Lecture 8 Historical Changes in Climate PDF

HISTORICAL CHANGES IN CLIMATE LECTURE 8  INTRODUCTION TO PALAEOCLIMATES  Global climates are generally stable with fairly predictable conditions occurring in a region from one year to the next  However, climatic conditions can and do fluctuate naturally  Natural fluctuations result from:...

HISTORICAL CHANGES IN CLIMATE LECTURE 8  INTRODUCTION TO PALAEOCLIMATES  Global climates are generally stable with fairly predictable conditions occurring in a region from one year to the next  However, climatic conditions can and do fluctuate naturally  Natural fluctuations result from:  Natural occurrences such as volcanic activity, which releases huge quantities of greenhouse gases and dust  Changing amounts of incoming solar radiation (sunspot activity)  Deviations from the perfect orbit that the Earth and other planets make around the sun Long-term climatic cycles caused by: Walker circulation (about every 3-5 years) Differences in intensity of sunspots (about every 11 years) Milankovitch cycles: differences in the Earth’s orbit and axial rotation due to planetary alignments in the solar system Ice ages, ‘Little Ice age’ (1300-1850) WHAT IS THE WALKER CIRCULATION  Two phases in the arrangement of convection zones and high pressure zones in Southern Hemisphere  Low phase means drought for South Africa, coincides with El Niño  High Phase means wetter years in South Africa, coincides with La Niña  Walker circulation was first described in Indian Ocean, with El Niño later described off the Pacific coast near Peru Milankovitch cycles Irregularities in Earth’s orbit causing long-term climatic changes HOW DO WE GET INFORMATION ON PAST CLIMATIC CHANGES ?  Scientists have studied earth's past climate extending back 106 years.  Generally, the further back in time, the less accurate For these time scales, known as geologic time, scientists have gathered clues from:  Ice cores the polar ice caps  Marine records  Pollen records  Fossil records  Tree ring records  Global climate patterns stretching back 740 000 years have been ICE CORES confirmed by a three kilometre long ice core drilled in the Antarctic  Inside the ice cores are tiny pockets of preserved air  Tell us the concentration of various gases and dust at the time the snow fell and was compacted.  Our planet has had eight glacial periods during the last 740 000 years, interspersed with brief warm spells – such as at present (interglacial)  Initial tests on gas trapped in the ice core show that current carbon dioxide (CO2) levels are higher than they have been in 800 000 years 9. INTRODUCTION TO CLIMATE CHANGE MARINE RECORDS  Deep sea cores from the ocean  Oxygen isotope analysis  The ratio of 18O to 16O in ice and deep sea cores is temperature dependent  During colder periods more 18O in the ocean, more 16O evaporates from the oceans  Dinoflagellates (phytoplankton), radiolarians (zooplankton), foraminifera (zooplankton) and diatoms (phytoplankton)  Specific habitat requirements  Temperature, pH, salinity etc MARINE PROTOZOA Radiolarian Diatom Dinoflagellate Foram PLANTS AS INDICATORS OF CLIMATE  Phytogeography: the study of the natural causes of the geographic distribution of plants  Palaeoclimatologists are interested in changes in plant distributions over time  Some plants are found only in specific climatic conditions  Temperature  Rainfall  Humidity etc PALYNOLOGY (POLLEN ANALYSIS)  Every plant species has a distinctive pollen shape  Botanists can identify from which plant type the pollen came  Often to species level  Sometimes only to family or genus level  Core into the ground  Subsampling of sediment  Pollen preparation  Counting  Dating of samples  Once pollen counts are completed, the palynologist will have a pollen spectrum PALYNOLOGY  Scientists can infer the climate of the layer being studied by relating it to the current climatic preferences of the same plants  For example:  A sediment layer with large amounts of Fynbos pollens (e.g. Ericacae) would imply a winter rainfall regime  A lot of water-lily pollen suggests a wet environment – such as a overlying lake.  Predominantly grass pollen with few tree grains could suggest a grassland PLANT AND ANIMAL MACROFOSSILS Plant Macrofossil – seeds, buds, capsules, Animal Teeth needles, wood TREE RINGS  Trees contain accurate evidence of the past  The growth layers make up rings in the cross section of the tree trunk  Record evidence of floods, droughts, insect attacks, lightning strikes, and even earthquakes  A tree ring consists of two layers  A light coloured layer in spring  A dark coloured layer in late summer  READING TREE RINGS  Wet, cool years: wider rings  Hot, dry years: narrower rings  Drought, or severe winter: narrower rings  Consistent widths: climate the same year after year