Understanding Climate Change #2

It’s week two of Climate Change: Challenges and Solutions and this week is a focus on how the Earth’s climate has changed over the past 4.5 billion years. Feedbacks and their effects are explored some more too. It’s a pretty information-heavy week with lots of theory and facts. I’ll provide a list of the most important points at the end as I did before.

Snowball Earth

As you can probably tell, a snowball earth is one which is completely covered in ice and snow, as opposed to just the poles. The video below from Science Channel gives a great overview and introduction to this process. Below that is a more in-depth explanation should you wish to know more. For an even more in-depth look the Snowball Earth website provides plenty of information.

The feedbacks explored last week are the key to understanding why a snowball Earth occurs. While these feedbacks usually act to stabilise the climate, they can have the opposite effect under certain conditions and lead to a destabilised one. There are a number of stages in this process:

Reduced greenhouse gasses

The reduction in atmospheric greenhouse gasses is the initial step, which leads to a temperature decrease. Carbon dioxide and methane are the ones to focus on here.

To understand why carbon dioxide levels decrease it is essential to understand the carbon cycle and the effects of silicate weathering. Large quantities of carbon dioxide are fed into the atmosphere by way of volcanic eruptions. To oppose this, carbon dioxide is removed by this silicate weathering. Carbon dioxide reacts with the water in the atmosphere to form carbonic acid, resulting in acid rain. This acid rain dissolves silicate rocks, forming bicarbonate ions that are carried into the oceans. The ions form calcium carbonate, which is used by organisms to form their shells. Eventually, these fall to the ocean floor and form carbonate rocks under high pressure. Hence a reduction in atmospheric carbon dioxide.

A reduction in methane levels can be the result of an increase in the amount of oxygen in the atmosphere. As oxygen levels rise it causes increased oxidation of methane (hence decreasing its abundancy).

As carbon dioxide and methane levels drop there is a reduction in the greenhouse effect and temperature falls.

Ice cover increases

Falling temperatures lead to greater areas of ice cover. The ice albedo effect then takes hold. More solar radiation is reflected away from the Earth’s surface, further cooling the planet.

Usually the climate system re-stabilises before anything overly bad happens, but if cooling is sufficient for ice cover to reach about 30 degrees’ latitude, a tipping point is reached. Here the feedback becomes self-sustaining, causing ice to spread all the way to the equator and result in an Earth that is completely covered in snow and ice – a snowball Earth.

Reversal

Considering that there have been snowball Earth’s in the past, there obviously must be a way for the process to reverse. I mentioned volcanic activity earlier and during a snowball Earth event this activity continues. Carbon dioxide builds up in the atmosphere since it cannot be removed by the process of silicate weathering. Eventually there is enough carbon dioxide to increase the temperature enough for the ice to melt. The albedo feedback works again – as more ice melts and exposes dark land and ocean more warming occurs. Eventually the ice recedes back to the poles and the system re-stabilises as the silicate weathering process is able to function normally again.

16.2

Photo: tiny bubbles by frankieleon (CC BY)

How is today’s warming different from the past?

This is an interesting question for those people who are still in doubt over whether today’s climate change is actually a result of human behaviour or is just another natural occurrence.

The climate has changed many times over the past 4.5 billion years. Global temperatures have risen to well beyond their current values and likewise have plummeted during ice-ages and snowball Earth events. We can track these changes by looking at the evidence found in ice-cores, coral reefs, sedimentary rocks, tree rings and fossilised pollen.

  • Ice cores provide a record of atmospheric composition. As the water freezes it traps tiny bubbles of air within it. These bubbles can be analysed to find out the composition of gasses present in the atmosphere at the time the ice froze. The texture of the fallen snow also changes as a result of sunlight exposure providing more information.
  • Sedimentary rocks and coral reefs can be analysed by looking at the atomic structure of oxygen atoms contained within calcium carbonate molecules. Oxygen-16 and oxygen-18 are two isotopes of oxygen. Looking at the ratio of these two isotopes gives an indication of the temperature of the water in which the calcium carbonate formed.
  • The growth of trees is affected by temperature and sunlight exposure; hence a tree’s rings give an indication of conditions as the tree was growing.

All these methods build up a picture of the past climate and allow us to compare it to today’s data. What scientists have found is that current global temperature rise is occurring at a much faster rate than ever before – this is the key difference. Currently, the rate of warming is 20 times greater than that of previous temperature increases. It is overwhelmingly likely that human behaviour is the reason for this sudden spike in global temperature.

Summary

An interesting look back at how the climate system has changed over the course of the Earth’s 4.5-billion-year history. In summary:

  • In perhaps the most severe of Earth’s climate shifts, there were times where it was covered completely in ice and snow. These ‘snowball Earth’ periods have occurred three times in the past.
  • Snowball Earth periods are a result of feedbacks that act to de-stabilise the climate system. A reduction in the abundancy of greenhouse gasses in the atmosphere leads to cooling and greater areas of ice cover. Eventually a tipping point will be reached and ice will converge at the equator.
  • Climate conditions in the past can be discovered by gathering data from ice-cores, coral reefs, sedimentary rocks, tree rings and fossilised pollen.
  • Global temperature rise is occurring at a much faster rate today than it ever has in the past. This is evidence that human activity is causing the climate to shift.

There you have it, week two is done and dusted. Yes, there was a lot of theory so if you read it all I congratulate you. If you’re super sciencey you can get pretty deep into this stuff. Personally I find the isotope analysis for palaeoclimatology highly interesting so that would be my recommendation of further reading. See you next week!

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Jack Nokes Written by:

Environmentalist and engineering student.