The mini-moocs continue and this week we looked at past climate change, ancient and not so ancient. Looking far back into the formation of the solar system 4.5 Billion years ago we learn that the sun was much cooler than it is today and yet the earth was much warmer. There is evidence that the atmosphere had a high CO2 content and that blanket effect was responsible for the warmer climate. The question then became how did it change because if the earth had the same atmosphere today then as the sun grew brighter the world would become much hotter but in fact it is cooler.
The answer lies in the “feedback loops” and the carbon cycle. As the world warmed evaporation, cloud formation and then rain reacting with the atmospheric CO2 created a weak acid. This acid rain, in turn, reacted with the rocks changing silicates to carbonates, removing carbon from the air into the earth via the rainfall, cooling the planet somewhat and balancing the climate. The carbon cycle also has a neat little trick that allows us to gauge the state of the ancient climate. It works like this, the carbonate that formed as part of the carbon cycle is used, together with calcium, to make shells for tiny sea creatures. When they die the shells fall to the ocean floor and build up into a deep layer of sediment where they can be preserved for millions of years. Now comes the clever bit that belongs to science – the ratio of the oxygen isotopes can reveal the temperature of the ocean when the shell was formed. Its all on web of course and too complex for a blog.
But feedback loops can destabilise the climate as well as balancing it. In fact it is postulated that about 2.2billion years ago and again at 700mllion years ago the earth became covered in ice and snow creating “snowball earth”. This time the feedback loops, rather than balancing the system, created runaway cooling. The trigger that set off the extreme cooling may not be clear but once the cooling got underway the mechanism is easy to grasp. Quite simply dark surfaces absorb heat and white ones reflect it. As the snow and ice spread more of the suns heat was reflected so the earth was cooling and more snow and ice were formed so more heat was reflected so…..
So how did we escape from the freezer? It seems probable that although the earth was frozen volcanic action was still putting CO2 into the air. Without rainfall there could be no weathering of the rocks (which were deep under the ice anyway) so the CO2 built up to a point where it was warm enough to start the melt. Once started the exposure of the sea and land absorbed the heat and raised the temperature by the same mechanism that caused the freeze but put in reverse.
The not so ancient bit of the story looked at how climate records can be inferred from data like isotopes (http://www.giss.nasa.gov/research/briefs/schmidt_01/), ice cores (http://www.earthobservatory.nasa.gov/Features/Paleoclimatology_IceCores/) and tree rings (http://web.utk.edu/~grissino/principles.htm#1), how the current warming differs from ancient warming events (it is happening much much faster) the effects of volcanic activity and aerosols in the modern era and archeological evidence of climate change.
The impact of volcanic action is interesting because it involves aerosols which vary greatly in their effect. They can block sunlight and cause cooling and light coloured particles can reflect the heat but black carbon can increase warming as can increased CO2 that may be pumped into the atmosphere. All a bit of a lottery I think.