By David Holland,
Grad. Dip. Environmental Management, B.A.S Env. Planning.
This article is part of research in a subject climate change impacts, mitigations and adaptation as part of a Masters of Environmental Management at CSU
Climate Change in the past has been caused by a range of natural causes and has been slow.
The primary drivers of natural climate change over the last 65 million years (known as the Cenozoic Era) is orbital forces, however there could have been considerable influence by any variability of the CO2 levels which would affect the glacial cycle processes. (Masson-Delmotte et al 188.8.131.52)
Other forcings such as solar and volcanic (heat) have more temporary effects. Volcanism can affect climate over a few years in general, where atmospheric particulates reduce the solar intensity.
The solar cycle generally takes a period of around 11 years over which solar flares rise in number and then decline. The solar cycle has had a marginal effect on climate over the period of Paleoclimate timeline.
There seems to have been a large range of CO2 concentrations over the Paleocene period which would suggest a large signal on the climate working with or against orbital forces to produce the glacial cycles. Within that period the Eocene Epoch may have been the most dynamic around 50,000 years ago after the major earth extinction around 65,000 years ago. (Masson-Delmotte et al 184.108.40.206).
It appears that over the last few thousand years, the Holicene Epoch stretching from the last Ice age about 11,700 years ago, that the main driver for the interglacial periods to the year 1900 AD has been orbital forcing. (Much of the pre-2,000 year ago data for temperature and ice sheet expansion was gained from tree rings and ice cores.)
Permanent weather patterns have had an influence on the variation of climate over the period which have produced a recent warm period between 950AD and 1400 AD (Medieval Climate anomaly (MCA)) and a cool period from 1450AD to 1850AD (Little ice Age (LIA)), however the largest part of these swings, and prior swings, are theorized by Masson-Delmotte (et al p.406) to be orbital forcing.
Temperatures were on average 1 degree hotter during the early to mid-holicene (8,000 to 6,000 years ago) than the pre-industrial period, then temperatures tendered to cool after 6,000 years ago as we move towards another glacial period due to orbital forces. Ice sheets expanded to about the middle of the first millennia AD, then contracted in the MCA and then expanded again in the LIA. This was probably caused by the Atlantic Ocean Meridional Overturning circulation (AMOC) changes as happened 8,200 years ago when a flood of freshwater entered the system from North America.
But what has caused the recent climate anomalies?
The LIA is suggested to be caused by a cluster of volcanos between 1275AD and 1300AD. (Byrd 2012)
The MCA was considered by Mann 2009 (at el) to be possibly a range of causes including La Nina, Atlantic Multi-Decadal Oscillation (AMO), the positive phase of the North Atlantic Oscillation (NAO) and related to the Arctic Oscillation (AO). They suggest that the La Nina effect coupled with high solar irradiance and inactive volcanism produced the MCA. (Mann et al 2009)
The ocean oscillations are drivers of our current climate
The relative importance of these drivers in determining our current climate, and the explanation of how amplification of temperature changes can occur naturally is related to the interaction of ocean warming and cooling causing natural circulation patterns.
For example, the various meridional overturning circulations (MOC) seem to be driven by not only surface winds but other inputs including warm, cool or fresh waters. Often warm water is produced by tropical solar radiation. This tends to driver macro weather patterns such as the La Nina, El Nino southern oscillation (ENSO) and the Indian Ocean Dipole (IOD).
Along with the seasons, driven by the solar radiation variations throughout the year, these patterns which last for several years change the prevalence of precipitation on land masses thus producing dryer conditions or wetter conditions. Dryer conditions tend to produce hotter conditions because of the lack of clouds and moisture in the air and are often accompanied by less wind which intensifies solar radiation.
Wetter conditions cause less solar radiation to reach the earth and the winds are stronger because of the variation locally between cloud covered land and non-cloud covered land (ie. areas of higher solar radiation) The wind in these systems is simply caused by hot air expanding and cool air contracting.
Through the above processes, natural variation of land and sea temperatures can occur. These natural variations drive local weather and depending on the degrees of warm and cool temperature dynamics, (ie. warm seas and cool land etc.) the intensity of the weather is often determined.
Determining ancient paleoclimates
To determine climates going back millions of years, scientists use surrogates or indirect measurements to theorise climates. However, there are limitations to these theories on climate for periods beyond the range of direct measurement methods like tree ring data.
Over the last thousand-year scale of collected data on temperatures it can be considered with a fair amount of confidence that it is accurate. Sources such as tree rings would be highly favoured to measure temperature over this period. Lake sediments would be a fairly reliable source by giving plant seeds as an indicator of the type of vegetation growing at a particular time in the paleoclimate time line.
There are many methods of finding temperature data by using indirect methods. These are called proxy data methods. It is data that indicates the level of temperature by a deduced estimate based on the knowledge of the organism or environment where the measurements were taken.
Ice cores can be used to gain data back several thousand years and by the indication of greenhouse gas (GHG) levels in the ice, some indication of average temperature can be made with moderate confidence by assuming a level of warming by the GHG concentrations in the atmosphere.
Leaf stomatal density reconstructions and boron isotope are some of these proxy data sets and produce data on CO2 levels, which can be interpreted to get temperature indications. Phytoplankton cell-size is an indication of carbon isotope fractionation giving an estimate of CO2 levels and in turn the deduced temperature levels.
Many of these proxy methods allow scientists to get indications of temperature ranges from millions of years ago. However, the degree of certainty drops due to the proxy nature of the data, the shear generality of the data and the time scale over which the data is purported to cover. Bivalves found in sediments are another proxy for temperature. The measurement of the growth year to year can indicate temperature by the size for the mollusk and the number of growth rings.
|Million Years||Thousand years||Level of certainty|
|Tree Ring Data||High|
|Ice Core Data||Ice Core Data||Medium|
|Seeds from sediments||Seeds from Sediments||Medium|
|Phytoplankton Cell-size||Phytoplankton Cell-size||Low|
|Leaf Stomatal Density||Low|
The limitations of using proxy kinds of data is that the data generally has low levels of certainty. However, if the data temperature results indicate a similarity to the results of other proxy data sets for a similar period and location by using a difference method and source there would be a tendency towards an increase in certainty of the data results.
By using any data that has uncertainties, by implication these uncertainties cause the data to be less valuable in convincing the community of a need to accept the findings of the data. Only when data corroborates other data can a case be put to make conclusions from the data.
The Quaternary period
Over the Quaternary period (last 2 million years), there were a range of physical effects of temperature changes on the global landscape, some of these global effects and landscape changes were related to temperature variations attributed to CO2 levels in the atmosphere.
Although during the Eocene temperatures spiked at somewhere in the order of 14 degrees higher than pre industrial temperatures, Pleistocene and Holocene temperatures (Quaternary Period temperatures) remained similar to pre-industrial levels in interglacial periods.
During the glacial phases over the quaternary period the temperatures dropped to an average of 3 to 8 degrees below pre-industrial levels.
During an interglacial period, CO2 concentrations were relatively static between 280- 300ppm, however during the glacial periods, Masson-Delmotte (et al p.391) suggests that CO2 concentrations could have been as low as 20 ppm.
The physical effects of these cooler temperature changes were simply to produce more ice, larger ice shelves and more glaciers. Erosion from these glaciers are evident in the geological record. At these times there was a retreat of the tropics with a commensurate change to weather patterns.
When the warmer temperatures kicked back into operation, the various MOC’s would start again and have the capacity to produce dry and arid conditions in some places and monsoon rains in others. But these changes would take a considerable amount of time in conjunction with a gradual atmospheric warming.
Masson-Delmotte (et al) suggests that 0.3 – 0.8 degrees per thousand years is the normal rate of change for average temperatures to move from a glacial to inter-glacial period with some faster periods from between 1 degree C to 1.5 degrees C recorded in ice cores.
Global Land- Ocean Temperature Index
Since solar, orbital and volcanic forcings are relatively stable presently, the only other possible influence affecting the global climate in any significant way would be GHG concentrations in the atmosphere. Carbon dioxide is the most prolific gas of these GHGs and has increased significantly in recent years. Two other gases also increase global warming but do not have the same high concentrations as CO2, although they are more effective at warming the planet than CO2 if they had the same concentrations as carbon dioxide has in the atmosphere. These gases are nitrous oxide and methane.
Below is a graph showing the concentrations of CO2 in the atmosphere derived from a range of data sets including proxy data. It shows how, in general, glacial periods and interglacial periods have different concentrations of the gas. Generally interglacial periods have higher concentrations of CO2.
It is evident from this graph that CO2 concentration levels have risen significantly since the LIA and the beginning of the industrial revolution.
PROXY (INDIRECT) MEASUREMENTS
Data source: Reconstruction from ice cores.
Current CO2 level 404.93 (NASA, October 2016)
It is evident from these graphs that the pre-industrial period starting from about 1850 AD to the present, being about 160 years, has produced a rate of change of temperature much higher than at any time in prehistory. The rate of change is approaching 1 degree Celsius over this period. (Matt McGrath, Nov 2015) This is about ten times faster than at any time in pre-history over the entire time period of the paleoclimate record.
Byrd Deborah, (Aug 2012), What Causes the Little Ice Age, Earth Sky News, earthsky.org/earth/volcanoes-might have -triggered-the-little -ice-age, cited July 2016.
Mann Micheal, Zhang Zhihua, Rutherford Scott, et al, (2009), Global Signatures and Dynamical Origins of the little Ice Age and Medieval Climate Anomaly, Science, American Association for the advancement of science, vol. 326, No 5957, (Nov 27 2009, pp. 1256-1260), cited 2016.
Masson-Delmotte, Valerie (France); Schulz, Micheal (Germany); Information from Paleoclimate Archives, AR5 WG1 Chapter 5: http://www.climatechange2013.org/images/report/WG1AR5_Chapter05_FINAL.pdf, cited July 2016.
McGrath Matt, Nov 2015, Warming set to breach 1C threshold, BBC News, Science and Environment, http://www.bbc.com/news/science-environment-34763036, cited Nov. 2016.
NASA, October 2016, Carbon Dioxide Measures Oct. 2016 404.93 ppm, Global Climate Change the vital signs of the planet, http://climate.nasa.gov/vital-signs/carbon-dioxide/, cited November 2016.
NASA, 2015, Global Temperatures, Global Climate Change the vital signs of the planet, http://climate.nasa.gov/vital-signs/global-temperature/, cited Nov. 2016