Paleoclimatology and the best climate change evidence are presented. Evidence of earth's constantly changing climate from the distant past has to be determined by methods other than direct observation, as you can guess.
Paleoclimatology is the science of piecing together the earth's past climate using various indirect or "proxy" methods. Some of the more useful proxy method employed by scientists include: sediments/geology, ice cores, tree rings, and coral/shells of past sea life. A method is developed to reconstruct the temperature and moisture conditions from the earth going back millions of years. Of course as one goes back in time, the estimates become less precise since less useful "material" exists further back in time.
Reconstructing past climates is more important than ever to help scientists better understand the abrupt climate change we are currently experiencing. To better understand the impacts of climate change it helps to see how past temperature and moisture conditions affected ancient life on earth and the make-up of the land forms and sea levels.
climate graph from noe21.org
The graph above shows different methods and different data employed to recreate the climate from the past 1,000 years. However the pattern is clear regardless of which method of reconstruction has been employed. Very clearly one can see the signature of the Medieval Warm Period and Little Ice Age (more on the causes later).
The methods of reconstruction are summarized below:
- Ice - can reconstruct climate back over 800,000 years. Air trapped during ice formation allows for determination of the air composition. Changes in layer thickness allows for determination of temperature and precipitation. O2 isotope changes, trapped pollen, and trapped volcanic ash allows for fine-tuning of the reconstruction.
- Tree Rings - trees respond to changes in temperature and precipitation by slowing down or speeding up growth. The rings can then be exploited to infer what the temperatures and/or precipitation was in the past.
- Sediment - and sedimentary rocks (formed when the sediment hardens) can provide useful information since they can provide remains of preserved vegetation, animals, or pollen that is representative of a given climate. Moreover, rock chemicals and O2 isotopes can allow for further details of the past to be reconstructed.
- Geology - rocks can show evidence of sea level rises and falls. Moraines and other geologic features can lend evidence to the past ice age.
- Corals - similar to tree rings, corals develop in different ways depending on the water temperature and wave action. Further analysis of coral oxygen isotopes can help with the climate reconstruction.
So what do the reconstructions of past climate (or the science of paleoclimatology) tell us? The graph to the left shows the reconstruction of the earth's temperature from the beginning of the Precambrian Period
when the earth formed, 4.6 billion years ago. The earth has gone through warm and cold periods - the majority of the time was warm, interrupted by "glacier epochs" (major glaciation period). There are many theories for the causes of climate change
Some "quick facts" about the distant past:
- For most of earth's history, world has been completely "ice free"
- Major glaciation periods (glacial epochs) have interrupted the warmth as can be seen from the graph above. "Drivers" include 100,000, 41,000, and 21,000 year cycles along with other possible factors.
- We are tectonically in an inter-glacial period of the Plio-Pleistocene Glacial Epoch that began around 3.2 million years ago. (If there's ice at the poles, we are in a glacial period)
- The last major advance of the ice sheets, peak around 18,000 years ago, is called the Ice Age by the media and even scientists, when talking of more recent climate.
- Our current inter-glacial period is warm; 90% of the last 2 million years have had more extensive ice on earth
Let's now look at the more recent past...
In the graph to the right you can see the rise in global temperatures since the last 18,000 years ago. More recently, the Medieval Warm Period ran from around 1000 to 1350 and was followed by the Little Ice Age from 1400 to 1850. The suggested causes of the Little Ice Age include: orbital cycles, lowered solar activity, above average volcanic activity, altered ocean currents, to name a few.
We have currently warmed up to temperatures comparable to the Medieval Warm Period. The future warming since the Industrial Revolution looks to exceed anything the earth has seen for thousands of years. Much of this latest warming can be attributed to the activities of mankind.
See the projected global temperatures (image at right) due the increases in carbon dioxide emissions. As you can see global temperatures followed concentrations of carbon dioxide (CO2) in lockstep. The global temperature and CO2 concentrations fluctuated for the past thousand years due to changes in solar energy and astronomical forcing along with volcanism. The recent changes are artificially enhanced by the activities of mankind, as can be seen from the striking curve on the graph.
The study of paleoclimatology is an important part of understanding what future climate might lie ahead. A healthy blend of climate modelling and the study of past climate causes and effects will probably yield the best projections for the future. No one method of determining the past is as good as a combination of strategies employed by climate scientists.(sources: wikipedia, lakepowell.net, and author's knowledge)