Possible Causes for Climate Changes

http://www2.sunysuffolk.edu/mandias/lia/possible_causes.html
Sunspot Variation
Because the sun is Earth's greatest source of energy and is the driving force behind its atmospheric circulation, any variation in solar output will influence the weather. Scientists have observed that the number of sunspots on the surface of the sun has been determined to correspond to solar output variability. More sunspots correspond to a higher solar energy output while fewer sunspots correspond to a lower solar output. A record of sunspot numbers has been recorded through time by various indicators including naked eye observations, auroral reports, and C14 isotope concentrations in tree rings (Schaefer, 1977.) Fig. 8 shows that during the MWP there was a high number of sunspots referred to as the Medieval Maximum, while during the LIA there were two periods of very low sunspot numbers called the Spörer Minimum and Maunder Minimum. Although a direct link has not yet been established between sunspot variability and climate change, the data is highly suggestive.

Volcanic Eruptions
Ash and other small particulate matter injected into the stratosphere can effectively reduce incoming solar radiation received at the earth's surface. Sulfur compounds from eruptions condense into very tiny sulfuric acid droplets that form clouds which may stay suspended in the stratosphere for years, further reducing incoming sunlight (Pollock et al., 1976.) Fig. 9 illustrates the process.

Large eruptions at low latitudes can cause the greatest global climate change. Weaker eruptions only send their eruptive materials into the troposphere where weather processes quickly remove them and high latitude eruptions only send their materials into one hemisphere. The explosion of Mt. Tambora in 1815 led to the year 1816 being called "the year without a summer" across much of Europe. The eruption of Mt. Pinatubo in 1991 provided a good example of how a large low-latitude eruption can quickly influence global climate.

Determining the Climate Record

http://www2.sunysuffolk.edu/mandias/lia/determining_climate_record.html
Tree Ring Data
By studying the width of tree rings (dendrochronology) history of annual tree growth can be determined. Individual rings represent individual years while the width of each ring shows the growth rate during that year. The width of rings from trees found at higher altitudes and higher latitudes is generally a function of temperature where wide rings indicate warm years and narrow rings indicate cool years (Lamb, 1995.) Because the pattern of rings is similar to a fingerprint, dendrochronologists are able to construct a chronology by matching similar ring patterns found in living trees, construction timbers, and fossil trees. Fig. 3 shows how a fossil timber in a river bed is dated by noting matching ring patterns found in building timbers and in living trees. LaMarche (1974), Lamb (1995), and Baillie (1982) have cited various tree-ring data that indicate warmer temperatures during the MWP and cooler temperatures during the LIA. Fig. 4 shows tree-ring widths of bristle cone pines in California during the end of the MWP [Medieval Warm Period] and through the LIA [Little Ice Age] that suggest warm and cool climates during the MWP and LIA, respectively.




Figure 4: Tree-ring widths vs. time from California Bristle Cone Pine trees. (Source: Tkachuk, 1983)

Ice Returns as Greenland Temps Plummet

Jan 17, 2008


On Disko Bay in western Greenland, where a number of prominent world leaders have visited in recent years to get a first-hand impression of climate change, temperatures have dropped so drastically that the water has frozen over for the first time in a decade. “The ice is up to 50cm thick,” said Henrik Matthiesen, an employee at Denmark’s Meteorological Institute who has also sailed the Greenlandic coastline for the Royal Arctic Line. ‘We’ve had loads of northerly winds since Christmas which has made the area miserably cold.’ Matthiesen suggested the cold weather marked a return to the frigid temperatures common a decade ago.

Temperatures plunged to -25C earlier this month, clogging the bay with ice and making shipping impossible for small crafts, according to Anthon Frederiksen, the mayor of the town of Ilulissat, where Disko Bay is located. The mayor cautioned against thinking that the freezing temperature indicated that global warming claims were overblown. He noted that a nearby glacier had retracted more in the past two decades than in recorded history. But he noted “‘We Greenlanders have acclimated to changing conditions over the past 1100 years,” said Frederiksen. “Temperatures change at regular intervals.”


NASA satellite image of Greenland. You clearly see the ice cap covering almost entire Greenland even in this non-winter photo. Only in summer are the coastal areas free of snow. On the far left you can see Baffin Island.