Friday, June 18, 2010

Six Minutes to Midnight - Part 3

Enhanced Greenhouse Effect

So as described in the previous section the greenhouse cycle is a finely balanced mechanism for the transfer of heat from the sun to the atmosphere and the ground here on Earth. What effect, if any, does human activity have on this cycle?

In the debate of climate change the key issue is whether human activity, such as the burning of fossil fuels which adds to the greenhouse gases, enhances this greenhouse effect. The effect of greenhouse gases is clearly understood but what is not understood is what affect adding to these gases will have on the climate. Will the climate act in a linear fashion or is there a threshold limit where we can trigger a runaway greenhouse effect?

There is clear evidence that industrial activity has added considerably to the concentration of greenhouse gases in our atmosphere.  The main change in these gases is the amount of carbon dioxide; current estimates put it at a 40% increase over the natural level since the pre-industrial era.

There is also the consideration of the feedback effect. An increase in global temperatures due to carbon dioxide increases ocean temperature, which in turn increases surface evaporation. This increase in evaporation leads to more water vapour which itself increases temperatures. This is an example of a positive feedback. These feedback effects can come in two forms:
  •   Positive feedback – drives global warming
  •   Negative feedback – reduces global temperatures

This makes the prediction of the average global temperature trend very difficult. To help illustrate this problem let’s take three simple scenarios:
  •  No change
  •  Double carbon dioxide levels
  • Double carbon dioxide + feedback effects

In the no change scenario the amount of solar energy incoming is balanced by the amount radiated back into space global average temperature remains at 15 oC.  If we double just the carbon dioxide we expect a global increase in the region of 1.2 oC. Now, using the same scenario, we add in the possible feedbacks and their net effect, causing an estimated increase of over 3 oC above the global average. This last scenario should cause great concern. A change such as this can lead to large scale changes to the environment and climate mechanics.

What is not clear from all the current climate models is whether we are reaching a tipping point or have we already passed it. The question of what is the lifetime of the current levels of the greenhouse gases in our atmosphere are, is one of the most important points of debate currently.  If we were to stop right now putting excess greenhouse gases into the atmosphere, how long would recovery take? The short answer is we don’t know. There are many feedback mechanisms in affect here, some of which we don’t fully understand and others which may be about to enter the fray.

The one undeniable fact is that carbon dioxide is very much linked with global temperatures; this link will be further explored in the next section where we return to ancient climates and see how changes in greenhouse gases are linked to changes in the climate. 

Past Climates

There is very clear evidence that the average global temperature has seen a rising trend over the last 100 years as the figure below illustrates.

Figure 3

Figure 3 represents the changes in average global temperature over the preceding 130 years. The zero on the scale represents the average temperature over the period of 1961-1990. There is a very obvious trend in the data presented here. However, as any good scientist knows, data can be misleading unless it is put in proper context. A good argument against such a set of data could be that we just may be in a current natural warming period. To counter such an argument, it is necessary to look at the climate over a far greater scale; to see the current climate in terms of a geological time scale.

To look back at previous climates in the time period of tens of thousands of years ago we can’t rely on direct measures like we have for the data in figure 3. A valuable source of evidence for past climates is the record which has being maintained by the ice. The ice which resides in Greenland and Antarctica provides scientists with a snapshot of previous climates. As the snow fell it trapped within it small pockets of air. As more snow fell it compacted into ice, and as more layers of ice were added it became compacted and formed ice sheets kilometres thick. Ice cores drilled into these ice sheets can reveal past climates. In one such recent ice core sample, data dating back almost 500,000 years was recovered. From the air trapped in these cores, scientists are able to extrapolate the carbon dioxide concentrations in the past climate. Also, from the isotopes of hydrogen and oxygen present, an estimate for the temperature can be calculated. These isotopes are very sensitive to small changes in temperature so can be used as a valid indicator of past global temperatures. 

Figure 4

As can be seen in figure 4, there is a very good correlation between carbon dioxide levels and temperatures in Antarctica. From the data the first obvious thing which pops out is the cyclic nature of the graph. There is a major cooling trend which appears to have a period of about 100,000 years. To account for this variation we have look beyond the terrestrial and examine our place in the solar system and more specifically our orbit around the sun.

The Earth’s orbit is elliptical by nature and this orbit varies periodically every 100,000 years. This variation manifests itself in the eccentricity of the orbit. The eccentricity of the orbit relates to the ratio of the maximum orbital distance compared to the minimum.  This precession of the orbit can account for this variation in the data. We can be quite confident in this proclamation, as we can be almost certain that the average solar radiation over this period was more or less constant. 

There are of course other disturbances in the data which we can also attribute to other periodic changes in the Earth’s orbit (see figure 5). The Earth's axis of rotation is tilted with respect to its orbital axis. This angle of tilt varies periodically at a rate of 41,000 years. This cyclic event can be seen in the data as a perturbation in between the major peaks.  There is also a smaller periodic shift in the Earth’s closest approach to the Sun (perihelion), the period of this event is about 23,000 years. 

Figure 5

All of these changes in axis orientation does not vary the amount of solar radiation incident on the Earth, but rather changes its distribution. For example, if the Earth’s Polar Regions are tilted further away from the Sun's rays, we can potentially enter an ice age, whereas if the Polar Regions are tilted towards the Sun, we enter a period of increased global temperatures. This effect is due to the seasonal variation we get. For example if the poles are tilted further away than the norm, the winter ice formed can spread much further south than normal. This further extension of the ice sheet can reduce global temperatures; the ice reflects more of the incoming rays so further reduces global temperatures.  This is an example of a feedback effect. This connection and subsequent theory relating to this phenomenon was developed by Yugoslavian climatologist Milutin Milankovitch in 1920.

An obvious question to ask now is: If we do enter an ice age what causes it to end? The natural cycles of the precession of Earth’s axis alone cannot undo a potential massive ice age. The Earth experiences smaller ice ages but these can be overcome as the Earth warms due to the natural cycle. As mentioned earlier ice is a very good reflector so even with the poles tilted towards the sun the incoming rays would increase melting by only a small amount. To find an answer to this almost paradoxical question we must again return to the greenhouse gases. As written earlier these gases act as a blanket around Earth. But the question arises where do these gases originate from?

These gases have a variety of sources such as; the animal life which continually takes in oxygen and breaths out carbon dioxide and the death of vegetation which releases carbon dioxide into the atmosphere, but these contributions are very small and do not account for the gas we see.  The Earth is highly geologically active with several active volcanoes at any one time. These volcanoes emit large quantities of these greenhouse gases and aerosols into the upper atmosphere.

A recent eruption in June 1991 of Mount Pinatubo released vast quantities of sulphur dioxide into the air, which in turn triggered a worldwide global cooling of about 0.5 K. This eruption did indeed cause a global cooling but under the right circumstances it can have the opposite effect. Such an event was believed to be the factor in bringing the Earth out of a major ice age which occurred around 700 million years ago. This ice age is referred to as the “Snowball Earth” theory, a paper first published in 1992 by Joseph Kirschvink. In his paper he was analysing the biology of this time period (Proterozoic Era). There was a mass extinction of the world’s species at this time and data from the geological recordings hinted that there was extensive ice covering the Earth’s surface which extended to the Equator.

The trigger for this event is unclear, it could be the culmination of the perturbation of the Earth axis relative to its orbit and a lower than normal concentration of natural greenhouse gases in the atmosphere. As the ice gathered it would reflect more sun light back and as a result we had a runaway cooling affect. So what triggered our release from this icy tomb?

It is believed that a major volcanic eruption triggered a series of events which released us from this icy tomb. The volcano that erupted released vast amounts of carbon dioxide and other greenhouse gases into the air. This massive release blanketed Earth and in doing so greatly increased the heat retention of the Earth. This in turn heated up the atmosphere which started to melt the ice sheets and liberated vast quantities of water. This water was loaded with rich nutrients which triggered a massive growth in the algae present in the world’s ocean at that time. This algae, which was now free to flourish at an unprecedented scale, released oxygen which aided new life and vegetation to grow. This positive feedback pulled the Earth from this ice age.

So as we can see there is a natural cycle of ice ages and warming periods which can happen in extremes. Understanding these past climates can help us understand how changes in greenhouse gases can effect climate change.

As can be seen in figure 4 there have also been eras where the global temperatures have been higher than they are now but what should be noted is the levels of carbon dioxide present as those times and compare those levels to what we have now. In the last interglacial period about 125,000 years ago global temperature were about 3-4 K higher than averages now but the carbon dioxide concentration was less than 300 parts per million (ppm), todays figure is about 385 ppm.

This figure should be a very worrying value. This value is outside the range of previous records so we have no recorded levels to compare this value to. This makes any prediction about future global climate trends very difficult to make. We could potentially see a runway greenhouse effect which could have drastic consequences for not only our civilisation but all life.

A possible indicator of what may happen is to look beyond our planet and out into the rest of the solar system. Our two neighbours are our best indicators of possible futures, both Venus and Mars have atmospheres which display similar greenhouse effects.

Our nearest neighbour Venus which is similar in size to the Earth has a thick atmosphere rich in carbon dioxide which accounts for about 96% of its composition. Its atmosphere is almost completely opaque to an observer on its surface. This thick blanket absorbs and retains far more heat than our own atmosphere. This thick atmosphere results in Venus having a surface temperature in excess of 500 oC. This high temperature means that water could never have existed as it does here on Earth. Water plays a key role in acting as a moderator, it has the ability to act as a sink for carbon dioxide and so can absorb excess carbon dioxide.

Mars is the polar opposite to Venus, Mars has a similar atmospheric composition but its atmosphere is far less thick. Due to its distance from the Sun it receives far less solar radiation and due to its thin atmosphere it retains far less heat. This means although Mars has a greenhouse effect in operation it can’t retain enough heat for the surface temperature to allow liquid water. This may not have been the case in its geological past; Mars lacks a magnetic shield such as the one which protects the Earth from the harsh solar wind. Mars’ core is no longer active so no magnetosphere is created, without this protection the solar wind can strip away gas from the planet. So with this lower temperature water could exist so it could help halt any over powering effects of the greenhouse cycle.  

Figure 6

From figure 6 we can see how water can act as a moderator of global temperatures. Venus was too hot so its greenhouse effect entered what is referred to as a runaway state. This is not possible on the Earth. While on Mars the greenhouse is halted as water can exist in its solid form ice. We are highly fortunate on the Earth that we habit a zone in the solar system where water can exist in a liquid form. This enables the Earth to enjoy a temperature climate which is favourable to the formation of complex life forms.