Monday, August 08, 2016

Could Recent Warming be Due to Natural Variation in the earth's climate? Part I A (very brief) history of the earth's climate

One explanation of recent global warming is natural variation in climate. The world has been warmer, colder, wetter, drier, and that what we are seeing is just part of that historic variation. So I’ve read a small book [1] and here goes …

According to the book “The climate system is very straightforward”. The key facts are that the equator gets much more heat from the sun per square meter than the poles, that the earth is spinning, and that the earth’s axis is at a small angle and rotates with respect to the sun giving us an annual cycle of seasons. Heat energy goes from the equator to the poles via the atmosphere and oceans to (partially) redress the differential in radiative energy. This has been the case since the creation of the earth, but climate has varied considerably over this time, and the main factor in driving this large scale change in climate has been the location of land mass on the earth’s surface. This determines how the oceans flow and distribute heat and has been the principal determinant in the earth’s climate history.

The current view is that there was a single supercontinent “Rodinia” formed 1100 million years ago, which broke up and reassembled to form “Pangea” about 300 million years ago. This started to fragment about 200 million years ago. About 100 million years ago Antarctica moved into position, but at this stage was warm and populated by dinosaurs. Finally about 50 million years ago the continents were in positions we recognise as being substantially the same as today.

The current position of the continents is the key determining factor in creating our polar-based energetic climate. When oceans had clear access to the poles ice forming at the poles was blown away from the poles and melted in the warmer latitudes, so polar regions remained temperate. About 2 million years ago a critical event happened. South America and North America joined at Panama. The closure of this gap cut off a key ocean flow from the equatorial regions to the poles. this reduced the ability to transport warmth to the heat-starved poles and brought in the period of the great ice-ages. The surrounding ocean takes heat away, so ice collects on the land-mass and adjacent ice shelves, and there is a temperature difference of 60C between equator and pole. In the northern hemisphere the pole is ocean but the surrounding ring of continents limits the movement south of ice and enables the ice to build up and a temperature difference of 40C results.[2]

When the continents first moved into the current configuration the world was a warmer place with much more carbon dioxide in the atmosphere. This period saw the creation of major mountain ranges, and it is believed that weathering of the mountains took carbon dioxide out of the atmosphere specifically through the reaction of carbonic acid in rain with silicates to form insoluble carbonates. This reduction in CO2 lead to cooling and a more familiar temperate climate. Overall CO2 levels have varied greatly over time but have “recently” decreased to historic lows – see the chart below.[3]

So, it's been hotter, colder, with much more CO2 but never less CO2. There are broad-brush explanations for most of this large-scale variation.

The next time-scale we need to look at is the more recent one of ice-ages, and I'll look at that next.

[1] “Climate. A very short Introduction” by Mark Maslin (Oxford)

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