Weather Eye with John Maunder

What causes the climate to change as well as understanding the methods for detecting changes in the climate are on-going research activities.

The following is a brief summary of the methods scientists and others, such as historians, employ in inferring and detecting changes in climate during a variety of time scales. I am indebted to a NIWA web site for much of this summary.

Instrumental measurements

Data from instruments and written or oral records provide quantitative records of temperature and other meteorological records for the last 150 years in New Zealand – and up to 350 years in a few other countries, such as summer, winter, and annual temperature charts for central England from 1659-2015 shown in the graph.

Such records must be analysed carefully, to identify the influence of any non-climate factors, such as changes in observing site or method, or encroaching urban development. Records of sea level and land movements are also important for assessing sea level change.

Proxy data

Beyond the scope of instrumental measurements, information about past climate can be obtained from natural proxy archives, as well as historical records of events, such as harvests etc.

Changes observed in these archives often identify so closely to climate variations they can be used as a substitute for climate records prior to the instrumental record after a careful calibration process has been undertaken.

Piecing evidence together from various natural proxy data sources includes:

Ice cores

Ice cores drilled in Greenland, the Antarctic ice sheets, the Himalayas, and in other alpine regions of the world comprise very important archives because they provide extensive detailed information about past climate variability and atmospheric composition

The ratio of oxygen isotopes in ice can indicate the temperature at the time ice was deposited as snow. Air bubbles can be analysed to measure atmospheric carbon dioxide and methane concentrations at the time the bubbles were trapped in the ice.

Dust trapped in the ice may indicate windy, arid conditions. Geochemistry, including trace elements and salts, can tell a story about regional atmospheric circulation. The core from the Russian Rostock station in Antarctica provides information back to at least 160,000 years ago, and when drilling is completed a climate record of the last 500,000 years is probable.

Fossil pollen and phytoliths

Different classes of plants produce pollen grains and phytoliths (siliceous formations precipitated by plants) that have distinctive shapes. Pollen grains and phytoliths are often found preserved in sediment cores from ponds, lakes and marine environments.

Lake sediments

Composition and sedimentation rates in lakes change in response to variations in environmental conditions during periods of wet and dry climate. Pollen in the sediments can indicate the type of vegetation present, and plankton biota indicates physical and chemical conditions in the lake water. In some cases, stark seasonal changes in lake inflows and sedimentation can cause annual layers to form in lake sediments.

Annual layers, or varves, commonly form in lakes fed by glacial meltwater, and can be used to infer the amount of melted ice and what past warm season temperatures were like. Within the sediment layers, microfossils like diatoms, bugs, and plant material are preserved. These fossils can also reveal information about what past environmental conditions were like, sometimes with incredible precision.

Ocean sediment cores

These cores contain primitive shelled animals (foraminifera) whose abundance in the surface layers of the ocean depends on surface water temperature and other conditions. Off New Zealand the rate and type of sediment deposition depends on factors such as the amount of glacial activity and on other climate-driven erosion processes.

Pollen types and the isotopic composition of material in the sediments provide further information on past climates. Cores obtained off New Zealand from the international deep sea drilling project provide information as far back as 6.3 million years, and drilling of more cores is planned.

New Zealand is coordinating an interesting international drilling project near Cape Roberts in Antarctica, to establish more information about past Antarctic climate and ice extent.

Loess

Loess are fine-grained wind-blown dust deposits on land. They typically accumulate during periods characterised by dry and windy conditions. In New Zealand, they are associated with cool and cold intervals that coincide with glacial advances. Numerous loess sections can be found on the South Island, particularly in eastern regions.

Glaciers

Variations in the past size of glaciers can be inferred from the location of moraines (rocks and debris deposited by glaciers that mark a former ice margin position), outwash fans, buried soils, and by the presence of glacial features in the landscape. In New Zealand, cool summer temperatures are only one factor in promoting ice accumulation on glaciers, and snow accumulation rates also respond to changes in the strength and direction of the westerly wind flow and sea level pressure in summer.

Speleothems

Speleothems are used to describe a stalactite, stalagmite or flowstone cave deposit of crystalline nature. These deposits occur within karst terranes in subterranean caverns mainly as calcite precipitated from groundwater that percolated through overlying limestone or marble rock.

Tree rings

Tree rings are some of the best resolved records of past climate in the world. This is because, in many cases, one tree ring is grown each year, allowing tree rings to be dated with great precision and with annual resolution.

Tree growth is dependent on many factors. However, common growth patterns often emerge at the regional scale between trees, suggesting there is a common growth response to climate changes. Correlations of tree ring data with soil moisture, temperature, and precipitation often enable tree ring records to be substituted for instrumental climate data into the distant past.

In the case of NZ, which has many long lived tree species suitable for dendrochronology, long climate reconstructions of droughts, storms, and even El Nino events are possible.

Boreholes

It is sometimes possible to deduce past surface temperatures going back several hundred years by measuring the way temperature varies with depth in a borehole several hundred metres deep (at a suitable site not disturbed by groundwater flow). This is because fluctuations in ground surface temperatures propagate slowly downwards into the earth as a “temperature wave”.

For further information see: https://sites.google.com/site/theweatherclimateeye/