Monitoring is the observation of a variable over a period of time, at regular intervals, usually using some form of instrumentation. Carrying this out over a long time period allows observers to measure, and understand, the parameters of the system being monitored. Monitoring related to climate science usually involves the study of one component of the Earth’s system, within the atmosphere, hydrosphere (and cryosphere), lithosphere, or biosphere. Due to the vast number of variables within each of these individual systems, detailed monitoring is an extremely valuable tool, and without it our understanding the Earth’s climate would be severely limited. Ever since climate was first studied, almost all of our understanding has been gained from monitoring modern processes.
It is only through the monitoring of climatic variables that long-term records can be constructed. Scientists can study the monitored past, compare this to the present, and make predictions about the future. Despite the major advances achieved in climate science using monitoring, any record is limited in length by how long it has been recorded. Most records began less than 200 years ago, with the majority of monitoring projects only being established in the past few decades due to advances in technology. Though longer records are useful to scientists, these too can have associated issues, and older records commonly have large errors associated with them. This can be due to less precise instrumentation in the past or a change in measurement method. This is why scientists often use monitoring records in conjunction with palaeo-data in order to extend the record.
At present, scientists are monitoring thousands of variables in thousands of locations around the world. These include aspects of the weather (e.g. temperature throughout the atmosphere, precipitation, wind), composition of the atmosphere (e.g. carbon dioxide, methane concentrations), characteristics of the oceans (e.g. sea-level, temperature through the water column, salinity, currents), state of the cryosphere (e.g. sea ice extent, glacier and ice sheet thickness) as well as biological information (e.g. changes in species’ abundance and extent). Below are explanations of a few of those:
Carbon dioxide (CO2) is one of the most important greenhouse gases in the world, and is monitored across the globe. However, the longest continuous record of direct CO2 measurement is from Mauna Loa Observatory, Hawaii, beginning in March 1958. Mauna Loa was chosen as an ideal location for CO2 monitoring as it is very remote from large urban centres, meaning readings are likely to be far more representative of the entire atmosphere. Additionally, Hawaii is within the Pacific Ocean, and prevailing winds bring well-mixed air from high in the atmosphere to the site. In conjunction with the high elevation of the observatory (3397 m above sea level), this limits the possible influence vegetation could have upon CO2 levels. Measurements are taken every hour using a CO2 analyser, though results are presented as monthly means.
Measuring the average sea-level, and how this changes through time is of vital importance to climate scientists, and the measurements have revealed that sea-level is rising. In order to accurately predict future sea-level changes and to mitigate against them, scientists need accurate and precise measurements. Sea-level itself varies across the globe which means that it needs to be measured in multiple locations across the globe to get an average global value. Sea-level is measured against a fixed point on land, usually between low and high tide.
Tide-gauges: The oldest, and most common instruments used for sea-level measurement are tide-gauges, of which 1,750 exist across the globe. The oldest of these is one from Amsterdam which began recording in 1682. Tide gauges are tubes attached to a fixed point on a harbour or sea wall. Water enters the base of the tube, and its level is recorded by a small computer against a ‘benchmark’, or known reference level on land. Before the development of computers, this was done by manual observation. In the United States of America, tide-gauges have been measuring sea-level for over 150 years, on all coastlines. However, though they remain exceedingly useful to scientists, tide-gauges actually record the movement of the sea-level relative to the land on which the tide-gauge is situated.
Satellite altimetry: Technological advances allowed the development of precise altimetry. This is a method by which satellites orbiting the Earth can measure changes in oceanic sea-level. The first of these (TOPEX/Poseidon) were launched in 1992, followed by the Jason-1 and Jason-2 satellites in 2001 and 2008 respectively. These have revolutionised sea-level observations, as measurements of the entire ocean surface is now possible.
The amount of mass glaciers are gaining or losing annually is an important observation, giving vital insight into the state of the cryosphere and also the input of freshwater into oceans. Measuring the mass of water gained (by snow falling on its surface during winter) or lost (by melting of the ice during the summer) by the glacier can be done in two ways.
Manual: The first is through relatively simple manual observations. Snow-stakes are drilled through the snow and ice of the glacier and left in place, or snow pits are dug. The locations are returned to monthly or annually, and the surface change is measured. These point measurements can be made across a glacier, and repeat measurements over multiple years can show if mass loss or gain is occurring. Storglaciären in northern Sweden represents the first, and longest continuous study of glacier mass, beginning in 1946. Manual observations can also be made through repeat photography. This is where photographs are taken from the same place over a long period of time (e.g. every 5 years). These give a visual representation of overall area change.
Satellites: As well as manual observations, recent developments in satellite technology have allowed remote observation of glacier gain or loss. These methods include a repeat satellite altimetry method (ICESat), and repeat satellite gravimetry (GRACE). The ICESat provides very precise altitude measurements of ice sheet surfaces by measuring the distance between itself and the surface. When repeated annually, evidence is provided of surface rising or lowering. The more recent satellite gravimetry (GRACE) method uses two satellites, separated by about 200 km. The distance between the two satellites varies, based upon the gravitational field associated with masses (continents, ice sheets) on the Earth’s surface below them. In contrast to satellite altimetry, this produces a direct record of ice mass loss, and has added enormously to glaciological study. These remote methods are useful as they allow for accurate and precise repeat measurement of large ice masses, without the need for extensive field research. As a result of these new methods, scientists have been able to calculate the global annual ice loss (in gigatonnes of ice mass!).