA surface temperature data, also known as GISSTEMP (Goddard Institute for Space Studies Surface Temperature Analysis). These charts, like the one below, are updated monthly. However, in order to define surface temperatures, the models use a baseline set of data, typically several decades in length, as an average for comparison.

Figure \(\PageIndex{3}\): GISSTEMP surface temperature anomaly analysis for January 2020. Average January 2020 temperatures for various surface temperatures around the globe are measured against a baseline average taken from records spanning 1951-1980. The map shows anomalies, or departures, from the average for this thirty year period. Which areas experienced a particularly warm winter? (NASA GISSTEMP)

Figure \(\PageIndex{4}\): Sea surface anomaly analysis for January 16, 2020. Which regions of the sea surface are warmer or cooler than normal? (NOAA map)

The sea surface temperature anomaly map above comes from satellite data collected by the National Oceanic and Atmospheric Administration. Using the GOES-15 satellite, is is possible to see which localized regions of the oceans are experiencing warmer temperatures than normal. For analyzing the degradation and bleaching of coral reefs, as one useful example, such data is critically important.