Sunday, June 23, 2013

What Causes Climate Change?

With all the massive publicity given to the issue of climate change, in particular global warming, over the last decade, it's an important exercise to look at the possible causes of this, particularly as it now has assumed major international political dimensions.

We can talk about “temperature change” in close parallel with “climate change” as temperature is the main driver of our climate system. Changes in temperature patterns produce a large variety of “knock on” effects such as variations in rainfall, humidity, snow, cloud cover and wind regimes around the world.

We know from so called proxy data that global temperatures have varied considerably across the known history of the Earth, over periods of millions of years. Climatologists studying such data as ice cores, rock structure, lake sedimentation, glaciers, fossils and tree rings have been able to build up at least a rough picture of our past temperature patterns.

From what we understand, the temperature time line of planet Earth looks something like this – where "mya" means “million years ago”. The reference line shown in red is the present global temperature and we see there are numerous instances where this line has been exceeded (warm epochs) and others when the reverse is the case, the times of Ice Ages.

The obvious question follows – what could be producing these variations?

In fact several likely causes have been identified, all of them entirely natural except one where climate scientists say there may be a human “footprint” – the so called anthropogenic effect.

This is a summary:

Milankovitch cycles: A sequence of “wobbles” of the Earths rotational axis and irregularities in its orbit around the Sun that occur over long periods and were first identified by the Serbian astronomer Milutin Milankovitch early in the 20th century. These cycles produce periods of warming and cooling in the Earth’s climate over time scales of several thousand years.

Serbian scientist Milutin Milankovitch - discoverer of the so - called Milankovitch cycles. (Image from Wikipedia commons)

Impact from extra terrestrial bodies: Large meteors and possibly even comets have collided with the Earth over past millions of years and these have strongly affected the climate. With a collision of this type a vast amount of debris is injected into the atmosphere and this can shield the surface of the Earth from solar radiation for extended periods, resulting in substantial cooling. It is suspected that one such collision around 65 mya produced a much cooler global climate for several thousand years, resulting in the demise of the dinosaurs.
A NASA impression of the giant collision of 65 mya that led to the extinction of the dinosaurs. (Image form Wikipedia Commons)

Continental drift: The world’s great continents actually move relative to each other at very slow rates of only a few millimetres a year. But over millions of years this movement becomes significant with different distributions of land and ocean resulting and changed patterns in ocean currents. These currents flowing around the world play a major role in transferring surface heat from tropical latitudes towards the polar regions and these have a major impact on climate patterns. The Gulf Stream, for example, produces much warmer weather for the United Kingdom and western areas of Scandinavia than areas of similar latitude in Canada. Disruption of these ocean currents, caused by continental drift, would certainly produce significant climate change.

Volcanoes: In a similar fashion to meteor impact, volcanic eruptions can inject massive amounts of dust into the atmosphere that result in solar shielding and global cooling. The 1815 eruption of Mount Tambora in Indonesia was a major cause of the so-called “Year without a summer” in 1816, when abnormally cold temperatures spreading across England and Europe produced widespread crop failures and famine.

The Mt. Pinatubo eruption in the Philippines, 1991. (Image from Wikipedia Commons)

Cosmic rays: These are high-energy particles that constantly bombard the Earth’s atmosphere. They are thought to originate from outside our solar system and could be produced by massive supernovae, or stellar explosions, from far distant stars. It has been speculated that the “gusts” and “lulls” in cosmic ray activity could produce climate change but this remains an area of controversy.

Variation in solar power: Observations in the power output of the Sun shows it to be variable with several known cycles. These include those of 11 years, 88 years, 208 years and 1000 years. There are probably more and these would have some type of impact on our temperature and climate system, although the specifics are far from clear

The Sun - power emitted varies over several different time scales (image from Wikipedia Commons)

Changing gas concentrations within the atmosphere: Our atmosphere contains a mixture of gases, with the so called “greenhouse gases “, including water vapour, carbon dioxide and methane, playing a major role. These gases prevent a total escape of solar energy back out into space and keep our global temperature warmer than it otherwise would be. Climatologists are worried that by the constant burning of fossil fuels we increase the concentration of carbon dioxide and thereby raise global temperatures. This mechanism for climate change, if real, is notably different to others in that it is produced by human activity rather than the natural processes already noted. This is known as the anthropogenic effect.
For information on how past climates have been measured go to:

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