Re: How has the climate changed in the past 100 years? 10,000years? 1,000,000yr

The question was: "How has the climate changed in the past 100 years? 10,000 years? 1,000,000 years?"

Past climates are not my speciality, so in what follows there may be some errors in names of geologic eras etc. That system of nomenclature is more than I can absorb to write this note.

The first question is the easiest to answer. I can give you the official word on it. In 1995 the United Nations organized the Intergovernmental Panel on Climate Change (IPCC). The members of the IPCC (many scientists from countries all around the world) agreed to make a statement about current climate, climate change, and human impact on climate. Summarized, this is what they said.

	Climate has changed in the past century.  

	Global mean surface air temperature has increased by 
	between about 0.3 and 0.6 C since the late 19th century.  

	Recent years have been among the warmest since 1860, despite 
	the cooling effect of the eruption of Mount Pinatubo in 1991.  

	Nightime average temperatures over land have increased more  
	than daytime temperatures.

	There have been regional changes.  For example, the recent 
	warming has been greatest over the mid-latitude continents 
	in winter and spring with a few areas of cooling, such as the 
	North Atlantic ocean.  Precipitation has increased over land 
	in high latitudes of the Northern Hemisphere, especially
	during the cold season.

	Global sea level has risen 10 to 25 cm in the past 100 years 
	and much of this rise may be due to the increase in global mean 
	temperature.

	There was insufficent data to determine whether there was a 
	change in the number of extreme events (ie. storms) in the 
	last century.

	Finally, the balance of evidence suggests a discernible 
	human influence on global climate.  Note that this is a vague 
	statement (that was intentional).  We know that we (human beings)
        can affect the climate, but exactly what the changes will be are 
	still largely unknown.

Now we move on the the recent past (geologically speaking). 10 000 years ago the Earth was coming out of a prolonged glacial period, when the mean temperature was much cooler than the present and great ice sheets covered much of North America and Fennoscandia. This transition lasted several thousand years, and represents natural climate variablity on long timescales. Basically, the mean global temperature increased until it was a bit warmer than present day conditions. Most of the glaciers melted because of this warming, causing the ocean level to rise. So conditions 10 000 years ago were not really that much different from today. One might actually argue that the change 10 000 years ago was actually a transition to the present climate.

The figure below shows the temperature at Greenland (actually it shows the difference from present day mean conditions, zero degrees on the y-axis) obtained from what is called a proxy record. The data comes from an ice core drilled in the Greenland ice cap. The temperature can be obtained from a ratio of oxygen isotopes obtained from each layer of the ice core. The figure shows the high degree of variablity that the recent climate has experienced (at least at this one northern location). The record covers 250 000 years. The current climate is at the 0 year mark (approximately).

It is believed that the transitions from major glacial (cold) to interglacial (warm) epochs are caused by the Milankovitch cycles. These cycles are periodic changes in the Earth's orbit around the sun. There are three important cycles that take place on 100 000, 23 000, and 4 000 year timescales. It is believed that these cycles were especially important during the glaciation cycles of the last million or so years.

For the more distant times I turned to a book called The Earth's Climate: Past and Future by M.I. Budyko. According to Budyko the climatic conditions of the Pleistocene (from 1.8 million years ago to the beginning of the Holocene, about 10 000 years ago) are more thoroughly studied than any earlier era. What is known is basically that the climate differed greatly from the earlier Mesozoic (230 to 65 million years ago) and Tertiary (65 to 1.8 million years ago) eras. The Mesozoic was warm. Most of the Earth was comparable to today's tropical temperatures. The poles were cool though still warmer than current conditions. In the mid-Tertiary, a climate similar to the current one at mid-latitudes began to appear and distinct temperature zones became more common. Seasonal snow cover was likely in high-latitudes and the continental climate became more extreme away from oceans. A general cooling trend persisted into the Pleistocene where the great ice sheets began to grow and recede. That brings us back to the present day again.

I hope that helps to answer your questions. I am sure that there are good reference books in your local library that will fill in many of the details I have glossed over. If you live in a city with a university, there is sure to be a lot of reference material at its library there. If I have not given you the answer that you wanted, please be sure to send me a message.

My email address is ewiebe@ocean.seos.uvic.ca.