Climate and Climate Change
Earth's astounding history
Earth has been bombarded by astronomical
crud and subjected to alternating freezing and roasting climates.
We live on a fragile planet in a violent cosmos. About every 100
million years, an object the size of Seattle has crashed into the planet,
sometimes vaporizing the oceans and sometimes producing a global freeze.
The presence of the planet Jupiter protects
Earth from such objects; without Jupiter, huge impacts would happen more
often. The Moon, an unusually large satellite for a planet the size
of Earth, helps stabilize the obliquity of Earth's orbit; without the moon,
Earth's axial tilt would at times approach 90 degrees and the seasons would
vacillate between searing heat and bitter cold everywhere on the planet,
making the evolution of higher life impossible.
Evidence of past climates
Isotopes play a very important role in determining what happened in
the past. Each element (hydrogen, helium, oxygen, etc.) is uniquely
identified by its "atomic number", the number of protons in its nucleus.
Elements also have "isotopes" which refer to the number of neutrons in
the nucleus. Isotopes are useful in two ways: for dating and for
determining some physical property. Dating is possible because some
isotopes undergo radioactive decay, like carbon-14 (14C), so the proportion
of that isotope provides an estimate of age. Physical properties
can be determined because some isotopes are "fractionated" through some
physical process; for example, the process of evaporation tends to leave
behind the heavier oxygen-18 isotope in favor of the lighter oxygen-16
isotope, so ocean water in hot times tends to have more oxygen-18 than
in cold times.
How did temperature change in the past?
One can look at variations of the temperature record (see Fig. 8-14 in textbook) over various timescales. Over the lifetime of the Earth (4.5 billion years), temperature has been remarkably stable and liquid water was present on the surface for most of the time. In contrast to our closest neighbours Venus (450°C) and Mars (-50°C) which are very hot and very cold, Earth has been able to maintain relatively pleasant temperatures. Over the last 4.5 billions years, global temperatures on Earth haven't varied by much more than 15-20°C, except during the interludes of "Snowball Earth".
Most of the time, the Earth had a warmer climate than today, with little or no polar ice. However there is evidence from geomorphology that during infrequent periods in Earth's history, the temperature was so cold that ice might have covered most of the surface (and sometimes the oceans). There are 5 such periods of glaciations (sometimes referred to as the "Great Ice Ages"):
- 2.2-2.4 billion years ago: Huronian glaciation
- 0.8-0.6 billion years ago: Late Proterozoic (or Neoproterozoic) glaciation
- 440 million years ago: Late Ordovician glaciation
- 280 millions years ago: Permian-Carboniferous glaciation
- 1.8 millions years ago: Pleistocene glaciation
Snowball Earth
There is substantial evidence that Earth's surface was completely ice-covered. Around 850-600 million years ago, the continents were apparently small and close to the equator, an arrangement that allowed faster weathering and growth of polar ice sheets. Combined with dropping CO2 concentrations (probably to about 100 ppm) and the ice-albedo feedback, this allowed ice sheets to grow until they covered nearly all land areas and froze the surface of the oceans. Evidence comes from glacial deposits which appear nearly everywhere that rocks date back that far. Atop these glacial deposits are deep layers of "cap carbonate", rock formations that apparently formed very quickly in warm shallow seas, suggesting that Earth's temperature rebounded very rapidly (perhaps from -50C to +50C). The best explanation for the rapid rebound is that on an icy Earth, no processes for removing CO2 remained, so a gradual buildup (over perhaps 10 million years) of CO2 from volcanoes probably raised CO2 to 100,000 ppm (10% of the atmosphere) and finally triggered a rapid melt.Although life had evolved only to very primitive levels by the Neoproterozoic, most life would have been wiped out by the combination of extreme temperatures and loss of oxygen. Global iron deposits suggest that the ocean became "anoxic" (nearly oxygen-free) at this time.
This alternating "freeze-fry" cycle may have happened as many as 3 times
during the Huronion glaciation and again at least twice during the Neoproterozoic
glaciation. At the end of the last Neoproterozoic glaciation - hothouse
cycle, evolution proceeded at a fantastic pace; this "Cambrian explosion"
provided all eleven of the major body designs (phyla) for animals.
The rise of vascular plants (those with the capability to transport fluids
internally, i.e., nearly all the complex plants we know today, like grasses,
bushes, and trees) followed, around 420 million years ago.
What are the causes of
climate change?
Long term climate change (hundreds
of millions of years)
The Earth managed to maintain liquid water
and life on its surface for the last 3.5 billion years despite changes
in solar luminosity over that time period because of the negative feedback
in the carbonate-silicate cycle: as the sun grew brighter, CO2 levels decreased
and temperature was maintained within a fairly stable range (see the Faint
young Sun discussion on pages 159-161).
Chapter 8 in your textbook has a good discussion of the causes of long-term climate change (see in particular pages 164-170). These causes involve changes in continental positions driven by plate tectonics and changes in the levels of atmospheric CO2 (driven by the carbonate-silicate cycle, and by the biosphere).
Causes of climate change over the
last 2 million years (timescale of hundreds of thousands of years)
The regular shifts in Earth's climate
over the past 2 millions years were initiated by small changes in the configuration
of Earth's orbit around the sun (Milankovitch cycles). Variations
in the orbital parameters of the Earth include obliquity variations,
eccentricity
variations, and precession variations (see pages 216-219 in textbook).
Each of these variations occur regularly and their respective time periods
are 41,000, 100,000, and 26,000 years. These small changes have affected
the warmth and length of northern hemisphere summers and have allowed ice
sheets to grow (see textbook, Chapter 11). The Milankovitch cycles
in and by themselves cannot explain global variations in temperature of
5C are recorded by ice cores, as they represent only minute changes in
the repartition of radiation over the surface of the Earth. What
the Milankovitch cycles do however is provide a regular trigger for the
glacial/interglacial switches. The climate system has amplified these small
changes through a number of positive feedback loops, such as the
ice-albedo feedback, changes in cloud properties, changes in levels of
atmospheric CO2 (through changes in marine productivity for example).
Causes of climate change over the last 2,000 years
The causes of short-term climate variability are presented in Chapter
12 of the textbook . These causes include solar variability and volcanic
activity.
Evidence of past climates: how do we know what climate was like in the past?
Direct measurements of temperature, such
as thermometer records, extend back about 150 years. Humans have also noted
aspects of climate change for about 1000 years in historical records (for
example looking at cherry blossoms in Japan and grape harvests in Europe).
For older evidence of past climate a wide variety of records span different
times and areas. The table below summarizes some of the records used to
look at conditions on Earth in the past. Tree rings
tell us about the changes in growing conditions that a tree might have
encountered over its lifetime (temperature and rainfall), pollen
from different plant species indicate shifts in vegetation patterns that
occured as a result of climate change, the shape of the landscape (geomorphology)
tells us about the extent of glaciers and ice sheets and sea level in the
past, ice cores record information about the conditions in
which the ice was formed and trap ancient air, corals give
us indications on sea surface temperature, and the shells of marine
organisms found in marine sediments tell us about past temperatures
and atmospheric CO2.
| Information | Time range | Areas | |
Tree rings
 |
Temperature, rainfall, wild fires | hundreds of years ago - present
(longest record extends 11,000 years) |
Continents (mostly Northern Hemisphere) |
Pollen
 |
Temperature, rainfall | several million years | Lake sediments, wetlands (mostly Northern Hemisphere) |
Geomorphology
 |
Extent of glaciers and ice sheets, sea level | billions of years | Worldwide |
Ice cores
 |
Surface temperature, snow accumulation, volume of continental ice, sunspot cycle, CO2, CH4, volcanic eruptions, sea-salt, wind speed (dust) | hundreds of thousands of years (longest record, in Antactica extends from 440,000 years ago to present) | Greenland, Antarctica |
Corals
 |
Sea surface temperature, sea level | each coral gives us information on hundreds of years, oldest corals recovered date back to 130,000 years ago | Tropical oceans |
| Marine sediments | temperature, salinity, ice volume, atmospheric CO2, ocean circulation, iceberg calving | up to 180 million years ago | Oceans |
Last
Updated:
02/11/2002