Paleoclimatology is the study of past climate, for times prior to instrumental weather measurements. 
Paleoclimatologists use information from natural climate "proxies," such as tree rings, ice cores, 
corals, and ocean and lake sediments, that record variations in past climate. Records of past climate 
from these proxy records are important for several reasons. Instrumental records of climate are limited 
in many parts of the world to the past 100 years or less, and are too short to assess whether climate 
variability, events, and trends of the 20th and 21st centuries are representative of the long-term 
natural variability of past centuries and millennia. For example, was the 1930s Dust Bowl drought, 
a widespread and severe event in the United States, a rare occurrence, or have similar events occurred 
in past centuries? Knowledge of the long-term natural variability of the Earth Climate system, 
and its causes, will also allow an understanding of the roles of natural climate variability 
and human-induced climate change in the current and future climate. In particular, reconstructed 
temperatures from proxy data for the past 1000 years have allowed an assessment of the warming 
over recent decades and indicate that the majority of this warming is due to the impact of human 
activities on climate, according to the Intergovernmental Panel on Climate Change (IPCC) 
(http://www.ipcc.ch).

Geological evidence demonstrates that the Earth's climate is dynamic, and has varied widely from our 
everyday experience. Over the past two million years, numerous glacial periods have covered much of 
the northern hemisphere in glacial ice, dropped sea level as much as 125 meters, and significantly 
cooled even tropical regions. In the more distant past, the Cretaceous Period was significantly warmer 
than today, with less polar ice, raising sea levels and allowing warm weather organisms to thrive even 
in near-polar regions. In the past 150 years, instrumental weather records indicate that the Earth has 
warmed by approximately 0.6C. Our goal is to provide an archive of past climate observations, derived from 
multiple proxies, that enables scientists to understand and model the many different states of the climate 
system that have existed in the past. 



Why Should We Care About Climate Change?

If there is one thing that the paleoclimatic record shows, it is the that the Earth's climate is always 
changing. Climatic variability, including changes in the frequency of extreme events (like droughts, 
floods and storms), has always had a large impact on humans. A particularly severe El Nino, or relatively 
short drought, can cost billions of dollars. For this reason, scientists study past climatic variability 
on various time scales to gain clues that will help society plan for future climate change.

Unfortunately, records of past climate change from satellites and human measurements (thermometers, 
rain gauges) generally cover less than 150 years. These are too short to examine the full range of 
climatic variability. For this reason, it is critical to examine climate change going back hundreds 
and thousands of years using paleoclimatic records from trees, corals, sediments, glaciers and 
natural or "proxy" sources.

The study of past climate change also helps us understand how humans influence the Earth's climate system. 
The climatic record over the last thousand years clearly shows that global temperatures increased significantly 
in the 20th Century, and that this warming was likely to have been unprecedented in the last 2000 years. 
The paleoclimatic record also allows us to examine the causes of past climate change, and to help unravel 
how much of the 20th century warming may be explained by natural causes, such as solar variability, 
and how much may be explained by human influences.

Lastly, most state of the art climate prediction is accomplished using large sophisticated computer models 
of the climate system. A great deal of research has been focused on ensuring that these models can simulate 
most aspects of the modern, present-day, climate. It is also important to know how these same models 
simulate climate change. This can be accomplished by comparing simulations of past climate change with 
observations from paleoclimatic records. Thus, paleoclimatology helps us improve the ability of computer 
models to simulate future climate.



How Do Scientists Study Past Climates?
There are several ways that scientists study how the Earth's climate is changing: satellites, instrumental records, 
historical records and proxy data.

Some scientists look to satellites to study the Earth's changing climate. However, the satellite record is too short 
(ca. 20 years) to provide much perspective on changing climate. The record of instrumental weather measurements, 
extending back into the 19th century, provides data from thermometers, rain gauges, historical documents and other 
instruments. However, this record is too short to study many climatic processes. Also, because we have few 
instrumented observations from before the major industrial releases of carbon dioxide began, it is difficult 
to separate human and natural influences on climate.

Paleoclimatologists find clues in natural records - proxy data. Proxy data are natural clues to past climate 
that are buried in sediments at the bottom of the oceans, locked in coral reefs, frozen in glaciers and ice caps, 
or preserved in the rings of trees. 


What Do We Know About the History of Climate?
Good weather records extend back less than 150 years in most places. In that time, the Earth's global average 
temperature has increased by approximately 0.7 degrees centigrade or 1.3 degrees Fahrenheit. Some of this warming 
is natural, however a large part is the result of human induced greenhouse warming.

Since the end of the last ice age occurred over 10,000 years ago, the planet has continued to undergo changes 
in climate. Warming during medieval times and cooling during the "Little Ice Age" a few centuries ago dominate 
the last millenia. From paleo records, we know that the climate of the past million years has been dominated by 
the glacial cycle, a pattern of ice ages and glacial retreats lasting thousands of years.  Eighteen-thousand years 
ago, at the peak of the last ice age, scientists estimate that nearly 32% of the earth's land area was covered 
with ice, including much of Canada, Scandinavia, and the British Isles. These glaciers developed because the 
earth was in the midst of an ice age. Today ice coversage about 10% of the Earth's land surface. 


Paleo Proxy Data
Paleoclimatologists gather proxy data from natural recorders of climate variability such as tree rings, ice cores, 
fossil pollen, ocean sediments, corals and historical data. By analyzing records taken from these and other proxy 
sources, scientists can extend our understanding of climate far beyond the 100+ year instrumental record. 
Listed below are some widely used proxy climate data types:

Historical documents contain a wealth of information about past climates. Observations of weather and climatic 
conditions can be found in ship and farmers' logs, travelers' diaries, newspaper accounts, and other written records. 
When properly evaluated, historical data can yield both qualitative and quantitative information about past climate.

Corals build their hard skeletons from calcium carbonate, a mineral extracted from sea water. The carbonate contains 
isotopes of oxygen, as well as trace metals, that can be used to determine the temperature of the water in which 
the coral grew. These temperature recordings can then be used to reconstruct climate when the coral lived.

All flowering plants produce pollen grains. Their distinctive shapes can be used to identify the type of plant 
from which they came. Since pollen grains are well preserved in the sediment layers in the bottom of a pond, 
lake or ocean, an analysis of the pollen grains in each layer tell us what kinds of plants were growing 
at the time the sediment was deposited. Inferences can then be made about the climate based on the types 
of plants found in each layer.

Since tree growth is influenced by climatic conditions, patterns in tree-ring widths, density, and isotopic composition 
reflect variations in climate.  In temperate regions where there is a distinct growing season, trees generally produce 
one ring a year, and thus record the climatic conditions of each year. Trees can grow to be hundreds to thousands 
of years old and can contain annually-resolved records of climate for centuries to millennia.


Located high in mountains and in polar ice caps, ice has accumulated from snowfall over many millenia. 
Scientists drill through the deep ice to collect ice cores. These cores contain dust, air bubbles, 
and isotopes of oxygen, that can be used to interpret the past climate of that area.


Billions of tons of sediment accumulate in the ocean and lake basins each year. Scientist drill cores of sediment 
from the basin floors. Ocean and lake sediments include tiny fossils and chemicals that are used to interpret past climates.




What is Glaciation?
What Causes it?

The Laurentide Ice Sheet covered most of Canada and northern areas of the United States 18,000 years ago. 
Geologists have found material left by the ice throughout this area, proving that virtually all of Canada 
and some of the northern US were once covered by thick glacial ice. Similar findings show that ice was also 
widespread in many other parts of the world. Scientists believe that changes in the earth's climate 
caused the ice sheets to grow.

The explanation for these climate changes is believed to be variations in the earth's orbit around the sun 
(The Milankovitch Theory), combined with positive feedbacks from albedo (reflection of sunlight from snow and ice) 
and decreases in Carbon Dioxide concentrations in the atmosphere. 


What is The Milankovitch Theory? 
The Milankovitch or astronomical theory of climate change is an explanation for changes in the seasons 
which result from changes in the earth's orbit around the sun. The theory is named for Serbian astronomer 
Milutin Milankovitch, who calculated the slow changes in the earth's orbit by careful measurements of the 
position of the stars, and through equations using the gravitational pull of other planets and stars. 
He determined that the earth "wobbles" in its orbit. The earth's "tilt" is what causes seasons, and changes 
in the tilt of the earth change the strength of the seasons. The seasons can also be accentuated or modified 
by the eccentricity (degree of roundness) of the orbital path around the sun, and the precession effect, 
the position of the solstices in the annual orbit. 

The earth wobbles in space so that its tilt changes between about 22 and 25 degrees on a cycle of about 
41,000 years. It is the cool summers which are thought to allow snow and ice to last from year to year 
in high latitudes, eventually building up into massive ice sheets. There are positive feedbacks in the 
climate system as well, because an earth covered with more snow reflects more of the sun's energy into space, 
causing additional cooling. In addition, it appears that the amount of Carbon Dioxide in the atmosphere falls 
as ice sheets grow, also adding to the cooling of the climate.

The earth's orbit around the sun is not quite circular, which means that the earth is slightly closer to 
the sun at some times of the year than others. The closest approach of the earth to the sun is called perihelion, 
and it now occurs in January, making northern hemisphere winters slightly milder. This change in timing of 
perihelion is known as the precession of the equinoxes, and occurs on a period of 22,000 years. 11,000 years ago, 
perihelion occurred in July, making the seasons more severe than today. The "roundness", or eccentricity, 
of the earth's orbit varies on cycles of 100,000 and 400,000 years, and this affects how important the timing 
of perihelion is to the strength of the seasons. The combination of the 41,000 year tilt cycle and the 22,000 year 
precession cycles, plus the smaller eccentricity signal, affect the relative severity of summer and winter, 
and are thought to control the growth and retreat of ice sheets. Cool summers in the northern hemisphere, 
where most of the earth's land mass is located, appear to allow snow and ice to persist to the next winter, 
allowing the development of large ice sheets over hundreds to thousands of years. Conversely, warmer summers 
shrink ice sheets by melting more ice than the amount accumulating during the winter. For more detailed explanations 
of orbital variations with graphic representations, please see WDC Paleo's educational slide set "The Ice Ages":
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/slidesets/iceage/



What does The Milankovitch Theory say about future climate change?
Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond 
to orbital forcing. Theory suggests that the primary driver of ice ages is the total summer radiation received 
in northern latitude zones where major ice sheets have formed in the past, near 65 degrees north. Past ice ages 
correlate well to 65N summer insolation (Imbrie 1982). Astronomical calculations show that 65N summer insolation 
should increase gradually over the next 25,000 years, and that no 65N summer insolation declines sufficient 
to cause an ice age are expected in the next 50,000 - 100,000 years.