Global warming and natural climate change in the past
What the science says...
The usual drivers of natural climate change have shown little to no warming trend since the 70's.
It's a well established fact that climate changes naturally and sometimes dramatically. The pertinent question isn't "has climate changed in the past?" (of course it has) but "what is causing global warming now?" To begin to answer that, it's helpful to look at the major causes of natural climate change in the past.
Solar variations have been the major driver of climate change over the past 10,000 years. When sunspot activity was low during the Maunder Minimum in the 1600's or the Dalton Minimum in the 1800's, the earth went through 'Little Ice Ages'. Similarly, solar activity was higher during the Medieval Warm Period.
However, the correlation between solar activity and global temperatures ended around 1975. At that point, temperatures started rising while solar activity stayed level. This led a team of scientists from Finland and Germany to conclude "during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source." More on the sun & global warming...
Earth's climate undergoes 120,000 year cycles of ice ages broken by short warm periods called interglacials. The cycle is driven by Milankovitch cycles. Long term changes in the Earth's orbit trigger an initial warming which warms the oceans and melts ice sheets - this releases CO2. The extra CO2 in the atmosphere causes further warming leading to interglacials ending the ice ages.
For the past 12,000 years, we've been in an interglacial. The current trend of the Milankovitch cycle is a gradual cooling down towards an ice age.
Volcanic eruptions spew sulfate aerosols into the atmosphere which has a cooling effect on global temperatures. These aerosols reflect incoming sunlight, causing a 'global dimming' effect. Usually, the cooling effect lasts several years until the aerosols are washed out of the atmosphere. In the case of large eruptions or a succession of eruptions such as in the early 1800's, the cooling effect can last several decades. Strong volcanic activity exacerbated the Little Ice Age in the 1800's.
The usual suspects in natural climate change - solar variations, volcanoes, Milankovitch cycles - are all conspicuous in their absence over the past 3 decades of warming. This doesn't mean by itself that CO2 is the main cause of current global warming - you don't prove anthropogenic warming by eliminating all other options. But the primary causes of commonly cited climate change in the past have played little part in the current warming trend.
As for CO2, empirical observations show that CO2 has a warming effect as a greenhouse gas, CO2 is increasing in the atmosphere and the expected warming you would get from greenhouse gases is occuring. Any alternative theory that found a different cause of global warming would also need to explain why the expected (and observed) warming from CO2 has not eventuated.
|© Copyright 2009 John Cook|